US20090165903A1

US20090165903A1 - Material Having Ultrafine Grained Structure and Method of Fabricating Thereof

Info

Publication number: US20090165903A1
Application number: US12/295,640
Authority: US
Inventors: Hiromi Miura
Original assignee: Individual
Current assignee: University of Electro Communications NUC
Priority date: 2006-04-03
Filing date: 2007-04-03
Publication date: 2009-07-02
Also published as: WO2007114439A1; JPWO2007114439A1

Abstract

In the present invention, there is provided a fabrication method of a material having an ultrafine grained structure characterized by including a step of providing a metal or an alloy having a stacking fault energy no greater than 50 mJ/mm²and a step of introducing deformation twins having a twin interval no greater than 200 nm into structures of the metal or the alloy by processing the metal or the alloy. Further, according to this method, a material having an ultrafine grained structure characterized in that twins are included in a crystal structure and the twins have a twin interval no greater than 200 nm.

Description

TECHNICAL FIELD

The present invention relates to a material having an ultrafine grained structure and a method of fabricating thereof.

BACKGROUND ART

As for methods of increasing mechanical strength of metals or alloys, a method of forming crystal grains inside the structures of a material into fine sizes is known. In addition, forming crystal grains into fine sizes has an advantage of facilitating processability of materials. Therefore, various studies on forming crystal grains into fine sizes have been made in the past. The most typical method of forming crystal grains into fine sizes is referred to as a thermomechanical treatment. With this method, crystal grains can be formed into fine sizes by thermally treating a processed material under various conditions. For example, one proposed method of forming crystal grains into fine sizes is performed by recrystalling a hot-rolled copper alloy at a temperature of approximately 300° C. through 400° C. (see International Publication WO 2004/022805). Another proposed method of forming crystal grains into fine sizes is performed by annealing a hot-rolled or a cold-rolled iron type metal at a temperature of approximately 750° C. However, the smallest crystal grain diameter which can be attained by using these methods of thermally treating processed materials is at best approximately 1 μm.
Recently, a method referred to as severe plastic deformation has been drawing attention as a method for fabricating ultrafine crystal grains A representative example of the severe plastic deformation method is an ECAP (Equal Channel Angular Press) method and an ARB (Accumulate Roll Bonding) method.
The ECAP method is performed by repeating the steps of forcing a metal material (referred to as a “billet”) through an L-shaped die channel and pressing out the metal material from an opening. This method enables crystal grains of the metal material to be formed into ultra-fine sizes without changing the shape of the metal material (see International Publication WO 2004/022805). The ARB method is performed by repeating numerous times the steps of rolling a sheet material to approximately 50%, cutting the sheet in half, stacking the two halves, and rolling the stacked sheets. By performing this series of steps, crystal grains of a material can be formed into ultra-fine sizes.
Patent Document 1: International Publication No. WO 2004/022805

Patent Document 2: Japanese Laid-Open Patent Application No. 62-182219

Non-Patent Document 1: R. Z. Valiev, R. K. Islamgaliev, I. V. Alexandrov, “Material Science” Vol. 45, p 103, 2000

Non-Patent Document 2: Nobuyasu Tsuji, “Iron and Steel”, vol. 88, p. 359-369, 2002

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

However, the ECAP method is unsuitable for industrial fabrication of ultra-fine crystal grains since the method requires many steps and is unable to manufacture very long materials. Although the ARB method is able to obtain crystal grains having a grain diameter of approximately 0.1 μm in a sheet-thickness direction the crystal grains are bulky with respect to an in-plane direction of a rolled sheet material. Thus, a material having a homogenous equiaxial ultra-fine grained structure cannot be obtained. This leads to problems such as heterogeneous distribution of strength of a material or inability to obtain a material having a desired strength. This method also requires many steps and is unsuitable for industrial production. Therefore, there is a demand for an ultra-fine crystal grain fabrication method that can easily form an equiaxial homogenous ultra-fine grained structure and increase material strength.
In view of the problems described above, the present invention provides a material having a high strength ultra-fine grained structure and a method that can easily fabricate such material.

Means of Solving the Problems

The present invention provides a material characterized by having an ultrafine grained structure including a metal or an alloy that has a stacking fault energy no greater than 50 mJ/mm², the material having twins included in a crystal structure, the twins have a twin interval no greater than 200 nm.
By fabricating a material having such an ultrafine grained structure, the maximum strength of the material can be improved.
Throughout this application, it is to be noted that a deformation twin is included in the term “twin”.
Further, the present invention provides a material characterized by having an ultrafine grained structure including a metal or an alloy that has a stacking fault energy no greater than 50 mJ/mm², the material including recrystallized grains having a crystal grain size ranging from 20 nm to 600 nm. By fabricating the structure of a material into such recrystallized structure, a material having high strength and homogenous ultra-fine crystal grains can be obtained.
Further, the present invention provides a fabrication method of a material having an ultrafine grained structure characterized by having: a step of providing a metal or an alloy having a stacking fault energy no greater than 50 mJ/mm²; and a step of introducing deformation twins having a twin interval no greater than 200 nm into structures of the metal or the alloy by processing the metal or the alloy. With this method, many deformation twins can be introduced into the structures of the material, and crystal grains can be formed into an ultrafine size by using the intersection of the deformation twins.
Here, the step of introducing the deformation twins may include a step of performing a multidirectional forging process (hereinafter also referred to as “MDF process”) on the metal or the alloy at a temperature no greater than room temperature. In this method, processing is simple. Thus, a material having an ultrafine grained structure can be fabricated with this simple process.
Particularly, the multidirectional forging process may include a step of performing a forging process on the metal or the alloy at a strain rate no less than 1×10⁻⁴/s. An MDF process at a high strain rate can increase deformation resistance of the material. Thus, deformation twins can be easily introduced into the material.
Alternatively or additionally, it is preferable for the temperature no greater than room temperature to be no greater than an absolute temperature of 223 K. An MDF process at such an ultra-low temperature can easily increase deformation resistance of a target process material. Thus, the same effect as increasing the strain rate can be easily obtained. Therefore, a material having an ultrafine grained structure can further easily be provided.
A step of performing an annealing process on the multidirectional-forged metal or alloy may also be included. Thereby, the structures of the material after forging can be made homogenous.
In an alternative method, the step of introducing the deformation twins may include a step of performing a rolling process on the metal or the alloy at a temperature no greater than room temperature. With this method, shear force can easily be applied to the material, and high density deformation twins can be relatively easily introduced into structures of the material. Therefore, in a case of applying this method, crystal grains can be formed into ultrafine size more easily than the MDF process.
A step of performing an annealing process on the rolled metal or alloy may also be included. Thereby, the structures of the material after forging can be made homogenous.
It is preferable for the step of performing the annealing process to include a step of performing an annealing process on the metal or the alloy at a temperature no greater than 0.5×Tm, wherein Tm is a melting point of the metal or the alloy. By performing the annealing process at this temperature, the structures can be made homogenous without causing the ultrafine grains obtained after forging or rolling to become bulky.
The step of performing the rolling process may include a step of rolling the metal or the alloy at a rolling rate no less than 5×10⁻¹cm/s. Because deformation resistance can be increased by increasing the rolling rate, many deformation twins can be introduced into the structures of the material.
The step of performing the rolling process may include a step of rolling the metal or the alloy so that a final draft becomes no less than 20%. Because deformation resistance can be increased by increasing the draft, many deformation twins can be introduced into the structures of the material.
Particularly, it is preferable for the step of performing the rolling process to include a step of performing a rolling process on the metal or the alloy at a temperature no greater than an absolute temperature of 223 K. By performing the rolling process a such an ultra-low temperature, the deformation resistance of the material can be increased. Therefore, many deformation twins can be introduced into the structures of the material without increasing the rolling rate and/or the draft during rolling. Thus, a material having an ultrafine grained structure can be provided more easily.
Further, the present invention provides a material having an ultrafine grained structure characterized by including: a first packet having a group of layered twins oriented substantially in a first direction, and a second packet including at least one of the twins inside the first packet that has a group of layered twins oriented substantially in a second direction, the first and second directions forming an angle other than 60 degrees. Here, a “packet” refers to a group of layered twins arranged towards the same crystal orientation as described below.
Further, the present invention provides a material having an ultrafine grained structure characterized by having: a first structure, the first structure including a first packet having a plurality of groups of layered twins oriented substantially in a first direction and a second packet including at least one of the twins inside the first packet that has a group of layered twins oriented substantially in a second direction; a second structure including recrystallized grains of the first packet; and a third structure including recrystallized grains formed of a plurality of layered twins arranged substantially in the same direction that are included in a single crystal grain.
The material having the ultrafine grained structure may be brass.

EFFECTS OF THE INVENTION

According to the present invention, a high strength material having an ultrafine grained structure is provided. Further, it is possible to relatively easily obtain such a material having an ultrafine grained structure.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram showing an example of a material having an ultrafine grained structure according to the present invention;

FIG. 2 is a schematic diagram for describing an MDF process method;

FIG. 3 illustrates a stress-strain curve for a copper-based alloy material containing 30 mass % zinc fabricated by a method of forming ultrafine grain crystals (MDF process method) of the present invention according to the present invention;

FIG. 4 illustrates TEM photographs showing structures of a copper-based alloy sample containing 30 mass % zinc after being subject to an MDF process at 77 K (a) and 300 K (b), respectively, and a schematic diagram showing an enlarged view of a packet portion of the photograph (b);

FIG. 5 illustrates a structure diagram (OIM map diagram) of a copper-based alloy sample containing 30 mass % zinc annealed for 8 hours at 503 K after an ultra-low temperature process according to a crystal orientation distribution analyzing apparatus;

FIG. 6 is schematic diagram showing an example of a configuration of an apparatus used for performing a method of forming ultrafine grain crystals according to the present invention;

FIG. 7 illustrates an TEN observation image of a structure of a copper-based alloy containing 30 mass % zinc after being MDF processed with a true strain rate of 1×10⁻³at a temperature of 77 K.;

FIG. 8 is a diagram showing changes of Vicker's hardness of a copper-based alloy material containing 30 mass % zinc annealed at temperatures of 503 K, 523 K, and 543 K after the material is rolled 60% at a temperature of 77 K.;

FIG. 9 are photographs of a copper-based alloy containing 30 mass % zinc annealed for 1000 seconds at a temperature of 523 K after being rolled 60% at an ultra-low temperature of 77 K, in which (b) is a magnified view of (a);

FIG. 10 shows a relationship between anneal time and a mean crystal grain size of a structure of a material in a case where a copper-based alloy material containing 30 mass % zinc is annealed at

temperatures

503 K and 523 K after the material is rolled 60% at a temperature of 77 K; and

FIG. 11 shows a stress-strain curve of copper at room temperature after being MDF processed at an ultra-low temperature of 77 K and at room temperature (300 K).

DESCRIPTION OF THE REFERENCE NUMERALS

- Apparatus
- 20 Roller
- 30 Conveying base
- 40 Material
- 50 Ultra-low temperature tank
- 100 Ultrafine grained structure
- 110 Twin
- 120 Packet

BEST MODE FOR CARRYING OUT THE INVENTION

According to the present inventions a material having an ultrafine grained structure is fabricated by a method characterized by the steps of providing a metal or an alloy having a stacking fault energy that is no greater than 50 mJ/mm², and introducing deformation twins having a twin interval no greater than 200 nm into the structures of the metal or alloy.
Throughout this application, the term “ultrafine grained structure” refers to a structure having a “mean crystal grain size” less than 1 μm. The mean crystal grain size of a deformed structure is measured by using cross-sectional images obtained from a Transmission Electron Microscope (TEM). The mean crystal grain size of annealed structures is measured by calculating a mean crystal grain area in a case where structures having a boundary with a misorientation no less than 3 degrees is assumed as a crystal grain based on a crystal orientation distribution image obtained from Orientation Imaging Microscopy (OIM).
Conventionally, it is considered that a twin does not contribute to the mechanical strength of a material and regarded that twins should be eliminated from crystal structures in order to improve the mechanical strength of the material. In contrast, according to an aspect of the present invention, crystal grains are formed into an ultrafine size by affirmatively introducing twins into structures of a material, to thereby improve the strength of the material. More specifically, according to an embodiment of the present invention, crystal grains are formed into ultrafine size by introducing many deformation twins into the structures of a target process material by performing a process accompanying a large deformation on the target process material and intersecting the deformation twins. According to studies by the inventor of the present invention, with this process, mechanical strength of a material significantly improves, particularly in a case % here an ultrafine grained structure including deformation twins having twin intervals no greater than 200 nm.
Here, the term “twin interval” refers to the distance “D” indicated with arrows according to an ultrafine grained structure 100 having plural twins 110 as depicted in FIG. 1, that is, the space between a twin line situated within one of the twins 110 and another twin line adjacent to the twin line. This twin interval cannot be measured by a normal optical microscope. Therefore, according to an embodiment of the present invention, a twin interval of a twin in an ultrafine grained structure of a material is measured by a structural image obtained from a transmission electron microscope at a magnification of approximately 8000-80000 times.
The processing method is not limited to that described above. Other processing methods may be used for introducing a large number of deformation twins into a material.
For example, as one processing method for introducing a large number of deformation twins, there is a Multi-directional forging (MDF) method. This method, which is also referred to as a multi-axial forging method, is performed by repeating the steps of applying compressive force with respect to a forging direction and changing the forging direction so that the longitudinal axis of the material is oriented in the compressing direction.
A detailed example of the MDF processing method is described with reference to FIG. 2. FIG. 2 is a schematic diagram of a MDF processing method. First, as depicted in (1) of FIG. 2, a bulk material having an aspect ratio corresponding to rectangular shape is prepared. The aspect ratio of the bulk material is determined according to a compression rate obtained by a forging operation in each axis direction (each forging operation is referred to as a “pass”) as illustrated in (2) through (4) of FIG. 2. In other words, the aspect ratio of the bulk material can be changed according to the compression rate of each pass that is used. In the example shown in FIG. 2, the process strain of a single pass is 0.4. Deformation twins appear more easily when the process strain of a single pass is increased since deformation resistance becomes larger. For example, in a case where the strain that can be introduced into the material is 0.8 (in this case, the aspect ratio is 1.0:1.49:2.22), a process of three passes is theoretically necessary in order to attain a total strain of 2.4.
With this MDF process, a large number of deformation twins can be introduced into a material. Further, the crystal grains of the material can be formed into ultrafine sizes by the intersection among the introduced deformation twins. Particularly, since compression of the material is performed from multiple directions with the MDF process, an equiaxial ultrafine grained structure can be obtained after performing the process. Further, this process enables forming a material having an ultrafine grained structure by a simple operation of repeating the steps of forging and changing the compressing axis. Therefore, a material having an ultrafine grained structure can be easily produced.
In general, in order to introduce a large number of deformation twins into a structure of a material at a single time, it is preferable to apply deformation to a material in a state where the materials resistance against deformation (hereinafter referred to as “deformation resistance”) is large. Therefore, in order to increase the deformation resistance, it is preferable to make the temperature of the material during the MDF process as low as possible and to make the strain to be applied to the material at a single time as large as possible.
From these aspects, in a case of using the MDF process as a method of introducing deformation twins, it is preferable to perform the MDF process at a temperature no greater than room temperature (300 K). This increases deformation resistance when performing the MDF process and allows more deformation twins to be introduced into a material at a single time. Further, in a case of performing the MDF process at an “ultralow temperature” no greater than 223 K (absolute temperature), it is preferable to select a processing rate so that the strain rate becomes approximately no less than 1×10⁻⁴/s. In a case other than such a case, it is preferable to select a processing rate so that the strain rate becomes approximately 5×10⁻⁴/s. By selecting a large strain rate, the amount of deformation applied to a material for each pass can be increased and deformation resistance can be raised. Therefore, even in a case where the temperature at which the process is performed is high (approximately room temperature), a large number of deformation twins can be introduced into the structures of a material.
Further, in a case industrially producing a material having an ultrafine grained structure by performing the MDF processing method, it is preferable to automatically rotate a target process material each time a process is performed on the target process material so that the longitudinal axis of the target process material is oriented in the compressing direction. This operation can easily be performed by using a motorized type or a mechanical type target process material position (or orientation) controlling part such as a manipulator. Thereby, the burden of changing the orientation of the target process material for each pass can be resolved.
Through these steps, a material having an ultrafine grained structure including many twins 110 can be obtained as exemplarily illustrated in FIG. 1. In the example of FIG. 1, the mean crystal grain size ranges from a size of approximately 500 nm to a size at most no greater than 1 μm, and the twin interval ranges from approximately 80 to 100 nm.
Although such deformation twins inside a single grain tend to be arranged in parallel towards a single direction, deformation twins oriented in another direction appear when the processing direction of the sample is changed and the sample is forged again, such that the deformation twins disconnect from each other, to thereby create further ultrafine crystal grains. The deformation twins exhibit misorientation of approximately 60 degrees formed by two crystals that sandwich the twin surface. By the generation of the deformation twins as well as the intersecting and disconnecting of the deformation twins, an ultrafine grained structure having its grain boundaries exhibiting high misorientation can be easily fabricated.
It is to be noted that an annealing process may be performed on the forged material. The ultrafine grained structure including many deformation twins generated by the deformation can be made homogeneous by performing the annealing process. It is preferable to perform the annealing process at a temperature as low possible. This is because there is a possibility that growth of ultrafine crystal grains is accelerated when the process temperature becomes high. Particularly, in a case where the melting point of the forged material is expressed as Tm (K), it is preferable that the temperature of the annealing process be no greater than 0.5 Tm. For example, in a case where a copper-based alloy containing 30 mass % zinc is used, the melting point of the alloy is 1223 K. Therefore, the process temperature is set no greater than 611 K.
FIG. 3 illustrates an example of a stress-strain curve for materials having an ultrafine grained structure fabricated by a method of the present invention at room temperature. The samples used in this example are fabricated by introducing a strain of 0.4 (1 pass), 2.4 (6 passes), and 6.0 (15 passes) to a copper-based alloy containing 30 mass % zinc. FIG. 3 shows results of the samples fabricated by performing an MDF process at room temperature (300 K). The maximum strength of a typical material which is not subject to a process of forming ultrafine crystal grains is approximately 500 MPa. On the other hand, a sample subject to the MDF process at a temperature of 77 K exhibits an increased maximum strength ranging from 600 Mpa to 900 MPa. Even a sample subjected to the MDF process at a temperature of 300 K exhibits an increased maximum strength ranging from 700 MPa to 800 MPa except for a sample to which a strain of 0.4 (1 pass) is introduced.
FIG. 4 shows photographs of the structures of samples after an MDF process. The samples are fabricated by performing the MDF process where a strain of 6.0 (15 passes) is introduced to a copper-based alloy containing 30 mass % zinc. The photograph (a) on the left side of FIG. 4 shows a structure after performing the MDF process at 77 K and the photograph (b) at the center of FIG. 4 shows a structure after performing the MDF process at 300 K. Further, the schematic diagram on the right side of FIG. 4 is an enlarged view showing a part of the photograph at the center of FIG. 4.
Particularly, as shown in (c) of the drawing, a group of layered twins arranged in the same crystal orientation is formed in a crystal grain of a sample to which the MDF process is performed. In the present application, this group of layered twins is referred to as a packet (packet 120). When observing each twin in the packet 120 in further details a second packet including a group of smaller layered twins is formed in a single twin. It can be understood that each of the twins included in the second packet is arranged in the same crystal orientation. It is believed that this structure is formed by dividing the twins in the packet 120 into a group of smaller twins by repeating a deformation process using the MDF process. According to this observation, whenever a deformation process is performed, the twins included in a group of twins in a single packet are divided into a group of finer layered twins. Accordingly, packets as well as their crystal grains can be formed into a fine size, to thereby form an ultrafine grained structure.
Typically, in a case of introducing plural twin groups into a structure by using, for example, thermomechanical treatment, the misorientation formed by the group of twins is 60 degrees. However, in a case of twins introduced by the MDF process, a packet, which exists before a deformation process is performed, becomes affected by repetition of the deformation process, to thereby cause crystal rotation of a group of twins in the packet. Therefore, the misorientation between a group of twins of a first packet generated by a first deformation process and a group of twins of a second packet generated by a second deformation process form an angle other than 60 degrees.
Therefore, by applying a method of fabricating a material having an ultrafine grained structure according to an embodiment of the present invention to a target process material, its crystal grains can be formed into ultrafine size and strength of the material can be improved.
Further, as another method of introducing many deformation twins, there is a roll process method. In order to introduce many deformation twins into a material, it is preferable to deform the material with shear. With the roll process method, shear strain can be easily applied to a material and high density deformed crystals can be relatively easily introduced in the material structure. Therefore, in a case of applying this method, crystal grains can be formed in an ultrafine size with a method easier than the MDF process method. Further, the roll process method is not confined by the size of the target process material. For example, the method can be applied to a large size material such as a board material having a large area.
As for process conditions when performing the roll process on a material, it is preferable to use one of an “ultra low temperature process”, a “low temperature-high rate process”, a “low temperature-high pressure process” or a “high rate-high pressure process”. Here, the “ultra low temperature process” refers to a method of rolling a target process material while maintaining the temperature of the target process material at an “ultra low temperature” no greater than 223 K (absolute temperature). As described above, in order to introduce many deformation twins in a single time, it is preferable to make the temperature of the material as low as possible and to make the strain to be applied to the material at a single time as large as possible, so that deformation resistance can be increased. In the “ultra low temperature process”, deformation resistance can be increased regardless of other process parameters, because a roll process is performed on a material having its temperature lowered to an ultra low temperature no greater than 223 K. In other words, with this condition of maintaining a material at an ultra low temperature, a deformation resistance sufficient for forming many deformation twins can be obtained. Therefore, unlike the other process conditions described below, precise control of other parameters (e.g., deformation rate) is unnecessary.
Further, the “low temperature-high rate process” and the “low temperature-high pressure” are methods of rolling a target roll material while maintaining the temperature of the target roll material at approximately 223-300K (room temperature). Unlike the “ultra low temperature process”, it is difficult to introduce high density deformation twins merely by performing a rolling process at this temperature. Therefore, with these conditions, high density deformation twins are introduced in a target roll material by combining with a parameter that increases the amount of deformation that can be applied at a single time. For example, in a case of the “low temperature-high rate process”, deformation resistance is increased by applying strain to a target roll material at a high rate, to thereby introduce many deformation twins in the target roll material. In order to apply strain to a material at a high rate, the rolling rate is to be greater than that of the “ultra low temperature process” and is preferred to be at least 5×10⁻¹cm/s. In a case of the “low temperature-high pressure process”, since deformation twins can be generated more easily as the amount of strain increases, draft of the target roll material is increased by processing the target roll material under a high pressure rolling condition, to thereby introduce high density deformation twins. In this case, it is preferable for the final draft of the material to be no less than 20%. Further, from the aspect of evenly distributing the deformation twins, it is preferable for the final draft of the material to be no less than 60%.
Further, the “high rate-high pressure process” is a method of rolling a target roll material in a non-low temperature range such as room temperature. The deformation resistance of the target roll material is increased by combining a high rate process and a high pressure process, to thereby introduce high density deformation twins. For example, in a typical case of rolling at room temperature, deformation twins appear by setting the rolling rate to 5×10⁻¹cm/s and the draft to 70% or more.
It is to be noted that, among these rolling process methods, the “ultra low temperature process” is preferred. This is because the other methods need to apply a large strain or pressure instantaneously to the material and require a specialized large size apparatus, which thereby leads to a problem where a process cannot be performed with a standard apparatus. This is also due to difficulty in applying a large strain instantaneously depending on the material such as a material having high ductility. Nevertheless, other than the constraints of the apparatus or the material, deformation twins can be introduced with any of the process conditions.
It is to be noted that an annealing process may be performed on the forged material. The ultrafine grained structure including many deformation twins generated by the deformation can be made homogenous by performing the annealing process. It is preferable to perform the annealing process at a temperature as low possible. This is because there is a possibility that growth in the size of ultrafine crystal grains is accelerated when the process temperature becomes high. Particularly, in a case where the melting point of the rolled material is expressed as Tm (K), it is preferable that the temperature of the annealing process be no greater than 0.5 Tm. For example, in a case where a copper-based alloy containing 30 mass % zinc is used, the melting point of the alloy is 1223 K. Therefore, the process temperature is set no greater than 611 K.
By this annealing process, a substantially homogenous ultrafine grained structure having a mean grain size ranging from approximately 20 nm to 600 nm can be obtained as shown in FIG. 5. FIG. 5 is a map diagram according to Orientation Imaging Microscopy (OIM) showing a sample in which a copper-based alloy containing 30 mass % zinc is rolled at an ultra low temperature of 77K (60% drift) and then annealed for 8 hours at a temperature of 503 K. According to the diagram, it is to be noted that the ultrafine grained structure fabricated by a method according to an embodiment of the present invention exhibits little progression of grain growth even after the annealing process. It is believed that the deformation twins oriented in different directions inside a structure confine the crystal grains and prevent the grain size growth. Therefore, the material having the ultrafine grained structure fabricated by the method according to an embodiment of the present invention has a significant characteristic of having satisfactory thermal stability.
It is to be noted that, according to research results of the inventor of the present invention, there is a tendency that the ultrafine grained structure containing many deformation twins having a twin interval no greater than 200 nm (as shown in FIG. 1) is easier to obtain as the stacking fault energy of a metal or an alloy is smaller. This is because, with a metal or an alloy having a large stacking fault energy, it is difficult to apply stress to a material beyond its critical stress for generating deformation twins since dislocation density of the material does not easily increase even if a process such as an MDF process is performed. Therefore, it is preferable to apply the present invention to a metal or an alloy having a stacking fault energy no greater than 50 mJ/mm². The metal or the alloy having a low stacking fault energy may be, for example, silver (stacking fault energy of approximately 22 mJ/mm²), copper (78 mJ/mm²), cobalt (15 mJ/mm²), nickel (128 mJ/mm²), brass (approximately 20 mJ/mm²), and stainless steel (211 mJ/mm²). It is to be noted that “brass” is a copper based alloy containing 20 mass % zinc (the stacking fault energy described above is a value of a copper-based alloy containing 30 mass % zinc). Even in a case where the stacking fault energy of the metal or the alloy itself is greater than 50 mJ/mm², the stacking fault energy can be significantly reduced by adding one or more impure elements to the metal or the alloy. For example, even with the above-described metal or the alloy as well as other alloys having a stacking fault energy no less than 50 mJ/mm², it may fall within the range of the present invention by adding an impure element thereto.
Next, an example of a method of fabricating a material having an ultrafine grained structure according to an embodiment of the present invention is described. The following example is described where the processing method for introducing deformation twins in the structure of a material is a method using a rolling process under the condition “ultra low temperature process”.
FIG. 6 schematically shows an example of a rolling apparatus for performing the method (rolling process under the condition of ultra low temperature) according to an embodiment of the present invention. With the method according to this embodiments a rolling apparatus 10 includes an ultra-low temperature tank 50, a conveying apparatus 30, and a pair of rollers 20. The conveying apparatus 30 is used for guiding a target roll material 40 towards the rollers 20. The ultra-low temperature tank 50 is used for cooling the target roll material 40 beforehand. The temperature of the ultra-low temperature tank 50 is no greater than 223 K. It is, however, preferable for it to be no greater than liquid-nitrogen temperature (77 K). In an alternative rolling apparatus 10, the ultra-low temperature tank 50 may be removed or have its position changed. What is important is that the target roll material 40 is to be cooled at the above-described temperature immediately before traveling through the rollers 20. For example, the apparatus may be configured having a cooling tank provided at the middle of a conveying path, so that the target roll material 40 can pass through the cooling tank before being conveyed to the rollers 20 and have its corresponding roll part cooled to the above-described temperature.
By performing the following steps, the rolling apparatus 10 can introduce deformation twins into the target roll material 40 such as a copper-based metal containing 30 mass % zinc. First, the target roll material 40, which is cooled beforehand in the ultra-low temperature tank 50, is placed on the conveying apparatus 30. Then, the conveying apparatus 30 is activated to convey the target roll material 40 in the direction of the rollers 20. The target roll material 40, when conveyed to the position of the rollers 20, is rolled by the rollers 20. Although it is preferable that the conveying rate (rolling rate) of the target roll material 40 to be no less than approximately 1×10⁻¹cm/s., it is not so limited. Further, although it is preferable that the drift for a single pass be approximately 10-20%, it is not so limited. As described above, under the condition of the ultra-low temperature process, the draft and the conveying rate does not have a large effect to the density of the generated deformation twins.
As these steps are repeated for a necessary number of times (passes), many deformation twins are introduced inside the material. It is preferable to cool the target roll material 40 again, each time a single pass of the rolling process is performed on the target roll material 40. This is to keep the target roll material 40 at a temperature suitable for generating deformation twins when repeating the rolling on the target roll material 40 since the rolling process causes the temperature of the target roll material 40 to increase. However, in a case where the temperature of the target roll material 40 can be maintained at a suitable ultra-low temperature range (e.g., a case where the entire rolling apparatus 10 is kept in a low temperature atmosphere), it is possible to repeat two or a few passes of the rolling process when the temperature of the target roll material 40 is within a predetermined range.
With the roll process method using the above-described apparatus, many deformation twins can be easily introduced because the target roll material 40 is maintained at an ultra-low temperature to have a sufficiently large deformation resistance. After the process, a material having an ultrafine grained structure can be obtained.
It is, however, possible to introduce deformation twins into the target roll material 40 at a non-ultra-low temperature such as room temperature (high rate-high pressure process). In this case, there is an advantage that the ultra-low temperature tank 50 of the above-described apparatus is unnecessary. Nevertheless, there is a need to devise a method of increasing deformation resistance such as by increasing the conveying rate of the target roll material 40 or increasing the draft during the rolling process. For example, with a high rate-high pressure process, the conveying rate of the target roll material is 5×10⁻¹cm/s., and the final draft after the rolling process is no less than 70%. Even with this rolling process, an ultrafine grained structure having homogenous and high density deformation twins can be obtained.
It is to be noted that an annealing process may be performed on the rolled material. As described above, it is preferable to perform the annealing process at a temperature as low possible. For example, in a case where the melting point of the material is expressed as Tm, with an annealing process performed at a temperature no greater than 0.5×Tm, an ultrafine grained structure having recrystallized grains with size ranging from 20 nm to 600 nm can be realized.
Next, evaluation experiment results of a material having an ultrafine grained structure obtained by applying the method of the present invention are described.

EXPERIMENT 1

In experiment 1, the MDF process was performed on a sample material and the strength of the processed material was evaluated. A copper based alloy containing 30 mass % zinc and having a stacking fault energy of 20 mJ/mm²was used as the material of experiment 1.
FIG. 7 shows an image of a structure of a copper based alloy containing 30 mass % zinc after being MDF processed with a true strain rate of 1×10⁻³at a temperature of 77 K. This process performs the MDF process on the material for 6 passes and the amount of cumulative strain introduced to the material is 2.4. From this image, it can be understood that many deformation twins are introduced into the material by performing the MDF process and that an ultrafine grained structure having a mean grain size no greater than 1 μm is formed by having the deformation twins intersecting each other. In the selected area diffraction regarding the 1 μm area encompassed by a circle of the image, a hollow ring (a phenomenon in which diffraction spots are connected to form a ring-like shape) appears. Normally, in a case where many crystals are oriented in different directions within a structure, diffraction spots corresponding to each of its grains are arranged extremely close to each other, to thereby create the hollow ring. Thus, from these results, it can be understood that many ultrafine crystal grains are contained inside the structure of the material obtained by the method of the present invention.
FIG. 3 illustrates a stress-strain curve of samples obtained at room temperature after the samples are subjected to the MDF process at temperatures 77 K and 300 K (room temperature). Strains of 0.4, 2.4, and 6.0 are introduced to the materials of the samples by performing the MDF process for 1 pass, 6 passes, and 15 passes, respectively. In a case of a sample obtained by performing the MDF process at an ultra-low temperature of 77 K, the maximum strength increases from 600 MPa to 900 MPa when the amount of strain is increased from 0.4 to 6.0 as shown in the upper part of FIG. 3. Each of the samples exhibited an elongation of approximately 20%. The maximum strength of the same material but using a conventional thermal mechanical treatment method is normally approximately 500 MPa. Therefore, from these results, it can be understood that strength of a material significantly improves by applying the MDF process method. Further, the elongation of the same material but using a conventional method of forming crystals having ultrafine grain size (e.g., ECAP method) is normally approximately 10%. Therefore, it can be understood that elongation can be improved by applying a method of forming crystals having an ultrafine grain size according to an embodiment of the present invention.
In a case of a sample obtained by performing the MDF process at 300 K (room temperature), the maximum strength increases from 500 MPa to 800 MPa when the amount of strain is increased from 0.4 to 6.0 as shown in the lower part of FIG. 3. This shows a tendency of strength improvement by increasing the amount of strain even in a case where the MDF process is performed at room temperature of 300 K. However, in performing the MDF process at room temperatures it is regarded to be more difficult to increase deformation resistance of the material compared to performing the MDF process at the ultra-low temperature process of 77 K because the number of deformation twins contained in a structure is smaller when compared with the same number of passes. However, according to the results of TEM observation with respect to a sample being MDF processed until the cumulative strain is 6.0 (maximum strength of approximately 800 MPa), homogenous ultrafine crystal grains can be formed even with a process performed at room temperature. Each of the three samples, which were fabricated with the above-described process conditions, exhibited an expansion of approximately 20%. This is an improvement compared to that of a conventional material (10%).

EXPERIMENT 2

In experiment 2, the status and stability of a process structure is evaluated in a case of using the same material (copper-based alloy containing 30 mass % zinc) as experiment 1 and performing an ultra-low temperature rolling process on the material. Firsts an ultra-low temperature rolling process using the apparatus shown in FIG. 6 is performed on this material. The temperature of the material during rolling is 77 K. The draft corresponding to each pass of the rolling process ranges from 10% to 20%. The structure of the material after a 60% rolling was observed by using a Transmission Electron Microscope (TEM). As a result, it is revealed that the same structure as that of FIG. 1 having ultrafine crystal grains having a grain size ranging from approximately 500 nm but less than 1 μm can be obtained. It is also revealed that many deformation twins are included in a crystal grain and that the twin interval between the deformation twins ranges from approximately 80-100 μm.
Next, changes of structure are examined in a case where a material rolled 60% at a temperature of 77 K is annealed with respect to each temperature. FIG. 8 is a diagram showing changes of Vicker's hardness of a material annealed at temperatures of 503 K, 523 K, and 543 K after the material is rolled 60% at a temperature of 77 K. From this diagram, although hardness may also depend on annealing temperature, it is understood that hardness steeply drops after 10³through 10⁴seconds and that static recrystallization occurs after this period.
In FIG. 9 are TEM photographs of a sample of a copper-based alloy containing 30 mass % zinc annealed for 1000 seconds at a temperature of 523 K after being rolled (60% draft) at an ultra-low temperature of 77 K. The photograph on the right side is a magnified photograph of the photograph on the left side. At the stage where the material is annealed for 1000 seconds at 523 K, it is revealed that, although some “packets” generated by the deformation still remain, static recrystallization is already started in which a part of the “packets” is recrystallized. Further, it is revealed that recrystallized grains at a portion inside a crystal grain, which are generated by plural layered deformation twins, are arranged in the same direction. The grain size of the recrystallized grains is approximately 20 nm. Therefore, it is believed that the deformation twins contained in the recrystallization grains have a grain size significantly smaller than 20 nm.
Accordingly, an ultrafine grained structure containing “packets”, recrystallized grains of the packets, and other recrystallized grains having plural layered deformation twins arranged in the same direction are fabricated by rolling a copper-based alloy containing 30 mass % zinc at an ultra-low temperature and annealing the rolled material.
FIG. 10 shows a relationship between anneal time and a mean crystal grain size of a structure of a material in a case where the material is annealed at temperatures 503 K and 523 K after the material is rolled 60% at a temperature of 77 K. The mean crystal grain size corresponding to data indicated with a bracket in FIG. 10 (anneal time=100 s) is calculated by obtaining the simple average of the interval and the length of the deformation twins (as shown in “D” and “L” of FIG. 1) because the grains are too fine to identify. The mean crystal grain size corresponding to other data is calculated by using photographs of a transmission electron microscope and performing a cross-sectional method assuming that structures having boundaries are crystal grains. Because deformation twins contained in the structure have a high misorientation angle no less than 60 degrees, it is evident that these structures are crystal grains surrounded by high misorientation boundaries. Therefore, structures having boundaries are assumed as crystal grains. It is to be noted that fine deformation twins developed inside the crystal grains are used for calculation. The actual crystal grain sizes are smaller than those shown in FIG. 10. From FIG. 10, it can be understood that, even after the annealing process, the crystal grains hardly become bulky and are approximately 0.6 μm at most.
FIG. 5 shows an example of a structure in a case where a copper-based alloy containing 30 mass % zinc is rolled at a temperature of 77 K and then annealed for 8 hours at a temperature of 503 K. Observation of this example is performed using an Orientation Imaging Microscopy (OIM) apparatus. From FIG. 5, it is understood that a substantially homogenous ultrafine crystal grains having a mean grain size of approximately 500 nm by the annealing process. The mean crystal grain size was calculated from mean crystal grain area where structures having boundaries with a misorientation angle no less than 3 degrees are assumed as crystal grains. Almost all of the structures had boundaries of high misorientation angles no less than 15 degrees. From this diagram, it is understood that crystal grains do not become bulky even after a long annealing time of 8 hours. From these results, it is understood that a homogenous ultrafine grained structure can be obtained by performing the annealing process on structures into which a large amount of deformation twins are introduced. It is also understood that the obtained structure is extremely thermally stable and difficult to become bulky. This is because growth of grains are restrained by forming crystal grains into an ultrafine size by introducing many twins into the structures of a material and generating many twins having different orientations.

EXPERIMENT EXAMPLE 3

Next, a similar experiment was performed on copper (stacking fault energy of 78 mJ/mm²) for comparison. FIG. 11 shows a stress-strain curve of copper at room temperature after being MDF processed at an ultra-low temperature of 77 K and at room temperature (300 K). The horizontal axis indicates cumulative strain. In a case where the MDF process was performed on copper at 300 K, the maximum stress becomes 380 Mpa when accumulative stress strain becomes greater than 2. After that, the maximum stress did not change where severe plastic deformation was performed until a cumulative strain of 6. According to the results of observing the structure of the experimented sample, hardly any deformation twins were generated. On the other hand, with a sample on which the MDF process was performed at a temperature of 77 K, the maximum stress was 590 Mpa where the cumulative strain was 2. It had been found that a few deformation twins were generated at some portions by observing the structure of the sample to which the cumulative strain of 2 was applied. However, the deformation twins were heterogenous and could not homogenously attain an ultrafine grained structure as the above-described copper-based alloy containing 30 mass % zinc. From this result, it can be understood that, even if deformation stress is large, deformation twins are difficult to obtain in a case of a material having a large stacking fault energy. This is because materials have their own unique critical stress for generating deformation twins with a high percentage. That is, in order to generate many deformation twins in a structure of a material by processing the material, the stress applied to the material is to surpass its critical stress (a critical stress of approximately 400-600 Mpa is estimated in a case of copper). In a case of copper having high stacking fault energy, the stress applied to the material cannot surpass its critical stress even it a process such as the MDF process is performed on the material due to difficulty in increasing its dislocation density. Thus, it is anticipated that deformation twins as many as the copper-based alloy containing 30 mass % zinc cannnot be generated with the same process conditions.
The present application is based on Japanese Priority Application Nos. 2006-102216 and 2006-120942 filed on Apr. 3, 2006 and Apr. 25, 2006, respectively, with the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.

Claims

1. A material comprising: an ultrafine grained structure including a metal or an alloy that has a stacking fault energy no greater than 50 mJ/mm², wherein structures that predominantly exist in the material have twins included in a crystal structure, wherein the twins have a twin interval no greater than 100 nm.

2. A material comprising: an ultrafine grained structure including a metal or an alloy that has a stacking fault energy no greater than 50 mJ/mm², wherein structures that are predominant in the material have recrystallized grains, wherein twin boundaries having a crystal grain size ranging from 20 nm to 600 nm predominantly exist in the recrystallized grains.

3. A fabrication method of the material having an ultrafine grained structure as claimed in claim 1 comprising the steps of:

providing a metal or an alloy having a stacking fault energy no greater than 50 mJ/mm²; and

introducing deformation twins having a twin interval no greater than 100 nm into structures of the metal or the alloy by processing the metal or the alloy.

4. The method as claimed in claim 3, wherein the step of introducing the deformation twins includes a step of performing a multidirectional forging process on the metal or the alloy at a temperature no greater than room temperature.

5. The method as claimed in claim 4, wherein the step of performing the multidirectional forming process includes a step of performing a forging process on the metal or the alloy at a strain rate no less than 1×10⁻⁴/s.

6. The method as claimed in claim 4, wherein the temperature no greater than room temperature is no greater than an absolute temperature of 223 K.

7. The method as claimed in claim 4, further comprising a step of performing an annealing process on the multidirectional-forged metal or alloy.

8. The method as claimed in claim 3, wherein the step of introducing the deformation twins includes a step of performing a rolling process on the metal or the alloy at a temperature no greater than room temperature.

9. The method as claimed in claim 8, further comprising a step of performing an annealing process on the rolled metal or alloy.

10. (canceled)

11. The method as claimed in claim 8, wherein the step of performing the rolling process includes a step of rolling the metal or the alloy at a rolling rate no less than 5×10⁻¹cm/s.

12. The method as claimed in claim 8, wherein the step of performing the rolling process includes a step of rolling the metal or the alloy so that a final draft becomes no less than 20%.

13. The method as claimed in claim 8, wherein the step of performing the rolling process includes a step of performing a rolling process on the metal or the alloy at a temperature no greater than an absolute temperature of 223 K.

14. A material having an ultrafine grained structure comprising:

structures predominantly existing in a single grain, the structures including

a first packet having a group of layered twins oriented substantially in a first direction, and

a second packet including at least one of the twins inside the first packet that has a group of layered twins oriented substantially in a second direction,

wherein the first and second directions form an angle other than 60 degrees.

15. The material having an ultrafine grained structure as claimed in claim 14, wherein the structures include

a first structure, the first structure including the first packet having a group of layered twins oriented substantially in the first direction and the second packet including at least one of the twins inside the first packet that has a group of layered twins oriented substantially in the second direction,

a second structure including recrystallized grains of the first packet, and

a third structure including recrystallized grains formed of a plurality of layered twins arranged substantially in the same direction,

wherein the first, second, and third structures predominantly exist in the single crystal grain.

16. (canceled)

17. The method as claim as claimed in claim 7, wherein the step of performing the annealing process includes a step of performing an annealing process on the metal or the alloy at a temperature no greater than 0.5×Tm, wherein Tm is a melting point of the metal or the alloy.

18. The method as claim as claimed in claim 9, wherein the step of performing the annealing process includes a step of performing an annealing process on the metal or the alloy at a temperature no greater than 0.5×Tm, wherein Tm is a melting point of the metal or the alloy.

19. The material having an ultrafine grained structure as claimed in claim 14, wherein the material is brass.

20. The material having an ultrafine grained structure as claimed in claim 15, wherein the material is brass.