JP2013001946A - Titanium alloy member having bidirectional shape-memory characteristic, and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a titanium alloy member having bidirectional shape-memory characteristics.SOLUTION: The titanium alloy member having bidirectional shape-memory characteristics contains 4.0 to <5.5% Al, 1.1 to <3.1% Fe, 1.0 to <4.0% Cr, 0.5 to <5.5% Mo. The contents of adding elements are characterized in that in the formula where Mo equivalent=2.9×[%Fe]+1.6×[%Cr]+[%Mo]-[%Al], Mo equivalent contains 4.5 to <9.4%, while restraining Si to be <0.1% and C to be <0.01%, and including the balance composed of Ti with inevitable impurities. Furthermore, a pro-eutectoid α-phase in an optical microscope structure is 45% or lower, by cooling with cooling speed of water-cooling or more in the temperature range of β single-phase zone upper limit from the β transformation point-100°C.

Description

  The present invention relates to a titanium alloy member suitable for use as a fastener for aircrafts, two-wheeled vehicles and four-wheeled vehicles, having a shape memory characteristic in two directions whose deformation direction is reversed depending on temperature, and a method for manufacturing the same.

  Shape memory alloy is a special alloy that returns to the shape before processing by applying heat after processing. In addition to the aerospace field, actuators for automobiles and home appliances, orthodontic wires, medical tools, mobile phone antennas, Widely used in eyeglass frames.

  One shape memory alloy that is practically used as a functional material is an alloy having a 1: 1 atomic ratio of titanium to nickel (Nitinol), which has excellent strength, heat resistance, wear resistance, and corrosion resistance. ing. Nitinol is a two-way shape with a reversible deformation direction that undergoes reversible changes at high and low temperatures only by changing the temperature by applying special treatments such as strong processing, restraint heating, training, and restraint aging. Memory characteristics can be developed.

  In contrast, titanium alloys having shape memory characteristics are also known. Examples of alloys having typical shape memory characteristics include Ti-10V-2Fe-3Al, Ti-15.4V-4Al, both titanium and zirconium, or at least one element containing niobium and tantalum in a total of 10 to 20 element%. And an alloy containing 3 to 6 elemental tin (Patent Document 1), an alloy containing 10 to 15 mass% molybdenum and 5% or less of aluminum in titanium (Patent Document 2), Ti-Sc-X alloy 1 at% ≦ Sc ≦ 30 at%, 1 at% ≦ X ≦ 15 at% (provided that X = V, Nb, Mo, Ta, or some combination thereof) (Patent Document 3), An alloy (Non-Patent Document 1) is known in which 8% by weight molybdenum in titanium and 5% by weight tin contain a total of 10% by mass of aluminum, zirconium, and tin.

Japanese Patent No. 3512253 Japanese Patent No. 1258024 Japanese Patent No. 4220772

SIXTH WORLD CONFERENCE ON TITANIUM 1988 P.1069

  A titanium alloy having shape memory characteristics needs to stabilize the β phase in order to develop shape memory characteristics, and a relatively expensive β stabilizing element such as Nb, V, or Mo is added in a large amount. Further, in most cases, the shape memory characteristic is only in one direction and does not have the shape memory characteristic in two directions.

  An alloy containing 10 to 20 element% in total of niobium and tantalum in both titanium and zirconium or at least one element and 3 to 6 element% in tin (see Patent Document 1) contains a large amount of niobium and tantalum. And the cost is very high.

  Further, even an alloy in which 10 to 15% by mass of molybdenum and 5% or less of aluminum are added to titanium (see Patent Document 2) contains 10% or more of molybdenum, and the cost is very high. Moreover, since molybdenum is easily segregated when added in a large amount, it is very difficult to dissolve.

Further, it is a Ti—Sc—X alloy, from 1 at% ≦ Sc ≦ 30 at%, 1 at% ≦ X ≦ 15 at% (provided that X = V, Nb, Mo, Ta, or a combination of several). In such an alloy (see Patent Document 3), since Sc is a very expensive and rare metal, it is considered industrially difficult to add it as an alloy element in% units.
An alloy in which 8% by weight molybdenum in titanium and 10% by weight of aluminum, zirconium and tin in total in 5% by weight tin (see Non-Patent Document 1) has a bi-directional shape whose deformation direction is reversed depending on the temperature. Although it has memory characteristics, it contains a total of 10% by mass of aluminum, zirconium, and tin, so the workability is very poor.

  An object of the present invention is to provide a titanium alloy member that has two-way shape memory characteristics, is inexpensive, and is easy to manufacture in terms of workability.

  The present inventor uses a relatively inexpensive element Fe, and adjusts the alloy composition with a formula consisting of Mo equivalents, thereby obtaining a titanium alloy having two-way shape memory characteristics whose deformation direction is reversed depending on the temperature. , Earnest research. As a result, it has been found that by setting the content of each element within a predetermined range, shape memory characteristics in two directions are expressed.

In order to solve the above problems, the gist of the present invention is as follows.
(1) 4.0% or more and less than 5.5% Al, 1.1% or more and less than 3.1% Fe, 1.0% or more and less than 4.0% Cr, 0.5% or more by mass% It contains less than 5.5% Mo, the Mo equivalent represented by the following formula is 4.5 or more and less than 9.4%, and Si: less than 0.1%, C: less than 0.01% Titanium alloy member having shape memory characteristics in two directions, characterized by comprising a balance Ti and inevitable impurities.
Mo equivalent = 2.9 × [% Fe] + 1.6 × [% Cr] + [% Mo] − [% Al]
(2) The optical microscope structure, wherein the α phase is 45 area% or less, and the remainder is a β phase or a β phase and a martensite phase, and an inevitable phase, as described in (1) above A titanium alloy member having shape memory characteristics in two directions.
(3) In the final annealing step, cooling is performed at a cooling rate equal to or higher than water cooling from within a temperature range from a β transformation point of −100 ° C. to an upper limit of a β single phase region, as described in (1) or (2) above The manufacturing method of the titanium alloy member which has the shape memory characteristic of two directions of these.

  Since the present invention can provide a titanium alloy that is less expensive than a general shape memory titanium alloy and has shape memory characteristics in two directions, industrial effects are immeasurable.

It is an optical microscope structure | tissue photograph of this invention material and a water-cooled sample. Chemical components are included in the present invention, but are optical micrographs of air-cooled samples that do not include bi-directional shape memory properties in the present invention. It is a figure which shows an example of the shape memory characteristic result by a bending test.

  The present invention is described in detail below. Hereinafter, the content of the additive element is indicated by “mass%”.

  The material index of the present invention will be described. Titanium alloys exhibit shape memory characteristics in β-type titanium alloys mainly composed of β-phase. As a method for securing the β phase, a large amount of substitutional solid solution elements such as eutectoid β stabilization elements Fe, Ni, Cr, Mn, and all-solid solution β stabilization elements V, Mo, etc. It has been added to. However, in order to ensure the β phase at room temperature in the Ti material, it is necessary to add nearly 8% in terms of Mo equivalent. Since Mo is a relatively expensive element, an attempt to secure a β phase with only Mo results in an extremely high alloy cost. Accordingly, the present inventor can maintain the Mo equivalent, which is a guideline for the addition amount of the β-phase stabilizing element, and add Fe, which is a relatively inexpensive element, as a guideline. In addition, since the shape memory characteristic is caused by the β-phase processing-induced martensitic transformation, it is necessary to secure the amount of β-phase while reducing the stability of the β-phase itself to some extent to facilitate martensitic transformation.

  As will be described later, in the present invention, as a result of earnest investigation, in order to ensure the β phase amount necessary to obtain the shape memory characteristics in two directions, 4.5% or more in Mo equivalent is necessary, and It has been found that the Mo equivalent needs to be less than 9.4% in order to make the stability of the β phase itself undergoing martensitic transformation moderate. In addition, as described above, the shape memory characteristics are generally expressed by the β phase, but the α phase does not contribute, and conversely plastic deformation, which hinders the expression of the shape memory characteristics. It was found that the area ratio in the optical microscopic structure of the product should be 45% or less.

[Indicator of additive element content]
In order to develop shape memory characteristics, it is necessary to stabilize a large amount of β phase at room temperature. On the other hand, if the amount of β-stabilizing element added is too large, the alloy cost will increase, segregation during solidification of the additive element, and the β phase will be too stable to express shape memory characteristics. It is necessary to add an appropriate amount of additive elements. In the present invention, the addition amount of the additive element is adjusted by the Mo equivalent shown below, which is generally used as an indicator of β-phase stability.
Mo equivalent = 2.9 × [% Fe] + 1.6 × [% Cr] + [% Mo] − [% Al]

[Indicator of Mo equivalent]
In a titanium alloy, a large amount of β phase can be left by cooling after heating in the α + β high temperature region or β single phase region. In general, in a titanium alloy, shape memory characteristics are caused by a β-phase deformed by processing-induced martensite transformation during processing, and reverse transformation by heat treatment to return to the β-phase. Therefore, it is necessary to reduce the stability of the β phase itself to some extent in order to secure the β phase amount to a certain amount or more and to easily cause the processing-induced martensitic transformation. However, if the above-mentioned Mo equivalent is too low, the necessary β phase amount cannot be ensured, and the majority of the β phase generates a martensite phase during cooling, which is not preferable. If the heat treatment temperature is lowered too much to leave the β phase during cooling, the area ratio of the α phase will increase. As will be described later, the area ratio of the α phase needs to be 45% or less in order to develop the shape memory characteristics in two directions. For this reason, in the examination using the laboratory-level 100 g vacuum arc melting sample 20 charge in the vicinity of and within the component system of the present invention, the lower limit of the Mo equivalent which is an indicator of β-phase stabilization is 4.5%. It became clear that it was necessary to do it above. On the other hand, if the Mo equivalent becomes too high, the β phase becomes too stable, and the processing-induced martensitic transformation does not occur at the time of deformation, and the shape memory characteristics in two directions are not exhibited. In the examination test, it was found that the upper limit of Mo equivalent needs to be 9.4%.

[Al addition amount]
Al is an α stabilizing element, and it is necessary to reduce the addition amount as much as possible in order to stabilize the β phase. However, since Al suppresses the generation of the ω phase in the β phase, the content is set to 4.0% or more. However, when the addition amount is increased, the addition amount of the β-stabilizing element is also increased, and the cold workability deteriorates, so the upper limit was set to 5.5%.

[Fe, Cr, Mo addition amount]
In order to stabilize the β phase at room temperature and to exhibit the shape memory characteristics in two directions, it is necessary to limit the addition amount of the β stabilizing element to an appropriate range. As a β-stabilizing element, V is famous as represented by Ti-6Al-4V, a general-purpose alloy, but V is an element that is toxic to the human body and is not particularly suitable for medical use. Therefore, in the present invention, Fe, Cr, and Mo are used as β stabilizing elements without using V. Fe is a β-stabilized substitutional solid solution element, and the degree of stabilization of the β phase at room temperature increases according to the amount added. In order to reduce the relatively expensive additive elements as much as possible, addition of 1.1% or more is necessary. However, since it is easy to segregate at the time of solidification, the effect becomes remarkable when the addition amount is increased. Therefore, the upper limit of the addition amount is 3.1%. Cr is an element having a higher β-stabilizing ability than Mo and can be added in a smaller amount. Therefore, 0.5% or more of Cr is contained. However, since Cr is easily segregated during solidification, the upper limit of addition is set to 4.0%. By adding 0.5% or more of Mo, the low β stabilizing ability of the present invention can be secured. On the other hand, Mo is easily segregated at the time of solidification, causing variations in characteristics depending on the site, and is a relatively expensive element, so the upper limit was set to 5.5%.

[Content of Si and C]
As a large amount of impurity elements Si and C, room temperature ductility, cold workability, and hot workability may be deteriorated. Si should be less than 0.1% and C should be less than 0.01%. For example, it was found that there was no problem level, and the upper limit was set for each. Since Si and C are unavoidable inclusions, the lower limit of the substantial content is usually 0.005% or more for Si and 0.0005% or more for C.

[Area ratio of α phase grains]
The shape memory characteristics in two directions vary depending on the residual amount of β phase. For example, when annealing is performed for a long time at a relatively low temperature in the α single-phase region that deviates from the industrial production conditions, the β phase may be much lower than 50%. It is plastically deformed (irreversible deformation) and is mechanically constrained by this, and the shape memory characteristics in two directions are not exhibited as a whole member.

  First, the index of the residual amount of β phase was examined. As a result, it was found that it is appropriate to use the area ratio of pro-eutectoid α phase grains. However, the pro-eutectoid α phase is an α phase generated during high-temperature heat treatment, and in order to obtain shape memory characteristics in two directions, no α phase is precipitated in the β phase grains, or the amount of precipitation It is assumed that there is very little area ratio. On the other hand, if a lot of fine α-phase is precipitated in the β-phase grains during cooling, it is etched at room temperature with a nitric hydrofluoric acid aqueous solution (nitric acid concentration is about 12%, hydrofluoric acid concentration is about 1.5%). When the obtained optical microscope sample is observed with an optical microscope at a magnification of 100 to 1000, the inside of the β-phase grains is black, and further, an acicular α-phase is observed. When such an acicular α-phase is observed, even if the area ratio of the pro-eutect α phase is small, the area ratio of the α-phase is obtained by adding the area ratio of the acicular α-phase to the area ratio of the pro-eutect α phase. When the area ratio of the α phase clearly exceeds 45%, the shape memory characteristics in the two directions are not expressed.

  In the examination using the laboratory-level 100 g vacuum arc melting sample 20 charge, when the α phase area ratio exceeds 45%, the α phase in the metal structure is plastically deformed due to the deformation applied to the member. As a result, the member is mechanically restrained, and the bidirectional shape memory characteristic is not developed. Therefore, in order to achieve the object of the present invention, the area ratio of the α phase needs to be 45% or less.

  In order to exhibit a bi-directional shape memory deformability of 5% or more in both directions, where bi-directional shape memory is practically meaningful, the area ratio of the α phase is desirably 35% or less.

  As shown in FIG. 1, when a fine α-phase is hardly precipitated in β-phase grains, the pro-eutectoid α-phase area ratio becomes equal to the α-phase area ratio. Therefore, a method for measuring the area ratio of pro-eutectoid α phase grains will be described. This pro-eutectoid α phase can be easily discriminated from an optical micrograph obtained by etching a full thickness embedded polishing sample of a plate thickness section with the aqueous fluoric acid solution. FIG. 1 shows an example of an optical micrograph. FIG. 1 shows a sample cooled with water from 900 ° C. as an example of claim 1 of the present invention. In FIG. 1, a nitric hydrofluoric acid aqueous solution having a nitric acid concentration of about 12% and a hydrofluoric acid concentration of about 1.5% was used for etching. In FIG. 1, a white portion having a particle size of about 5 μm indicated by a solid line arrow (a contrast is lower than the β phase of the parent phase) is an α phase crystal grain. Using a general image analyzer, a 1000-times observation field of view (plate surface direction 88 μm × plate thickness direction 71 μm) was randomly observed from each part of the full-thickness embedded polishing sample in total 25 fields. The total area ratio occupied by the deposited α-phase grains was measured, and the average of the values was defined as the area ratio of the α-phase grains.

  In the present invention, as shown in FIG. 1, since the fine α-phase is hardly precipitated in the β-phase grains, the β-phase grains do not appear black in the cross-sectional structure etched with the aqueous hydrofluoric acid solution. The α phase area ratio becomes equal to the α phase area ratio. On the other hand, in the cross-sectional structure air-cooled from 900 ° C. shown in FIG. It appears black, and acicular α-phase is precipitated. Since the area ratio of the α phase is the value obtained by adding the area ratio of the acicular α phase to the area ratio of the pro-eutectoid α phase, the area ratio of the α phase is 45% together with the area ratio of the pro-eutect α phase. Clearly exceeded.

[Β-phase and martensite structure]
As described above, it is necessary to make the β phase unstable to some extent in order to develop shape memory characteristics. For this reason, the β-phase martensitic transformation temperature (Ms temperature) needs to be close to room temperature depending on the alloy composition or the heat treatment temperature. However, when it becomes a martensite single phase, the shape memory characteristic does not appear. Therefore, depending on the alloy component system or heat treatment conditions, the phases other than the pro-eutectoid α phase must be a β phase single phase or a β phase and a martensite phase. The presence or absence of a β phase or a martensite phase can be easily determined by an optical microscope or X-ray diffraction.

[Production method of titanium alloy]
The titanium alloy of the present invention contains the composition of the above titanium alloy, and in the final annealing step, it is cooled at a cooling rate higher than β transformation point −100 ° C. to a cooling rate of water cooling or more, and less than 45% proeutectoid The α phase and the balance can be a β phase single phase, or a β phase, a martensite phase, and an inevitable phase. The β transformation temperature can be determined using a differential thermal analyzer.

  Titanium alloys having the components shown in Table 1 were arc-melted to prepare about 100 g ingots, which were heated to 900 to 930 ° C. and hot forged into plate materials having a thickness of about 3 mm. Further, Table 2 shows the structural structure and the area ratio of the pro-eutectoid α phase when the material is air-cooled at a temperature higher than the β transformation point −100 ° C. for 30 minutes and then cooled with water. In this heat treatment condition, No. 1-6 and no. In any of the 8 to 10 titanium alloys, the water is cooled from a temperature higher than the β transformation point −100 ° C. On the other hand, no. 7 is water-cooled from a temperature lower than the β transformation point −100 ° C. In Tables 1 and 2, numerical values outside the scope of the present invention are underlined.

  Each measurement condition and test condition will be described below. A resin-embedded polishing material for optical microscope observation of a full-thickness cross section was observed after etching at room temperature using a nitric hydrofluoric acid aqueous solution (nitric acid concentration: about 12%, hydrofluoric acid concentration: about 1.5%). The constituent phases were identified by X-ray diffraction. The β transformation temperature was measured by a test in which each sample was gradually heated using a differential thermal analyzer. In addition, it was confirmed using the X-ray-diffraction apparatus that the phase which actually precipitates more than (beta) transformation temperature is (beta) phase, hold | maintained the sample powdered separately at the temperature of (beta) transformation temperature + 30-100 degreeC.

  Table 1 shows the area ratio of the β phase when the hot forged material in Table 1 is air-cooled at each temperature shown in Table 2 and then water-cooled. Each measurement condition and test condition will be described below. The area ratio of the β phase was measured with a general image analyzer using the etched embedded sample.

  A method for measuring the shape memory characteristics in two directions will be described. In the present invention, a bending test was conducted in order to examine whether or not the shape memory characteristics are in two directions. After cutting out the plate-like test piece, it was bent into a U-shape at room temperature so that the diameter was 5 mm. Then, after hold | maintaining to 100-500 degreeC for 5 minutes for every 50 degreeC, the shape memory characteristic of two directions was evaluated by measuring the bending angle of a bending test piece.

  From Table 2, No. of the Example which is an alloy component of this invention of Claim 1 is shown. In 1 to 5, those subjected to heat treatment at a temperature higher than the β transformation point −100 ° C. all have an area ratio of the pro-eutectoid α phase of less than 45%. Moreover, as shown in Table 2, the shape memory characteristics are shown by performing heat treatment between 100-250 ° C. in all cases. In addition, by performing heat treatment at a higher temperature of 300 to 500 ° C., shape memory characteristics in the same direction as the bending direction are shown, and shape memory characteristics in two directions are shown.

  On the other hand, No. of the comparative example of Table 2. No. 6 has a low Mo equivalent, so that the optical microstructure is a martensite single phase and does not have shape memory characteristics in two directions as shown in Table 2.

  Further, in the comparative example of Table 2, No. In No. 7, the Mo equivalent is within the predetermined value, but the heat treatment temperature is lower than the β transformation point −100 ° C., and the area ratio of the pro-eutectoid α phase is as high as 55%. Does not show shape memory characteristics of direction.

  Further, Comparative Example No. 2 in Table 2 was used. No. 8 has a very low Mo equivalent of 2.9, and all β phases are transformed into a martensite phase during cooling, and as shown in Table 2, it does not exhibit a bi-directional shape memory characteristic.

  Further, Comparative Example No. 2 in Table 2 was used. No. 9 has a high Mo equivalent of 12.9 and the β phase is too stable, and does not have a bi-directional shape memory characteristic as shown in Table 2.

  Further, Comparative Example No. 2 in Table 2 was used. No. 10 has a low Fe content, and accordingly, the Mo equivalent is -2.2, which is very small. Therefore, as shown in Table 2, the shape memory characteristics in two directions are not shown.

  The titanium alloy having shape memory characteristics of the present invention exhibits two-way shape memory characteristics by changing the heating conditions. In addition, the amount of additive elements such as Nb, V, and Mo, which are more expensive additive elements than conventional shape memory titanium alloys, is small, which is very advantageous in terms of cost. Therefore, it is suitable for use as a fastener for automobiles or motorcycles, and structural materials, and contributes to weight reduction of these component materials.

Claims (3)

4.0% or more and less than 5.5% Al, 1.1% or more and less than 3.1% Fe, 1.0% or more and less than 4.0% Cr, 0.5% or more and 5.5% by mass Containing less than% Mo, the Mo equivalent represented by the following formula being 4.5 or more and less than 9.4%, and suppressing Si to less than 0.1% and C to less than 0.01%, A titanium alloy member having a shape memory characteristic in two directions, characterized by comprising a balance Ti and inevitable impurities.
Mo equivalent = 2.9 × [% Fe] + 1.6 × [% Cr] + [% Mo] − [% Al]   The bi-directional shape according to claim 1, characterized in that the α phase is 45 area% or less in the optical microscope structure, and the balance is the β phase or β phase and martensite phase, and the inevitable phase. Titanium alloy member with memory characteristics.   The two-way shape memory according to claim 1 or 2, wherein in the final annealing step, cooling is performed at a cooling rate equal to or higher than water cooling from within a temperature range from a β transformation point of -100 ° C to an upper limit of a β single phase region. A method for producing a titanium alloy member having characteristics.