CN114619024A

CN114619024A - Method for simultaneously improving strength and toughness of ultra-fine grain Al-Mg alloy

Info

Publication number: CN114619024A
Application number: CN202210209689.3A
Authority: CN
Inventors: 林耀军; 刘志波
Original assignee: Wuhan University of Technology WUT
Current assignee: Wuhan University of Technology WUT
Priority date: 2022-03-04
Filing date: 2022-03-04
Publication date: 2022-06-14

Abstract

The invention discloses a method for simultaneously improving the strength and toughness of an ultrafine-grained Al-Mg alloy, which comprises the steps of firstly preparing equiaxed nanocrystalline powder of Al-Mg by adopting a ball-milling process, and forming a small amount of nano-sized nitride particles and oxide particles in the ball-milling process; then, consolidation is carried out to form compact block equiaxial superfine crystal Al-Mg alloy; and finally, rolling to obtain the nano-layer lamellar tissue with the nano-layer lamellar spacing and the micro-layer lamellar length. The method can synchronously improve the strength and toughness of the obtained Al-Mg alloy product, and provides a new idea for preparing high-performance Al-Mg alloy materials; and the related preparation process is simple, convenient and easy to control, the preparation conditions are mild, the requirement on preparation equipment is low, and the method is suitable for large-scale industrial production.

Description

Method for simultaneously improving strength and toughness of ultra-fine grain Al-Mg alloy

Technical Field

The invention belongs to the technical field of ultrafine grained metal materials, and particularly relates to a method for simultaneously improving the strength and toughness of an ultrafine grained Al-Mg alloy.

Background

The superfine crystal metal material is a novel structural material, and compared with the crude crystal metal material with the same components, the strength of the superfine crystal metal material can be improved by 2-10 times. Currently, ultra-fine grained metal materials are generally prepared in two ways: 1) severe plastic deformation of the macrocrystalline metal material; such as equal channel angular extrusion, high-pressure torsion, repeated overlapping rolling, high-strain rolling and the like; 2) consolidation of nanocrystalline powder obtained by ball milling: usually, hot isostatic pressing or vacuum hot pressing is used to obtain a dense bulk material, and then the bonding between the powders is improved by subsequent plastic deformation at higher temperatures, so as to achieve complete metallurgical bonding between the powders.

However, plastic deformation during the preparation of the ultra-fine grained metal material introduces high density of dislocations in the material, so that the ultra-fine grained metal material cannot further store dislocations (lacks work hardening capability), resulting in lower toughness of the ultra-fine grained metal material. In order to improve the toughness of the ultra-fine grain metal material, modification means such as annealing at a higher temperature are generally adopted at present: the ultra-fine grain metal material is kept at a higher temperature for a period of time to reduce the dislocation density. However, the reduction in dislocation density, while improving toughness, also reduces the dislocation strengthening effect; while annealing at higher temperatures results in grain growth. The decrease in dislocation strengthening and grain growth results in a decrease in strength of the ultra-fine grained metal material. Therefore, the toughness and the strength of the obtained ultrafine grained metal material cannot be effectively considered by the commonly adopted modification means such as higher-temperature annealing and the like at present.

Disclosure of Invention

The invention mainly aims to provide a method for simultaneously improving the strength and toughness of an ultrafine-grained Al-Mg alloy aiming at the problems and the defects in the prior art, wherein by changing the geometric form of the ultrafine grains, the equiaxial ultrafine grains are rolled and elongated to form a nano lamellar structure with the lamellar spacing of nanometer size and the lamellar length of micron size, so that the strength and the toughness of the nano lamellar structure are synchronously improved; and the related preparation process is simple, convenient to operate and suitable for popularization and application.

In order to achieve the purpose, the invention adopts the technical scheme that:

a method for simultaneously improving the strength and the toughness of an ultra-fine grain Al-Mg alloy comprises the following steps:

1) the equiaxed nanocrystalline powder of the Al-Mg alloy prepared by the consolidation ball milling method obtains block equiaxed superfine crystal Al-Mg alloy:

ball-milling the Al-Mg alloy powder to prepare equiaxed nanocrystalline powder of the Al-Mg alloy, and then consolidating the equiaxed nanocrystalline powder of the Al-Mg alloy to form compact isometric superfine crystal Al-Mg alloy blocks;

2) rolling the isometric superfine crystal Al-Mg alloy of the block to obtain a nano lamellar structure with the lamellar spacing of nano size and the lamellar length of micron size:

rolling the obtained block equiaxed superfine crystal Al-Mg alloy, and controlling the macroscopic orientation of the block equiaxed superfine crystal Al-Mg alloy to be always kept unchanged in the rolling process to obtain a nano lamellar structure with the nano-size lamellar spacing and the micron-size lamellar length.

In the scheme, the content of the main alloy element Mg adopted in the Al-Mg alloy is 1-10 wt%.

Furthermore, the Al-Mg alloy also comprises one or more of trace alloy elements such as Mn, Cr, Fe and the like.

Preferably, the Mn, Cr or Fe is contained in the Al-Mg alloy in an amount of 0.05 to 0.8 wt%, respectively.

In the scheme, the grain diameter of the Al-Mg alloy powder is 10-200 mu m; the material is prepared by inert gas atomization, rotary electrode atomization or centrifugal atomization and other processes, or a commercially available product is directly adopted.

Preferably, the Al — Mg alloy powder is a spherical powder.

In the scheme, the average grain size of the isometric nanocrystalline powder of the Al-Mg alloy is 10-100 nm; the average grain size of the block isometric ultrafine crystal Al-Mg alloy is 100-500 nm.

In the scheme, the interlayer spacing of the nano lamellar tissue is 10-100nm, and the length of each layer is 1-5 μm.

In the scheme, the ball milling process adopts a room temperature condition or a low temperature condition of-150 to-200 ℃.

Preferably, the low temperature condition is liquid nitrogen temperature or liquid argon temperature; the time required for ball-milling the Al-Mg powder to reach the nanocrystalline is short, and is 8-12 h.

Preferably, when the ball milling is carried out at room temperature, nitrogen or argon is filled in a ball milling tank; the content of oxygen in nitrogen is controlled to be 0.1-0.5 vol%, the content of nitrogen in argon is controlled to be 0.1-0.5 vol%, and the content of oxygen is controlled to be 0.1-0.5 vol%.

Preferably, when ball milling is performed under a low temperature condition, the Al-Mg alloy powder is completely immersed in liquid nitrogen or liquid argon for ball milling, wherein the content of oxygen in the liquid nitrogen is controlled to be 0.05 to 0.1 vol%, the content of nitrogen in the liquid argon is controlled to be 0.1 to 0.2 vol%, and the content of oxygen in the liquid argon is controlled to be 0.05 to 0.1 vol%.

In the scheme, the consolidation step adopts a step-by-step consolidation process: firstly, hot isostatic pressing or vacuum hot pressing technology is adopted for primary consolidation, and then hot extrusion technology is adopted for secondary consolidation.

In the above scheme, the hot isostatic pressing or vacuum hot pressing process parameters include: the temperature is 350-500 ℃, the pressure is 50-200MPa, and the time is 2-5 h; the hot extrusion process parameters comprise: the temperature is 300 ℃ and 450 ℃, the area shrinkage ratio is 6-36, and the movement speed of the extrusion punch is 0.01-10 mm/s.

In the scheme, the rolling is carried out at room temperature or at the low temperature of-50 to-200 ℃, the rolling thickness reduction is 50-95 percent, and the rolling strain rate is 1-20s^-1。

In the scheme, the low-temperature condition can be liquid nitrogen temperature, liquid argon temperature or alcohol temperature condition of-100 to-50 ℃ and the like.

The principle of the invention is as follows:

the method comprises the steps of firstly preparing equiaxed nano-crystalline powder of the Al-Mg alloy by adopting a ball milling process, wherein after nano-sized nitride particles and oxide particles formed in the ball milling process are consolidated into a massive equiaxed superfine-crystalline Al-Mg alloy, part of the nano-sized nitride particles and the oxide particles are distributed on a grain boundary, in addition, Mg solute atom segregation and solute atom segregation of one or more elements of Mn, Cr and Fe also exist on the grain boundary. In the subsequent rolling process of the equiaxed ultrafine grain Al-Mg alloy, the nanometer-sized nitride particles and oxide particles distributed on the grain boundary and Mg solute atom segregation, solute atom segregation of one or more elements of Mn, Cr and Fe can effectively inhibit stress-induced grain boundary migration and grain rotation in the rolling process, and equiaxed ultrafine grains are prevented from growing into equiaxed grains with larger size; meanwhile, as the rolled fine equiaxed ultrafine grains are fine, the formation of smaller equiaxed grains due to intracrystalline decomposition can be effectively prevented. The two effects act together to cause that the equiaxed ultra-fine crystal grains can be elongated only along the rolling direction to form a nano lamellar structure with the lamellar spacing of nano size and the lamellar length of micron size. During rolling, the dislocation density is increased along with rolling deformation, and the nano-scale lamellar spacing of the formed lamellar structure is smaller than the size of equiaxed ultrafine grains before rolling, so that the strength is further increased by the two factors. In addition, the length of the micron-sized lamina in the lamellar structure is beneficial to storing dislocation, so that the work hardening capacity and the toughness of the obtained alloy product can be synchronously improved.

Compared with the prior art, the invention has the beneficial effects that:

1) compared with the existing means of reducing dislocation density by annealing at higher temperature to improve the toughness of the ultrafine grained metal material, the invention firstly provides a method for converting an equiaxial form of ultrafine grains into a nano lamellar form with a nano-size lamellar spacing and a micron-size lamellar length by rolling the ultrafine grained metal material at room temperature or low temperature, synchronously improves the toughness and strength of the ultrafine grained metal material, and can provide a brand new idea for preparing a high-performance ultrafine grained metal material;

2) the atomization, ball milling, hot isostatic pressing or vacuum hot pressing consolidation, hot extrusion consolidation, rolling and the like adopted by the invention are conventional processes, the related preparation conditions are mild, the requirement on preparation equipment is low, the operation is convenient and easy to control, and the method is suitable for large-scale industrial production.

Drawings

FIG. 1 is a TEM micrograph of equiaxed ultrafine Al-4.5% Mg-0.7% Mn-0.15% Cr-0.25% Fe (mass%) alloy microstructure obtained in step 2) of example 1;

FIG. 2 is a TEM image of the Al-4.5% Mg-0.7% Mn-0.15% Cr-0.25% Fe (mass%) alloy with a nanoplatelet lamellar microstructure obtained in step 3) of example 1;

FIG. 3 is a three-dimensional atomic probe full elemental map of the Al-4.5% Mg-0.7% Mn-0.15% Cr-0.25% Fe (mass%) alloy with a nanolaminate-like microstructure obtained in step 3) of example 1 (for clarity of presentation, Al atoms show 1%, Mg atoms show 10%, other Mn, Cr, Fe atoms, etc. show 100%);

FIG. 4 is a graph of the distribution of oxides in a three-dimensional atom probe map of an Al-4.5% Mg-0.7% Mn-0.15% Cr-0.25% Fe (mass%) alloy with a nanoplatelet-like microstructure obtained in step 3) of example 1;

FIG. 5 is a distribution diagram of nitrides in a three-dimensional atom probe map of an Al-4.5% Mg-0.7% Mn-0.15% Cr-0.25% Fe (mass%) alloy with a nanoplatelet sheet-like microstructure obtained in step 3) of example 1;

FIG. 6 is a graph of tensile engineering stress-strain curves of the alloy obtained in step 2) of example 1.

FIG. 7 is a graph of tensile engineering stress-strain curves of the alloy obtained in step 3) of example 1.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail with reference to the following embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

Example 1

A method for simultaneously improving the strength and the toughness of Al-Mg ultrafine grained alloy comprises the following steps:

1) firstly, preparing Al-Mg alloy powder by adopting an argon atomization method, wherein the specific preparation steps comprise: according to the component requirements of Al-4.5% of Mg-0.7% of Mn-0.15% of Cr-0.25% of Fe (mass percent) alloy, putting pure Al, pure Mg, intermediate alloy Al-15% of Mn (mass percent), Al-3% of Cr (mass percent) and Al-5% of Fe (mass percent) in corresponding proportions into a graphite crucible, smelting into alloy at 900 ℃ under the protection atmosphere of argon, and preserving heat for 30min to obtain uniform components; the stopper rod is pulled out, the obtained liquid alloy flows out along the backflow pipe from the bottom of the crucible, high-pressure argon is sprayed out from an atomizing nozzle tightly coupled with the flow guide pipe to atomize the liquid alloy into fine liquid drops, the nozzle is provided with 12 gas nozzles, and the total area of the gas nozzles is 8.5mm²The gas pressure is 1.2MPa, and fine spherical coarse-grained powder with the average diameter of about 45 mu m is formed after the liquid drops are cooled;

putting the obtained spherical powder into a ball milling tank, putting a ball milling rod into the ball milling tank, stirring stainless steel balls for ball milling, wherein the ball-material ratio is 32:1, and the rotating speed of the ball milling rod is 180 r/min; continuously introducing liquid nitrogen into a ball milling tank in the ball milling process, controlling the content of oxygen in the liquid nitrogen to be 0.1 vol%, keeping the balance between the introduction amount of the liquid nitrogen and the volatilization amount of the liquid nitrogen, soaking the powder in the liquid nitrogen all the time in the ball milling process, performing ball milling at the liquid nitrogen temperature for 12 hours to obtain equiaxed nanocrystalline powder of an alloy with the average grain size of 26nm, wherein the content of O, N in the obtained powder is 0.45% and 0.06% (mass percentage) respectively;

2) performing primary consolidation on equiaxed nanocrystalline powder of an Al-4.5% Mg-0.7% Mn-0.15% Cr-0.25% Fe (mass percentage) alloy obtained by ball milling by adopting a hot isostatic pressing process, wherein the consolidation temperature is 400 ℃, the pressure is 175MPa, and the time is 5 hours, so as to obtain a compact hot isostatic pressing blank; secondly, performing secondary consolidation on the hot isostatic pressing blank by adopting a hot extrusion process to obtain an equiaxial superfine crystal Al-4.5% of Mg-0.7% of Mn-0.15% of Cr-0.25% of Fe (mass percentage), wherein the adopted hot extrusion temperature is 400 ℃, the area shrinkage ratio is 8, and the movement speed of an extrusion punch is 1 mm/s;

3) rolling the obtained equiaxial superfine crystal Al-4.5% Mg-0.7% Mn-0.15% Cr-0.25% Fe (mass percentage) alloy at room temperature to the thickness reduction of 80%, and the rolling strain rate of 6s^-1To obtain Al-4.5% Mg-0.7% Mn-0.15% Cr-0.25% Fe (mass percentage) alloy with nano-size lamellar spacing and micron-size lamellar length and nano-lamellar microstructure.

The microstructure of the equiaxed ultrafine Al-4.5% Mg-0.7% Mn-0.15% Cr-0.25% Fe (mass%) alloy obtained in step 2) of this example is shown in FIG. 1, and has an average grain size of about 215nm and an equiaxed grain structure. The microstructure of the nanolaminate Al-4.5% Mg-0.7% Mn-0.15% Cr-0.25% Fe (mass%) alloy obtained in step 3) is shown in FIG. 2, which shows an average interplay distance of about 46nm and an average interplay length of about 1.5 μm.

Fig. 3 is a three-dimensional atomic probe full elemental map (containing all elements Al, Mg, Mn, Cr, Fe, etc.) of the resulting Al-4.5% Mg-0.7% Mn-0.15% Cr-0.25% alloy (mass%) with a nanoplatelet microstructure, showing segregation of alloying elements at grain boundaries, measured concentrations of Mg, Mn, Cr, Fe at grain boundaries of 5.19%, 0.42%, 0.27%, 0.11% (mass%), in-grain concentrations of 3.28%, 0.27%, 0.018%, 0.08% (mass%), resulting in dissolved alloying element concentrations (solute concentrations) lower than the corresponding alloying element content in the alloy due to precipitation of a portion of the alloying elements as a second phase. The three-dimensional atom probe maps of fig. 4 and 5 show that a portion of the oxide and nitride particles are distributed at the grain boundaries.

FIGS. 6 and 7 are the tensile engineering stress-strain curves of the Al-4.5% Mg-0.7% Mn-0.15% Cr-0.25% Fe (mass%) alloys obtained in steps 2) and 3) of this example, respectively, and the results show that: the tensile yield strength of the equiaxial ultra-fine grain Al obtained in the step 2) before rolling is about 515MPa, the tensile strength is 640MPa, and the uniform elongation is about 2.0 percent, wherein the equiaxial ultra-fine grain Al is 4.5 percent of Mg, 0.7 percent of Mn, 0.15 percent of Cr and 0.25 percent of Fe (mass percent); after the room-temperature rolling treatment in the step 3), a nano lamellar microstructure with the average lamellar spacing of about 46nm and the average lamellar length of about 1.5 mu m can be formed, the tensile yield strength of the obtained alloy product is improved to about 625MPa, the tensile strength is 765MPa, and the uniform elongation is improved to about 3.7%.

Example 2

1) firstly, preparing Al-Mg alloy powder by adopting an argon atomization method, wherein the specific preparation steps comprise: according to the component requirement of Al-10% Mg (mass percent) alloy, putting pure Al and pure Mg (mass percent) in corresponding proportion into a graphite crucible, smelting the alloy at 750 ℃ in an argon protective atmosphere, keeping the temperature for 30min to obtain uniform components, pulling out a plug rod, enabling liquid alloy to flow out along a backflow pipe from the bottom of the crucible, simultaneously spraying high-pressure argon from an atomizing nozzle tightly coupled with a flow guide pipe to atomize the liquid alloy into fine liquid drops, wherein the nozzle is provided with 12 gas nozzles, and the total area of the gas nozzles is 8.5mm²The gas pressure is 1.5MPa, and fine spherical coarse-grained powder with the average diameter of about 40 mu m is formed after the liquid drops are cooled;

putting the obtained coarse-grained spherical powder into a ball milling tank, filling nitrogen into the ball milling tank, sealing, carrying out planetary ball milling, wherein the nitrogen contains 0.4 vol% of oxygen, the rotating speed of a main disc is 175r/min in the planetary ball milling process, the rotating speed of a planetary disc for placing the ball milling tank is 350r/min, the ball-material ratio is 25:1, carrying out ball milling at room temperature for 60 hours to obtain equiaxial nanocrystalline powder of Al-10% Mg (mass percent) alloy with the average grain size of about 20nm, and detecting that the O, N content in the obtained powder is 0.62% and 0.08% (mass percent) respectively;

2) performing primary consolidation on equiaxed nanocrystalline powder of an Al-10% Mg (mass percentage) alloy obtained by ball milling by adopting a vacuum hot pressing process, wherein the adopted consolidation temperature is 440 ℃, the pressure is 150MPa, and the time is 3h, so as to obtain a compact vacuum hot pressed blank; then, continuously carrying out secondary consolidation on the vacuum hot-pressed blank by adopting a hot extrusion process to obtain equiaxial superfine crystal Al-10% Mg (mass percentage) alloy, wherein the adopted hot extrusion temperature is 450 ℃, the area shrinkage ratio is 16, and the movement speed of an extrusion punch is 0.05 mm/s;

3) rolling the obtained equiaxial superfine crystal Al-10% Mg (mass percentage) alloy at the temperature of liquid nitrogenUntil the thickness reduction is 95 percent, and the rolling strain rate is 2s^-1The Al-10% Mg (mass percentage) alloy with the nano lamellar microstructure is obtained.

Tests prove that the equiaxial ultra-fine grain Al-10% Mg (mass percentage) alloy obtained in the step 2) of the embodiment has the average grain size of about 650nm, the tensile yield strength of about 420MPa, the tensile strength of 530MPa and the uniform elongation of about 3.5%; after rolling treatment at the liquid nitrogen temperature in the step 3), a nano lamellar microstructure with the average lamellar spacing of about 62nm and the average length of about 1.8 mu m can be formed, and the existence of oxide particles and nitride particles with nano sizes in a crystal boundary is observed by a transmission electron microscope; the tensile yield strength of the obtained alloy product is improved to about 655MPa, the tensile strength is improved to 730MPa, and the uniform elongation is improved to about 5.2 percent.

Comparative example 1

A rolling process of an equiaxed ultra-fine grain metal material without nanometer-sized second-phase particles in grain boundaries comprises the following steps:

1) according to the component requirements of Al-4.5% of Mg-0.7% of Mn-0.15% of Cr-0.25% of Fe (mass percent) alloy, putting pure Al, pure Mg, intermediate alloy Al-15% of Mn (mass percent), Al-3% of Cr (mass percent) and Al-5% of Fe (mass percent) in corresponding proportions into a graphite crucible, smelting under the protection atmosphere of high-purity argon, keeping the smelting temperature of 875 ℃ for 30 minutes to obtain uniform components, and then casting the uniform components into a steel mould. After the cast ingot is subjected to heat preservation for 24 hours at 500 ℃, carrying out component homogenization treatment, and carrying out 8-pass severe plastic deformation on an as-cast Al-4.5% Mg-0.7% Mn-0.15% Cr-0.25% Fe (mass percentage) alloy after the component homogenization treatment by using a Bc way of equal channel angular extrusion (after each pass of equal channel angular extrusion is finished, the extruded material rotates clockwise by 90 ℃ and the next pass of equal channel angular extrusion is carried out) at 100 ℃ to obtain an equiaxial ultrafine crystal Al-4.5% Mg-0.7% Mn-0.15% Cr-0.25% Fe (mass percentage) alloy, wherein the average grain size is about 190nm, the content of O, N in the detected alloy is below 0.005% (mass percentage), and the transmission electron microscope observation shows that no nanometer-sized second phase particles exist on a grain boundary; the tensile yield strength is about 510MPa, the tensile strength is 610MPa, and the uniform elongation is about 1.8%;

2) at room temperature, the obtained equiaxial ultra-fine grain Al-4.5% Mg-0.7% Mn-0.15% Cr-0.25% Fe (mass percentage) alloy is rolled to a thickness reduction of 80%. Because the grain boundary does not have the second phase particles with the nanometer size, the grain boundary migration and the grain rotation occur under the induction of external stress caused by rolling deformation in the rolling process, so that the grains grow up; after the rolling is completed, the crystal grains are still equiaxed, but the crystal grains grow up to an average grain size of 220 nm. Although the dislocation density is increased after rolling, the tensile yield strength after rolling is reduced to about 480MPa and the tensile strength is reduced to 590MPa due to the increase of the grain size, but the grain is still equiaxed ultrafine crystal after rolling, the micron-sized grains capable of effectively storing dislocations are lacked in the grains, and the uniform elongation is only 2.2%.

The results show that after rolling the equiaxed superfine crystal metal material without the second phase particles with nanometer sizes in the crystal boundary, the grain growth speed caused by the grain boundary migration and the grain rotation induced by stress is higher than the grain refinement speed caused by rolling deformation, so that the equiaxed superfine crystal further grows to generate equiaxed superfine crystal with larger size, and the nanometer lamellar structure with the interval of nanometer-sized lamellar sheets and the length of micron-sized lamellar sheets can not be obtained; the strength and toughness of the obtained product cannot be effectively considered.

Comparative example 2

A rolling process of an equiaxed coarse-grained metal material with a grain boundary containing nano-sized second-phase particles, wherein the coarse grains refer to grains with the size of more than 1 mu m, and the rolling process specifically comprises the following steps:

1) firstly, for the equiaxed coarse-grained Al-4.5% Mg-0.7% Mn-0.15% Cr-0.25% Fe (mass percentage) alloy prepared in the step 2) of the example 1, the temperature of the alloy starting to melt is 574 ℃, the alloy is kept at the high temperature of 550 ℃ for 6 hours, the ultra-fine grains grow up at the high temperature for a longer time, the equiaxed coarse-grained Al-4.5% Mg-0.7% Mn-0.15% Cr-0.25% Fe (mass percentage) alloy is obtained, the volume fraction of crystal grains with the size of 5-20 mu m is more than 96%, and almost all crystal boundaries are distributed with nano-sized oxide particles and nitride particles;

2) rolling the equiaxed coarse-grained Al-4.5% Mg-0.7% Mn-0.15% Cr-0.25% Fe (mass percentage) alloy with nano-sized oxide particles and nitride particles distributed in the grain boundary at room temperature until the thickness reduction is 80%; in the rolling process, because of the existence of the oxide particles and the nitride particles with the nanometer size of the crystal boundary, the original coarse crystal boundary does not migrate and does not rotate; however, under the action of rolling plastic deformation, a large number of dislocations are generated in the equiaxed coarse grains, and the dislocations are rearranged to form new grain boundaries, so that the original equiaxed coarse grains are decomposed to form new finer equiaxed grains instead of being elongated into a lamellar structure along the rolling direction.

Transmission electron microscope analysis shows that the Al-4.5% Mg-0.7% Mn-0.15% Cr-0.25% Fe (mass percentage) alloy obtained after rolling is composed of equiaxed superfine crystals containing high-density dislocation, the average grain size is about 250 mu m, the tensile yield strength reaches about 485MPa, the tensile strength is 625MPa, but the superfine crystal has no micron-sized size capable of storing dislocation, so the toughness is lower, and the uniform elongation is only 2.2%.

The above results indicate that after rolling an equiaxed coarse grain metal material containing second phase particles with nanometer size in the grain boundary, a new equiaxed ultrafine grain is formed due to decomposition of the coarse grain, and a nanometer lamellar structure with nanometer size lamellar spacing and micron size lamellar length cannot be obtained.

The above embodiments are merely illustrative of the technical ideas and features of the present invention, and the purpose thereof is to enable those skilled in the art to understand the contents of the present invention and implement the present invention, and not to limit the protection scope of the present invention. All equivalent changes and modifications made according to the spirit of the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for simultaneously improving the strength and the toughness of an ultra-fine grain Al-Mg alloy is characterized by comprising the following steps:

1) ball-milling the Al-Mg alloy powder to prepare equiaxed nanocrystalline powder of the Al-Mg alloy; then solidifying the isometric nanocrystalline powder of the Al-Mg alloy to form a block isometric superfine crystal Al-Mg alloy;

2) rolling the obtained block isometric ultrafine crystal Al-Mg alloy to obtain a nano lamellar structure with nano lamellar spacing and micro lamellar length.

2. The method according to claim 1, wherein the Al-Mg alloy powder has a content of the main alloying element Mg of 1-10 wt%.

3. The method according to claim 2, wherein the Al-Mg alloy comprises one or several of the minor alloying elements Mn, Cr, Fe.

4. The method according to claim 1, wherein the Al-Mg alloy powder has a particle size of 10-200 μ ι η; the average grain size of the equiaxed nanocrystalline powder of the Al-Mg alloy is 10-100 nm; the average grain size of the block isometric ultrafine crystal Al-Mg alloy is 100-500 nm.

5. The method of claim 1, wherein the nano-lamellar tissue has a lamella spacing of 10-100nm and a lamella length of 1-5 μm.

6. The method according to claim 1, wherein the ball milling process adopts a room temperature condition or a low temperature condition of-150 to-200 ℃.

7. The method of claim 6, wherein the ball milling is performed at room temperature by filling a ball milling tank with nitrogen or argon; the content of oxygen in nitrogen is 0.1-0.5 vol%, the content of nitrogen in argon is 0.1-0.5 vol%, and the content of oxygen is 0.1-0.5 vol%; when ball milling is carried out under the low temperature condition, the Al-Mg alloy powder is completely soaked in liquid nitrogen or liquid argon for ball milling, the content of oxygen in the liquid nitrogen is 0.05-0.1 vol%, the content of nitrogen in the liquid argon is 0.1-0.2 vol%, and the content of oxygen is 0.05-0.1 vol%.

8. The method of claim 1, wherein said consolidating step employs a step consolidation process: firstly, hot isostatic pressing or vacuum hot pressing technology is adopted for primary consolidation, and then hot extrusion technology is adopted for secondary consolidation.

9. The method of claim 8, wherein the hot isostatic pressing or vacuum hot pressing process parameters comprise: the temperature is 350-; the hot extrusion process parameters comprise: the temperature is 300 ℃ and 450 ℃, and the area shrinkage ratio is 6-36.

10. The method according to claim 1, wherein the rolling is performed at room temperature or at a low temperature of-50 to-200 ℃ with a rolling reduction of 50 to 95%.