CN113444946A

CN113444946A - High-strength and high-toughness rare earth magnesium alloy and treatment method thereof

Info

Publication number: CN113444946A
Application number: CN202110536044.6A
Authority: CN
Inventors: 薛勇; 张治民; 王强; 郑杰; 于建民; 闫钊鸣; 任贤魏; 徐健; 张慧芳
Original assignee: North University of China
Current assignee: North University of China
Priority date: 2021-05-17
Filing date: 2021-05-17
Publication date: 2021-09-28
Anticipated expiration: 2041-05-17
Also published as: CN113444946B

Abstract

The invention relates to a high-strength and high-toughness rare earth magnesium alloy and a processing method thereof, wherein the rare earth magnesium alloy is Mg- (7-12) Gd- (3-6) Y- (1-3) Zn- (0.2-1) Zr (wt.%), and comprises uniform fine grains with the average grain size of not more than 35 mu m, and dense short rod-shaped LPSO phase and cylindrical surface beta' nano strengthening phase precipitated from the uniform fine grains; the treatment method comprises the steps of deformation rare earth magnesium alloy with a bimodal microstructure, first aging, annealing and second aging. According to the invention, by carrying out alternate aging treatment and annealing treatment on the rare earth magnesium alloy, a fine crystal structure with uniformly distributed and dense short rod-shaped long-range ordered phase and beta' strengthening phase is obtained, the initiation of cracks can be inhibited, the mechanical property of the alloy can be improved, and the rare earth magnesium alloy with high strength and toughness is finally prepared.

Description

High-strength and high-toughness rare earth magnesium alloy and treatment method thereof

Technical Field

The invention belongs to the field of metal heat treatment, in particular to the field of magnesium alloy heat treatment, and particularly relates to a high-strength and high-toughness rare earth magnesium alloy and a treatment method thereof.

Background

The magnesium alloy has the characteristics of light weight, high specific strength and the like, and is widely applied to the fields of aerospace, automobile production, medical instruments, electronic equipment and the like. Meanwhile, the magnesium alloy has poor plasticity and difficult molding characteristics, which prevents the magnesium alloy from being widely applied. Compared with other types of magnesium alloys, the addition of the rare earth elements obviously improves the heat resistance and corrosion resistance. The rare earth element yttrium is beneficial to forming a second phase, thereby achieving the purpose of weakening the texture and improving the mechanical property. The second phase formed is most commonly a bulk phase and a lamellar phase. The lamellar phase can be adjusted to deform in a twisting mode, the blocky phase is favorable for forming recrystallized grains so as to refine the grains and improve the grain orientation, and the beta' precipitation phase is easy to form and strengthen and improve the mechanical strength. In addition, the grains of the alloy can be refined by the thermal deformation, and the ductility and strength can also be improved by the fine grain strengthening mechanism.

While the alloys deformed by conventional extrusion and rolling tend to have strong basal plane texture orientations, which result in mechanical property anisotropy and reduced ductility. For this reason, a large plastic deformation technique has been proposed, which has promoted the widespread use of magnesium alloys in practical production and application activities. Such as equal channel angular extrusion and high-pressure torsion, can significantly refine grains and weaken basal plane texture, but the prepared sample has small specification and high equipment requirement, so that the application to industrial production is difficult.

At present, the heat treatment methods of wrought magnesium alloys mainly include traditional aging treatment, deformation aging treatment and double-stage aging treatment (for example, patent CN 101191168A), the above aging treatment is based on the microstructure after original deformation, the aging strengthening effect of the alloy is enhanced by increasing the dislocation density in the alloy, and these methods can only increase the strength unilaterally but reduce the ductility. Therefore, there is a need for an optimized heat treatment process to improve the ductility of the alloy while increasing the strength of the alloy.

Disclosure of Invention

Aiming at the problems that the existing metal heat treatment process has inapplicability to the deformation rare earth magnesium alloy, mainly shows that the performance is not optimal, the ductility of the alloy cannot be improved while the alloy strength is improved, the energy consumption is high, the working procedure is complex and the like, the invention provides a method for treating the deformation rare earth magnesium alloy, and the problems that the microstructure is optimized, the structure and the size are stable, and a better match between the mechanical properties is obtained, so that the requirements in the actual production are met.

Specifically, according to the treatment process and the optimized microstructure of the deformation state rare earth magnesium alloy, the high-strength and high-toughness rare earth magnesium alloy is prepared through alternate aging and annealing treatment, and the precipitated phase and the structure in the rare earth magnesium alloy are regulated and controlled through the heat treatment process, so that the peak aging hardness is achieved, the crystal grain arrangement is improved, and the toughness is improved.

One of the technical schemes of the invention is to provide a high-strength rare earth magnesium alloy, which is Mg- (7-12) Gd- (3-6) Y- (1-3) Zn- (0.2-1) Zr (wt%), and the microstructure of the rare earth magnesium alloy comprises uniform fine grains with the average grain size not more than 35 mu m, preferably the average grain size not more than 25 mu m, more preferably the average grain size not more than 10 mu m, wherein the uniform fine grains account for more than 85 vol%, preferably more than 90%, more preferably 92-97.7 vol%; and precipitating dense short rod-shaped LPSO phase and cylindrical beta ' nano strengthening phase from the uniform fine crystal grains, wherein the average diameter of the cylindrical beta ' nano strengthening phase is less than 50nm, preferably less than 35nm, more preferably less than 25nm, and the proportion of the cylindrical beta ' nano strengthening phase is 2-7.6 vol%; the dense short rod-like LPSO phase is uniformly distributed inside the fine crystal grains.

Furthermore, the yield strength of the high-strength and high-toughness rare earth magnesium alloy is not lower than 400MPa, the ultimate tensile strength is not lower than 478MPa, and the elongation is more than 10%.

The second technical scheme of the invention provides a method for preparing high-strength and high-toughness rare earth magnesium alloy, which comprises the following steps:

step one, blanking: taking a blank from the as-cast bar;

step two, homogenization treatment: carrying out homogenization treatment on the as-cast blank, and then cooling the blank at room temperature; preferably, the homogenization treatment temperature is 450-530 ℃, and the time is 12-24 h;

step three, extrusion deformation: preheating and insulating the blank subjected to the homogenization treatment in the step two for a certain time, and then putting the blank into an extrusion die which is preheated and insulated for extrusion deformation to obtain an extrusion deformation blank with a bimodal microstructure; preferably, the extrusion deformation temperature is 350-480 ℃, the extrusion ratio is 4-30:1, the extrusion rate is 0.5-1.5mm/s, preferably 0.6-1.0mm/s, and the blank heat preservation time is 15-45 min;

step four, first aging treatment: sampling the extruded blank in the third step, putting the sampled blank into a preheated aging furnace, performing aging treatment, taking out the sample every 0.5-2h, performing water cooling, and then performing hardness test until the hardness reaches the highest value, namely reaching peak aging, wherein the time for obtaining the peak aging is aging time, the temperature range of the aging treatment is 100-250 ℃, the aging time is 0.5-256h, and preferably 2-120 h;

step five, annealing treatment: putting the peak aging sample obtained in the step four into a preheated heat treatment furnace, keeping the temperature for a certain time, taking out the sample, performing room temperature water cooling, taking out the sample every 0.5-2h, performing water cooling, and performing metallographic structure observation until a uniform microstructure is obtained; the annealing temperature is 300-500 ℃, and the annealing time is 1-20 h, preferably 5-10 h; further, with the increase of annealing time, the average grain size in the sample tends to be reduced, the average KAM value is increased and then decreased, the texture strength of the extrusion basal plane tends to be reduced, and the texture is changed into a weak texture which is randomly distributed after the annealing is finished;

step six, re-aging treatment (secondary aging treatment): and placing the annealed blank into a preheated aging furnace, performing aging treatment, wherein samples are taken out every 0.5-2h, performing water cooling, and then performing hardness test until the hardness reaches the highest value, namely reaching peak aging, and the time for obtaining the peak aging is aging time, wherein the aging temperature range is 100-250 ℃, and the aging time is 0.5-256h, preferably 5-150 h.

Further, the extrusion-deformed billet having a bimodal microstructure in the third step is a microstructure in which fine recrystallized grains and coarse deformed grains are combined, and the proportion of the fine recrystallized grains and the coarse deformed grains is determined by the extrusion-deformation temperature and the extrusion ratio (deformation amount);

furthermore, the peak aging in the fourth step is a state that the strengthening effect of the fine precipitated phase is optimal and most intensive;

in a preferred embodiment, the heating speed of the heating furnace in the second step to the sixth step is 0.3-0.5 ℃/s;

in a preferred embodiment, the aging blank is ensured to enter water within 1min in the fourth step, and the temperature is reduced to the room temperature within 3 min;

in a preferred embodiment, the temperature of the extruded billet in step three is the same as the extrusion die.

In a preferred embodiment, the temperature of the first aging is the same or different from the temperature of the second aging, and the second aging time is greater than the first aging time.

The purpose/reason for determining the steps one to six and the corresponding processing conditions of the invention is that:

the first step to the third step are mainly to prepare an extrusion deformation blank with a bimodal microstructure, i.e. the microstructure of the extrusion deformation blank is a combination of fine recrystallized grains and coarse deformation grains, and the specific proportion is mainly determined by the extrusion deformation temperature and the extrusion ratio (deformation amount).

The peak aging treatment in the fourth step is mainly to perform low-temperature aging treatment on the blank after the extrusion deformation treatment to precipitate a large amount of nano-scale fine strengthening phases, wherein the peak aging treatment is a state that the strengthening effect of the fine precipitated phases is optimal and most dense, and relatively low aging temperature hardly influences the grain size distribution of the alloy.

Annealing treatment in the fifth step, which is mainly to carry out high-temperature (relative to the aging temperature) annealing treatment on the sample in the first step peak aging state, and to enable the coarse deformation crystal grains to generate static recrystallization and particle excitation nucleation through higher temperature, refine the crystal grains and reduce the size of the crystal grains; in addition, the dense strengthening phase precipitated by the first aging treatment is partially coarsened in the annealing treatment and then serves as a nucleation site to form fine recrystallized grains through a particle-excited nucleation mechanism, and the other dense strengthening phase pins dislocations and grain boundaries in a fine grain region to prevent the fine grains from growing in the annealing treatment. Furthermore, the higher the annealing temperature, the higher the solubility of solute atoms in the magnesium matrix. Further research shows that the average grain size of the sample tends to decrease with the increase of annealing time, the average KAM value increases first and then decreases, the texture strength of the extrusion basal plane tends to decrease, and weak textures which are randomly distributed are obtained after the annealing is completed, wherein the average grain size of the sample after 5h of annealing is less than or equal to 10 mu m, the average KAM value is less than 0.6, and the texture strength is less than or equal to 2.5.

And in the step six, the re-aging treatment is mainly to perform the re-aging treatment on the blank with uniform and fine structure after the annealing treatment, so as to precipitate a dense strengthening phase and improve the strength and the ductility of the blank. On the one hand, the uniform microstructure is formed due to the previous annealing treatment, which changes the precipitation position of the strengthening phase precipitated by aging again, namely, the strengthening phase is uniformly distributed in a fine grain region (compared with an extruded alloy with coarse deformation grains); on the other hand, higher annealing temperatures increase the solubility of solute atoms to allow for the precipitation of denser strengthening phases upon re-aging (as compared to the first aging).

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

1. the cast rare earth magnesium alloy blank obtains uneven bimodal microstructure and strong base surface texture after being extruded and deformed, so that the initiation and growth of cracks are easy to have great influence on the performance of the blank, but the high dislocation density in the coarse grains has a key effect on a particle excited nucleation mechanism and static recrystallization in the annealing process.

2. The alternate aging treatment and the annealing treatment in the invention can refine crystal grains and improve the distribution of the second phase, optimize the overall structure, facilitate the elimination of the overall performance difference of the blank, ensure the stability of the performance by more uniform structure distribution, and on the other hand, adjust the overall performance of the blank and prevent the situation that the local stress is too high to influence the reduction of the performance such as the plasticity of the blank.

3. The secondary aging treatment provides guarantee for the overall performance of the blank to reach the optimal state, annealing brings optimization on the aspects of structural organization and toughness, the solubility of solute in the alloy can be improved to a certain extent, and the increase of precipitated phases during secondary aging is facilitated; in addition, the uniformly distributed fine grains can change the distribution position of a precipitated phase and improve the serious segregation phenomenon.

4. For the extruded blank with precipitated phases, the precipitated quantity and distribution position of the second phase have great influence on the overall performance of the blank, and the temperature and the time are important factors influencing the precipitation of the second phase.

5. The method is simple to implement, can be operated in large batch, and can obviously improve the toughness of the alloy compared with the traditional heat treatment mode of matching solid solution with aging; compared with large plastic deformation such as equal channel angular extrusion, high-pressure torsion and the like, the processing method can obviously reduce equipment requirements and increase sample specifications. Furthermore, the treatment method of the invention is more cost-effective from an economic point of view, and the improvement of the subsequent heat treatment process optimizes the overall production route for the complete production of the product.

6. The sample subjected to the alternate aging and annealing treatment of the invention has excellent strength-ductility synergistic effect which is mainly attributed to the coordinated deformation of homogeneous tissues and the pinning effect of short rod-shaped LPSO phase and prismatic beta' relative dislocation and grain boundary, so that the strength can be obviously improved and the generation of cracks can be inhibited, and the treated sample has excellent comprehensive mechanical properties, the yield strength is not less than 400MPa, the ultimate tensile strength is not less than 478MPa, and the elongation is more than 10%.

Drawings

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings needed to be used in the embodiments will be briefly described below, it should be understood that the following drawings only illustrate some embodiments of the present invention and therefore should not be considered as limiting the scope, and for those skilled in the art, other related drawings can be obtained according to the drawings without inventive efforts.

FIG. 1 is a structural morphology diagram of an extruded Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy before heat treatment.

FIG. 2 is a diagram of the peak aging structure of an extruded Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy after conventional heat treatment.

FIG. 3 is a structural morphology diagram of a peak-aged Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy after annealing treatment in the heat treatment mode of the invention.

FIG. 4 is a diagram of the peak aging structure of the extruded Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy after the second aging treatment in the heat treatment mode of the invention.

FIG. 5 is an Electron Back Scattering Diffraction (EBSD) structure morphology of the Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy with different annealing times;

wherein:

FIG. 5a is the EBSD structure morphology of the Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy sample after annealing for 1 h;

FIG. 5b is a graph of local stress orientation (KAM) corresponding to the EBSD structure morphology of the Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy sample after annealing for 1 h;

FIG. 5c is an EBSD pole figure of the Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy sample after annealing for 1 h;

FIG. 5d is the EBSD structure morphology of the Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy sample after annealing for 3 h;

FIG. 5e is a KAM diagram corresponding to the EBSD structure morphology of the Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy sample after annealing for 3 h;

FIG. 5f is an EBSD pole figure of the Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy sample after annealing for 3 h;

FIG. 5g is the EBSD structure morphology of the Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy sample after annealing for 5 h;

FIG. 5h is a KAM diagram corresponding to the EBSD structure morphology of the Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy sample after annealing for 5 h;

FIG. 5i is an EBSD pole figure of the Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy sample after annealing for 5 h.

FIG. 6 is a line graph showing the aged hardness in the heat treatment method of the present invention and the conventional heat treatment method.

FIG. 7 is a stress-strain diagram of the heat treatment method of the present invention and the conventional heat treatment method.

FIG. 8 is a TEM structural morphology of the Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy after the heat treatment of the present invention and the conventional heat treatment;

wherein:

FIG. 8a is a TEM structure morphology of coarse grains in a Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy after a conventional aging treatment;

FIG. 8b is a Selected Area Electron Diffraction (SAED) corresponding to a TEM texture map of coarse grains in a conventional aging treated Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy;

FIG. 8c is a TEM structure morphology of fine grains in the Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy after the conventional aging treatment;

FIG. 8d is a Selected Area Electron Diffraction (SAED) corresponding to a TEM texture map of fine grains in a conventional Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy after aging treatment;

FIG. 8e is a TEM texture map of fine grains after the second aging in the heat treatment of the present invention;

FIG. 8F is an enlarged view of area F in TEM texture map (e) of fine grains after a second aging in a heat treatment of the present invention;

FIG. 8g is a SAED plot of an enlarged view of the F region in the TEM texture profile (e) of fine grains after a second aging in a heat treatment of the present invention;

FIG. 8H is an enlarged view of the area H in the TEM texture profile (e) of fine grains after the second aging in the heat treatment of the present invention;

FIG. 8i is a SAED plot of an enlarged view of the H region in the TEM texture profile (e) of fine grains after the second aging in the heat treatment of the present invention;

FIG. 9 is a cross-sectional structure of a conventional heat treated Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy;

wherein:

FIG. 9a is an EBSD structure pattern diagram of a conventional heat-treated Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy;

FIG. 9b is a KAM diagram corresponding to the EBSD structure morphology diagram of the conventional heat treated Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy;

FIG. 9c is an SEM image corresponding to an EBSD structure morphology of the Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy after the conventional heat treatment;

FIG. 9d is the average KAM value corresponding to the EBSD structure morphology chart of the conventional heat treated Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy;

FIG. 9e is the average KAM value corresponding to the fine grain region in the EBSD structure morphology of the conventional heat treated Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy;

FIG. 9f is the average KAM value corresponding to the coarse grain region in the EBSD structure morphology of the conventional heat treated Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy.

FIG. 10 is a drawing of the tensile cross-sectional structure of the Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy after heat treatment according to the invention:

wherein:

FIG. 10a is an EBSD structure pattern diagram of the Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy after the second aging treatment of the heat treatment of the present invention;

FIG. 10b is a KAM diagram corresponding to the EBSD structure morphology diagram of the Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy after the second aging treatment of the heat treatment of the present invention;

FIG. 10c is an SEM image corresponding to the EBSD structure morphology of the Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy after the second aging treatment of the heat treatment of the invention

FIG. 10d is a polar diagram corresponding to the EBSD structure morphology diagram of the Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy after the second aging treatment of the heat treatment of the present invention;

FIG. 10e is the average KAM value corresponding to the EBSD structure morphology chart of the Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy after the second aging treatment of the heat treatment of the invention.

FIG. 11 is a cross-sectional view of Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy with heat treatment according to the present invention and with conventional heat treatment;

wherein:

FIG. 11a is a cross-sectional view of a conventional heat treated Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy;

FIG. 11b is a corresponding enlarged view in a cross-sectional profile of a conventional heat treated Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy;

FIG. 11c is a corresponding enlarged view in a cross-sectional profile of a conventional heat treated Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy;

FIG. 11d is a corresponding enlarged view in a cross-sectional profile of a conventional heat treated Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy;

FIG. 11e is a corresponding enlarged view in a cross-sectional profile of a conventional heat treated Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy;

FIG. 11f is a cross-sectional view of the Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy after the second aging treatment of the heat treatment of the present invention;

FIG. 11g is a corresponding enlarged view in a cross-sectional profile of the Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy after the second aging treatment of the heat treatment of the present invention;

FIG. 11h is a corresponding enlarged view in a cross-sectional profile view of the Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy after the second aging treatment of the heat treatment of the present invention;

FIG. 11i is a corresponding enlarged view in the cross-sectional morphology of the Mg-9Gd-4Y-2Zn-0.5Zr rare earth magnesium alloy after the second aging treatment of the heat treatment of the present invention.

Detailed Description

To further explain the technical means and effects of the present invention adopted to achieve the predetermined object, the following detailed description of the embodiments, structures, features and effects according to the present invention will be made with reference to the accompanying drawings and preferred embodiments. In the following description, different "one embodiment" or "an embodiment" refers to not necessarily the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

The invention is further illustrated by the following specific examples:

example 1

The material adopted in the embodiment is an as-cast Mg-9Gd-4Y-2Zn-0.5Zr (wt.%) rare earth magnesium alloy rod with the diameter of 440mm and the height of 660 mm.

The invention adopts a heat treatment mode combining multi-step aging and annealing, which comprises the following specific steps:

step one, blanking: a blank with the diameter of 60mm and the height of 50mm is machined from an as-cast bar with the diameter of 440mm and the height of 660 mm;

step two, homogenization treatment: homogenizing the as-cast bar stock at 520 ℃ for 24h, and taking out the blank to be cooled by water at 25 ℃;

step three, extrusion deformation: preheating and insulating the homogenized blank at 450 ℃ for 30min, putting the preheated and insulated blank into an extrusion die preheated to 459 ℃ in advance and insulated for 8h for extrusion deformation, wherein the extrusion deformation temperature is 450 ℃, and the extrusion ratio is 16: 1, the extrusion rate is 0.8 mm/s;

step four, first aging treatment: putting the extruded blank into a preheated aging furnace for aging treatment at 200 ℃, wherein samples are subjected to room temperature water cooling every 1h, and then performing hardness test until the hardness reaches the highest value, wherein the sample with the highest hardness is a peak aging sample, and the sample in the step reaches peak aging within 4 h;

step five, annealing treatment: putting the sample subjected to the four-peak aging in the step into a preheated heat treatment furnace, and carrying out annealing treatment at 420 ℃, wherein the sample is taken out every 1h and is subjected to room temperature water cooling, and then metallographic structures are observed, and a better uniform microstructure is obtained after 5h of annealing treatment;

step six, secondary aging treatment: putting the blank subjected to the annealing treatment in the fifth step into a preheated aging furnace for aging treatment at 200 ℃, wherein samples are subjected to room temperature water cooling every 1 hour, and then performing hardness test until the hardness reaches the highest value, wherein the sample with the highest hardness is a second peak aging sample, and the sample in the step reaches peak aging within 9 hours;

in addition, the tensile bar after the second aging treatment is subjected to a room temperature tensile test to obtain the tensile property of the sample after the two aging and annealing treatments, and the yield strength is 425MPa, the tensile strength is 493.2MPa, and the elongation is 11.2%.

In addition, in order to compare the advantages of the process of the invention, the first aging temperature is the same as the second aging temperature, so that the comparison is easier and the strengthening and toughening effects of the invention are more prominent.

Example 2

The procedure was the same as in example 1, except that the annealing time was 10 hours, the yield strength was 400MPa, the tensile strength was 478MPa, and the elongation was 10.5%.

Example 3

The procedure was the same as in example 1 except that the annealing temperature was 400 ℃. The yield strength is 412MPa, the tensile strength is 485MPa, and the elongation is 9%.

Example 4

The procedure is the same as in example 1 except that the secondary aging temperature is 175 ℃, the yield strength is 436MPa, the tensile strength is 512MPa, and the elongation is 10.9%.

The mechanical properties of the samples prepared in inventive examples 1-4 are shown in table 1.

TABLE 1

Examples	Yield strength (MPa)	Tensile strength (MPa)	Elongation (%)
				Example 1	425	493.2	11.2
Example 2	400	478	10.5
				Example 3	412	485	9
Example 4	436	512	10.9

Table 2 shows the micro vickers hardness test data obtained for the samples of example 1 at different heat treatment steps.

TABLE 2

Comparative example 1

The adopted material is an as-cast Mg-9Gd-4Y-2Zn-0.5Zr (wt.%) rare earth magnesium alloy rod with the diameter of 440mm and the height of 660mm, and the preparation method specifically comprises the following steps:

step 1, blanking: a blank with the diameter of 60mm and the height of 50mm is machined from an as-cast bar with the diameter of 440mm and the height of 660 mm;

step 2, homogenization treatment: homogenizing the as-cast bar stock at 520 ℃ for 24h, and taking out the blank to be cooled by water at 25 ℃;

step 3, extrusion deformation: preheating and insulating the homogenized blank at 450 ℃ for 30min, putting the preheated and insulated blank into an extrusion die preheated to 459 ℃ in advance and insulated for 8h for extrusion deformation, wherein the extrusion deformation temperature is 450 ℃, and the extrusion ratio is 16: 1, the extrusion rate is 0.8 mm/s;

and (3) carrying out room-temperature tensile test on the tensile bar of the sample subjected to extrusion deformation to obtain the tensile property of the sample subjected to traditional extrusion deformation, wherein the yield strength is 307MPa, the tensile strength is 377.3MPa, and the elongation is 8.84%.

Comparative example 2

Steps 1 to 3 are the same as comparative example 1, except that the conventional aging treatment of step 4 is carried out on the sample after extrusion deformation, specifically:

and 4, step 4: and (3) putting the extruded blank into a preheated aging furnace for aging treatment at 200 ℃, wherein samples are subjected to room temperature water cooling every 1h, then hardness testing is carried out, the hardness reaches the highest value after 5h, and the sample with the highest hardness is called as a peak aging sample.

And (3) carrying out room-temperature tensile test on the tensile bar of the extrusion sample subjected to the traditional aging treatment to obtain the tensile property of the sample subjected to the traditional extrusion deformation, wherein the yield strength is 370MPa, the tensile strength is 440MPa, and the elongation is 7.95%.

Comparative example 3

Steps 1-4 are the same as comparative example 2, except that step 5 is performed on the time-effect sample: annealing treatment is carried out for 5h at 420 ℃.

And (3) carrying out room-temperature tensile test on the tensile bar of the extrusion sample after annealing treatment to obtain the tensile property of the sample after annealing treatment, wherein the yield strength is 338MPa, the tensile strength is 409MPa, and the elongation is 14.2%.

The mechanical properties of the samples obtained in comparative examples 1 to 3 are shown in Table 3.

TABLE 3

Comparative example	Yield strength (MPa)	Tensile strength (MPa)	Elongation (%)
				Comparative example 1	307	377.3	8.84
Comparative example 2	370	440	7.95
				Comparative example 3	338	409	14.2

Table 4 shows the micro vickers hardness test data of comparative example 2 after the conventional heat treatment.

TABLE 4

Procedure (ii)	Aging treatment (200 ℃ for 5h)
		Vickers Hardness (HV)	96.45

Table 5 shows the tensile property test data of the samples subjected to different treatment modes.

TABLE 5

Further analysis showed that: as shown in fig. 1-2, the samples of the extruded rare earth magnesium alloy after the conventional aging treatment still have the typical bimodal microstructure, and coarse deformed grains are surrounded by fine recrystallized grains; as shown in fig. 3-4, the samples treated by the method of the present invention had a microstructure of uniform fine grains. Further, the average grain size of the extruded rare earth magnesium alloy is about 48.4 μm, and the fine grain percentage is about 35.5 vol%; due to the fact that the aging temperature is low, dislocation motion is difficult to activate, the average grain size and the percentage of fine grains of a sample subjected to the traditional aging treatment are almost unchanged and are respectively 48.3 mu m and 36.5 percent; the coarse grains in the sample treated by the present invention almost disappeared, and the percentage of uniform fine grains having an average grain size of about 8.2 μm was about 92 to 97.7 vol%.

The effect of different annealing times on the alloy microstructure and local stress concentration (KAM) and grain orientation according to the present invention is shown in fig. 5, where: the average grain size of the samples after annealing for 1h was about 34.9 μm; and the average KAM value rises from 0.78 to 0.98 of the extruded sample, since dislocations are activated in large quantities at the time of initial heating; and a basal texture intensity of 17.2; with increasing annealing time, the average grain size of the samples after 3h of annealing was about 22.7 μm, the average KAM value decreased to 0.62, and the texture strength was 10.1. It is clear that as the annealing time increases, the coarse deformed grains in the sample become increasingly engulfed by the fine grains, and the high stress concentration in the coarse grains is also depleted. With the completion of the annealing process, the typical extrusion basal plane texture in the final sample evolves into weak texture which is randomly distributed; the average grain size of the samples after 5h of annealing had dropped to about 8.2 μm, the average KAM value to 0.57, and the texture strength was only 2.1.

As shown in FIG. 7, the strength, ultimate tensile strength and ductility of the as-extruded samples were 307MPa, 377.3MPa and 8.84%, respectively; the strength of an extruded sample subjected to traditional heat treatment is improved, but the ductility is reduced, and the strength, the ultimate tensile strength and the ductility are respectively 370MPa, 440MPa and 7.95 percent; the strength and the ductility of the sample treated by the method are obviously improved, and the strength, the ultimate tensile strength and the ductility of the sample respectively reach 425MPa,493.2MPa and 11.2 percent.

As shown in fig. 2, the average length of the metastable gamma' strengthening phase of the base surface precipitated from the fine crystal grains of the sample after the traditional heat treatment is 178nm, the thickness is 3.4nm, and the percentage is 3.4 vol%; the coarse grains are mainly internally provided with longer and thicker LPSO phases, the average length is more than 1 μm, and the average thickness is more than 50 nm. As shown in fig. 8e-8i, the sample treated by the present invention separated out dense rod-like LPSO phase and cylindrical β ' strengthening phase from the uniform fine grains, the average diameter of the cylindrical β ' strengthening phase was only about 23nm, the percentage of the cylindrical β ' strengthening phase was about 2-7.6 vol%, and the dense short rod-like LPSO phase was almost densely and uniformly distributed in the uniform fine grains.

As shown in fig. 9-10, the magnitude of the average KAM value reflects the magnitude of the dislocation density. As shown in fig. 9a to 9f, the dislocation distribution in the cross-sectional view of the conventional heat-treated tensile specimen was not uniform, and the average KAM value was only 1.09, which was improved by 0.31 relative to the average KAM value in the extruded sample; as shown in fig. 10a-10e, the dislocation density of the tensile specimens after heat treatment of the present invention is uniformly distributed, and the average KAM value is 1.22, which is 0.65 higher than the average KAM value of the annealed specimens, indicating that the dislocation density of the tensile specimens after treatment of the present invention is higher, and the uniform and fine grains can accommodate more dislocations, while improving the strength and ductility of the alloy.

As shown in FIGS. 11a-11e, the tensile cross-section of the conventionally treated sample had a large number of cracks, concentrated in coarse grains and massive long-range ordered phases and in the boundary between them, with most of the cracks being longer than 50 μm and wider than 5 μm; as shown in FIGS. 11f to 11i, the tensile cross-sectional profile of the sample treated by the present invention had fewer cracks and was substantially concentrated around the bulk long-range ordered phase, and the dense short rod-like LPSO phase was uniformly distributed inside the uniform fine grains, with most of the cracks being no more than 10 μm and the width being only 3 μm. This shows that cracks in the samples treated by the traditional aging treatment are easy to initiate and expand, while cracks in the samples treated by the invention are shorter and thinner, are not easy to expand and have better ductility.

In summary, the present invention provides a process combining alternating aging and annealing treatments. The result shows that after alternate aging and annealing treatment, the short rod-shaped long-period stacking ordered phase (LPSO) phase and the cylindrical surface beta 'nano strengthening phase are densely and uniformly distributed in a sample with a uniform microstructure, and only the gamma' phase is sparsely distributed in the sample after the traditional aging treatment. The conventional aged sample has a fine grain region with a bimodal microstructure, and high local stress concentration is easily generated by deformation incompatibility among deformed grains with strong matrix structure and different regions (deformed grain region, fine grain region and bulk LPSO phase), so that the alloy is broken and cannot be fully work-hardened. The samples subjected to the alternate aging and annealing treatments of the present invention have excellent strength-ductility synergy. This is mainly due to the coordinated deformation of the homogeneous structure and the pinning effect of the short rod-like LPSO phase and the prismatic surface β' nano-reinforcement with respect to dislocations and grain boundaries, which can significantly improve the strength and suppress the generation of cracks. In conclusion, the samples after heat treatment according to the invention have excellent overall mechanical properties, a yield strength of 425MPa, an ultimate tensile strength of 493.2MPa and an elongation of 11.2%. The method is beneficial to the wide application of the magnesium alloy in various fields and promotes the saving of petroleum resources.

The foregoing detailed description of the preferred embodiments of the invention has been presented. It should be understood that numerous modifications and variations could be devised by those skilled in the art in light of the present teachings without departing from the inventive concepts. Therefore, the technical solutions available to those skilled in the art through logic analysis, reasoning and limited experiments based on the prior art according to the concept of the present invention should be within the scope of protection defined by the claims.

Claims

1. The high-strength and high-toughness rare earth magnesium alloy is Mg- (7-12) Gd- (3-6) Y- (1-3) Zn- (0.2-1) Zr (wt.%), and is characterized in that: the rare earth magnesium alloy comprises uniform fine grains with the average grain size not more than 35 mu m, and a dense short rod-shaped LPSO phase and a cylindrical beta' nano strengthening phase which are precipitated from the uniform fine grains; preferably, the average crystal grain size is not more than 25 μm, more preferably, the average crystal grain size is not more than 10 μm, and the uniform fine crystal grains account for 85 vol% or more, preferably 90 vol% or more, more preferably 92 to 97.7 vol%; the average diameter of the cylindrical beta 'nano reinforcing phase is less than 50nm, preferably less than 35nm, more preferably less than 25nm, and the proportion of the cylindrical beta' nano reinforcing phase is 2-7.6 vol%; the dense short rod-shaped LPSO phase is uniformly distributed in the uniform fine crystal grains.

2. The high strength and toughness rare earth magnesium alloy as claimed in claim 1, wherein: the yield strength of the high-strength and high-toughness rare earth magnesium alloy is not lower than 400MPa, the ultimate tensile strength is not lower than 478MPa, and the elongation is more than 10%.

3. A method for preparing high-strength and high-toughness rare earth magnesium alloy comprises the following steps:

step one, blanking: taking a blank from the as-cast bar;

step three, extrusion deformation: preheating and insulating the blank subjected to the homogenization treatment in the step two for a certain time, and then putting the blank into an extrusion die which is preheated and insulated for extrusion deformation to obtain an extrusion deformation blank with a bimodal microstructure; preferably, the extrusion deformation temperature is 350-480 ℃, and the extrusion ratio is 4-30:1, the extrusion rate is 0.5-1.5mm/s, preferably 0.6-1.0mm/s, and the blank heat preservation time is 15-45 min;

step four, first aging treatment: sampling the extruded blank in the third step, putting the sampled blank into a preheated aging furnace, performing aging treatment, taking out the sample every 0.5-2h, performing water cooling, performing hardness test until the hardness reaches the highest value, namely peak aging, wherein the time for obtaining the peak aging is aging time, the temperature of the aging treatment is 100-250 ℃, and the aging time is 0.5-256h, preferably 2-120 h;

step five, annealing treatment: putting the peak aging sample obtained in the step four into a preheated heat treatment furnace, keeping the temperature for a certain time, taking out the sample, performing room temperature water cooling, taking out the sample every 0.5-2h, performing water cooling, and performing metallographic structure observation until a uniform microstructure is obtained; the annealing temperature is 300-500 ℃, and the annealing time is 1-20 h, preferably 5-10 h;

step six, secondary aging treatment: and placing the annealed blank into a preheated aging furnace, performing aging treatment, wherein samples are taken out every 0.5-2h, performing water cooling, and then performing hardness test until the hardness reaches the highest value, namely peak aging, wherein the time for obtaining the peak aging is aging time, the aging temperature range is 100-250 ℃, and the aging time is 0.5-256h, preferably 5-150 h.

4. The method for preparing the high-strength and high-toughness rare earth magnesium alloy according to claim 3, wherein the method comprises the following steps: in the third step, the bimodal microstructure is a structure combining fine recrystallized grains and coarse deformed grains, and the proportion of the fine recrystallized grains and the coarse deformed grains is determined by the extrusion deformation temperature and the extrusion ratio.

5. The method for preparing the high-strength and high-toughness rare earth magnesium alloy according to claim 3 or 4, wherein the method comprises the following steps: the peak aging in the fourth step is a state that the strengthening effect of the precipitated phase is optimal and most intensive.

6. The method for preparing the high-strength and high-toughness rare earth magnesium alloy according to any one of claims 3 to 5, wherein the method comprises the following steps: the heating speed of the heating furnace from the second step to the sixth step is 0.3-0.5 ℃/s.

7. The method for preparing the high-strength and high-toughness rare earth magnesium alloy according to any one of claims 3 to 6, wherein the method comprises the following steps: and step four, ensuring that the aging blank is soaked in water within 1min and the temperature is reduced to the room temperature within 3 min.

8. The method for preparing the high-strength and high-toughness rare earth magnesium alloy according to any one of claims 3 to 7, wherein the method comprises the following steps: in the third step, the temperature of the extruded blank is the same as that of the extrusion die.

9. The method for preparing the high-strength and high-toughness rare earth magnesium alloy according to any one of claims 3 to 8, wherein the method comprises the following steps: the first time aging temperature is the same as or different from the second time aging temperature, and the second time aging treatment time is longer than the first time aging treatment time.

10. The method for preparing the high-strength and high-toughness rare earth magnesium alloy according to any one of claims 3 to 9, wherein the method comprises the following steps: in the fifth step, along with the increase of annealing time, the average grain size of the sample tends to be reduced, the average KAM value is increased first and then decreased, the texture strength of the extrusion basal plane tends to be reduced, and weak textures which are randomly distributed are obtained after annealing is completed, wherein the average grain size of the sample after annealing for 5 hours is less than or equal to 10 microns, the average KAM value is less than 0.6, and the texture strength is less than or equal to 2.5.