CN114540688A - A kind of ultra-high pressure heat treatment method of Mg-Zn-Zr-Gd alloy - Google Patents

Abstract

The invention relates to the field of alloy processing, and discloses a method for ultra-high pressure heat treatment of Mg-Zn-Zr-Gd alloys. The treatment method comprises the following steps: (1) using high-purity magnesium ingots, high-purity zinc ingots, and Mg-30% Gd intermediate The alloy and the Mg-30% Zr master alloy are used as raw materials, and the magnesium alloy is smelted in a resistance furnace to obtain a Mg-Zn-Zr-Gd alloy; (2) softening heat treatment is performed on the extruded alloy; (3) the steps are (1) extruding the obtained alloy; (4) pretreating the extruded alloy; (5) performing ultra-high pressure heat treatment on the pretreated alloy in step (4). The invention effectively solves the problems that the number of internal defects increases significantly after the magnesium alloy is deformed, the crystal grains are broken and deformed, and the diffusion of alloy atoms is obviously inhibited. The advantage of the present invention is that the strength, toughness and corrosion resistance are simultaneously significantly improved.

Description

A kind of ultra-high pressure heat treatment method of Mg-Zn-Zr-Gd alloy

technical field

The invention belongs to the field of alloy processing, and more particularly relates to a method for ultra-high pressure heat treatment of Mg-Zn-Zr-Gd alloy.

Background technique

As a research hotspot of biomedical materials in recent years, magnesium alloys have received extensive attention, mainly due to the following aspects: 1) the elastic modulus (20-40GPa) of magnesium alloys is close to that of human cortical bone (20GPa), The "stress shielding" effect of other metallic bone implant materials can be avoided. 2) Magnesium alloy has good biocompatibility and can promote the attachment of osteocytes on its surface to form bone. 3) Magnesium alloys are the most active metal structural materials currently used, especially in solutions containing Cl- ions, they are easily corroded and degraded. If magnesium alloy is used as implant material, secondary surgery can be avoided.

However, the problem that has been perplexing researchers at present is that magnesium alloys have poor resistance properties, which lead to too fast degradation speed, which makes the concentration of magnesium ions around the bone tissue too high, which leads to osteolysis and affects the healing of fractures. , resulting in a rapid decrease in the mechanical properties of magnesium alloys, which immobilizes the fractures before they heal.

The methods of improving the corrosion resistance of magnesium alloys are divided into three categories: one method is to not change the composition of magnesium alloys, mainly including surface treatment, deformation treatment and heat treatment to improve the corrosion resistance of magnesium alloys. Another method is to improve its corrosion resistance by optimizing the composition. Through this method, researchers have developed a large number of new medical magnesium alloys, including: Mg-Mn-Zn, Mg-Zn-Ca, Mg-Zn-Zr, Mg-Zn, Mg-Nd-Zn-Zr, etc. In addition, the rare earth elements Y, Nd, Ce and In can also significantly improve the corrosion resistance of magnesium alloys; the third method is to purify the alloy components. The iron, nickel and copper impurity elements in magnesium alloys can form cathode phases in magnesium alloys. It forms galvanic corrosion with the magnesium matrix, and elements such as Mn and Zr have a significant purification effect.

High-pressure heat treatment is a material treatment method that has emerged in recent years. Under the action of GPa-level high pressure, the diffusion process of atoms and the driving force of phase transition will change. Therefore, compared with the conventional heat treatment of metal materials, the structure and properties are significantly improved, such as The use of high pressure can improve the solid solubility of the alloy, and can control the morphology and size of the precipitated phase. But the research in this area is still limited.

The recrystallization process and the nucleation and growth process of the precipitation phase changed significantly, the grain size was refined, and the precipitation phase became finer, thus improving the mechanical properties and corrosion resistance of the alloy. After the magnesium alloy is deformed, the number of internal defects increases significantly, the grains are broken and deformed, and subsequent annealing can cause recrystallization to proceed. Under the action of high pressure, the diffusion of alloy atoms will be significantly inhibited, so the recrystallization process will be affected, but at present The mechanism of change in the recrystallization process of wrought magnesium alloys under high pressure is rarely reported.

SUMMARY OF THE INVENTION

In order to solve the problems in the prior art that the number of internal defects of magnesium alloys increases significantly after deformation, the crystal grains are broken and deformed, the diffusion of alloy atoms is significantly inhibited, the corrosion resistance of the alloy is reduced, and the mechanical properties are not ideal. - Zn-Zr-Gd alloy ultra-high pressure heat treatment method.

The specific scheme adopted in the present invention is: a Mg-Zn-Zr-Gd alloy ultra-high pressure heat treatment method, the treatment method comprises the following steps:

(1) Using high-purity magnesium ingot, high-purity zinc ingot, Mg-30% Gd master alloy and Mg-30% Zr master alloy as raw materials, smelting magnesium alloy in resistance furnace to obtain Mg-Zn-Zr-Gd alloy;

(2) softening heat treatment of the extruded alloy;

(3) extruding the alloy obtained in step (1);

(4) Pretreatment of the extruded alloy;

(5) Ultra-high pressure heat treatment is performed on the alloy pretreated in step (4).

In the step (1), the magnesium alloy smelting is carried out under the mixed gas of CO2 99.5 at.% and SF6 0.5 at.%.

In the step (3), before extrusion, the alloy is poured after smelting, and the pouring temperature is 700-800°C, poured into a metal mold preheated to 200-250°C, and under the condition of 400-500°C, 8- 12 hours of softening treatment.

Cool the softened alloy and extrude it on a hydraulic press to extrude a bar with a diameter of 10-20mm. The extrusion speed is 22-25mm/s, the extrusion ratio is 10:1-50:1, and the die temperature is 350 -380℃, extrusion temperature 300-350℃.

The pressure condition of the ultra-high pressure heat treatment in the step (5) is 2-6GPa.

The temperature condition of the ultra-high pressure heat treatment in the step (5) is 300-400°C.

The step of pre-processing the extruded alloy in the step (4) is to use a wire cutting machine to cut the magnesium alloy into a cylinder with a diameter of 10 mm and a height of 8 mm; After immersed in anhydrous alcohol, the surface of the sample was cleaned with ultrasonic waves and dried.

In the step (5), before the ultra-high pressure heat treatment, the magnesium alloy sample of Φ10mm×8mm is covered with tantalum sheets, and placed in a boron nitride crucible with an inner diameter of 10mm, an outer diameter of 12mm, and a height of 8mm, and the crucible is placed as a whole. Into a graphite furnace with an inner diameter of 12mm, an outer diameter of 14mm and a height of 16.6mm, put boron nitride sheets of Φ12mm×2mm and Φ12mm×2.3mm of boron nitride on the top and bottom of the boron nitride crucible placed in the graphite furnace. Pyrophyllite flakes; put the graphite furnace as a whole into the pyrophyllite tank, and put Φ14mm×1mm graphite flakes and steel caps on the upper and lower sides of the graphite crucible in the pyrophyllite tank.

Before the ultra-high pressure heat treatment, put the boron nitride crucible, boron nitride sheet, pyrophyllite sheet, graphite furnace, graphite sheet, pyrophyllite block, and steel cap used to encapsulate the sample into the dryer for 20-24 hours in advance. .

During the ultra-high pressure heat treatment in the step (5), the pressure is first raised to the preset pressure, then rapidly heated to the preset temperature, kept for 1-1.5 hours, the power is turned off to stop heating, and the heating is naturally cooled to room temperature, and taken out after pressure relief. sample. The present invention has the following beneficial effects with respect to the prior art:

The invention realizes large deformation for magnesium alloy extrusion. After extrusion, the alloy is heat treated at 300 DEG C and 1H, the grain size of the alloy is slightly increased, and the extruded Mg-Zn-Zr-Gd alloy is subjected to ultra-high pressure heat treatment. , the grain size of the alloy decreased, the strength of the alloy increased, and the elongation increased; the electrochemical corrosion results showed that the corrosion resistance of the alloy was significantly improved after high pressure heat treatment. The invention solves the problem that the number of internal defects increases significantly after the magnesium alloy is deformed, the crystal grains are broken and deformed, the corrosion resistance is reduced, and the diffusion of alloy atoms is bound to be obviously inhibited under the action of ultra-high pressure, so that the size of the crystal grains is reduced. Refined, the aging precipitate phase (cathode phase) becomes more fine and uniform, reducing the local corrosion tendency, and the dispersion strengthening effect of fine particles significantly improves the strength of the alloy.

Description of drawings

1 is a diagram of a high-pressure heat treatment experimental device in the present invention;

Fig. 2 is the metallographic structure diagram of the sample 100 times in Example 1;

Fig. 3 is the metallographic structure diagram of the sample 100 times in Example 2;

Fig. 4 is the metallographic structure diagram of the sample 100 times in Comparative Example 1;

Fig. 5 is the metallographic structure diagram of the sample 100 times in Comparative Example 2;

Figure 6 is a cross-sectional topography of the Mg-Zn-Zr-Gd alloy after extrusion;

Fig. 7 is the EDS analysis figure of the precipitated phase in Fig. 6;

Fig. 8 is the morphology of precipitates in the alloy of Example 1 under the action of ultra-high pressure;

Fig. 9 is the relative morphology diagram of precipitation in the alloy under the action of normal pressure of Example 1;

Among them, the reference signs are:

1- steel cap; 2- graphite sheet; 3- pyrophyllite sheet; 4- boron nitride sheet; 5- boron nitride crucible; 6- graphite furnace; 7- pyrophyllite block.

Detailed ways

The present invention will be described in further detail below with reference to the accompanying drawings, and it should be clearly understood that the described embodiments are not all embodiments, and are only used to illustrate the present invention, but not to limit the present invention.

The present invention provides a method for ultra-high pressure heat treatment of Mg-Zn-Zr-Gd alloy, which comprises the following steps:

(1) Using high-purity magnesium ingot, high-purity zinc ingot, Mg-30% Gd master alloy and Mg-30% Zr master alloy as raw materials, smelting magnesium alloy in resistance furnace to obtain Mg-Zn-Zr-Gd alloy;

(2) softening heat treatment of the extruded alloy;

(3) extruding the alloy obtained in step (1);

(4) Pretreatment after extrusion

(5) performing ultra-high pressure heat treatment on the alloy pretreated in step (3).

In the step (1), the magnesium alloy smelting is carried out under the mixed gas of CO2 99.5 at.% and SF6 0.5 at.%.

In the step (1) before extrusion, the alloy is poured after smelting, and the pouring temperature is 700-800°C, poured into a metal mold preheated to 200-250°C, and under the condition of 400-500°C, 8- 12 hours of softening treatment.

(3) Cool the softened alloy and extrude it on a hydraulic press to extrude a bar with a diameter of 10-20mm. The extrusion speed is 20-30mm/s, and the extrusion ratio is 10:1-50:1. Die temperature 350-400℃, extrusion temperature 300-350℃.

(4) The step of pretreating the extruded alloy is to use a wire cutting machine to cut the magnesium alloy into a cylinder with a diameter of 10 mm and a height of 8 mm; the surface of the alloy is smoothed with water sandpaper, and the oxide skin formed by the wire cutting is removed. After immersion in anhydrous alcohol, the surface of the sample was cleaned with ultrasonic waves and dried.

(5) The pressure condition of the ultra-high pressure heat treatment in the step (4) is 2-6GPa.

Among them, high-purity magnesium ingot (99.99wt.%), high-purity zinc ingot (99.99wt.%), Mg-30%Gd master alloy and Mg-30%Zr master alloy were used as raw materials, and the magnesium alloy was smelted in a resistance furnace. , to obtain a Mg-Zn-Zr-Gd alloy; the obtained alloy is subjected to softening heat treatment; the alloy after softening heat treatment is subjected to extrusion large deformation treatment; the pretreated alloy is subjected to ultra-high pressure heat treatment. The step of pretreatment of the extruded alloy is to use a wire cutting machine to cut the magnesium alloy into a cylinder with a diameter of 10mm and a height of 8mm; In anhydrous alcohol, ultrasonically clean the surface of the sample and dry it.

In step (1), the magnesium alloy smelting is carried out in a mixed gas of CO2 99.5 at.% and SF6 0.5 at.%. In the step (3), before extrusion, the alloy is poured after smelting, and the pouring temperature is 700-800 °C, poured into a metal mold preheated to 200-250 °C, and under the condition of 400-500 °C, 8- 12 hours of softening treatment. Cool the softened alloy and extrude it on a hydraulic press to extrude a bar with a diameter of 10-20mm. The extrusion speed is 22-25mm/s, the extrusion ratio is 10:1-50:1, and the die temperature is 350 -380℃, extrusion temperature 300-350℃.

Table 1 Chemical composition of as-extruded Mg-Zn-Zr-Gd alloy (wt.%)

In the step (5), before the ultra-high pressure heat treatment, the magnesium alloy sample of Φ10mm×8mm is covered with tantalum sheets, and placed in a boron nitride crucible with an inner diameter of 10mm, an outer diameter of 12mm, and a height of 8mm, and the crucible is placed as a whole. Into a graphite furnace with an inner diameter of 12mm, an outer diameter of 14mm and a height of 16.6mm, put boron nitride sheets of Φ12mm×2mm and Φ12mm×2.3mm of boron nitride on the top and bottom of the boron nitride crucible placed in the graphite furnace. Pyrophyllite flakes; put the graphite furnace as a whole into the pyrophyllite tank, and put Φ14mm×1mm graphite flakes and steel caps on the upper and lower sides of the graphite crucible in the pyrophyllite tank.

The tantalum sheet can protect the magnesium alloy and play a role in protecting and preventing pollution; the boron nitride in the boron nitride crucible protects the magnesium alloy and transmits pressure; the graphite in the graphite furnace is the heating source, and the temperature is controlled by adjusting the input voltage ; Pyrophyllite flakes play the role of protection and pressure transmission, and the pyrophyllite is the pressure transmission medium; the steel cap plays a conductive role.

Before the ultra-high pressure heat treatment, put the boron nitride crucible, boron nitride sheet, pyrophyllite sheet, graphite furnace, graphite sheet, pyrophyllite block, and steel cap used to encapsulate the sample into the dryer for 20-24 hours in advance. . During the ultra-high pressure heat treatment in the step (5), the pressure is first raised to the preset pressure, then rapidly heated to the preset temperature, kept for 1-1.5 hours, the power is turned off to stop heating, and the heating is naturally cooled to room temperature, and taken out after pressure relief. sample.

Example 1

A Mg-Zn-Zr-Gd alloy ultra-high pressure heat treatment method, the treatment method comprises the following steps:

(1) Using high-purity magnesium ingot (99.99wt.%), high-purity zinc ingot (99.99wt.%), Mg-30%Gd master alloy and Mg-30%Zr master alloy as raw materials, the magnesium alloy was prepared in a resistance furnace. Smelting to obtain a Mg-Zn-Zr-Gd alloy; (2) pouring at a temperature of 750°C, poured into a metal mold preheated to 200°C, softened at 420°C for 10 hours, and extruded on a 3150KN hydraulic press after cooling. Press and extrude a bar with a diameter of 15mm, the extrusion speed is 22mm/s, the extrusion ratio is 10:1, the die temperature is 360°C, and the extrusion temperature is 340°C; (3) Pretreatment of the extruded alloy, The step of pre-processing the extruded alloy in the step (3) is to use a wire cutting machine to cut the magnesium alloy into a cylinder with a diameter of 10 mm and a height of 8 mm; After immersed in anhydrous alcohol, the surface of the sample was cleaned with ultrasonic waves and dried. (4) Ultra-high pressure heat treatment is performed on the alloy pretreated in step (3), the temperature of the ultra-high pressure heat treatment is 300° C., the pressure is 2 GPa, and the time is 1 hour.

In the step (4), before the ultra-high pressure heat treatment, the magnesium alloy sample of Φ10mm×8mm is covered with a tantalum sheet, and placed in a boron nitride crucible with an inner diameter of 10mm, an outer diameter of 12mm and a height of 8mm, and the crucible is placed as a whole. Into a graphite furnace with an inner diameter of 12mm, an outer diameter of 14mm and a height of 16.6mm, put boron nitride sheets of Φ12mm×2mm and Φ12mm×2.3mm of boron nitride on the top and bottom of the boron nitride crucible placed in the graphite furnace. Pyrophyllite flakes; put the graphite furnace as a whole into the pyrophyllite tank, and put Φ14mm×1mm graphite flakes and steel caps on the upper and lower sides of the graphite crucible in the pyrophyllite tank.

Before the ultra-high pressure heat treatment, the boron nitride crucible, boron nitride sheet, pyrophyllite sheet, graphite furnace, graphite sheet, pyrophyllite block, and steel cap used to encapsulate the sample were placed in a dryer for 20 hours in advance to dry. During the ultra-high pressure heat treatment in step (4), the pressure was first increased to the preset pressure, then rapidly increased to the preset temperature, kept for 1 h, the power was turned off to stop heating, and the sample was taken out after the pressure was released after being cooled to room temperature naturally.

Example 2

The difference from Example 1 is that in step (4) of this embodiment, the alloy pretreated in step (3) is subjected to ultra-high pressure heat treatment, the temperature of ultra-high pressure heat treatment is 400 ° C, the pressure is 2 GPa, and the time is 1 Hour.

Comparative Example 1

This comparative example provides a Mg-Zn-Zr-Gd alloy, the Mg-Zn-Zr-Gd alloy adopts high-purity magnesium ingot (99.99wt.%), high-purity zinc ingot (99.99wt.%), Mg-30% Gd master alloy and Mg-30% Zr master alloy are used as raw materials to smelt magnesium alloy in a resistance furnace. (2) The pouring temperature is 750°C, poured into a metal mold preheated to 200°C, softened at 420°C for 10 hours, and then extruded on a 3150KN hydraulic press after cooling to extrude a bar with a diameter of 15mm. The pressing speed is 22mm/s, the extrusion ratio is 10:1, the die temperature is 360°C, and the extrusion temperature is 340°C;

(1) Comparative Example 2

The difference between this comparative example and Example 1 is that in step (4), the alloy pretreated in step (3) is subjected to heat treatment. .

In Example 1-2, the high-pressure experimental equipment was a CS-1B artificial diamond hydraulic press (high-pressure six-sided top press). The main technical parameters of the press are shown in Table 2.

Table 2: Main technical parameters of the press

Electrochemical polarization experiments were carried out on the above examples and comparative examples.

Electrochemical samples were cut from the above-mentioned extrusion rods, and the samples were taken perpendicular to the extrusion direction. The above samples were sealed and mounted with epoxy resin, the exposed surface area was 1 cm ² , the surface was polished with 200-1000# sandpaper in turn, and then polished to 1 μm. The electrochemical polarization experiment adopts a standard three-electrode system: the reference electrode is saturated calomel electrode, the auxiliary electrode is platinum electrode, and the sample is used as working electrode. The polarization experiment was carried out in a beaker filled with 300ml of normal saline, and the temperature of the solution was controlled at 37±1℃. The scanning speed is 0.3mV/s.

Microstructure observation was carried out on the above examples and comparative examples.

The microstructures of the as-extruded alloy and the alloy after high pressure treatment were observed on ZEISS-Axiovert 200MAT metallographic microscope (OM) and MX2600 field emission scanning electron microscope (SEM). The samples were corroded with 5wt.% picric acid ethanol solution . The microhardness sample is a Φ10×8mm columnar sample cut from the above-mentioned bar. The test is carried out on a microhardness tester, and the average value of 5 points is taken for each sample, and the temperature is 20 °C. TEM analysis JEM-2010 high-resolution transmission electron microscope was used in this experiment to study the precipitation phase and recrystallization state inside the material. Its lattice resolution is 0.14nm, and the point resolution 5 is 0.23nm.

With reference to Figure 4, it can be seen that there are a large number of deformed grains on the original alloy matrix of Comparative Example 1, and there are fine equiaxed grains in local areas, and the grain size is quite different; it can be seen that dynamic recrystallization occurs during the hot extrusion process, The average grain size of the alloy measured by the nodal method was 23 μm. Combining with Figure 5, it can be seen that the grain size of the alloy matrix increases significantly and the number of fine grains decreases significantly after 1 hour of treatment at 300 °C. The calculated average grain size of the alloy is 25 μm. Combined with Figure 2, under the action of 2GPa high pressure and 300 ℃ heat, a large number of distorted grains appeared on the alloy matrix, and the number of fine granular grains was significantly more than that of the samples of Comparative Example 1 and Comparative Example 2. The average grain size was 20 μm. With reference to Fig. 3, it can be seen that under the action of 2GPa high pressure, when the heat treatment temperature reaches 400 °C and the temperature is maintained for one hour, compared with the sample in Example 1, the distorted grains on the alloy matrix are significantly reduced, and the fine granular The number of grains was also significantly reduced, and a large number of slender deformed grains appeared. The average grain size was calculated to be 22 μm.

From the above results, it can be seen that under the condition of hot extrusion, there are a large number of deformed grains. After heat treatment at 300 ° C for 1 hour, the grains in the local area will grow under the action of heat, and when the temperature is 2GPa Under the action of high pressure, under the action of heat at 300 °C and 400 °C, respectively, after 1 h of heat treatment, the grain size and morphology did not change much compared with the original as-extruded state.

The SEM morphology of the samples in Example 1-2 and Comparative Example 1-2 is similar, as shown in FIG. 2 . As can be seen in conjunction with accompanying drawing 6, the precipitation phase is distributed on the matrix, and the precipitation phase is dispersed distribution; Table 3 is the EDS analysis result of the alloy matrix, it can be found that the alloy matrix contains Zn element and a small amount of Zr element, wherein the content of Zn element It reaches 2.67at%, which proves that 2.67at% of Zn element is dissolved in the magnesium alloy, and the solid solubility of Zn element in other heat treatment states is also about 2.67at%, the alloy is in a supersaturated state; the energy spectrum of the alloy precipitation phase The analysis results are shown in Table 4. It can be seen that the composition is Mg, Zn and Gd elements, of which the Mg element content is 54.86at%, the Zn element content is 30.52at%, the Gd element content is 13.72at%, and a small amount of Zr element, through research, found that the composition of the precipitate phase is similar.

Table 3 Alloy matrix energy spectrum analysis table

Table 4 Energy spectrum analysis table of alloy precipitates

Referring to Figure 8, it can be seen that the calculated mechanical properties parameters are shown in Table 5. It can be seen that the compressive strength of the alloy in Comparative Example 1 is 507 MPa, the yield strength is 209 MPa, and the elongation is 13%, which is similar to that of the sample in Comparative Example 1. Compared with the sample of Comparative Example 2, the compressive strength and yield strength of the sample were significantly improved, reaching 558 MPa and 242 MPa, respectively, and the elongation was also increased, reaching 14%, which was mainly due to the precipitation of the extruded sample after heat treatment. A large number of strengthening phases play a role in dispersion strengthening, and at the same time, a large number of dislocations existing in the extruded state are significantly reduced after heat treatment, which promotes the improvement of elongation; The tensile strength has been improved, and the yield strength and elongation have been significantly improved. This is because under the action of 2GPa ultra-high pressure, the grain growth caused by heat treatment is hindered, and under the action of pressure, a large number of fine recrystallized crystals are produced. Compared with Example 1, the tensile strength, yield strength and elongation of Example 2 are all reduced. It can be seen that under the ultra-high pressure of 2GPa, the heat treatment temperature increased to 400 At ℃, the grains and precipitates in the alloy will grow, resulting in a decrease in tensile strength, yield strength and elongation.

Table 5 Compression performance parameters of heat treatment in different processes

Testing of Corrosion Behavior of Mg-Zn-Zr-Gd Alloys.

Referring to Figure 9, the electrochemical polarization curve, after fitting calculation and electrochemical parameter values are shown in Table 8, the self-corrosion potential of the extruded alloy No. 1 sample is -1.4364V, relative to Comparative Example 1 , the self-corrosion potential of the samples of Comparative Example 2, Example 1 and Example 2 increased to -1.3923, -1.4214 and -1.4115 respectively; it can be seen that both normal pressure heat treatment and high pressure heat treatment can improve the thermodynamic stability of the alloy, which is mainly It is because the structural defects such as dislocations on the matrix are significantly reduced after heat treatment, which improves the stability of the alloy; the corrosion rate of the alloy is also significantly reduced after heat treatment, and the corrosion rate of the samples of Example 1 and Example 2 after high pressure heat treatment is the most significant decrease. This is because under high pressure, heat preservation can inhibit the growth of the precipitation phase during the heat treatment process. Comparing the samples of Example 1 and Example 2, it can be seen that heat treatment at 300 °C under high pressure, although the grains are refined, the lattice distortion is relatively serious. And the number of deformation twins or dislocations that may be generated is significantly higher than that of heat treatment at 400 °C and 300 °C.

Table 6 Electrochemical parameter values of different heat treatment processes

It can be seen from the microstructure observation of the three recrystallization processes of the extruded Mg-Zn-Zr-Gd alloy that the recrystallization behavior is relatively sufficient under normal pressure. However, due to the abnormal growth of grains, coarse grains appear in local areas. During the crystallization annealing process, 2GPa high pressure treatment is introduced, and the increase in the internal distortion of the alloy structure caused by the high pressure provides a favorable site for the formation of new crystal nuclei. At the same time, high pressure can also reduce the critical free energy required to form nuclei, thereby increasing the nucleation rate of new grains. On the other hand, the diffusion of atoms is more difficult under too high pressure, which inhibits the growth of crystal nucleus, so the grain size of the alloy obtained after 2GPa high pressure treatment is smaller than that obtained under normal pressure. . Annealing at 300°C under high pressure of 2GPa will cause the lattice of the alloy to be distorted under the action of high pressure, and this distortion will be more significant under the interaction with the particles.

The corrosion resistance of the as-extruded Mg-Zn-Zr-Gd alloy was significantly improved after heat treatment at 300℃ for 1 hour under normal pressure and high pressure.

Combined with the accompanying drawings, the annealing process under normal pressure and ultra-high pressure conditions, the morphology, size and volume fraction of the aging precipitated particles are compared:

First of all, after 2GPa, 300 ℃ holding pressure for 1h, it can be seen that the size of the high-pressure annealing precipitates is significantly lower than that of the normal-pressure annealing precipitates; under general conditions, the fine precipitates have a lower cathodic effect than the coarse precipitates, and The finely dispersed precipitates will have stronger dispersion strengthening effect. For Mg-Zn-Zr alloy, the aging precipitation process is as follows: SSSS→GP region→β'1→β'2, where β'2 is MgZn2 phase, adding rare earth or Cu and other elements to promote this process, high pressure makes the precipitation phase The size of the alloy is significantly reduced, thus improving the strength and corrosion resistance of the alloy.

Secondly, during the extrusion deformation process, there are dislocation and substructure-enriched regions in the deformation region. Under the action of ultra-high pressure and heat, its configuration and motion form will change, which has a significant impact on the recrystallization process. , under the action of ultra-high pressure, nanocrystals will appear in the dislocation plugging area, which has the characteristics of continuous recrystallization.

To sum up, the present invention through the ultra-high pressure heat treatment of the extruded Mg-Zn-Zr-Gd alloy, the strength of the alloy increases, the elongation increases, the strength increases, and the elongation decreases significantly; the electrochemical corrosion results show that the alloy After heat treatment, the corrosion resistance is significantly improved. The invention solves the problem that after the magnesium alloy is deformed, the number of internal defects increases significantly, the crystal grains are broken and deformed, the subsequent annealing can cause recrystallization to proceed, and the diffusion of alloy atoms is obviously inhibited under the action of high pressure. The advantage is that strength, toughness and corrosion resistance are significantly improved.

The above drawings and explanations are only a specific embodiment of the present invention, but the specific protection scope of the present invention is not limited to the above explanations. Any substitutions or changes should fall within the protection scope of the present invention.

Claims (10)

1. The ultrahigh-pressure heat treatment method of the Mg-Zn-Zr-Gd alloy is characterized by comprising the following steps of:

(1) smelting magnesium alloy in a resistance furnace by using high-purity magnesium ingots, high-purity zinc ingots, Mg-30% Gd intermediate alloy and Mg-30% Zr intermediate alloy as raw materials to obtain Mg-Zn-Zr-Gd alloy;

(2) softening and heat treating the extruded alloy;

(3) extruding the alloy obtained in the step (1);

(4) pretreating the extruded alloy;

(5) and (4) carrying out ultrahigh pressure heat treatment on the alloy pretreated in the step (4).

2. The ultra-high pressure heat treatment method of Mg-Zn-Zr-Gd alloy according to claim 1, wherein in the step (1), the magnesium alloy is smelted in CO₂99.5 at.% with SF₆0.5 at.% of mixed gas.

3. The Mg-Zn-Zr-Gd alloy ultra-high pressure heat treatment method according to claim 1, wherein before the extrusion in step (3), the alloy is poured after the smelting, the pouring temperature is 700-.

4. The Mg-Zn-Zr-Gd alloy ultra-high pressure heat treatment method according to claim 3, characterized in that the alloy after softening treatment is cooled and extruded on a hydraulic press to extrude a rod with a diameter of 10-20mm, the extrusion speed is 22-25mm/s, the extrusion ratio is 10: 1-50:1, the mold temperature is 350-380 ℃, and the extrusion temperature is 300-350 ℃.

5. The Mg-Zn-Zr-Gd alloy ultra-high pressure heat treatment method according to claim 1, wherein the pressure condition of the ultra-high pressure heat treatment in said step (5) is 2-6 GPa.

6. The method for ultrahigh pressure heat treatment of Mg-Zn-Zr-Gd alloy according to claim 1, wherein the temperature condition of the ultrahigh pressure heat treatment in the step (5) is 300-400 ℃.

7. The Mg-Zn-Zr-Gd alloy ultra-high pressure heat treatment method according to claim 1, wherein said step (4) of pretreating the extruded alloy is cutting the magnesium alloy into a cylinder having a diameter of 10mm and a height of 8mm using a wire cutting machine; and (3) polishing the surface of the alloy by using water sand paper, removing oxide skin formed by linear cutting, soaking the alloy in absolute alcohol, cleaning the surface of the sample by using ultrasonic waves, and drying.

8. The Mg-Zn-Zr-Gd alloy ultra-high pressure heat treatment method according to claim 7, wherein before the ultra-high pressure heat treatment in the step (5), a magnesium alloy sample of phi 10mm x 8mm is coated with a tantalum sheet, and is charged into a boron nitride crucible of 10mm in inner diameter, 12mm in outer diameter and 8mm in height, the whole crucible is put into a graphite furnace of 12mm in inner diameter, 14mm in outer diameter and 16.6mm in height, and the boron nitride sheet of phi 12mm x 2mm and the pyrophyllite sheet of phi 12mm x 2.3mm are respectively put on and under the boron nitride crucible put into the graphite furnace; the graphite furnace is integrally placed in a pyrophyllite groove, and graphite flakes and steel caps with the diameter of phi 14mm multiplied by 1mm are respectively placed on the upper side and the lower side of a graphite crucible in the pyrophyllite groove.

9. The method for ultrahigh-pressure heat treatment of Mg-Zn-Zr-Gd alloy according to claim 8, wherein the boron nitride crucible, the boron nitride sheet, the pyrophyllite sheet, the graphite furnace, the graphite sheet, the pyrophyllite block and the steel cap used for packaging the sample are put into a dryer in advance and dried for 20 to 24 hours before the ultrahigh-pressure heat treatment.

10. The Mg-Zn-Zr-Gd alloy ultra-high pressure heat treatment method according to claim 9, wherein in the step (5), during the ultra-high pressure heat treatment, the pressure is first raised to a preset pressure, then the temperature is rapidly raised to a preset temperature, the temperature is kept for 1-1.5h, the power is turned off, the heating is stopped, the sample is taken out after natural cooling to room temperature and pressure is released.

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