CN114918430A - Design method of super-solid-solution heat-resistant magnesium rare earth alloy based on non-equilibrium solidification - Google Patents
Design method of super-solid-solution heat-resistant magnesium rare earth alloy based on non-equilibrium solidification Download PDFInfo
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- 229910052761 rare earth metal Inorganic materials 0.000 title claims abstract description 64
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- 238000000034 method Methods 0.000 title claims abstract description 40
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Abstract
The invention belongs to the technical field of alloy design, and relates to a method for designing a super-solid-solution heat-resistant rare earth magnesium alloy based on non-equilibrium solidification, which comprises the following steps of 1, selecting appropriate elements as multi-element synergistic alloy design elements by taking solid solution strengthening, second phase strengthening and grain boundary strengthening of the magnesium alloy under a high-temperature condition as design targets; and 2, based on the characteristics of selective laser melting and instantaneous non-equilibrium solidification, calculating the influence of the mass fraction of the selected synergistic alloy design elements on the alloy thermal sensitivity index and the critical temperature range, and determining the optimal alloying component range of each element by taking the minimized alloy thermal sensitivity index and the critical temperature range as targets. The alloy designed by the method has good high-temperature performance, is more suitable for the manufacturing process with the characteristic of instantaneous non-equilibrium solidification, namely selective laser melting, and can meet the requirement of high density and no crack in additive manufacturing.
Description
Technical Field
The invention belongs to the technical field of alloy design, and relates to a design method of a super-solid solution heat-resistant rare earth magnesium alloy based on non-equilibrium solidification.
Background
Magnesium alloy has attracted much attention from researchers as one of the lightest metal structural materials, because its density is equivalent to that of engineering plastics, but its strength and rigidity are much higher than those of engineering plastics. In the past decades, magnesium alloys have been widely used in fields where weight reduction is required, such as aerospace, weaponry, and transportation. However, the poor high temperature mechanical properties of magnesium alloys have been a key issue limiting their application as high temperature structural components.
The rare earth elements have unique physical and chemical properties, so that atoms in the magnesium matrix diffuse slowly, a high-melting-point precipitated phase and an intermetallic compound can be formed, and the high-temperature service performance of the magnesium alloy can be improved. The WE series rare earth alloy mainly formed by adding Y/Gd and combining other beneficial elements has obvious advantages in high-temperature mechanical property. However, in the conventional solidification methods such as powder metallurgy, stir casting and spray deposition, the rare earth element has certain limitations in solid solution of the matrix, and the formation of a stable second phase is limited, so that the preparation of magnesium alloy parts with good high-temperature mechanical properties is difficult to realize.
Compared with the traditional forming manufacturing technology, the instantaneous solidification speed of the laser additive manufacturing technology can obviously change the supersaturation degree, phase formation and element diffusion segregation behavior of the forming material solute, thereby influencing the mechanical strengthening and toughening mechanism of the forming material solute and being hopeful to improve the high-temperature performance of the alloy. The selective laser melting technology is one of laser additive manufacturing, modeling is carried out by utilizing CAD/CAE software, a complex model is sliced in the software, each slice layer comprises all geometric information of the slice layer, a file is stored in an STL format, and then the slice layer is guided into a printing device, and metal powder is melted layer by high-energy laser beams until a complete part is finally formed.
In conclusion, magnesium alloy is one of the lightest metal structural materials, and the lightweight structural component manufactured by using the material brings great weight-reducing benefits. However, the magnesium alloy has fast atomic diffusion at high temperature, easy substrate slippage and unstable second phase, and the application of the magnesium alloy as a high-temperature structural component is severely limited. And the traditional manufacturing method is difficult to realize the preparation of the magnesium alloy part with good high-temperature mechanical property, and the selective laser melting has great potential in the aspect of improving the high-temperature mechanical property of the alloy due to the higher cooling speed. However, the mechanical behavior, the crystal grain and the precipitated phase strengthening mechanism of the magnesium alloy under the non-equilibrium solidification condition are different from those of the equilibrium state, and when the magnesium alloy designed by the method in the prior art is applied to selective laser melting manufacturing, the problems that the high-temperature mechanical property cannot be expected, cracks are easy to occur and the like can occur due to lack of targeted performance strengthening.
Disclosure of Invention
In view of this, the invention provides a method for designing a supersoluble heat-resistant rare earth magnesium alloy based on non-equilibrium solidification, so as to solve the problems that a magnesium alloy designed in the prior art is not suitable for additive manufacturing and has poor high-temperature mechanical properties.
The method comprises the following steps:
and 2, based on the characteristics of selective laser melting and instantaneous non-equilibrium solidification, calculating the influence of the mass fraction of the selected synergistic alloy design elements on the alloy thermal sensitivity index and the critical temperature range, and determining the optimal alloying component range of each element by taking the minimized alloy thermal sensitivity index and the critical temperature range as targets.
Further, the design target of solid solution strengthening under the high temperature condition is higher solid solubility;
further, the second phase strengthening is designed to target a higher melting point of the high temperature resistant second phase;
further, the design goals of grain boundary strengthening are that the second phase lattice constant and atomic bonding slip resistance are greater, and that the phase stability is higher.
Further, the step 1 comprises selecting a plurality of candidate main rare earth elements X by combining a magnesium alloy phase diagram and aiming at high solid solubility;
combining a rare earth element solubility curve and a magnesium alloy phase diagram, and selecting a plurality of candidate cooperative rare earth elements M by taking the design target of being beneficial to high temperature resistance but having limitation on solid solubility performance;
and selecting a plurality of candidate modifying elements N with the aim of precipitating a second phase beneficial to high temperature resistance;
constructing a Mg-X-M-N multi-element collaborative alloying candidate system.
Preferably, the candidate primary rare earth elements X include Gd and Y.
Preferably, the candidate synergistic rare earth elements M include Nd, Ce, Sm.
Preferably, the candidate modifying elements N include Zr, Zn, Ca.
Further, step 1 further comprises:
establishing a typical WE-series rare earth magnesium alloy model, and simulating a fusing process time-temperature curve of the WE-series rare earth magnesium alloy formed by selective laser melting and rapid forming;
calculating an alloy solidification interval and an element solid solubility change curve after adding various candidate X elements with different contents into Mg under the nonequilibrium solidification condition of Scheil-Gulliver and a high-temperature resistant second-phase melting point corresponding to the candidate X elements on the basis of the time-temperature curve of the fusing process;
selecting one of the candidate main rare earth elements X as a main rare earth element by taking the conditions of simultaneously or as much as possible that the solid solubility of the element is high, the solidification interval of the alloy is narrow, and the high-temperature resistant second phase melting point is high as standards;
similarly, on the basis of the selected Mg-X alloy, adding different candidate cooperative rare earth elements M to perform the same calculation and selection, and selecting a final element M from the candidate cooperative rare earth elements M;
if more than one element simultaneously meets two or three conditions in the selection process of the element M, the element is selected according to the weight of the solid solubility of the element, the melting point of the high-temperature resistant second phase and the solidification interval of the alloy.
Further, the standard WE rare earth magnesium alloy is Mg-3.77Y-2.46Nd-1.23Gd-0.21Zn-0.4Zr wt (%) alloy.
Further, in the step 1, based on the selected Mg-X-M alloy, the influence of adding different candidate modified elements N on the second phase lattice constant, the phase structure stability and the atomic bonding slip resistance of the prepared alloy is calculated by utilizing VASP (ViennaaB-initio Simulation Package);
selecting final element N from candidate modified element N by simultaneously or as much as possible according to the standards of large second-phase lattice constant, strong phase stability and large atom bonding slip resistance;
if more than one element simultaneously appears in the selection process of the N element and simultaneously meets two or three conditions, the elements are selected according to the weight of atomic bonding slip resistance > phase stability > second phase lattice constant.
Further, in step 2, firstly, the solid fraction f of the alloy formed by the main rare earth element X and Mg with different mass fractions in the solidification process is calculated s Changes with temperature T, and then the T-f corresponding to each mass fraction is drawn according to the changes s 1/2 A curve;
selecting the element mass fraction corresponding to the curve meeting the following conditions as the mass fraction of the main rare earth element X:
get T on the curve E At 568 deg.C f s 1/2 <0.99；
At temperature T E The absolute value of the slope, i.e. the thermal sensitivity index | dT/d (f) s ) 1/2 L is a minimum relative to the other curves;
③ Critical temperature range DeltaT CTR Relative to othersThe curve is the minimum;
the critical temperature range DeltaT CTR Is f on the curve s 1/2 Point and f at 0.31 s 1/2 The difference in temperature corresponds to the point 0.99.
Secondly, according to the same idea, taking 0.5 wt% as the minimum value, taking the rest mass fraction obtained by subtracting the mass fraction of the selected element Y from 15 wt% as the maximum value, sampling and calculating the thermal sensitivity index and the critical temperature range of Mg-X-M alloys with different contents of various synergistic rare earth elements M to determine the component range of the element M, wherein the mass fraction of the main rare earth element X is the mass fraction selected previously, and the rest of the alloy is Mg;
finally, in the same way, by taking 0.5 wt% as the minimum value and taking the rest mass fraction obtained by subtracting the mass fraction of the selected element Y and the element M from 15 wt% as the maximum value, sampling and calculating the thermal sensitivity index and the critical temperature range of Mg-X-M-N alloys with different element N contents to determine the component range of the element N, wherein the mass fraction of the main rare earth element X and the mass fraction of the synergistic rare earth element M are the mass fractions selected previously, and the rest parts of the alloys are Mg;
in each of the above options, if two curves appear, the thermal sensitivity index | dT/d (f) is satisfied s ) 1/2 Minimum and critical temperature range Δ T CTR The minimum criterion is to set the coefficient K ═ f s 1/2 (T＝TE) /ΔT CTR And selecting the element mass fraction corresponding to the curve with the maximum K value.
The invention has the advantages that aiming at the problems that in the design of the non-equilibrium super-solid solution synergic alloying of the rare earth magnesium alloy under the rapid laser solidification, the design target of specific parameters which are used for improving the high-temperature mechanical property and can be calculated is not clear, and the blindly designed alloy has poor high-temperature mechanical property, a new strengthening mechanism is provided, namely strengthening of the solid solubility, strengthening of the second phase (melting point) and strengthening of the grain boundary (second phase lattice constant, phase structure stability (forming heat and bonding energy) and atomic bonding slip resistance) under the high-temperature condition are carried out on the magnesium alloy, so that the high-temperature mechanical property can be improved, firstly, rare earth elements and modified elements are selected based on a magnesium alloy phase diagram, and the alloying design rule of the magnesium alloy is established by calculating the influence of the selected elements on the solidification parameter and the thermophysical property parameter of the alloy under the non-equilibrium condition, thereby obtaining the optimal alloying component range, and the designed alloy is suitable for the selective laser melting manufacturing process. The strengthening target selected by the method enables the designed alloy to have good high-temperature performance, and meanwhile, the optimal alloying component range is determined by taking the minimized alloy thermal sensitivity index and the critical temperature range as targets, so that the designed alloy is more suitable for the manufacturing process with the characteristic of instantaneous non-equilibrium solidification, namely selective laser melting, and can meet the requirement of high density and no crack in additive manufacturing.
Drawings
FIG. 1 is a flow chart of preliminary screening of alloying elements in the embodiment of the present invention.
Fig. 2 (a) is a schematic diagram of a model of a typical WE-based rare earth magnesium alloy for finite element temperature field simulation in an embodiment of the present invention, and (b) is a time-temperature curve of a melting process of the typical WE-based rare earth magnesium alloy obtained by simulation in the embodiment of the present invention.
FIG. 3 is a graph of the solidification characteristics of the alloy calculated in an example of the present invention, wherein (a) is a graph of the solidification interval and rare earth second phase content of the alloy as a function of the mass fraction of the rare earth alloying element Y or Gd under the Scheil non-equilibrium solidification condition; (b) the part is an alloy element solid solubility change curve chart after changing the content of Y or Gd element in pure Mg.
FIG. 4 is a comparative graph of T-fs1/2 curves for alloys formed by adding different mass fractions of Y element to Mg in examples of the present invention.
Detailed Description
The technical solution of the present invention will be further described in detail with reference to specific embodiments. It is to be understood that the following examples are only illustrative and explanatory of the present invention and should not be construed as limiting the scope of the present invention. All the techniques realized based on the above-mentioned contents of the present invention are covered in the protection scope of the present invention.
The design method in the embodiment comprises the following steps of 1, selecting appropriate elements as multi-element synergistic alloy design elements by taking solid solution strengthening, second phase strengthening and grain boundary strengthening of the magnesium alloy as design targets under a high-temperature condition;
and 2, calculating the influence of the selected design elements of the same alloy on the alloy thermal sensitivity index and the critical temperature range based on the characteristics of selective laser melting and instantaneous non-equilibrium solidification, and determining the optimal alloying component range by taking the minimized alloy thermal sensitivity index and the critical temperature range as targets.
Specifically, in this embodiment, firstly, the alloying elements are primarily screened, and the process is basically as shown in fig. 1; according to a magnesium alloy phase diagram, aiming at high solid solubility, firstly, a commonly used Mg-Gd/Y-based rare earth magnesium alloy is taken as a design object, namely Gd and Y are taken as candidate main rare earth elements X, then, commonly used candidate rare earth elements M which are beneficial at high temperature and have limited solid solubility are screened based on a rare earth element solubility curve and the magnesium alloy phase diagram, such as Nd, Ce, Sm and the like, and then, beneficial candidate modified elements N, such as Zr, Zn, Ca and the like, are screened based on a design principle of separating out a high-temperature resistant second phase, so that an Mg-X-M.
Next, in this embodiment, it is necessary to calculate the influence of the alloy elements on the solidification characteristics and the thermophysical characteristics of the alloy.
Firstly, simulating a time-temperature curve of a melting process of melting and rapidly forming WE-series rare earth magnesium alloy in a selective laser area by adopting finite element temperature field simulation.
In this example, the fusion process time-temperature curve of Mg-3.77Y-2.46Nd-1.23Gd-0.21Zn-0.4Zrwt (%) alloy was simulated using the model as shown in FIG. 2, which can be generalized to this example for further calculations with a uniform fusion process time-temperature curve due to its typicality in magnesium alloys. Part a in fig. 1 is a single-layer multichannel magnesium alloy selective laser melting model, a laser heat source with gaussian distribution is used as an incident heat source, and the process parameters comprise the laser scanning speed of 1000mm/s, the scanning distance of 70um, the power of 125W and the laser spot radius of 0.04 mm. By placing a point probe at the model (250, 800, 35), a temperature profile at that point can be obtained, as shown in section b of fig. 1. As can be seen from the figure, the material is typically solidified non-equilibrium within 0.3ms from melting to solidifying in the additive manufacturing process.
Under this time-temperature curve of the fusing process, the temperature magnitude corresponding to each time point in the solidification process can be obtained. Considering the diffusion behavior of the multi-component magnesium alloy, correspondingly setting time and temperature one by one, then introducing the time and temperature into a Thermo-Calc software package, and then calculating the solidification characteristics of alloy elements by combining a Scheil-Gulliver non-equilibrium solidification calculation module in the software, wherein the solidification characteristics comprise the element solid solubility, the alloy solidification interval and the high-temperature resistant second phase type;
FIG. 3 is a calculation result of different mass fractions of Gd or Y added to pure magnesium, according to the magnesium alloy high temperature strengthening principle, according to the criteria of simultaneously or as much as possible satisfying the selected elements with large element solid solubility, narrow alloy solidification interval and high temperature resistant second phase melting point. As can be seen from FIGS. 2a and b, the Y element satisfies both the narrow solidification range of the alloy and the Mg as the second phase 24 Y 5 Has a melting point (848.44K) higher than that of the second phase Mg 5 The Y element is selected according to the condition of the melting point (805.02K) of Gd.
Similarly, different candidate M elements can be added on the basis of the Mg-Y alloy for calculation, and finally the M element is determined. If more than one element simultaneously meets two or three conditions in the selection process of the M element, the element is selected according to the weight of the solid solubility of the element, the melting point of the high-temperature resistant second phase and the solidification interval of the alloy.
Then, a 3X 2 Mg alloy (hcp) super-cell model with 36 atoms is established by utilizing VASP (Viennaab-initio Simulation Package) first principle software, and a multi-element magnesium rare earth alloy model is established by adopting a replacement method. The exchange correlation effect between electrons is processed by Perew-Burke-Ernzerhof functional and generalized gradient approximation, a Brillouin zone K point is subjected to Monkhorst-Pack grid division of 5 multiplied by 4, the plane wave truncation energy is set to be 520eV, and the total energy error of system convergence is less than 1 multiplied by 10 -5 eV is formed. Calculating the influence of the alloy elements on the thermophysical characteristics including the second phase lattice constant, the phase structure stability (forming heat and binding energy) and the atomic bondResultant slip resistance. In the same way, according to the high-temperature strengthening principle of the magnesium alloy, elements which meet the requirements of large second-phase lattice constant, strong phase stability and large atomic bonding slip resistance are selected simultaneously or as much as possible. If more than one element simultaneously appears in the selection process of the N element and simultaneously meets two or three conditions, the sliding resistance is determined according to atomic bonding>Phase stability>The weight of the lattice constant of the second phase selects the element.
Next, in this example, the component contents of the selected elements were varied, and the cracking tendency of the above-mentioned alloying element components to the laser selective melting rare earth magnesium alloy was calculated by combining thermodynamic modeling and first principle calculation to evaluate the formability of the laser selective melting, in this example, in terms of the heat sensitivity index (| dT/d (f dT/d) s ) 1/2 I) and critical temperature range (Δ T) CTR ) Both parameters serve as indicators for assessing the susceptibility of the alloying design element constituents to hot cracks in the print.
Firstly, for selected main rare earth element Y, using thermo-calc to execute Scheil-Gulliver nonequilibrium solidification module to calculate solid fraction (f) of alloy formed by elements Y and Mg with different mass fractions in the solidification process s ) As a function of the temperature (T), the T-f for each content is then plotted s 1/2 A curve. When f is s 1/2 At 0.99 the alloy solidifies sufficiently (no correlation anymore beyond 0.99) so that only the T is considered E 568 ℃ f s 1/2 <Curve 0.99 (T on the curve in the figure) E Point is at f s 1 /2 Before the 0.99 vertical line). And a thermal sensitivity index (| dT/d (f) s ) 1/2 L) can be expressed as a point on the curve at a temperature T E The absolute value of the slope of time is determined by the smaller value of | dT/d (f) s ) 1/2 A smaller crystal growth rate and more liquid at the grain boundaries, reducing crack resistance. To sum up, T is reached on the curve E At temperature f s 1/2 <0.99 and at T E The element mass fraction corresponding to the curve with the absolute value of the slope being the minimum value relative to other curves is the mass fraction of the selected element Y; for example, FIG. 4 shows that the content of Y element in pure Mg is 5 wt% from 1 wt%As an end point, the T-f during solidification of various alloys formed by sampling with a step size of 0.5 wt% s 1/2 Graph is shown. According to the selection standard, the mass fraction of the Y element is selected to be 5%. And Δ T CTR Can also be made of T-f s 1/2 Graph acquisition, Δ T CTR Is on the curve f s 1/2 Point and f at 0.31 s 1/2 The difference in temperature corresponds to the point 0.99. Delta T CTR Higher values of (A) indicate longer time spent by the alloy in the crack susceptible phase and the presence of higher shrinkage strains detrimental to crack resistance, hence Δ T CTR The curve that is the minimum relative to the other curves is then the selected level. As shown in fig. 3, according to this principle, the mass fraction of the Y element in pure magnesium should be selected to be 5%.
Meanwhile, considering economic cost, according to the principle that the mass fraction of the total amount of the rare earth elements in the alloy is not more than 15%, according to the same idea, taking 0.5 wt% as the minimum value, taking the rest mass fraction of 15 wt% minus the mass fraction of the selected element Y as the maximum value and taking 0.5 wt% as the step length, sampling and calculating the thermal sensitivity index and the critical temperature range of Mg-X-M alloys with different contents of various synergistic rare earth elements M to determine the component range of the element M, wherein the mass fraction of the main rare earth element X is the mass fraction selected previously, and the rest part of the alloy is Mg;
and finally, in the same way, taking 0.5 wt% as the minimum value and taking the mass fraction of the rest of 15 wt% minus the mass fractions of the selected element Y and the element M as the maximum value, sampling and calculating the thermal sensitivity index and the critical temperature range of the Mg-X-M-N alloy with different element N contents to determine the composition range of the element N, wherein the mass fractions of the main rare earth element X and the synergistic rare earth element M are the mass fractions selected previously, and the rest of the alloy is Mg.
If two curves (different components) appear when selecting the components, the curves respectively satisfy | dT/d (f) s ) 1/2 Minimum and Δ T CTR When the criterion is minimum, the coefficient K is set to f s 1/2 (T＝TE) /ΔT CTR The curve with the largest K value is the selected component.
In the method in the embodiment, based on the strengthening mechanism of solid solution strengthening, second phase strengthening (melting point) and grain boundary strengthening (second phase lattice constant, phase structure stability (heat formation and binding energy) and atomic bonding slip resistance) of the magnesium alloy under a high temperature condition, elements beneficial to the high temperature of the magnesium alloy are selected by combining thermo-calc with first principle calculation software VASP. Meanwhile, in order to enable the designed alloy to meet the standard of additive manufacturing for high compactness and no crack, the thermal sensitivity index | dT/d (f) of the alloy elements with different compositions is also calculated s ) 1/2 And critical temperature interval DeltaT CTR As an indicator to assess the susceptibility of the alloying design element constituents to thermal cracking of the print to determine the optimum compositional range for the selected element. Ensuring that the designed alloy is suitable for non-equilibrium solidification conditions of additive manufacturing and good high temperature performance.
Claims (10)
1. A design method of a super-solid solution heat-resistant rare earth magnesium alloy based on non-equilibrium solidification is characterized by comprising the following steps:
step 1, selecting appropriate elements as multi-element synergistic alloy design elements by taking solid solution strengthening, second phase strengthening and grain boundary strengthening of the magnesium alloy under the high-temperature condition as design targets;
and 2, based on the characteristics of selective laser melting and instantaneous non-equilibrium solidification, calculating the influence of the mass fraction of the selected synergistic alloy design elements on the alloy thermal sensitivity index and the critical temperature range, and determining the optimal alloying component range of each element by taking the minimized alloy thermal sensitivity index and the critical temperature range as targets.
2. The method according to claim 1, wherein the design goal of solid solution strengthening under high temperature conditions is higher solid solubility;
the second phase strengthening is designed to target a higher melting point of the high temperature resistant second phase;
the design goals of grain boundary strengthening are that the second phase lattice constant and atomic bonding slip resistance are larger, and the phase stability is higher.
3. The method according to claim 1, wherein step 1 comprises, in combination with a magnesium alloy phase diagram, selecting a plurality of candidate primary rare earth elements X with the objective of having a high solid solubility;
selecting a plurality of candidate synergistic rare earth elements M by combining a rare earth element solubility curve and a magnesium alloy phase diagram and taking the design goal of being beneficial to high temperature resistance but having limitation on solid solubility performance;
and selecting a plurality of candidate modifying elements N with the aim of precipitating a second phase beneficial to high temperature resistance;
and constructing a Mg-X-M. -N multi-element synergistic alloying candidate system.
4. The method of claim 3, wherein the candidate primary rare earth elements X comprise Gd and Y.
5. The method of claim 3, wherein the candidate cooperating rare earth elements M comprise Nd, Ce, Sm.
6. The method of claim 3, wherein the candidate modifying elements N comprise Zr, Zn, Ca.
7. The method of claim 3, wherein step 1 further comprises:
establishing a typical WE-series rare earth magnesium alloy model, and simulating a fusing process time-temperature curve of the WE-series rare earth magnesium alloy formed by selective laser melting;
calculating an alloy solidification interval, an element solid solubility change curve and a high-temperature resistant second-phase melting point corresponding to each candidate X element after each candidate X element with different contents is added into Mg under the non-equilibrium solidification condition of the Scheil-Gulliver based on the time-temperature curve of the fusing process;
selecting one of the candidate main rare earth elements X as a main rare earth element by taking the conditions of simultaneously or as much as possible that the solid solubility of the element is high, the solidification interval of the alloy is narrow, and the high-temperature resistant second phase melting point is high as standards;
similarly, different candidate cooperative rare earth elements M are added on the basis of the selected Mg-X alloy for carrying out the same calculation and selection, and the final element M is selected from the candidate cooperative rare earth elements M;
if more than one element simultaneously appears in the selection process of the element M and meets two or three conditions, the element is selected according to the weight of the solid solubility of the element, the melting point of the high-temperature resistant second phase and the solidification interval of the alloy.
8. The method of claim 7, wherein the standard WE-based rare earth magnesium alloy is Mg-3.77Y-2.46Nd-1.23Gd-0.21Zn-0.4Zr wt (%) alloy.
9. The method of claim 7, wherein in step 1, based on the selected Mg-X-M alloy, the influence of adding different candidate modifying elements N on the second phase lattice constant, phase structure stability and atomic bonding slip resistance of the prepared alloy is calculated by using VASP (Vienna Ab-initio Simulation Package);
selecting a final element N from the candidate modified element N by taking the criteria of simultaneously or as much as possible that the second phase lattice constant is large, the phase stability is strong and the atom bonding slip resistance is large as the criteria;
if more than one element simultaneously appears in the selection process of the N element and simultaneously meets two or three conditions, the elements are selected according to the weight of atomic bonding slip resistance > phase stability > second phase lattice constant.
10. The method of claim 9, wherein in step 2, first, the solid fraction f of the alloy of the main rare earth elements X and Mg during solidification is calculated for different mass fractions s Changes with temperature T, and then the T-f corresponding to each mass fraction is drawn according to the changes s 1/2 A curve;
selecting the element mass fraction corresponding to the curve meeting the following conditions as the mass fraction of the main rare earth element X:
get T on the curve E At 568 deg.C f s 1/2 <0.99；
At temperature T E The absolute value of the slope, i.e. the thermal sensitivity index | dT/d (f) s ) 1/2 I is a minimum relative to the other curves;
③ Critical temperature range DeltaT CTR Minimum relative to other curves;
the critical temperature range DeltaT CTR Is f on the curve s 1/2 Point and f at 0.31 s 1/2 The difference in temperature corresponds to the point 0.99.
Secondly, according to the same idea, taking 0.5 wt% as the minimum value, taking the rest mass fraction obtained by subtracting the mass fraction of the selected element Y from 15 wt% as the maximum value, sampling and calculating the thermal sensitivity index and the critical temperature range of Mg-X-M alloys with different contents of various synergistic rare earth elements M to determine the component range of the element M, wherein the mass fraction of the main rare earth element X is the mass fraction selected previously, and the rest of the alloys are Mg;
finally, in the same way, by taking 0.5 wt% as the minimum value and taking the rest mass fraction obtained by subtracting the mass fraction of the selected element Y and the element M from 15 wt% as the maximum value, sampling and calculating the thermal sensitivity index and the critical temperature range of Mg-X-M-N alloys with different element N contents to determine the component range of the element N, wherein the mass fraction of the main rare earth element X and the mass fraction of the synergistic rare earth element M are the mass fractions selected previously, and the rest parts of the alloys are Mg;
in each selection, if two curves appear, the thermal sensitivity index | dT/d (f) is satisfied s ) 1/2 Minimum and critical temperature range Δ T CTR The minimum criterion is to set the coefficient K ═ f s 1/2 (T＝TE) /ΔT CTR And selecting the element mass fraction corresponding to the curve with the maximum K value.
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