Study on Mechanical Properties and Deformation Mechanism of TWIP Stainless Steel

: In this study, based on the sensitivity of the chemical composition ﬂuctuation to the thermodynamic parameter, which controls the level of the stacking fault energy (SFE), a series of high Cr–Mn–N twinning-induced plasticity (TWIP) stainless steels are designed by using a sublattice model, and their mechanical properties and micro deformation mechanism are analyzed The formation of mechanical twins (Mts) during the deformation makes the test steel show a perfect combination of strength and ductility after different solution treatments. Among them, after a solution treatment at 950 ◦ C and 1050 ◦ C, the 19Cr–0.7N and 19CrSi–0.7N samples have the maximum value with the product of the strength and plasticity reaching 60.7% and 64.6%, and 12Cr–CN has the maximum value after the solution treatment at 1200 ◦ C, reaching 81.3%. The SFE values of the 19Cr–0.7N and 19CrSi–0.7N samples were relatively high, 48 mJ · m − 2 and 45 mJ · m − 2 , respectively. The SFE of 12Cr–CN is 37 mJ · m − 2 , and the Mts grow rapidly during the deformation and maintain the highest twinning density under the same strain conditions. The characterization of the tensile samples occurs under different deformations by electron backscattered diffraction (EBSD) and transmission electron microscope (TEM). The results of the EBSD local misorientation difference angle analysis showed the Silicon element addition with a good Mts saturation rate. It is observed from the TEM that the nucleation process of the Mts with a high SFE is difﬁcult, and the Mts emit and grow inward along the grain boundary during the tensile process and present a cross shape with the increase in strain. The contribution of the grain boundary strengthening ( σ 0 ), dislocation strengthening ( σ f ), and twinning strengthening effect ( σ t ) under dynamic micro-reﬁnement to stress were calculated. It is known that under a certain amount of strain, the ratio of σ t and σ f changes with increasing, and when the contribution of the twinning deformation to the stress exceeds about 25%, the reinforcement of the plastic deformation is dominated by the plane of σ f .


Introduction
The TWIP steel has great potential for application in automotive structural components, the construction industry, and oil and gas exploration due to its excellent tensile strength, ductility, and high energy absorption capability [1].In FCC crystal structure alloys, the SFE of the alloy system is the key factor affecting the deformation mechanism.The SFE in the range of 20 mJ•m −2 to 40 mJ•m −2 is accompanied by the twinning effect, while the Mts with an SFE less than 20 mJ•m −2 are replaced by martensite sheets in the deformation process, that is, phase transformation-induced plastic steel (TRIP).A dislocation slip is dominant when the SFE is greater than 40 mJ•m −2 ; typical Cr-Ni austenitic stainless steel belongs to this mechanism [2,3].The chemical driving force for the transformation of metastable austenite into twins is proportional to the intrinsic SFE of the alloy.It is explicit that being closely related to the alloy components and their contents was the most important parameter for controlling the transformation behavior of the metastable austenitic steels.The research shows that the addition of manganese can increase the SFE in a certain range and provide a plastic effect [4,5].Nitrogen, as an interstitial strengthening element, can expand the austenite phase zone, and the addition of nitrogen and chromium will improve the SFE of the system [5].It is found that when the manganese content is greater than 25 wt.%, the martensitic transformation is completely suppressed, and the deformation mode is mainly twinning and slip deformation, such as typical Fe-25Mn-3Si-3Al and Fe-22Mn-0.6CTWIP steels for automotive [6,7].Carbon is very important for SFE control, but in stainless steel, only ultralow-carbon (<0.02 wt.%) stainless steel content has good acid corrosion resistance, while the previous SFE calculation research has focused on high-carbon structural steel [8][9][10], how to adjust the Cr, Mn, Ni, C, N, and Si elements to match the TWIP stainless steel design method with an SFE between 20 mJ•m −2 and 40 mJ•m −2 has not been mastered.The researchers of this paper, combined with typical automotive TWIP steel, introduced the twinning effect to an ultralow carbon and developed a stainless TWIP steel with a high resistance to intergranular corrosion, which has both high corrosion resistance and high strength properties.
However, it should also be noted that TWIP steels are based on dislocations slip and twinning mechanisms that induce high strength and plasticity, but the contribution of the different mechanisms to the flow stress and hardening behavior varies for a new stainless steel.For example, the Bauschinger effect in Fe-22Mn-0.6CTWIP steel studied using reverse shear showed that the maximum contribution of twins to flow stress during plastic deformation is only 8.4%.In the tensile process of 18Mn-0.75C-1.7AlTWIP steel, the forest dislocation-related stress accounts for 60% of the flow stress, the contribution of twins to the flow stress does not exceed 10%, and more than 90% of the stress increment comes from dislocations [11][12][13][14][15].The essence is that the initial state of the sample is different, the dislocation density of the solid solution-strengthened austenitic stainless steel after the solid solution is low, which is different from the warm rolling state of the sample used in [12][13][14][15], and a higher dislocation density is introduced in advance.The calculation result is naturally that dislocation strengthening accounts for a higher proportion.In this study, a series of high corrosion-resistant austenitic steels with a TWIP effect are obtained by calculating the relationship between the alloy composition of ultralow-carbon TWIP stainless steel, and adjust the phase balance (ferrite and austenite) and ensure the full austenite structure, the corresponding element range was optimized.The microstructure characteristics of this kind of steel at different strain stages were analyzed and studied by tensile tests and advanced characterization methods.The effects of grain, twinning, and dislocation in TWIP stainless steel on preventing dislocation slip and their contributions to flow stress were calculated.

Experimental Procedures
According to the composition calculation results in Section 3.1, Fe-xCr-25Mn-xC-xN series (x is wt.%)TWIP ingots are smelted in a 50 kg medium-frequency vacuum induction furnace.Then, the bar with a diameter of 16 mm is forged at 1200 • C, and the final forging temperature is greater than 980 • C.After wire cutting, the material is kept at 950 • C, 1000 • C, 1050 • C, 1100 • C, 1150 • C, and 1200 • C for 60 min and then cooled to obtain subsequent test analysis samples.The tensile test is completed by CMT5205 testing machine produced by Shenzhen Xinsansi material testing Co., Ltd., Guangdong Province, Shenzhen, China, and the strain rate is 0.1 mm/min, during the tensile deformation process, the load-displacement curve is recorded on the computer screen, and the different deformation samples are prepared by displaying the curve vernier caliper to determine the difference.XRD and EBSD samples were longitudinal anatomical samples, and the pretreated surface was electropolished with 10 vol% perchloric acid ethanol for 20 s.
In this experiment, the dislocation density under different strain was measured by XRD.After the sample is cut by wire cutting, the block sample with smooth surface is made by mechanical polishing.The experimental parameters of tube voltage and current are 40 kV and 150 mA.Through continuous scanning mode, the scanning speed is 0.5 • /min, including (111)γ, (200)γ, (220)γ, (311)γ, and (222)γ.Then preprocess the diffraction peak, such as eliminating background noise and smoothing, to obtain the data required for calculation.The measured diffraction peak width is caused by both geometric broadening and physical broadening, but the change of geometric broadening calculated by this test equipment is less than 1.5%, which is very small compared with the dislocation density measured by XRD.EBSD model is FEI Quanta 650 thermal field scanning electron microscope, provided by Hillsboro FEI Company, MA, USA, with an operating voltage of 20 kV and a scanning microstructure area of 500 µm × 500 µm.
The equipment used for EBSD analysis is Zeiss Auriga focused ion beam field generator with HKL-Channel 5 EBSD system, scanning step is 0.5 µm, the measurement system is based on Brandon criterion (∆θ ≤ 15 • ∑ −1/2 ) determine the coincidence site lattice.The data were processed and analyzed by TSL-OIM software.The misorientation angles of 5~15 • are low angle grain boundaries (LAGBs), and greater than 15 • are high angle grain boundaries (HAGB).For the TEM required observation sample, use wire cutting to cut a 5 mm thin sample, and rough grind it to 50 µm.A disc with a diameter of 3 mm is prepared by punching mechanism for double spray thinning.The double-jet thinning solution is 8 vol% perchloric acid alcohol, the double-jet temperature is between minus 15 • C and 30 • C, and the thinning current is 40 mA.After thinning, the microstructure and deformation twin evolution were analyzed by JEM-2100F field emission transmission electron microscope, JEF-2100F provided by JEOL, Tokyo, Japan, with an operating voltage of 200 kV.

SFE Calculation
The TWIP effect is the focus of high Mn (more than 15 wt.%) steel research, and SFE (γ) is the internal factor affecting the deformation mechanism.However, there is no perfect and unified SFE calculation method, most of which are summarized as empirical formulas of component type and content, and the application of the newly established component system is not accurate.The calculation error of the SFE value of different models for the same material can reach 3-30 times.In this study, an ideal stacking fault on FCC crystal was composed of a thin layer of closely arranged hexagonal lattice, take widely used thermodynamic calculation sublattice model to calculate the SFE in TWIP stainless steels [16,17].This model assumed that Fe, Cr, Mn, Ni, Si, C, N, and vacancy (Va) separately occupy the substitutional and interstitial sites with random mixing only in take up one sublattice.Based on this assumption, the SFE of austenite can be expressed as [16][17][18]: where interface energy is a constant, generally taken 5~10 mJ•m −2 [18], the ρ is the molar surface density of the atomic dense plane 4/(a γ 2 N A ), N A is Avogadro constant, a r is the lattice parameter, which is represented the change of composition as by Equation ( 2) [19]: Bulk atoms form a sublattice, which can be regarded as a solid sublattice, and the other sublattice is composed of octahedral holes in the lattice, which can be regarded as a void sublattice or a void lattice.The main elements Fe, Mn, Cr, Ni, Si, C, and N added to TWIP stainless steel are as follows: X is the ratio of the nodes of two sublattices, where x = 1 for γ, x = 3 for α, and x = 0.5 for ε [17], respectively.
In Equations ( 4) and ( 5), y i is the site fraction of each element, L i,j is the interaction parameter for the elements (i and j) of C, N, Si, Va in the substitutional sublattice, while G γ→ε ex is the excess Gibbs free energy related to the original grain size, a = b = 1 for γ phase, a = 1, b = 0.5 for ε phase, R is the gas constant, the lack of relevant parameters is also a factor affecting the accuracy of SFE calculation.Ref [20] studied how the G γ→ε ex reduces the SFE of the system by 7 mJ•m −2 , when the ultrafine austenite grain increases from 5 nm to 150 nm.Fine crystal structure can improve the SFE of the system, but it can inhibit the TWIP effect.G m:n is composed of several solid solution elements Gibbs free energy of phase, (i and j) of C, N, Si, Va in the substitutional sublattice, the phase of γ and ε Gibbs in the magnetic state is represented by Equation ( 6) T neel is the phase of ϕ Neel temperature, β is a component-dependent magnetic moment.The lattice fraction y i of each element satisfies Equation ( 7) in the sublattice model: C, N, and Si content satisfy Equation (8) in the sublattice gap: where x i is the mole fraction of each element, where x i is the mole fraction of solid solution element, λ is 1 in γ phase, λ is 0.5 in ε phase, respectively.The data in the research results of [21][22][23][24][25] are referred, and the calculation parameters are listed in Table 1.The influence of items not included in SFE calculation is small and can be neglected, and some parameters are incomplete and untraceable.Each lattice fraction satisfies formula (9) as: According to the above equations, the SFE of the different systems of the alloys at room temperature 298 K were calculated, and then the measured chemical composition was calculated, and the results were obtained, as shown in the Table 2.

Tensile Test
The service performance of the material depends on the solid solution strengthening effect after the addition of the multi-component alloy and the optimization of the microstructure in the subsequent process, including the strong allowable stress value of the material contributed by the grain boundary, dislocation, precipitated phase, and phase transformation [25].The contribution of strength can be obtained indirectly by subtracting the contribution of each part from the measured stress value.The samples designed in this study do not contain precipitation strengthening elements, and the individual strengthening contribution of the TWIP stainless steel can be quantified by iteratively adding various strengthening increments, with the total stress value σ ss consisting of the following partial: In Equation (10), σ 0 for solid solution strengthening, σ g for grain boundary strengthening, σ f for dislocation strengthening, and twinning strengthening σ t .
The three TWIP steels have similar microstructure characteristics after the solution treatment.Typically, as shown in Figure 1, in the 19CrSi-0.7Nsample after the solution treatment at 1050 • C, they are all equiaxed grains with a certain number and thickness of annealing twins in the grain distribution.With the increase in the solution temperature, the austenite grains grow gradually, and the annealing twins distributed in the grains become wider and thicker.The grain size and performance parameters of the three steels treated at different solution temperatures are shown in Table 3.The three test steels' mechanical values are summarized in Table 3.It is generally accepted that the adequacy of the grain size plays an important role in affecting the strength and ductility of TWIP steels.The maximum total elongation and strength of the 19Cr-0.7N,19CrSi-0.7N,and 12Cr-CN steels are 69.2%,73%, and 96.5%, respectively, and the Rm is 868 MPa, 880 MPa, and 843 MPa, respectively.The fine Mts formed from Figure 2a,b, after the deformation average width of about 85 nm shown in Figure 2b.The Mts as the grain boundary play a strong role in hindering the movement of the dislocation.During the deformation, the Mts continuously nucleate and grow, which leads to the continuous strengthening effect of the dynamic Hall-Petch effect and the high strain-hardening rate of the alloy, thus showing excellent strength and plasticity, A > 55%, Rm > 800 MPa.The deformation storage energy during the forging process causes the alloy to undergo static recrystallization during the solution treatment, and the grains grow gradually with the extension of the heat-treatment time, and the dislocation density in the grains continues to decrease.Therefore, the driving force of the Mts nucleation is small at the initial stage of the plastic deformation.The contribution is negligible in the calculation, and numerous dislocation arrangements can be observed in the few annealing twins, as shown in Figure 2a.
The yield stress (YS) of the fully recrystallized (σ y,recrystallized ) is controlled by the solid solution and grain boundary hardening, i.e., σ y,recrystallized = σ 0 + σ g .For alloys with low SFE, the higher the critical shear stress of twins, it is difficult to form deformation twins [26,27].Compared with 19CrSi-0.7Nand 19Cr-0.7NTWIP stainless steel, the strength is reduced after the addition of the Si element, but the plasticity is greatly improved, which is related to the low SFE of the system.As mentioned, the SFE of the system with the addition of 1.5 wt.% Si is reduced by 3 mJ•m −2 , and the deformation process is easier to induce Mts and further improve the plasticity.The relationship between the grain size and yield strength for evaluating the strengthening effect of the solid solution is described in formula (11): In Equation (11), σ y , σ 0 , k y , and D are yield strength (MPa), lattice friction resistance (MPa), Hall-Petch coefficient (MPa•mm 1/2 ), and grain size (mm), respectively.The data fitting relationship is shown in Figure 3.The Hall-Petch relationship for the yield strength had a similar slope in Figure 3.The recrystallized grains and the intracrystalline annealing twin boundary or original austenite boundary hinder the dislocation movement to achieve the grain boundary strengthening effect, which can be achieved through σ y,recrystallized = σ 0 + σ g indicates.By fitting the Hall-Petch formula to the R p0.2 corresponding to the grain size at different solution temperatures, different test materials can be obtained, respectively, by σ y,recrystallized Equation ( 12): 12Cr-CN, σ y (MPa) = 260.58+ 26.86D −1/2 r 2 = 0.93 19Cr-0.7N,σ y (MPa) = 361.44+ 27.09D −1/2 r 2 = 0.91 (12) 19CrSi-0.7N,σ y (MPa) = 365.46+ 21.94D −1/2 r 2 = 0.91 The test sample was solution treated at 1050 • C, and the corresponding D value was put into formula (12) to obtain the parameter values of the three samples: 12Cr-CN σ 0 and σ g are 260.58MPa and 131.31MPa, 19Cr-0.7Nσ 0 and σ g are 361.44MPa and 131.08 MPa, and 19CrSi-0.7Nσ 0 and σ g are 365.46MPa and 106.17MPa.Compared with 19Cr-0.7N,Si as a solid solution strengthening element reduces the SFE of the system within the alloy system, the frequency of excited dislocations decreases, and the GBs strengthening coefficient, ky.In high Mn-N austenitic stainless steel, the solid solution strengthening effect of nitrogen can be summarized as σ 0.2 = 150 + 500 √ N [28].According to the tensile test results, the yield strength value of the strengthening effect achieved by a single nitrogen addition is about 568.33 MPa.While the maximum values of the strength sample of the 19Cr-0.7Nand 19CrSi-0.7Ntest materials with ultralow-carbon content are 533 MPa and 492 MPa after the solid solution at 950 • C, respectively, which also shows that the silicon addition will reduce the solid solution strengthening effect of Cr-Mn-Ni-N TWIP steel but can improve the plastic toughness.

Dislocation Strengthening Calculation
The measured diffraction peak line shape is used to characterize the broadening caused by microdistortion.The lattice plane of (111) γ curves with different true strains are shown in Figure 4.The intensity of the diffraction peaks of the three steels becomes shorter and wider with the increase in strain.It clearly indicates that the evolution of the peak profiles with plastic strain are linked to the microstructure in terms of several parameters, including crystal size and dislocation density.The full width at half maximum is found in the jade software database and brought into the calculation formula of dislocation density as [29]:  4. The increase in the flow stress should be dominated by the dislocation evolution and deformation twinning after the yield strain.For the evaluation of the forest hardening with Equation ( 14) [30]: where M is the average Taylor factor, obtained through the EBSD test, and 3 is taken as the calculated value, α is the interaction strength between dislocations, and the corresponding theoretical derivation and extensive experimental results, α is about 0.2 in the FCC structure, G is the shear modulus of elasticity, GPa, derived G = E/2(1 + ν) from the value of E in tensile deformation in Table 4,where ν is Poisson's ratio, taken as 0.3 and obtain the value of G as 70, b is Burgess vector, the value is 0.2587 nm.The stress values for σ f are given in Table 5 based on the above calculations.Dislocation multiplication is strong in TWIP steels, which is usually attributed to the continuous reduction in the dislocation mean free path by the deformation twinning during the plastic deformation.

Twinned Strengthening Calculation
Figure 5 shows the TEM images of the specimens under different tensile strains, which clearly captures the morphologies of the medium Mts of different SFE specimens under varying strains.
As shown in Figure 5a, for the 12Cr-CN specimen with a low SFE, a small amount of sparsely distributed and small and transparent Mts were observed in the strain at the time of yielding.The 19Cr-0.7N and 19CrSi-0.7Nspecimens have a higher SFE and only a small amount of twin growth emanating from the grain boundaries.Incomplete Mts can be observed at the grain boundaries, emitting along the grain boundaries, and they are small and mostly terminated within the grain with a high dislocation density between the twins in Figure 5b,c, which is seen as the nucleation stage of the Mts.The Mts and grain boundaries also act as barriers to hinder dislocation movement.High-density twin grain boundaries can effectively reduce the mean free path of dislocation movement, resulting in a large number of dislocations accumulating at the Mts interface, thereby achieving the purpose of strengthening.The Mts became more complete when the tensile strain increased to 10%, as shown in Figure 5d, but the number of Mts in the 19Cr-0.7Nand 19CrSi-0.7Nsamples with a relatively high SFE is small, and there is a large width between the Mts, and there is a staggered trend from Figure 5e,f.6d.In the local grain fine area, the Mts density is higher, which was consistent with the result of the TEM image shown in Figure 5. Compared with the 19Cr-0.7Nand 19CrSi-0.7Nsteel which has a smaller difference in SFF, the trend of the Mts in the test material with the 1.5 wt.% Si addition is more obvious, and it was observed that the amount of Mts generated in the local grains was more as shown by the blue circle in Figure 6f.The twinning process is not only related to the advantage of the Mts orientation, that is, the crystal orientation in the {111} and {101} directions also has a higher Mts density, as shown by the white circle in Figure 6c.In addition, it also shows that the addition of the Si element promotes the twinning effect.The grain size and grain orientation have a certain dependence on the generation of Mts; the results are similar to what Gutierrez et al. [12] found in their study of Fe-22Mn-0.6CTWIP steel that grain orientation influenced the twinning process at a small strain.The frequent formation of nano Mts is considered to be an important reason for the enhancement of the strain hardening ability, which leads to high strength and large tensile ductility in TWIP stainless steel samples.When the strain was increased from 0.14 to 0.3 (Figure 7), the density of the Mts increased significantly, the Mts formed by dislocations and transformed into HAGB, represented by black lines, as marked by blue circles in Figure 7d.The Mts are only generated in some of the specially oriented grains at small deformation volumes, which easily reach the twinning critical shear stress to satisfy the twinning condition with the test steel Schmid coefficient.However, the effect of these {111} and {101} orientations of grains on mechanical twinning disappears rapidly with increasing deformation variables [31].With deformation twinning occurring mainly in the <111> direction, while other directions have less deformation twinning, when the strain is 30%, the Mts are generated in almost all directions except for the <001> direction of the grain.If the strain continues to increase to the fracture stage, the plane slip characteristics, such as the typical Taylor lattice, will appear.
As can be seen from the Figure 8 orientation difference distribution diagram, the high percentage of LAGBs for the different composition specimens after the tensile deformation, except for the weak change in specimen 19CrSi-0.7N,show an increasing trend with the degree of deformation, Figure 8c,f.This high density and saturation of the Mts is due to the Si addition to promote the twinning process, which shortens the growth time of the Mts and reaches a higher saturation state more quickly in a larger strain range.By means of the above statistics, the value at each strain is recorded according to the true stress-strain curve, and the specific values of each strengthening effect at different strains can be calculated according to σ ss = σ 0 + σ g + σ f + σ t .Figure 9 shows the percentage of each strengthening term.It can be seen from the histogram of each enhancement ratio in Figure 9, the stressstrain curve of 12Cr-CN was at the highest level, and the σ 0 and σ g contributions are fixed, and the percentages of the twinning strengthening and dislocation strengthening increase with the increase in strain.The percentages of the Mts strengthening contribution of 12Cr-CN are 11.74%, 22.4%, and 24.7%, respectively, and the percentages of the Mts strengthening of 19Cr-0.7Nare 16.6%, 25.6%, and 25.9%, respectively.The Mts strengthening percentages of 19CrSi-0.7Nare 10.4%, 23.1%, and 27.1%, respectively.Unlike the pre-deformed Fe-Mn-C TWIP steel [32], the twin contribution is only 3% of the total plastic deformation (under 35% of the total strain).Refs.[33,34] studied the role of twins in TWIP steel which is to form the strengthening effect by continuously reducing the mean free path of the dislocations during the plastic deformation.The solid solution specimens studied in this experiment had a twinning effect as the main deformation mechanism, especially at low strains, with the volume fraction of the deformation twins accounting for a large proportion of the total grain volume, and the high density of the dislocations during the continued increase in the strain of the dislocations in the continued deformation process continuing to become secondary twinning, and the effect of the dislocation strengthening and twinning strengthening ratio will also be increased.

Conclusions
In this study, the SFE of the Fe, Cr, Mn, Ni, Si, C, and N system is calculated through thermodynamic calculation.After the Cr content is mainly considered, a relatively conservative low Cr TWIP stainless steel 12 wt.%Cr is designed, which has a sufficient TWIP effect under strain.However, 12 wt.%Cr content is too low, and the high C content is difficult to meet the high corrosion-resistance standard.The ultralow C 19 wt.% stainless steel is added with a N element to adjust the ferrite and austenite balance and stabilize the SFE and attempts to add Si to regulate the SFE (about 40 mJ•m −2 ) to meet the TWIP occurrence during the deformation.The high Mn-N type 19Cr-0.7Nand 19CrSi-0.7Ntest steels are designed.The mechanical property parameters of the samples with different grain sizes were obtained by the solution treatment at 900-1200 • C, and the microstructure changes and corresponding strengthening mechanism of the three test steels after the solution treatment at 1050 • C were studied by controlling the tensile strain.The main conclusions are as follows: 1.In this study, it was assumed that the Fe, Cr, Mn, Ni, Si, C, N, and vacancy (Va) site separately occupy the substitutional and interstitial position sublattice model to calculate the design of new TWIP stainless steels' SFE of the 12Cr-CN and ultralow carbon 19Cr-0.7Nand 19CrSi-0.7Nsamples with 35 mJ•m −2 , 48 mJ•m −2 , and 45 mJ•m −2 , respectively.
2. In the solution temperature range of 1000-1200 • C, the grain size and annealing twin thickness tend to grow with the increase in the solution temperature, which is more conducive to the nucleation and growth of the Mts, and a large amount of the Mts is generated under the tensile load, which plays a role in dynamically refining the structure, so that the designed test steel has good plastic toughness, strength, and plasticity, which is more than 50 GPa•%.The Mts of 12Cr-CN with the lowest SFE is the most sufficient, the strengthening effect is the best, and the product of the strength and plasticity can even reach 80 GPa•%.
3. The deformed structure of the low SFE TWIP stainless steel shows a different twinning process and dislocation movement characteristics at different strain stages.The deformation mechanism of the TWIP stainless steel is controlled by the range of the SFE.The strength of 19CrSi-0.7N is lower than that of 19Cr-0.7N,but its plasticity is improved, which indicates that the silicon addition will reduce the SFE of the system and promote the twinning process.Under the same strain, the lower the SFE, the smaller the twin-induced critical stress and the higher the twin density.With the continuous increase in strain, the tendency of the preferential generation of the Mts by the grain orientation decreases rapidly, and the strengthening effect of the Mts also decreases.

Figure 4 .
Figure 4. XRD pattern of (111)γ at different strain of (a) 12Cr-CN, (b) 19Cr-0.7N,(c) 19CrSi-0.7N.ρ is average dislocation density in material (m −2 ), which can be measured by X-ray diffraction (XRD) experiments or simulated based on the models.The β is the full width at half-maximum of the XRD diffraction peak on the crystal plane, b is the burger vector.The calculations corresponding to the 2θ crystal plane positions (111) γ , (200) γ , (220) γ , and (311) γ results are shown in Table4.

Figures 6
Figures 6 and 7 are the EBSD characterization.The red line in Figure 6 represents that the Mts are at LAGB (orientation difference of 2~15 • ), the density distribution of the LAGB corresponds to the change in the Mts, and the black line represents that the original austenite grain boundary is at HAGB (orientation difference over 15 • ).It can be directly observed from Figure 6d-f that the Mts grow along the grain boundary and have a high density at the grain boundary, and the low SFE of the 12Cr-CN sample twinning process is more adequate at the same amount of deformation, as shown in Figure6d.In the local grain fine area, the Mts density is higher, which was consistent with the result of the TEM image shown in Figure5.Compared with the 19Cr-0.7Nand 19CrSi-0.7Nsteel which has a smaller difference in SFF, the trend of the Mts in the test material with the 1.5 wt.% Si addition is more obvious, and it was observed that the amount of Mts generated in the local grains was more as shown by the blue circle in Figure6f.The twinning process is not only related to the advantage of the Mts orientation, that is, the crystal orientation in the {111} and {101} directions also has a higher Mts density, as shown by the white circle in Figure6c.In addition, it also shows that the addition of the Si element promotes the twinning effect.The grain size and grain orientation have a certain dependence on the generation of Mts; the results are similar to what Gutierrez et al.[12] found in their study of Fe-22Mn-0.6CTWIP steel that grain orientation influenced the twinning process at a small strain.
Fe y Si G Fe:Si + y Cr y Si G Cr:Si + y Mn y Si G Mn:Si +y Ni y Si G Ni:Si + y Al y Si G Al:Si + y Fe y Va G Fe:Va + y Cr y Va G Cr:Va + y Mn y Va G Mn:Va + y Ni y Va G Ni:Va +y Al y Va G Al:Va + aRT(Fe ln y Fe + y Cr ln y Cr + y Mn ln y Mn + y Ni ln y Ni + y Al ln y Al ) + bRT(y C ln y C +y N ln y N + y Va ln y Va + y Si ln y Si ) + G γ→ε Fe yCr (y C L Fe,Cr:C + y N L Fe,Cr:N + y Va L Fe,Cr:Va + y Si L Fe,Cr:Si ) + y Fe yMn (y C L Fe,Mn:C + y N L Fe,Mn:N +y Va L Fe,Mn:Va +y Si L Fe,Mn:Si ) + y Fe yNi (y C L Fe,Ni:C + y N L Fe,Ni:N + y Va L Fe,Ni:Va +y Si L Fe,Ni:Si ) + y Cr yMn (y C L CrMn:C + y N L CrMn:N + y Va L CrMn:Va + y Si L CrMn:Si ) +y Cr yNi (y C L CrNi:C + y N L CrNi:N + y Va L CrNi:Va + y Si L CrNi:Si ) +y Mn yNi (y C L Mn,Ni:C + y N L Mn,Ni:N + y Va L Mn,Ni:Va +y Si L Mn,Ni:Si ) + y C yN (y Fe L Fe:C,N + y Cr L Cr:CN + y Mn L Mn:C,N +y Ni L Ni:C,N ) + y C ySi (y Fe L Fe:C,Si + y Cr L Cr:C,Si + y Mn L Mn:C,Si +y Ni L Ni:C,Si ) + y C yVa (y Fe L Fe:C,Va + y Cr L Cr:C,Va + y Mn L Mn:C,Va + y Ni L Ni:C,Va ) +y N ySi (y Fe L Fe:N,Si + y Cr L Cr:N,Si + y Mn L Mn:N,Si +y Ni L Ni:N,Si ) + y N yVa (y Fe L Fe:N,Va + y Cr L Cr:N,Va + y Mn L Mn:N,Va + y Ni L Ni:N,Va ) +y Si yVa (y Fe L Fe:Si,Va + y Cr L Cr:Si,Va + y Mn L Mn:Si,Va +y Ni L Ni:Si,Va ) + y Fe y Cr yMn (y C L Fe,Cr,Mn:C + y N L Fe,Cr,Mn:N + y Si L Fe,Cr,Mn:Si + y Va L Fe,Cr,Mn:Va ) +y Fe y Cr yNi (y C L Fe,Cr,Ni:C +y N L Fe,Cr,Ni:N + y Si L Fe,Cr,Ni:Si + y Va L Fe,Cr,Ni:Va ) +y Fe y Cr y Mn yNi (y C L Fe,Cr,Mn,Ni:C + y N L Fe,Cr,Mn,Ni:N + y Si L Fe,Cr,Mn,Ni:Si +y Va L Fe,Cr,Mn,Ni:Va)

Table 1 .
The values used for the estimation of the SFE in the investigated TWIP steel.

Table 2 .
Measured composition (mass %) of the main alloying elements of the investigated steels.

Table 3 .
Performance parameters of samples at different heat-treatment temperatures.

Table 4 .
Comparison of the rate of dislocation accumulation during deformation, m −2 .