| Low cost Fe-Mn-Si-based shape memory alloys(SMAs)have great potential for engineering applications due to excellent mechanical properties.However,their poor shape memory effect(SME)limits their practical applications.To achieve excellent SME,the Fe-Mn-Si-based SMAs must undergo special heat treatments,including training,thermomechanical treatment and ausforming.Based on the presence of a large number of uniformly distributed stacking faults in alloys undergoing the above special heat treatments,scholars suggested that the pre-existing of a large number of stacking faults is the prerequisite for excellent SME of Fe-Mn-Si-based SMAs.However,some studies have shown that the alloys under hot-rolling and annealing can achieve comparable SME to that of the ones undergoing special heat treatments despite the absence of large stacking faults.Recent results revealed that the solutiontreated Fe-Mn-Si based SMAs have a large number of annealing twins due to low stacking fault energy,but the length fraction of the annealing twin boundary is significantly reduced after the special heat treatments mentioned with the optimal parameters,and there is a significant negative dependence of SME on the length fraction of the annealing twin boundary.However,the relationship between the length fraction of annealing twin boundaryand SME in the alloy under special heat treatments with the non-optimal parameters is not clear at present.In addition,the previous study only focused on the alloy without the participation of thermal martensite during the special heat treatments.For the alloy with the participation of thermal martensite,the relationship between the length fraction of annealing twin boundary and SME in the special heat treatment process with the participation of thermal martensite remains to be clarified.The mechanism by which the length fraction of the annealing twin boundary is reduced by the special heat treatments and its evolution in relation to the martensitic transformation has not been reported in domestic or foreign literature.In addition,how to reduce the fraction of the annealing twin boundary of Fe-MnSi based SMAs by a simple process to improve the SME is also the focus of this paper.On the one hand,is it possible to suppress the generation of annealing twins in deformation-processed alloys by simple microalloying and thus achieve excellent SME in deformation-processed SMAs?On the other hand,can low fraction of annealing twin boundary and excellent SME be achieved in deformation processed SMAs by controlling the industrial hot rolling process?The study of the above problem is of great scientific significance and engineering value.On the one hand,this study can further validate our proposed conditions for obtaining excellent SME in Fe-Mn-Si-based SMAs:low annealing twin fraction;on the other hand,this study will provide the opportunity to prepare Fe-Mn-Si-based SMAs with excellent SME by conventional industrial techniques.To answer the above questions,we specifically investigated(1)the relationship between the annealing twin boundary and the shape memory effect in alloys with and without the participation of thermal martensite during special heat treatments;(2)the relationship between B and Nb microalloying on the the evolution of the annealing twin boundary and its relationship with the shape memory effect;(3)the relationship between the hot rolling process on the the evolution of the annealing twin boundary and its relationship with the shape memory effect;(4)the relationship between the evolution of the annealing twin boundary and the martensitic transformation.From the above studies,we have obtained the following important conclusions.(1)Independent of special treatments methods,the SME of Fe-Mn-Si-based alloys showed a strong negative dependence on the length fraction of the annealing twin boundary and no significant relationship with the length fraction and grain size of low-angle grain boundaries(<15°).The SME of Fe-18.8Mn-5.0Si-8.2Cr-5.0Ni alloy(Ms=238 K)without thermal martensite participation also showed a negative dependence on the length fraction of the annealing twin boundary,regardless of the annealing temperature,and no significant dependence on the grain size and the length fraction of the low-angle grain boundaries.The SME of Fe-15Mn-5.5Si-8.2Cr-5.2Ni alloy(Ms=313 K)with thermal martensite participation also showed a negative dependence on the length fraction of the annealing twin boundary,but no obvious dependence on the grain size,the amount of thermally induced martensite and the length fraction of low-angle grain boundary.(2)The SME of alloys without the participation of thermal martensite after training for over three cycles is independent of the annealing temperature,but an optimum annealing temperature exists for alloys with thermal martensite.For Fe15Mn-5.5Si-8.2Cr-5.2Ni alloys with thermal martensite participation(Ms=313 K),the optimal annealing temperature after training for over three cycles is consistent with the currently reported 923 K.However,for Fe-18.8Mn-5.0Si-8.2Cr-5.0Ni alloy(Ms=238 K)without thermal martensite participation,the improvement of SME after annealing at 923 to 1123 K is almost equal.(3)By suppressing the generation of annealing twins during high-temperature annealing,both B and Nb microalloyed Fe-Mn-Si-based SMAs can achieve excellent SME.Compared with the alloy without B addition,the addition of trace B can significantly reduce the generation of annealing twins during high-temperature annealing and achieve higher SME.After annealing at temperatures above 973 K,the length fraction of the annealing twin boundary in the Fe-16.8Mn-5.8Si-9.3Cr-5.7Ni0.0089B alloy with the addition of trace B was significantly lower than that in the Fe17.0Mn-5.2Si-9.0Cr-4.8Ni alloy without the addition of B and Nb,and its shape memory effect was also significantly higher than that in the alloy without the addition of B and Nb.The addition of Nb can increase the recrystallization temperature,increase the optimal annealing temperature correspondingly and remarkably inhibits the grain growth.The recrystallization annealing temperature of the cold-rolled 10%Fe-14.1Mn-5.0Si-8.3Cr-5.1Ni-1.0Nb alloy was increased by 100 K compared with that of the unalloyed one.This leads to a corresponding increase in the optimum annealing temperature,which retards the generation of annealing twins during hightemperature annealing.Moreover,the addition of Nb substantially suppressed the grain growth during high-temperature annealing.Even after 1373 K annealing,the grain size of the alloy with Nb is only 16 μm,which is significantly lower than that of 83 μm and 123 μm for the alloy with and without B.(4)The industrial method of hot rolling combined with annealing can prepare alloys with low annealing twin boundary length fraction and achieve the same excellent SME as after thermomechanical treatment.After annealing at the optimum temperature of 973 K,hot-rolled Fe-17.0Mn-5.2Si-9.0Cr-4.8 Ni alloys all have low length fractions of annealing twin boundary,and their SME is comparable to that of the alloys undergoing thermomechanical treatment at optimal temperature annealing.(5)The special heat treatments severely distorted the boundaries of the annealing twins as a result of their reduced length fraction of the annealing twin boundary,and the large number of distorted boundaries or reduced length fraction of the annealing twin boundary significantly promoted the generation of thermal and stress-induced martensite.Ex-situ EBSD observation revealed that the Schmid factor of the grains did not change significantly after the special heat treatment,indicating that the grains did not undergo meritocratic orientation.After the special heat treatments,the annealing twin boundary of the high-temperature solid solution alloy was severely distorted and no longer satisfied the twin orientation relationship with the matrix,but was transformed into a normal large-angle grain boundary.At the same time,a large number of low-angle grain boundaries or subgrain boundaries are introduced into the grains of the original orientation relationship.The introduction of a large number of low-angle grain boundaries and subgrain boundaries results in a more uniform generation of thermal and stress-induced martensite,which is also significantly higher than that of solution-treated alloys with a large number of flat annealing twin boundary present. |
PDF Full Text Request