| Fe-Mn-Si based shape memory alloys (SMAs) have good workability, goodmachinability, low cost and large thermal hysteresis (Af-Ms). However, Fe-Mn-Sialloys have got no important industrial applications. Under analyzing the researchesof Fe-Mn-Si-Cr-Ni alloys, this paper concludes three problems existing in Fe-basedSMAs as: (1) the poor shape memory effect (SME) and low recovery strain; (2) thelow recovery stress and (3) the higher recovery stress relaxation at low temperature.To develop one kind of training-free FeMnSiCrNi shape memory alloy (SMA) andprocessing technology with better SME, higher recovery stress and lower recoverystress relaxation at low temperature, the paper put forward two conceives of: (a)improving SME and recovery stress by directional precipitation of second phase and(b) improving SME and recovery stress and depressing recovery stress relaxation bygrain refinement. The following research was carried out.Through ageing after deformation, conceive of directional precipitation ofsecond phase was realized. In Fe13.53Mn4.86Si8.16Cr3.82Ni0.16C alloy aged after10% deformation, a great amount of directional Cr23C6 carbides precipitate insidethe grains. The amount of precipitates is much more than direction ageing alloys andthe size of the precipitates is only half of those in direction ageing alloys. Investigated by in situ microstructure analysis on SEM and TEM, it is determinedthat directional precipitates are mainly induced byγ/εphase interfaces. In situmicrostructure analysis showed that directional precipitates have perfect coincidencerelation with theγ/εphase interfaces before ageing. TEM analysis showed thattempering at 1073K, part ofγ/εphase interfaces need longer time to recover andeliminate. During the reversion of theseγ/εphase interfaces, second phase isnucleated and precipitated preferentially at the interfaces. In fact, nucleation ofsecond phase on interfaces is very soon (beginning at 2min). With the timeincreasing, moreγ/εinterfaces recover and at the same time, more second phaseprecipitates nucleate. When the ageing time increasing to 60 minutes, almost all oftheγ/εinterfaces vanish and nucleation of second phase finish. The Gibbs energyproduced by recover ofγ/εinterfaces is used in nucleation of second phase and thusdecreases the nucleation energy needed.Ageing after deformation remarkably improves SME of Fe-based SMA. Theshape recovery ratio in the alloys aged after deformation reaches 89% (withpre-strain 5%), being 63% higher than as-quenched alloys and 26% higher thanmaximum value in direction ageing alloys. XRD and electrical-resistivity analysisshow that in the alloys subjected to same pre-strain, the amount of stress-inducedεmartensite in alloys aged after deformation is much more than both as-quenched anddirection ageing alloys. After heating to recovery temperature, the amount ofreversedεmartensite in alloys aged after deformation is also much more than thelatter and thus the alloys obtain the better SME. The principals for the SMEimprovement attribute to: (a) second phase precipitates induced by ageing afterdeformation strength the matrix more effectively and meantime, precipitation of thegreat amount of Cr23C6 carbides decreases the critical stress for inducing martensitephase transformation and thus enlarged the decrement△σbetween the critical slipstressσp and the critical stressσSIM for inducingγ→εphase transformation and (b)directional precipitates divide the matrix into many small areas and duringdeforming these precipitates prevent the collisions between martensite bands belongto different areas. The amount and direction of second phase precipitates can be controlled bycontrolling the amount and direction ofγ/εinterfaces by adjusting the pre-strain,deforming temperature before ageing, the ageing temperature and time. The paperfound that precipitates with single direction can divide the matrix into more smallareas without intersection, being favorable for decreasing the collisions betweenmartensite bands and increasing the reversibility of the martensite. The optimalparameters are suggested as: deforming 10% at room temperature and subsequentageing at 1073K for 300 minute.Ageing after pre-deformation also improves the recovery stress and reasons aresummarized as that (a) shape recovery ratio is improved and (b) middle-temperaturestrength is improved by the great amount of precipitates. The paper also found thatrecovery stress relaxation ratio in alloys aged after deformation is higher than bothas-quenched and direction ageing alloys, but the value of recovery stress of alloysaged after deformation is still higher than the latter even being cooled to lowtemperature (240K). To reveal the effects of temperature change on recovery stressin Fe-based SMA, the paper simulated the natural temperature change and found thatafter recovery stress relaxation occurs at low temperature, recovery stress keepsdecreasing when the temperature rising again due to the thermal expansion. As aresult, the paper defined this stress decreasing as thermal expansion relaxation andadvice to take the recovery stress after thermal expansion as a critical parameter forengineering applications.To produce the fine-grained FeMnSiCrNi bulk alloys, the paper applied theequal channel angular pressing (ECAP) technology to introduce severe strain after1～2 passes pressing. After subsequent annealing, the as-pressed structures willrecover and recrystallize and the fine grains with average size of about 5.0μm areobtained. In fine-grained alloys, with the increasing pre-strain from 4.33% to 12%,decreasing of shape recovery ratio is little and this illustrates that plastic slip iseffectively suppressed. After deforming at 200K with pre-strain 4.33%, shaperecovery ratio reaches 95%. The effects of grain refinement on SME including that(a) grain refinement increases both the critical slip stressσp and the critical stress σSIM for inducingγ→εphase transformation, while enlarges the decrement△σbetweenσp andσSIM and (b) grain refinement decreases collisions betweenmartensite bands and thus increases their reversibility.Grain refinement can remarkably improve recovery stresses in FeMnSiCrNialloys. The room recovery stress in fine-grained alloys reaches 460MPa, 3 times ofthe as-quenched alloys. Grain refinement improves the recovery stress by improvingthe middle-temperature strength and suppressing decease of recovery stress inducedby plastic slip during cooling under constraints. On the other hand, grain refinementdecreases the Ms temperature and increases the critical stress for inducing martensite,suppresses the secondγ→εphase transformation and thus effectively resolves therecovery stress relaxation at low temperature in FeMnSiCrNi alloys.At last, the paper drew conceive of ageing after deformation into ECAPedalloys, applying the great amount of dislocations induced by ECAP to prompt theprecipitation of second phase. The result showed that precipitates prefer to nucleateat the dislocations and sheafing bands and the nucleation accompanies with therecover and recrystallization of the as-pressed structures. The precipitation of secondphase can inhibit the recrystallization and especially the grains' coarsening at hightemperature. After pressing and ageing, shape recovery ratio reaches 89% with4.33% pre-strain at room temperature in Fe19.04Mn4.98Si8.50Cr4.59Ni0.12C alloydue to the synthetically strengthening of matrix by grain refinement and secondphase precipitation.
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