Study Of Improving Shape Memory Effect In Fe-based Alloys Through Controlling Directional Precipitation Of Second Phase

FeMnSiCrNi shape memory alloys have a great prospect of application in petroleum, chemical engineering, mechanical industry etc. However, FeMnSi alloys have got no important industrial applications. The main reason is its low shape memory effect, the recovery strain is only 2% to 3%. To improve shape memory effect in FeMnSi alloys, a new method of strain ageing has been put forward based on the principle of improving shape memory effect in FeMnSiCrNi alloys by controlling precipitation of second phase. Directionalγ/εinterfaces and stacking fault can be produced by tensile deformation at room-temperature, then ageing followed deformation, the second phase paticles precipitate along some directions inside austenite grains. As these directional second phase particles can subdivide the grains into some smaller areas and prevent stress-inducedεmartensite to grow up, laminar stress-inducedεmartensite can be attained in these smaller areas and shape memory effect in FeMnSiCrNi alloy be improved remarkably. Based on this principle, we have studied influence of ageing mode on the directionality, quantity and distribution of second phase particles and shape memory effect. Some creative research achievements are obtained as follows.The shape memory effect of FeMnSiCrNi alloy can be improved remarkably through strain ageing. After 10% tensile strain at room-temperature and then ageing at 800°C for 300min, shape memory effect of the specimen alloy gets maximum( 89% ), 41% higher than that after direct ageing and 217% higher than that after solution treatment, reaching the value after thermo-mechanical treatment.The mechanism that the shape memory effect of FeMnSiCrNi alloys can be improved remarkably through strain ageing is that lots of second phase precipitated directionally in grain after strain ageing. These second phases can strengthen austenite, suppressing the occurrence of permanent deformation. On the other hand, they can subdivide the grains into some smaller areas and prevent stress-inducedεmartensite to grow up, thus laminar stress-induced￡martensite can be attained in these smaller areas and the reversibility of reverse transformation can be improved.Both shape memory effect and recovery stress can be improved by strain ageing. To increase the shape memory effect remarkably and the recovery stress of FeMnSiCrNi alloy to promote engineering application, we optimize the processing parameters of the strain ageing. We think that after 10% tensile strain at room-temperature and ageing at 800°C for 180min is the optima processing parameters of strain ageing for Fel3.53Mn4.86Si8.16Cr3.82Ni0.16C alloy. After this strain ageing, shape memory effect of the specimen alloy gets to 87% at 5% pre-strain and recovery stress at room temperature is 339MPa. These results offers reliable theoretical basis for engineering application of FeMnSi shape memory alloys.