Effect Of Ageing After Pre-deformation On Shape Memory Effect And Recovery Stress In FeMnSiCrNi Alloys

Fe-Mn-Si based shape memory alloys have a great prospect of application in engineering because of their lower cost, good workability, good machinability, good weldability and great thermal hysteresis which makes it possible to deform and store them at room temperature. Now there are some reports about their application in petroleum, chemical engineering, mechanical industry etc and drawn special attention of the scientists all over the world.However, Fe-Mn-Si alloys have got no important industrial applications. The main reason is its low shape memory effect, whose recovery strain is only 2% to 3%. To improve shape memory effect in Fe-Mn-Si based alloys, a new method of ageing after pre-deformation has been put forward based on the principle of improving shape memory effect in Fe-Mn-Si-Cr-Ni alloys by controlling precipitation sites of second phase. Directionalγ/εinterfaces and stacking fault can be produced by tensile deformation at room-temperature, then ageing followed a 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 Fe-Mn-Si-Cr-Ni alloy can be improved remarkably. Based on this principle, we have studied influence of ageing mode on the shape memory effect and recovery stress and the size, quantity and distribution of second phase particles and its influence on shape memory effect. Some creative research achievements are obtained as follows.Both the shape memory effect and the recovery stress of Fe-Mn-Si-Cr-Ni alloys can be improved remarkably through ageing after pre-deformation. Ageing at 800℃for 180 min after 10% tensile pre-deformation at room-temperature is the optima processing parameters for Fe15.2Mn5.01Si8.14Cr4.20Ni0.12C alloy. After this treatment, the shape recovery ratio of the specimen alloy gets to 89% and recovery stress at room temperature 264MPa when the pre-strain ratio is 5%. The mechanism that the shape memory effect of Fe-Mn-Si-Cr-Ni alloys can be improved remarkably through ageing after pre-deformation is that lots of second phase precipitated directionally inside grains 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-inducedεmartensite can be attained in these smaller areas and the reversibility of reverse transformation can be improved. We found that the second phase was much more in amount, smaller in size and better in direction while ageing after 10% of tensile pre-deformation at room temperature. A great number of Cr2 3 C6 precipitated in alloys contained carbon element after ageing with pre-deformation. The shape memory effect and recovery of the specimen alloy contained 0.12% C element were better compared to the specimen alloy contained 0.18% C element because the more and smaller and more directional Cr2 3 C6 . We can know from Orowan mechanism that the increase of number and decrease of size of the Cr2 3 C6 particles are good to the strengthening of austenite.The shape memory effect and recovery stress of Fe-Mn-Si-Cr-Ni alloy with the addition of Nb were improved much more evidently after ageing with pre-deformation than ageing without pre-deformation and the second phase was much more and smaller. But the aligned NbC carbides can not be obviosly presented due to rather small amount of NbC carbides.These results offers reliable theoretical basis for engineering application of Fe-Mn-Si based shape memory alloys.