Recently our group has succeeded, by producing very small particles of NbC carbides in austenite, in improvement of shape memory effect (SME) of the low-cost conventional Fe-Mn-Si based SMAs to such an extent that the so-called "training" treatment is no longer necessary. It was also found that the shape memory properties of the Fe-Mn-Si based SMAs were further improved by pre-rolling at 870K. The present paper describes similar improvement of shape memory properties of an Fe-15Mn-5Si-9Cr-5Ni-0.5NbC (mass %) by more convenient way of pre-extension at room temperature. This alloy is high corrosion-resistant (equivalent to SUS430) as well as low cost material, which is also one of the important requisites for industry application in various fields. A nearly perfect shape recover (90%) of an initial 4% strain was achieved when the alloy was pre-extended 12% at room temperature and then aged at 1070K for 10min. The origin of this improvement of SME has been studied by atomic force microscopy (AFM) and trasmission electron microscopy (TEM). It is concluded that uniform distribution of fine martensite plates with the same variant on the primary system is the key factor to obtain a perfect shape memory recovery.
By addition of small amount of Nb and C to the conventional Fe-Mn-Si based shape memory alloys, shape memory properties are greatly improved in such an extent that the costly 'training' heat treatment is no more necessary. The key to this remarkable improvement of shape memory effect is to produce small NbC precipitates of about several nm in size in austenite. In order to generate such very small NbC particles, the sample is firstly rolled at 870 K and then aged at 1070 K. An example of Fe-28Mn-6Si-5Cr-0.53Nb-0.06C (mass %) alloy is shown; 95% shape recovery for initial strain of 4% is obtained and the shape recovery stress of about 300 MPa is attained for the sample pre-rolled 14%, which is well above the criterion for industry application of pipe jointing. A pipe jointing with this material is demonstrated.
Ausforming treatment can improve the shape memory effect of Fe-Mn-Si based shape memory alloys, however, the mechanism of this improvement is not so clear. In this paper, the influence of ausforming treatment on stress-induced martensitic transformation and its reverse transformation in an Fe-28Mn-6Si-5Cr shape memory alloy has been studied using atomic force microscope and TEM aiming to clarify the origin of this improvement. It was found that the ausforming treatment at 970K by 9 percent pre-straining, which is the optimum condition for improving shape memory recovery, can introduce many uniformly distributed stacking faults on the same slip plane in austenite. When an external stress is applied to such an ausformed specimen for shape change, uniformly distributed martensite bands with the same variant are produced in a grain due to the assistance of those preexisted stacking faults. When being heated over Af, these martensite bands are nearly completely reverse-transformed to parent phase through the same atomic pass as for the forward transformation, so a nearly perfect shape memory effect is obtained.
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