Adjustment On Martensitic Transformation Of Ni-Mn-based Ferromagnetic Shape Memory Alloys

The off-stoichiometric Ni-Mn-X-based (with Z=In, Sn and Sb) Heusler Ferromagnetic shape memory alloys (FSMAs) have attracted considerable attentions for their rich and special properties and potential application across the martensitic transition (MT) or under martensitic phase, owing to the strong coupling between structural and magnetic degrees of freedom. Effective controlling the working-temperature interval and enhancing the properties are important for these FSMAs. The main researches include:1. Effect of electron density in martensitic transition temperatures in Ni-Mn-Z(Z=In, Sn, Sb) alloysValence electron concentration (e/a) and cell volume are two main factors to effect the MT temperatures in Ni-Mn based Heusler alloys. Usually, the MT temperatures increase with the increase of e/a or the cell volume decreasing. However, neither e/a nor size factor can explain the variations of the MT temperatures in some cases. In our experiment, we found that electron density, combining the two factors, could be more appropriate for describing the critical MT temperatures. In order to investigate the mechanism of the variations of MT temperatures in Ni-Mn-X (X=In, Sn, and Sb) alloys, in this work, a systematic substitution of III main group elements Al, Ga and In for Sn in the off-stoichiometric Ni4Mn4Sn11alloys was performed. The results indicated that the Ni44Mn45Sn10R (R=Sn, Al, Ga and In) alloys have smaller value of e/a as well as smaller cell volume compared with Ti44Mn45Sn11alloy, owing to their fewer electrons and smaller radius. The MT temperatures do not increase or decrease monotonously with the variation of e/a and cell volume. In the Ni44Mn45Sn10R (R=A1, Ga and In) alloys system, the e/a effect effect plays the leading role in case of In doping while cell volume effect plays the leading role in case of A1and Ga doping. Hence, neither e/a or cell volume effect can explain the variations of MT temperatures singly. We can find that the MT temperatures increase with the increasing of electron density. Hence, combining the two factors, electron density might be a more appropriate parameter to describe the variations of MT temperatures.2. The study of Martensitic Transition and Magnetic Entropy Change in Ni-Mn-Sn Ferromagnetic shape memory Alloys Recently, there has been a great deal of attention in materials showing magnetic magnetocaloric effect (MCE) due to their potential application in magnetic refrigeration. MCE can be characterized by isothermal magnetic entropy change△SM. Much effort has been made to increase△SM-According to the Maxwell relation, the value of△SM is determined not only by magnetization change△AMsf but also by martensitic transition temperature range(MTTR). A series of Mn5oNi4oSn10-xAlx(x=0,1,2)ferromagnetic shape memory alloys have been prepared. The results indicated that though the magnetization decreases due to the introduction of A1, the MTTR decreases greatly with increasing of Al, leading to the more sharp varies of magnetization. A large△SM can be obtained due to the increase of AMsf/MTTR. Hence, reducing the MTTR is another effective method to gain large inverse MCE in addition to enlarging the△Msf...