Study On Microstructure And Properties Of NiMnSn(Co) High Temperature Shape Memory Alloys

In the present paper, the fourth element, Co, is added into Ni50Mn41Sn9by substituting Ni, Mn and Sn, respectively, to form high temperature shape memory alloy. Martensitic transformation, microstructure, mechanical and magnetic properties of Ni50-xMn41CoxSn9(x=5,7,9,10,12), Ni50Mn41-xCoxSn9(x=1,2,3,4,5) and Ni50Mn41Sn9-xCox(x=0.5,1,2,3,4) are studied systematically by using optical microscope, SEM, XRD, DSC and compression test.The results show that, the substitution of Co for Ni lead to the decrease of martensitic transformation temperature;respectively; the substitution of Co for Mn and Sn increase martensitic transformation temperature initially and then decrease.all the alloys exhibit one step thermalelastic martensitic forward and reverse transformation upon cooling and heating, respectively.With the increase of Co content, the second phases with different morphology and amount precipitate in the case of the substitution of Co for Ni, Mn and Sn. When the Co substitutes for Ni, the second phase with FCC structure distributes along the grain boundary or inside the martensite. The second phases exhibit banded feature and initially increase then decrease with the increase of Co content. The characterization of second phase in the case of Co substituting for Mn is similar to the case of Co substituting for Ni. When Co substitutes Sn, the second phases exhibit complex morphology, some are granulated and others are like a straight line. In this case, the amount of second phases is increased with increasing Co content.Regardless of the substitution method, the addition of Co improves the ductility and strength of the alloys. While the effect of Co substitution on shape memory effect is different. The substitution of Co for Ni increases the shape recovery ratio, however, the substitution of Co for Mn and Sn deteriorates the shape memory effect.Fracture analysis confirms the results of mechanical properties caused by the addition of Co. All the alloys possess typical brittle fracture feature when Co substitutes Ni. In the case when Co replaces Mn and Sn, the fracture feature changes from the combination of intergranular crack and transgranular crack to the combination of transgranular crack and dimple fracture. Magnetic measurements show that, Co substitute Ni changes the magnetization of alloy from anti-ferromagnetic state to ferromagnetic state; Co substitute Mn just increases the magnetization of anti-ferromagnetic state; Co substitute Sn changes the magnetization of alloy from paramagnetic state to anit-ferromagnetic state.