Influence of Ni/Mn concentration ratio on microstructure and martensitic transformation in melt spun Ni-Mn-Sn Heusler alloy ribbons

摘要

The paper describes the effect of Ni/Mn concentration ratio on microstructure and martensitic transformation in melt spun Ni_(50-x)Mn_(37+x)Sn_(12.5) Heusler alloy ribbons in the composition range between 0 ≤ x ≤6. The four alloys with the linear change of Ni/Mn ratio and constant Sn content were induction melted, homogenized and subsequently rapidly solidified on a rotating copper wheel. The ribbons featured a single or a two phase structure composed of the L2_1 parent phase and the martensite phase. The type of martensite differed depending on the composition. The increase of the martensite start (M_s) temperature with the increase of the Ni/Mn ratio, corresponds to the increase of the valence electron concentration ratio (e/a). The cellular type microstructure composed of grains featuring the L2_1 Heusler structure and martensite grains appearing at grain boundaries were confirmed in all the studied ribbons. The slight changes of the chemical composition of the parent and martensite phases were noticed. Such segregation was introduced by rapid quenching in response to different melting points of each element. This then had an effect on the local changes in the e/a ratio, effectively leading to nucleation of martensitic transformation in the affected areas. The 10 M and 40 type modulated martensites were indentified depending on the alloy composition.