Effect of quenching on martensitic transformation course in non-stoichiometric NiMnGa alloy

Tomasz Goryczka 1Marek Gigla Henryk Morawiec 

1. University of Silesia, Institute of Material Science, Bankowa 12, Katowice 40-007, Poland

Abstract

Since the time when a Ni-Mn-Ga alloy with composition near to the stoichiometric Heusler structure has been recognized as material revealing ferromagnetic shape memory effect, the number of experimental results and its practical application still increases. Especially, non-stoichiometric composition attracts its attention due to a course of martensitic transformation. Several types of martensite can be found such as non-modulated tetragonal martensite (T), five-layerd martensite (10M) as well as seven-layerd martensite (14M). Above the martensitic transformation, the parent phase undergoes ordering transformation from B2’ to the Heusler’s structure L21.

The purpose of presented work was to study the correlation between structure of the martensite type and ordering of the parent phase in the alloy with chemical composition: 51.2at%Ni, 27.6at%Mn and 21,1at% Ga. From the polycrystalline bulk material samples were cut and quenched after annealing at 700oC, 800oC and 900oC for one hour. Increasing of quenching temperature results in lowering the characteristic temperatures of the martensitic transformation and simultaneously broadens its thermal hysteresis. The parent phase, in sample quenched from 900oC, reveals the B2’ structure whereas in sample quenched from 700oC - the Heusler’s structure L21. The ordering transformation was confirmed by the results obtained from high temperature DSC measurement. On a heating curve broad thermal peak appeared in the range between 750 and 850oC.

Independently on the quenching temperature and the ordering of the parent phase the martensite shows the 10M structure type. However, in electron diffraction pattern additional effects of the diffusion electron scattering have been observed as a results of a short-range ordering caused by the Ni exceeding atoms.

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Presentation: poster at E-MRS Fall Meeting 2005, Symposium C, by Tomasz Goryczka
See On-line Journal of E-MRS Fall Meeting 2005

Submitted: 2005-05-20 11:58
Revised:   2009-06-07 00:44
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