Effect of deformation on structure and mechanical behavior of plycrystalline Ni-Mn-Ga alloys

Henryk Morawiec ,  Tomasz Goryczka ,  Jozef Lelatko ,  Krystian Prusik 

University of Silesia, Institute of Materials Science, 12, Bankowa Str., Katowice 40-007, Poland

Abstract

The purpose of this work was to investigate the stress-strain behavior of the Ni-Mn-Ga alloys with nickel excess. The deformation was introduced in prismatic specimens by compression test for both parent and martensitic phase. The effect of the deformation on structure was studied by means of X-ray diffraction, transmission and electron scanning microscopy (TEM and SEM).

The martensitic structure, dependently on chemical composition of the alloys, was 10M or non-modulated tetragonal. The non-modulated martensite was observed in the alloys with higher nickel content. The deformation of the martensitic samples up to the fracture does not change their structure. The stress-strain curve shows that the initial elastic deformation stage terminates at a strain of about 2% and critical stress 70 MPa, which corresponds to the start of martensite reorientation.

Several of studied alloys, obtained by fast solidification, before homogenization show a broad temperature interval of the martensitic transformation and reveal a small uniform grain size. The limited deformation of these alloys in the L21 parent phase show the pseudoelastic stress-strain behavior. The volume of deformation up to the fracture is in the range of 20-25%. TEM studies of the fractured specimen reveal a small amount of the induced martensitic phase. The mechanical behavior of the specimens, homogenized at 9000C for 50 hours, varies in comparison to those specimens, which were not homogenized. The long annealing at high temperature causes not only the compensation of the chemical composition but also the increase of the grain size, which may affect the mechanical behavior of the alloys.

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Presentation: Oral at E-MRS Fall Meeting 2007, Symposium E, by Henryk Morawiec
See On-line Journal of E-MRS Fall Meeting 2007

Submitted: 2007-05-14 10:01
Revised:   2009-06-07 00:44
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