Mechanical Anisotropy in Austenitic NiMnGa Alloy: Nanoindentation Studies

Abstract

Abstract:Mechanical anisotropy in an austenitic ferromagnetic shape memory alloy (SMA), Ni 50 Mn 26.25 Ga 23.75 , is investigated along (010), 120 , 121 , 231 and (232) using nanoindentation. While (010) exhibits the highest reduced modulus, E r , and hardness, H, (232) shows the lowest amongst the grain orientations examined in this study. The significant elastic anisotropy measured is attributed to differences in planar packing density and number of in-plane Ni-Mn and Ni-Ga bonds, whereas the plastic anisotropy is due to the differences in the onset of slip, which is rationalized by recourse to Schmid factor calculations. This would help determine the grain orientations in austenitic NiMnGa which exhibit better mechanical properties for SMA applications such as improving vibration damping characteristics of the alloy.

New Metastable Baro- and Deformation-Induced Phases in Ferromagnetic Shape Memory Ni2MnGa-Based Alloys

“…Alloys undergoing thermoelastic martensitic phase transformations (TMPTs) have a great innovative potential for a variety of structural and functional applications due to the effects of shape memory (SM), giant superelasticity (GS), elastic- and magnetocaloric (EMC) and other phenomena [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 ]. Notable among these alloys, of course, are the atomically ordered L 2 1 Heusler alloys, which exhibit a ferromagnetic ordering below the Curie temperature T C [ 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 ]. For alloy compositions with the electron concentration e / a < 7.7, TMPT occurs from a ferromagnetic parent phase (i.e., M s < T C , where M s is the temperature at the start of martensitic transformation).…”

“…During cooling below M s , these alloys exhibit a sequence of first-order phase transitions from the parent cubic L 2 1 phase to long-period modulated intermediate martensite structures (denoted as 10 M and 14 M ), as well as to tetragonal 2 M martensite without lattice modulation [ 5 ]. A significant key limitation for wide practical application is the brittleness and poor machinability of ferromagnetic and other polycrystalline SM alloys, with the exception of titanium nickelide [ 1 , 2 , 3 , 4 , 16 , 18 , 20 ]. The high brittleness of Ni 2 MnGa-based alloys even in a single-crystalline state represents an obvious obstacle for realization of SM, GS, and other related effects.…”