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Title: Effects of the interplay between atomic and magnetic order on the properties of metamagnetic Ni-Co-Mn-Ga shape memory alloys

Ni-Co-Mn-Ga ferromagnetic shape memory alloys show metamagnetic behavior for a range of Co contents. The temperatures of the structural and magnetic transitions depend strongly on composition and atomic order degree, in such a way that combined composition and thermal treatment allows obtaining martensitic transformation between any magnetic state of austenite and martensite. This work presents a detailed analysis of the effect of atomic order on Ni-Co-Mn-Ga alloys through the evolution of structural and magnetic transitions after quench from high temperatures and during post-quest ageing. It is found that the way in which the atomic order affects the martensitic transformation temperatures and entropy depends on the magnetic order of austenite and martensite. The results can be explained assuming that improvement of atomic order decreases the free energy of the structural phases according to their magnetic order. However, it is assumed in this work that changes in the slope—that is, the entropy—of the Gibbs free energy curves are also decisive to the stability of the two-phase system. The experimental transformation entropy values have been compared with a phenomenological model, based on a Bragg–Williams approximation, accounting for the magnetic contribution. The excellent agreement obtained corroborates the magnetic origin of changes in transformation entropymore » brought about by atomic ordering.« less
Authors:
 [1]
  1. Departament de Física, Universitat de les Illes Balears, Campus universitari. Cra. de Valldemossa km 7.5, 07122 Palma de Mallorca (Spain)
Publication Date:
OSTI Identifier:
22271318
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Applied Physics; Journal Volume: 115; Journal Issue: 11; Other Information: (c) 2014 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; APPROXIMATIONS; AUSTENITE; COBALT; COMPARATIVE EVALUATIONS; ENTROPY; FREE ENERGY; FREE ENTHALPY; GALLIUM; HEAT TREATMENTS; INTERMETALLIC COMPOUNDS; MANGANESE; MARTENSITE; NICKEL; PHASE DIAGRAMS; PHASE STABILITY; PHASE TRANSFORMATIONS; SHAPE MEMORY EFFECT; TEMPERATURE DEPENDENCE