A range of Ni–Mn–Ga alloy compositions close to the stoichiometric Heusler composition, Ni2MnGa, has been reported to show field-induced strains of several percent. Such observations, and the magnitude of the strain observed, depend on the values of several critical material parameters, most importantly the martensitic transformation temperature (Tmart), Curie temperature (TC), saturation magnetization (Ms), strength of the magnetocrystalline anisotropy, and the details of the martensite structure. Here, data collected from a variety of sources are plotted and their variations are fit with empirical formulas to afford a better overall picture of the behavior of this system. It is found that the martensitic transformation temperature is the parameter most sensitive to the composition; saturation magnetization appears to peak sharply at 7.5 valence electrons/atom, finally the composition field over which the saturation magnetization exceeds 60 emu/g, and 300 K <Tmart<TC, has been explored only at the margins.

A modified heat treatment process designated quenching–partitioning–tempering (Q–P–T) process is developed based on the quenching and partitioning process proposed by J.G. Speer et al. [Acta Mater.51, 2611 (2003)] and D.K. Matlock et al. [Mater. Sci. Forum426–432, 1089 (2003)]. A Fe–0.485C–1.195Mn–1.185Si–0.98Ni–0.21Nb steel after Q–P–T process satisfies the designed requirement of tensile strength over 2000 MPa and elongation over 10%. The microstructure characterization indicates that this ultrahigh-strength steel consists of nanomicrostructures including lath martensite, filmlike retained austenite, and dispersive Nb-containing carbides. The effect of tempering time on the mechanical properties is analyzed based on microstructures.

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For steel with combination of high strength (~2000MPa) and toughness, along with low cost, the designed structure should be low-temperature tempered, fine lath martensite with high density of dislocation, coated by film of austenite with considerable thickness and distributed with fine ε (η) or (and) complex carbide. Correspondently, the steel should contain less than 0.5 (wt%) of carbon, certain amount of alloying elements for lowering Ms, such as Ni, Mo and (or) Mn, carbide forming element, e.g. Nb, as well as Si or (and) Al, the element depressing the formation of cementite, the brittle phase in high strength steel. The heat treatment process is suggested as: austenitizing at a temperature slightly above Ac3, followed by quenching at Ms-Mf, partitioning either at quenching temperature or at slightly above Ms for a few minutes, cooling down to room temperature and tempering at low temperature about half an hour.

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