Effect of upsetting deformation temperature on the formation of the fine-grained cast alloy structure of the Ni–Mn–Ga system

Abstract

The plastic behavior during deformation by upsetting and its effect on the microstructure in the polycrystalline Ni_2.19Fe_0.04Mn_0.77Ga alloy are studied. The temperatures of martensitic and magnetic phase transformations were determined by the method for analyzing the temperature dependence of the specific magnetization as M_F = 320 K, A_S = 360 K, and T_C = 380 K. Using differential scanning calorimetry, it is shown that the phase transition from the ordered phase L2₁ to the disordered phase B2 is observed in the alloy during sample heating in the temperature range of 930–1070 K. The melting temperature is 1426 K. An analysis of the load curves constructed for sample deposition at temperatures of 773, 873, and 973 K shows that the behavior of the stress–strain curve at a temperature of 773 K is inherent to cold deformation. The behavior of the dependences for 873 and 973 K is typical of hot deformation. After deforming the alloy, its microstructure is studied using backscattered scanning electron microscopy. Plastic deformation of the alloy at study temperatures results in grain structure fragmentation in the localized deformation region. At all temperatures, a recrystallized grain structure is observed. It is found that the structure is heterogeneously recrystallized after upsetting at 973 K due to the process intensity at such a high temperature. The alloy microstructure after plastic deformation at a temperature of 873 K is most homogeneous in terms of the average grain size.