Development of post heat treatments for a hot extruded polycrystalline NiMnGa alloy
In this article we present the results of microstructure and texture analysis of hot extruded polycrystalline Ni50Mn29Ga21 after different heat treatments. The aim is to produce a strong textured and coarse grained structure. The alloy was conventionally casted and hot extruded at 1000 °C afterwards. It shows a 5M modulated martensitic structure at ambient temperature. To optimize the microstructure concerning grain size and texture treatments on samples were performed by using a deformation/quenching dilatometer. To characterize the microstructure optical microscopy and electron backscatter diffraction (EBSD) were applied. Additionally, measurements with high-energy synchrotron radiation (HESR) were conducted to analyze the texture of the samples. The initial microstructure of the hot extruded alloy is dynamically recrystallized with a grain size of 100 ìm. It shows a <100>/<110>/<111> cyclic fibre texture along extrusion direction with <110>/<111> clearly dominating in the off-central region of the extruded rod . To increase the grain size the cube-shaped samples with an edge length of 5 mm were heated to different temperatures (800 °C, 900 °C, 1000 °C), annealed for different holding times (from 10 min to 48 h) and finally cooled down with a continuous rate of 10 K/s. The conclusion is that only heat treatments at 1000 °C increase the grain size. Long-time annealing of 48 h is not required, because the grain growth remains static after short-time annealing of 60 min. The maximum grain size was about 300 ìm. Neutron texture measurements showed a reduced fibre texture. To achieve a texture after heat treatment the samples were cut orientated from off-central regions and heat treated under load with compression direction perpendicular to ED. This treatments cause an additional grain growth depending on the degree of deformation. A <111> fibre texture along compression direction was found. There is no increase of grain size with a compression about 3 %. The grain growth reaches a maximum of 600 ìm at a deformation of 15 %. Higher deformations do not induce an additional grain growth as it's shown in Fig. 1. In summery the conclusion was found, that to improve microstructure with regard to grain size and texture post heat treatments at 1000 °C with defined compression of 15 % perpendicular to ED and annealing under load for 60 min are necessary. Thus a grain size of about 600 ìm and a <111> fibre texture along the compression direction can be achieved.