Improving microstructure, texture and mechanical behaviour of polycrystalline ferromagnetic shape memory alloys: Poster at Junior Euromat, 23.-27.07.2012, Lausanne

Abstract

Ferromagnetic shape memory alloys are new materials which deform reversibly under a magnetic field. This magnetic field induced strain (MFIS) caused by twin boundary motion in the martensitic phase reaches values of about 10 % depending on composition, microstructure and texture of the material. To enable this effect special characteristics in the mechanical behaviour of the material, directly affected by microstructure and texture, are needed. Formerly it was possible to produce large strains in moderate magnetic fields only in single crystals. In this work we present results of investigations to obtain MFIS also in polycrystalline alloys. The aim is to produce strongly textured coarse grained material that shows proper mechanical behaviour for MFIS using heat treatment and training methods. The initial state forms hot extruded Ni50Mn29Ga21 with a 5M modulated martensitic structure at ambient temperature. The microstructure of the material shows a grain size of 100 µm and a cyclic fibre texture along extrusion direction. The stress-strain curve shows no martensitic plateau. To increase grain size and texture different heat treatments were applied. The results show that only annealing at 1000°C leads to increased grain size but also reduces texture. To achieve a stronger texture the samples were compressed by 5 % at 1000°C and annealed without load for 60 min afterwards. Additionally, a second treatment step under load at low temperatures was applied. This procedure leads to maximum grain size of 4 mm and sharp texture. Now the stress-strain curves demonstrate a martensitic plateau at stress levels of about 10 MPa. For further mechanical improvements thermo-mechanical training was used including compression in two directions and heating the material to 80°C for thermal resetting. This training leads to stress-strain behaviour with more reduced stress levels of the martensitic plateau and maximum residual strain of 6 %. Finally MFIS should be accomplished