Grain Growth of Nickel-Manganese-Gallium
Dr. Peter Mullner
Magnetic shape memory alloys (MSMA) strongly deform (up to 10%) when exposed to an external magnetic field. The alloy must be in single crystalline form to demonstrate these properties. Growth of single crystals from the molten phase is a time consuming and expensive process. We conduct grain growth experiments to produce single crystals from the solid state. This production route is comparatively inexpensive and avoids disadvantages such as chemical segregation. To process these single crystals, alloys with various compositions were cast, from bulk materials, into cylindrical copper ingot molds with diameters ranging from 0.9 mm to 1.9 mm. Disks were cut from the cast ingot rods and the sample disks were polished to a smooth reflective finish. The disks were then subjected to heat treatments with varying temperatures and dwell times. These heat treatments induced grain growth in the samples. The main goal of these experiments is to grow large grains that can be ‘cut out’ of the parent samples to extract single crystals. The magneto-mechanical properties of the single crystals will be studied with an emphasis on magnetic-field-induced strain and twinning stress.