Microstructure and martensitic transformation in Si-coated TiNi powders prepared by ball milling

Abstract

Si was coated on the surface of Ti–49Ni (at%) alloy powders by ball milling in order to improve the electrochemical properties of the Si electrodes of secondary Li ion batteries and then the microstructure and martensitic transformation behavior were investigated by means of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Ti–Ni powders coated with Si were fabricated successfully by ball milling. As-milled powders consisted of highly deformed Ti–Ni powders with the B2 phase and amorphous Si layers. The thickness of the Si layer coated on the surface of the Ti–Ni powders increased from 3–5μm to 10–15μm by extending the milling time from 3h to 48h. However, severe contamination from the grinding media, ZrO2 occurred when the ball milling time was as long as 48h. By heating as-milled powders to various temperatures in the range of 673–873K, the highly deformed Ti–Ni powders were recovered and Ti4Ni4Si7 was formed. Two-stage B2–R–B19′ transformation occurred when as-milled Si-coated Ti–49Ni alloy powders were heated to temperatures below 873K, above this temperature one-stage B2–B19′ transformation occurred.