Thermal and mechanical properties of Ti-Ni-Cu-Sn amorphous alloys with a wide supercooled liquid region before crystallization

A wide supercooled liquid region exceeding 50 K before crystallization was observed for melt-spun Ti50Ni25-xCu25Snx (x=3 and 5 at%) amorphous alloys. The temperature interval of the supercooled liquid region defined by the difference between crystallization temperature (T-x) and glass transition temperature (T-g), Delta T-x(=T-x-T-g) is 40 K for the 0%Sn alloy and increases to 50 K for the 3%Sn alloy and 60 K for the 5%Sn alloy. With increasing Sn content, the Delta T-x value decreases significantly. A similar increase in thermal stability of the supercooled liquid was also recognized for the 3 at%Sb-containing alloy in the Ti50Ni25-xCu25Sbx system. The replacement by 3 to 5 at%Sn for Ni also induces an increase of mechanical strength, in addition to the increase in Delta T-x. The tensile fracture strength (sigma(f)), Young's modulus (E) and Vickers hardness (H-v) increase from 1800 MPa, 93 GPa and 530, respectively, for the Ti50Ni25Cu25 alloy to 2050 MPa, 102 GPa and 650, respectively, for the Ti50Ni20Cu25Sn5 alloy. The crystallization takes place through a single exothermic reaction, accompanying the simultaneous precipitation of multiple (CuTi + Cu4Ti3 + NiTi) phases. The crystallization mode requires long-range atomic rearrangements for precipitation of the crystalline phases, leading to the increase in the stability of supercooled liquid against crystallization. The high thermal stability of the supercooled liquid enabled the production of bulk amorphous alloys in the diameter range up to about 6 mm by copper mold casting. There is no appreciable difference in the stability of the supercooled liquid region between the melt-spun and cast bulk amorphous alloys. The first synthesis of the Ti-based amorphous alloys with high glass-forming ability and good mechanical properties allows us to expect the future development of bulk amorphous alloys as a new type of high specific strength material.