Development of Vickers-Type Hardness Tester at Cryogenic Temperature

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In the design of a superconducting magnet the mechanical properties of magnet structural materials at cryogenic temperature should be considered. The hardness value of the materials at cryogenic temperature is required to know for considering the tribological properties. A Vickers-type hardness tester, therefore, have been developed in this research project.In the first the design of a tester was made, followed by producing it. The tester was a Vickers-type hardness tester, wich consisted of four systems. A driving system of an indenter is made of a DC motor and a ball screw. A moving system of specimens is to rotate a cylindrical holder by a motor. Two cryostats are prepared for cooling specimen with liquid nitrogen and liquid helium, and the temperature of a specimen was measured with Au-0.07at% Fe vs.chromel thermocouple. The controlling and measuring system for indentation, applied load and moving specimens consists of a personal computer and other measuring and controlling devices.The measurement of hardness was carried out from room temperature down to 4.2 K for six materials of JN1, JN2, SUS304, SUS316L,Ti-6A1-4V,OFHC Cu. In general, hardness increases with decreasing temperature from room temperature to 77 K.However, hardness decrease and increases again between 77 and 4.2 K for all metals examined. As a result, the hardness at 4.2 K becomes lower than at 77 K for JN2, SUS316L and Cu. Ahrrenius plot of 1/Hv and 1/T shows that there are two fit lines for temperature ranges of from 4.2 K to 70 K,and above 77 K.At present the reason for it is not clearly understood. The deformation process might change at these temperature ranges. The comparisons of hardness at room temperature, 77 K and 4.2 K with yield stress and tensile strength are also made.