Development of Vickers-Type Hardness Tester at Cryogenic Temperature

在超导磁体的设计中，需要考虑磁体结构材料在低温下的力学性能。为了考虑摩擦学性能，需要知道材料在低温下的硬度值。为此，本课题研制了一种维氏硬度计。首先进行了试验机的设计，然后进行了样机的研制。该试验机为维克斯型硬度计，由四个系统组成。压头传动系统由直流电机和滚珠丝杠组成。试件的移动系统是通过电机旋转圆柱形支架。制备了两个液氮和液氦冷却样品的恒温器，并用Au-0.07at%Fe与铬的热电偶测量了样品的温度。压痕、加载和移动试件的控制与测量系统由一台PC机和其他测量和控制设备组成。对JN1、JN2、SUS304、SUS316L、Ti-6A1-4V、OFHC Cu6种材料进行了从室温到4.2K的硬度测量。一般来说，从室温到77K，硬度随着温度的降低而增加，但在77K到4.2K之间，所有被测金属的硬度都下降并再次上升。结果表明，JN_2、SUS316L和铜在4.2K时的硬度低于77K时的硬度。1/Hv和1/T的Ahrrenius曲线表明，在4.2~70K和77K以上的温度范围内有两条拟合线，其原因目前还不清楚。在这些温度范围内，变形过程可能会发生变化。比较了室温、77K和4.2K下的硬度与屈服应力和抗拉强度的关系。

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.