Development of Vickers-Type Hardness Tester at Cryogenic Temperature

在超导磁体的设计中，必须考虑磁体结构材料在低温下的力学性能。为了考虑材料的摩擦学性能，需要知道材料在低温下的硬度值。因此，本课题研制了一种维氏硬度计。首先进行了测试仪的设计，然后进行了生产。该硬度计为维氏硬度计，由四个系统组成。压头的驱动系统由直流电机和滚珠丝杠组成。试样移动系统是用马达转动圆柱形支架。制备了两个低温恒温器，分别用液氮和液氦冷却试样，用Au-0.07at% Fe对铬镍合金热电偶测量试样的温度。压痕、外加载荷和移动试样的控制和测量系统由一台个人计算机和其他测量和控制装置组成。对JN1、JN2、SUS304、SUS316L、Ti-6A1-4V、OFHC Cu 6种材料在室温至4.2 K范围内进行了硬度测量。从室温到77 K，硬度随温度的降低而增大。然而，所有金属的硬度在77 - 4.2 K之间又下降又上升。结果表明，JN2、SUS316L和Cu在4.2 K时的硬度低于77 K时的硬度。1/Hv和1/T的Ahrrenius图显示，在4.2 K ~ 70 K和77 K以上的温度范围内存在两条拟合线。目前，其原因尚不清楚。在这些温度范围内，变形过程可能发生变化。并对室温、77 K和4.2 K下的硬度、屈服应力和抗拉强度进行了比较。

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.