The Effects of Sintering Parameters and Molybdenum Addition on the Microstructure and Dynamic Mechanical Behaviour of Tungsten Heavy Alloys

• Main Authors: Tien-YinChan, 詹添印
• Other Authors: Woei-Shyan Lee
• Format: Others
• Language: zh-TW
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/85942384527859045757

博士 === 國立成功大學 === 機械工程學系碩博士班 === 100 === In this study, tungsten heavy alloys in the form of split-Hopkinson pressure bar, SHPB, were prepared by powder metallurgy process. The effects of five parameters on the microstructural evolution and dynamic mechanical behaviors of tungsten heavy alloys were investigated, including tungsten volume fraction, molybdenum addition, nickel to iron weight ratio, isothermal hold time at the sintering temperature, and sintering route. By using a universal material testing machine and a dynamic test device, the sintered specimens were tested under six strain rates (0.1s-1, 1500 s-1, 2200 s-1, 4500 s-1, 6500 s-1, 7500 s-1) to examine their dynamic mechanical behaviors. The mechanical behaviors were closely related to the details of the microstructure, including tungsten grain size, contiguity of tungsten grains, precipitation of intermetallic phases, dissolved concentration of solute in the matrix phase, and sintered density. The appropriate adjustment of processing parameters or the addition of alloy elements could be effective in controlling the sintered microstructure and, therefore, promoting dynamic mechanical properties to some extent. However, the limitation of the power efficiency of typical sintering furnaces led to some limitations on the control of microstructures in a liquid phase sintering process, while the addition of other alloy elements could either increase the cost to the alloys or could cause the precipitation of intermetallic com-pounds. Therefore, in addition to the traditional sintering process, a two-stage sintering practice to improve the microstructure was illustrated. The two-stage sintering practice included first sintering the alloys by solid phase sintering, reaching a sintered density of about 99% theoretical and no obvious grain growth, and subsequently fast sintering the alloys by induction heating to the formation of a liquid phase at 1470 ℃ , with a short isothermal hold and then water quenched. In the secondary liquid phase sintering stage, the alloys could be completely densified and the contiguity of grains could be substantially reduced, with minimal grain growth, while a high dissolved concentration of W in the matrix phase could be maintained and the precipitation of intermetallic phase could be avoided. All of these beneficial consequences led to more satisfactory dynamic mechanical behaviors.