| Summary: | 碩士 === 遠東技術學院 === 機械研究所 === 93 === Abstract
Two tungsten-based heavy alloys of W-3.99Ni-1.71Fe and W-9.79-Mo-4.35Ni-1.81 Fe (wt%) in composition were investigated based on our previous research results. These two powder mixtures were subjected to sintering at 1520℃ for different thermal hold times, including 1 min, 5mins , 15mins, 30mins, 60mins and 120mins. The sintered specimens were then exposed to pseudo-static test using MTS and dynamic test using Hopkinson bar, whose strain rates ranged from 0.01s –1 to 1 s –1 and from 1500 s –1 to 7170s –1, respectively. The experimental results indicated that, along with the variation of matrix composition, the percentage of the matrix area and the mean grain size increased while the contiguity of grains decreased with the increase of isothermal hold time. The changes of microstructure and composition in matrix phase resulted in very different mechanical properties in different strain rates. Deformation in pseudo-static test took place primarily by twinning, resulting in negative strain sensitivity, such that the flow stress decreased with the increase of strain rate. Solution strengthening of the matrix phase by the addition of molybdenum along with the decrease in tungsten grain size resulted in the increase of the ultimate tensile strength of the alloy by more then 172MPa (13.7%) in the pseudo-static test. In addition, the strain rate sensitivity decreased with the increase of strain and isothermal hold time, where the alloy without the addition of molybdenum exhibited a more significant trend than that with molybdenum. The hardness and flow stress increased with the increase of strain rate, almost in a linear trend. Molybdenum-rich phases possibly formed around the tungsten grains in the molybdenum-added alloy, which resulted in brittleness of materials and deteriorated the mechanical properties at high strain rates.
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