| Summary: | 碩士 === 義守大學 === 材料科學與工程學系 === 90 === Abstract
Intermetallic compounds based on Ni3Si (ordered fcc L12 structure) offer a number of interesting properties for structural applications in aggressive environments at elevated temperature. These interesting properties include the excellent corrosion and oxidation resistance at both ambient and elevated temperatures, good specific strength in comparison with other materials, increasing yield strength as a function of temperature, and a peak yield strength occurring at about 700℃. However, similar to the Ni3Al, binary Ni3Si suffers from grain boundary embrittlement at room temperature. Therefore, how to improve its grain boundary adhesion will be the critical point of increasing the ductility and widespread the applications of Ni-Si intermetallic alloys.
According to the pre-studies, we select the Ni-19Si-3Nb alloy as the based alloy to investigate the effect of adding boron and Chromium by means of arc melting and drop casting under argon atmospheres, and then homogenizing and aging in a vacuum annealing furnace with atmosphere of 5×10-5 torr at 1080℃ for 4 hours and 700℃ for 10 hours respectively.
The main part of heat-treated ingots were machined into the tensile test specimen with the gauge dimension of 3mmx2mmx20mm..Then the specimen were tensile test under different atmosphere, [such as air (14000ppm H2O vapor), vacuum(2×10-4torr),and pure water vapor(850ppm)] at different temperature(25℃、500℃、600℃、700℃、800℃). The evolution of microstructure change and mechanical properties of the Ni-19Si-3Nb-0.15B-XCr alloys were characterized by x-ray diffraction, SEM, TEM, EPMA, microhardness test, Vickers hardness test, and tensile tests.
According to the result of this study. The optimum condition of the mechanical properties occurs at the composition of Ni-18Si-3Nb-0.15B-1.0Cr. It can improve elongation on high temperature due to add Cr. The elongation reaches 23﹪for the specimen tested under the vacuum at 800℃ temperature.
This reason that with Cr additions the hydrogen atoms were adsorption on the alloy surface increased the possibility of adsorption of hydrogen species onto the surface. Furthermore it can form a chromium oxide layer to enhance corrosion and oxidation resistance. The results of this study showed that with Cr addition can improves ductility and suppress environment embitterment on high temperatures.
|
|---|
