The effect of Boron and Chromium addition on the microstructrue and mechanical behavior of nickel silicide based intermetallic alloy
| Autor: | 鍾志明 |
|---|---|
| Rok vydání: | 2005 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 93 Ni3Si alloys (Ni3Si, ordered fcc, Ll2) have been considered to be a candidate compound, which could be used as the basis of high-temperature structural materials and chemical parts, because Ni3Si displays an increasing strength with increasing temperature and also shows excellent oxidation and corrosion resistance over a wide range of temperatures. As a result of the solid solution hardening and also high tensile ductility, adding Ti to Ni3Si resulted in an increased strength owing to reducing the propensity to intergranular fracture at ambient temperatures. Therefore, the doping such as niobium and boron and carbon has been conducted and shown to further improve the ambient temperature ductility by means of preventing intergranular fracture. Thus, how to improve Ni3Si alloys grain boundary adhesion and resistance of environment embrittlement will be the pressing point of increasing the ductility, and widespread the applications of Ni-Si intermetallic alloys. In the present study, Ni-18Si-3Nb alloy was selected to investigate the effect of doping boron and chromium by means of arc melting and drop casting under argon atmosphere. The casting ingot was treated in vacuum furnace under the condition of homogenizing (1080℃, 24hrs) and then aging (700℃, 10hrs). Part of the heat treated ingot was machined into tensile test specimens with the gauge dimension of 3mm×2mm×20mm using WEDM method. Part of the ingot was sampled and used as specimens for several different testing. (Such as GDS、EDS、XRD and hardness analysis). The specimens of tensile test of metallographic microstructure and fracture surface were observed by TEM and SEM respectively, the observation can be contrasted with the results of tensile test. The ductility and fracture mechanism of Ni-18Si-3Nb intermetallic alloy under different testing combinations of temperatures, strain rates and atmospheres will be discussed. The purpose of the present study are to understand the relationship between mechanical properties of Ni-18Si-3Nb intermetallic alloy and different testing environments (atmosphere：air (14000 ppm H2O Vapor) and Vacuum, temperature：25℃、500℃、600℃、700℃ and 800℃, strain rate：2×10-2 S-1、2×10-3 S-1 、2×10-4 S-1 and 8×10-5 S-1 ) and find out the ideal testing environment. In accord with the experimental results, the chromium was added to the Ni-18Si-3Nb intermetallic alloy that showed excellent oxidation resistance in Vacuum at 1073K and ductility values was 14%. In addition, Boron doped to the alloy resulted in increasing grain boundary adhesion and decreasing environment brittlement. Thus, Ni-18Si-3Nb alloy has the tensile strength of 1270 Mpa in Vacuum at 298K. The minimum elongation (about 4%) occurs in air at 973K. When the test is conducted in Vacuum, the fracture surface tested at 873K exhibits the intergranular cracking mixed the partial dimpled fracture. However, fracture surface tested at 1073K in Vacuum, presents a typical ductile fracture surface, fully fine dimpled fracture pattern. This observation appears to be well correlated with those results of tensile elongation, which idicates that the additions of chromium may suppress the moisture-induced embrittlement at low temperature as well as at high temperature. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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