| Summary: | 碩士 === 義守大學 === 材料科學與工程學系 === 91 === Strain Rate and Environment Dependence on the Mechanical Behavior of the Ni-19Si-3Nb Based Intermetallic Alloy Doped with Boron and Carbon.
Abstract
Intermetallic compounds based on Ni3Si (order 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 temperature, good specific strength in comparison with other materials, increasing yield strength as a function of temperature. However, binary Ni3Si, similarity the Ni3Al suffers from grain boundary embrittlement at room temperature. In addition, the Ni3Si intermetallic alloy also exhibits, brittle behavior at mid-high temperature due to the environment embrittlement. Therefore, how to improve its grain boundary adhesion and resistance of environment embrittlement will be the critical points of increasing the ductility and widespread the applications of Ni-Si intermetallic alloys.
According to the previous studies, we select the Ni-19Si-3Nb alloy as the based alloy to investigate the effect of adding boron and carbon by means of arc melting and drop casting under argon atmospheres. The casting ingot was treated in vacuum furnace under the condition of homogenizing （1080℃，4hrs）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、EPMA、DTA and hardness analysis）.The metallographic microstructure and fracture surface were observed by TEM and SEM respectively ,The observations can be contrasted with the results of tensile tests. The ductility and fracture mechanism of Ni-19Si-3Nb intermetallic alloy under different testing combinations of temperature. strain rate and atmosphere will be discussed.
The purpose of this study is to understand the relationship between mechanical properties of Ni-19Si-3Nb-0.15B-0.1C intermetallic alloy and different testing environments （atmosphere：air （14000 ppm H20 vapor ）,vacuum and pure water vapor （850 ppm H20 vapor ）,temperature：25℃、500℃、600℃、700℃ and 800℃,strain rate：2×10-2S-1、2×10-3S-1 2×10-4S-1and 8×10-5S-1 and find out the ideal testing environment under which the optimal property of Ni-19Si-3Nb intermetallic alloy can be achieved.
According to the experimental results, we found that Ni-19Si-3Nb-0.15B-0.1 intermetallic alloy has the tensile strength of 1335Mpa under the testing environment（600℃,vacuum,2×10-2S-1 strain rate）and the tensile strength of 1255 Mpa（600℃,vacuum,8×10-5S-1 strain rate）.The ductility values under above two testing environments were 14.2%（2×10-2S-1）and 17%（8×10-5S-1）respectively. This result have indicated that for a very wide range of metal there exists a rather simple relationship between the dislocation density in a metal increases as it is strained plastically . This increase in dislocation density is accompanied by an increase in the flow-stress, that is, the stress required to deform the metal further. In other words, as the metal is deformed it work hardens, and the work hardening is associated with an increase dislocation density .However, the dislocation density at a given strain is frequently a function of the deformation temperature, so the increase in strength of a metal not only depends on the size of the strain but also on the temperature at which it is strained.
In most cases, work hardening decreases with increasing temperature at a fixed strain rate. Furthermore, at a fixed temperature, the tensile strength increases and ductility decrease with increasing strain rate. On the contrary, at a fixed temperature , the tensile strength decreases and ductility increases with decreasing strain rate. The experimental results of this study is consistent with the theory.
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