In Part I of the paper, tensile and fracture-toughness tests on a heat-resistant alloy were conducted in the temperature range from 24 to 1,093 deg C in vacuum. Note that the tests at 24 deg C were done in air. It was found that both the yield strength and ultimate tensile strength of the heat-resistant alloy decreased, but its elongation first increased slightly, then decreased with increasing the test temperature from 24 to 1,093 deg C. The scanning-electron-microscopy (SEM) observations on the fracture surfaces of tension-tested samples revealed that the amount of cleavages dropped, and that of dimples increased, as the test temperature increased. When increasing the test temperature, fracture toughness increased. At or below 177 deg C, valid fracture-toughness (K_(IC)) experiments were obtained. At or above 204 deg C, it is necessary to use elastic-plastic fracture mechanics in order to evaluate the fracture toughness of the heat-resistant alloy, as presented in Part II. Based on the load versus displacement curves of the fracture-toughness tests, the ductile-to-brittle transition temperature (DBTT) is between 177 and 204 deg C for this thickness and geometry, as also clearly demonstrated in Part II.
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