Isothermal constant-displacement-rate compression tests of superalloy IN718 were conducted using a computer-controlled MTS machine, at temperatures from 960 ЊC to 1040 ЊC, initial strain rates from 0.001 to 1.0 s Ϫ1 , and true strains from 0.105 to 1.204 (corresponding to a relative height reduction from 10 to 70 pct). The flow stress was obtained directly from compression tests using the controlling computer. The grain size was obtained by the intercept method and by comparing microstructures to the ASTM standard grain chart. Based on the results of these tests, a mathematical model showing the hot-deformation behavior of superalloy IN718 was developed in the form of a constitutive equation and a Hall-Petch relationship. Both the flow stress and the grain size calculated from the model were compared to the experimental results. Meanwhile, the effect of hot-deformation parameters such as strain, strain rate, and temperature on the flow stress and on the grain size was analyzed according to the model. It was found that these parameters influenced the flow stress and the grain size. The model developed in this work can be used to explain such effects.
Delta-Phase behaviour has important effect on mechanical properties in Alloy718. Structure control on a-phase behaviour by forging and followed heat treatment processing has been conducted in this research, simultaneously different quantity, morphology and distribution of b-phase have been achieved in turbine disk fabrication. Experimental results indicate that 6-phase behaviour has obvious influence on crack propagation. In certain range a-phase has retardation effect on crack propagation. Crack propagation mechanism and failure analyses under creep and fatigue conditions have been discussed also.
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