Elevated Temperature And Superplastic Deformed Behavior And Mechanism In NiAl Intermetallic Alloys

Ni-28. 5at-Al20. 4at%Fe, Ni-20at%Al-30at%Fe and NiA1/Cr(Mo)-Hf alloys have been successfully fabricated by casting, hot梕xtrusion. The microstructures, superplasticity and elevated temperature deformation are studied with XRD, SEM and TEM. The relationship between the microstructure and mechanical properties as well as the elevated temperature deformed mechanism is discussed. The point defects in NiA1 and NiA1-Fe alloys are examined by means of positron annihilation. The microstructure and superplastic deformed behavior and mechanism of a casting + extruded Ni-28. 5at%A1-20. 4at%Fe alloy are examined. Its tensile properties were determined at temperature from 850C to 9800C and initial strain rates from 1.04×l0-2s-1 to l.04×10-4s-1 A maximum elongation of 233% was obtained at 850C and a strain rate of .5. 2 × 10-4s-1 Furthermore, microstructural features, such as decrease in the average grain size after deformation at 850℃ and 980℃, the presence of many dislocation-free grains adjacent to grains with a high dislocation density, and the subgrains and suhgrain boundaries, indicate that dynamic recrystallization has occurred as an efficient accommodation mechanism. SEM examination of the fracture sample after superplastic deformation reveals many voids on the fracture surface. By correlating with the results of TEM observation, it is suggested that the superplastic deformation in this alloy should be controlled by a grain boundary sliding-hased mechanism accommodated by the movement of dislocation and dynamic recrystallization. With the objective of exploring the superplasticity of the NiA1-based alloys, a multi-phase Ni-20at%Al-30at%Fe alloy was chosen for studied, which was processed through casting followed by hot-extrusion route. This alloy consists of β+γ/γphases with an average linear intercept grain size of 10?0 tim. Its tensile properties were examined at temperature from 750℃ to 980℃ and Iv initial strain rate from l.67X1O4s1 to 3. 34X1O3s? It was shown that the superplastic elongation to fracture increased as decreasing strain rate and increasing temperature. A maximum elongation of 468% was obtained at 9800C and a strain rate of 1. 67 × 10-4s-1 while a maximum strain rate sensitivity index of 0. 53 was obtained at 900C and 8. 35X 104s? TEM observation showed that the γ/γ?phase grains contained a high density of dislocation and dynamic recrystallization occurred in the f3 phase. SEM examination of the fracture samples after superplast.ic deformation revealed many voids and cavities on the fracture surface. By correlating with the results of TEM observations, it was suggested that the superplastic deformation in this alloy should he controlled by a grain boundary sliding-based mechanism accommodated by the movement of dislocation and the dynamic recrystallization. The effect of the alloying element hf added to substitute for the Mo element in NiA1?8Cr?Mo alloy on the microstructure and mechanical properties are investigated systematically. It is found that the Heusler phase existing in the casting alloy depends on the Hf content. The alloy of NiA1?8Cr?. 8Mo-0. 2Hf consists of N1AJ, Cr(Mo) and Hf solid solution which exist at NiA1 and Cr(Mo) phase boundary. After various heat treatments, the Hf solid solution could exist in different phase, that is , it remains unchanged above 10000C heat treatment (air co...