Hot Deformation Behaviors And Ageing Characteristics Of Spray Formed GH742y Alloy

The billet of GH742y superalloy was prepared by the spray forming processing, and it was consolidated by hot isostatic pressing (HIP) (hereafter referred to spray formed alloy). The workability and deformation characteristics were investigated by means of isothermal hot compression testing. To optimize hot deformation processing variables, the constitutive equations and processing maps of the spray formed alloy were established on the basis of testing data and dynamic materials model. Subsequently, the spray formed billet was hot forged by means of above-mentioned processing parameters. The deformation mechanism and the ageing characteristic of the spray formed and forged alloy were by studied SEM, TEM, electrical resistivity and positron annihilation technology, and ageing kinetics and the optimum heat treatment procedure were determined. The mechanical properties of heat treated alloy were examined at room and high temperatures. By contrast with the spray formed alloy, the HIPed as-cast GH742y (hereafter referred to as-cast alloy) alloy was also investigated.The compression testing results show that the curve shapes of true stress-true strain of two alloys were similar. The peak stresses of two alloys decreased with the increase of temperature or the decrease of strain rate. It is found that the spray formed alloy exhibits an excellent hot-workability compared to the as-cast counterpart. The spray formed specimens didn't crack at the engineering strain up to 80%, but all as-cast specimens cracked at engineering strain beyond 35%. Also, the result shows that the higher the strain rate used, the better is the deformability of the spray formed alloy. However, the as-cast alloy is reverse.The hyperbolic sine function of sinh(0 .0031σp)instead of exponent equation of ln Z = 72+0.018σP, fitted for the spray formed alloy.The activation energy map and processing map of the spray formed alloy were established according to the constitutive equations. A small platform region exhibits at 1110~1140℃,1.0~10s-1 in the activation energy map. However, flow stability occurs at 1110~1140℃, 1.0~10s-1 in processing map, with to efficiency power dissipation being max. in this region. The flow stability region corresponds to the small platform region, and the spray formed alloy possesses better workability in the region. Flow instability occurs at 1050~1080℃, 0.01~0.1s-1 in processing map, the deformed specimens cracked in this region.The microstructure observation indicates that the dynamic recrystallization (DRX) of the spray formed alloy occurs in deformation, which related with temperature, strain rate and strain. With increasing of strain rate, testing temperature and strain, the extent of DRX is increased, which causes development of deforming workability with increasing of strain rate. Nevertheless the DRX extent of the as-cast alloy decreased with the increase of strain rate, resulting in the worse workability.In contrast to as-cast alloy, the peak aging time of the spray formed alloy was significantly shortened, and the peak ageing hardness of the former was lower than that of the latter. The difference of two alloys in ageing characteritics was attributed to high solid solubility, high vacancy concentration, low vacancy migration energy and high dislocation density for the spray formed alloy.For the spray formed alloy, it is found that theγ′size growth fits with LSW ripening theory, viz r 3∝trelationship. The calculation shows that, theγ′phase growing activation energy is 133.9kJ/mol, thereby theγ′phase growth kinetics equation as follows:Two heat treatment procedures are determined according to solution and ageing treatment tests. In the first treatment, the alloys were solutionized at 1140℃for 6h and air cooled, and aged at 850℃for 6h and then air cooled. In the second treatment, aging at 780℃for 8h and air cooling was added on the basis of the first treatment. The mechanical properties of the heat treated the spray formed alloy were studied. The results show that its properties are better than those of the as-cast, which is attributed to cooperation among the primary, secondary and tertiaryγ′phases. The fracture surfaces are characterized by dimple aggregation.