Hot Deformation Behavior Of Nickel During Compression At Elevated Temperature

Pure nickel is mainly shaped by rolling, forging and extrusion. However, it is easyto produce crack, brittle fracture problems when being upsetted and rolled, which leadsto low production of final products, poor product quality, high cost and other problems.In order to better solve these problems, it is necessary to understand the flow behaviorof pure nickel during thermal process.In this paper, the hot deformation behavior of pure nickel with coarse, columnargrains was investigated by isothermal hot compressive testing on the Gleeble-3500thermal and mechanical simulator, the flow behavior was studied under variousdeformation parameters to reveal its dynamic softening behavior of microstructureevolution and dynamic recrystallization nucleation mechanisms. The relationshipbetween the performance and the microstructure of nickel was studied bymicro-hardness test.The major conclusions of this investigation are as follows:(1)The flow curves were prone to peak or even multiple peaks at the condition ofhigh temperatures and low strain rates. But it did not exhibit peak at high strain ratedue to the effect of work hardening which far exceeds that of softening; Withincreasing the deformation temperature or decreasing the strain rate, the flow stressdecreased.And the effects of the strain rate was greater than that of temperature duringhot deformation. The apparent activation energy can be calculated as312.4KJ/mol andvariation of activation energy was sensitive to strain rate rather than temperature. Thedependence of flow behavior on deformation temperature and strain rate can berepresented by introducing the Zener-Hollomon parameter. And the relative errorbetween measured stress and calculated stress was within±10%, indicating that theflow behavior of pure nickel at elevated temperature can be described by theconstitutive equation.(2)The changes of processing map based on the principle of dynamic materialsmodel reflected that the strain rates, deformation temperatures and strains had asignificant impact on thermal deformation behavior of pure nickel. The instabilityregions mainly concentrated in the high strain rates. With increasing the strain, theinstability regions increased and migrated from high temperature to low temperature.The optimum processing parameters of deformation temperature and strain rateobtained from processing map and microstructural observation were1060-1120℃and of0.03-0.2s-1.(3)The micro-hardness change of nickel during hot deformation was mainlydepended on the change in dislocation density due to the effects of work hardening anddynamic softening. At the deformation temperature of1000℃, too low or too highstrain rates the micro-hardness decreased, and were not beneficial for formation of fineequiaxed recrystallized grains, at the strain rate of0.1s-1the micro-hardness increasedto159.2HV. At the strain rate0.01s-1, with deformation temperature increasing, therecrystallized grains grew gradually, but the micro-hardness decreased. However, attemperature of1150℃, it was prone to DDRX and resulted in the coarsing ofrecrystallized grains, and the hardness increased. With strain increasing, the hardnessincreased, and formed fine recrystallized grains, at the strain of0.7the hardnessincreased to150.1HV.(4)The bulging process along markedly serrated and bulged grain boundarieswas an important nucleation mechanism of dynamic recrystallization. Bulgingnucleation also led to the formation of fine recrystallized grains. In addition, highergrain boundary mobility resulted in the nucleation of the twins which played animportant role in the generation of new grains during DRX and refined the initialgrains. The grain boundary bulging and repeated (growth) twinning were veryimportant and distinct mechanisms of new grains nucleation of nickel.