Study On Hot Deformation Behavior And Mechanisms Of Microstructure Evolution For Nimonic80A

The hot deformation behavior and microstructure evolution ofNimonic80A was studied via hot compression experiments in thetemperature range from800oC to1180oC, at strain rates of0.01s-1,0.1s-1,1s-1and strain from0.1to1.1. Further study was made onthe transition of different dynamic recrystallization (DRX) mechanismsunder hot working (1100oC-1180oC) and warm working (800oC-950oC) atstrain rate of1s-1, which include discontinuous DRX, continuous DRXand DRX on microbands within initial grains. The relationship betweenpeak stress (σp) and DRX grain size (DR) was established to find thetransitional temperature and to support the grain refinement inmanufacture.The hot-compression tests were performed at a Gleeble3500system.Central regions of specimens with largest deformation were examinedusing optical microscopy (OM). Some of them were investigated byscanning electron microscopes (SEM) equipped with EBSD system.EBSD results were analyzed by Channel5.True stress-true strain curves exhibit a broad peak representing arapid strain hardening at the initial deformation stage followed by DRX as the deformation proceeds. The peak stress (σp) decreases as thetemperature increases and increases with the increasing of strain rate.Transitions of deformation behavior and microstructure evolutionwere observed around1000oC at a constant strain rate of1s-1. Therelationship between the peak stress and the deformation temperature wasfound to be bilinear with the slope of-4.0under warm working and-1.0under hot working, showing that the peak stress is much moretemperature-dependent in warm working range than in hot working range.At strain of1.1, homogeneous grain structures obtained above1000oCgradually changed into inhomogeneous ones consisting of deformedinitial grains as temperature decreases into warm working range.The DRX grain size can be related to the peak stress by an empiricalpower-law function. The grain size exponents (n) are-0.39and-0.71forwarm compression and hot compression, respectively. DRX grain sizeincreases as the temperature increases with the minimum value of0.3μmand the maximum value of25μm.These transitions of deformation behavior and microstructureevolution with the temperature variation are associated with the change ofthe operating DRX mechanism. In hot working, new grains form via thebulging of initial grain boundaries (i.e. discontinuous DRX) with theactivation energe (Q) of175KJ/mol. While in warm working, new grainsdevelop from subgrains via the gradual increase of sub-boundarymisorientations into high angle type upon straining (i.e. continuous DRX)with Q of932KJ/mol. There is a temperature range for the transition inNimonic80A, which is approximately around1000oC where both DDRXand CDRX take place during the deformation. Besides, DRX at microbands within the initial grains at800oC is another deformaitonmechanism observed in this work.