Zhang Zhefeng

MICROSTRUCTURE EVOLUTION AND MECHANICAL PROPERTIES OF FCC METALLIC MATERIALS SUBJECTED TO EQUAL CHANNEL ANGULAR PRESSING

Microstructure evolution,grain refinement mechanism and mechanical properties of face-centered cubic(fcc) metallic materials,subjected to equal channel angular pressing(ECAP),were systematically investigated.According to the special shear deformation mode of ECAP,Al single crystals with different orientations and Cu bicrystals with different initial grain boundary directions were subjected to ECAP for one pass,and it is found that shear deformations both parallel and perpendicular to intersection plane play important roles in the ECAP process.Moreover,Al single crystals,Cu single crystals and polycrystalline Cu-3%Si(mass fraction) alloy with different stacking fault energies (SFEs) and special crystallographic orientations,subjected to ECAP for one pass,were selected to experimentally and analytically explore the combined effects of crystallographic orientation,SFE and grain size on deformation twinning behaviors in several fcc crystals.Furthermore,ultrafine grained (UFG) or uanocrystalline(NC) Cu-Al alloys with different Al contents were prepared using multiple passes ECAP.The results show that the grain refinement mechanism is gradually transformed from dislocation subdivision to twin fragmentation,and the equilibrium grain size decreases with lowering the SFE of Cu-Al alloys.Meanwhile,the homogeneous microstructures of materials with high or low SFE are much more readily gained than those of medium-SFE metals.More significantly,the strength and uniform elongation can be simultaneously improved with lowering the SFE,i.e.,the better strength ductility combination is achieved in the Cu-Al alloy with lower SFE.

EFFECT OF MISORIENTATION ON THE TENSILE PROPERTIES OF SRR99 SUPERALLOY BICRYSTALS

The tensile deformation behavior,surface morphology,fracture mode and fracture morphology of SRR99 superalloy bicrystals were systematically investigated at room temperature in air.It is shown that the bicrystal with misorientation of 4°has little influence on the tensile properties, however the effect of grain boundary(GB) becomes obvious when the misorientation is 10°,for the bicrystals with misorientations of 16°and 18°the effect of GBs becomes much more serious.It is experimently found that slip is the most important deformation mode in the SRR99 superalloy bicrystals at room temperature.For the bicrystal with misorientation of 4°,slip bands would transfer through GB easily,and finally the specimens fractured along slip bands with typical slip characteristic on their fractography.In the bicrystal with misorientation of 10°,cracks nucleated and propagated either along slip bands or along GBs due to the impingement of slip bands and the strain accumulations, and finally the fracture instability happened along slip bands.The fracture morphology showed not only slip characteristic,but also the features of interdendritic cracks.In the bicrystals with misorientations of 16°and 18°,cracks initiated after yielding,and then propagated along GBs and the fracture surfaces mainly consisted of interdendritic cracks.