Zhou Yizhou

The effect of high current pulsing on persistent slip bands in fatigued copper single crystals

Copper single crystals fatigued along [(1) over bar 23] loading axis were treated by high current pulsing after the formation of persistent slip bands (PSBs). The results show that the treatment by electropulsing made the PSBs vanish locally. This is due to the thermal compressive stress induced by electropulsing, which reduced the stress concentration at the interface between PSBs and matrix. The annihilation of edge dislocations reduced the width of PSBs and made the PSBs vanish locally. The main factor that made the dislocations move was the thermal compressive stress. The velocity of the dislocation motion is about (10(3)-10(2)) cm s(-1)

STRUCTURE EVOLUTION IN DIRECTIONALLY SOLIDIFIED BICRYSTALS OF NICKEL BASE SUPERALLOYS

Bicrystal samples of Ni base superalloy IN792 were produced to study the structure evolution in directional solidification process. In the case of converging grains, the misaligned dendrites were able to block the better aligned dendrites at the grain boundary. The dendrite blocking leaded to inclination of the grain boundary from the misaligned grain to the better aligned grain. Thus, the better aligned grain was overgrown by the misaligned grain. In the case of diverging grains, dendrite development at the grain boundary leaded to inclination of the grain boundary from the better aligned grain to the misaligned grain. Consequently, the better aligned grain overgrew the misaligned grain.

Effects of Substrate Crystallographic Orientations on Microstructure in Laser Surface-Melted Single-Crystal Superalloy：Theoretical Analysis

A geometric analysis technique for crystal growth and microstructure development in single-crystal welds had been previously developed. And the effect of welding conditions on the tendency of stray grains formation during solidification was researched. In the present work, these analytical methods were further extended. Combined with an original vectorization method, a 3D Rosenthal solution was used to determine thermal conditions of the welds. Afterward, the dendrite growth orientation, the dendrite growth velocity and the thermal gradient along dendrite direction were calculated and lively plotted. Finally, the tendency of stray grains formation in the solidification front was forecasted and its distribution was presented with a 3D plot. The results indicate that substrate orientation has some impacts on the crystal growth pattern, dendrite growth velocity, distribution of thermal gradient and stray grain. Based on the research methods proposed in this work, any substrate crystallographic orientation can be studied, and predicted stray grains distribution can be visualized.

INFLUENCE OF Ru AND Cr ON SOLIDIFICATION PROCESS IN A Re-FREE Ni-BASED SINGLE CRYSTAL SUPERALLOY

The effect of addition of Ru and change of Cr content on solidification behaviors are systematically investigated in a Re-free Ni-based single crystal superalloy with a fixed mass fraction additions of Ru and Cr,with the use of DTA,OM,SEM and EPMA.The results show that the addition of Ru can increase the eutectic fraction,facilitate the formation of Ru-rich phase which would increase largely at the time of Ru and Cr addition synchronously.Ru addition can promotes W segregate to the dendrite arm,and inhibits Mo to segregate to the interdendritic region.The increasement of Cr addition can decrease the segregation ratio of W,but have little effect on Mo.The combination of Ru and Cr influences the solidification microstructures and segregation behaviors corporately,and thus influences heat treatment processes and the microstructure stability.

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.

EFFECT OF Hf ON THE INTERFACIAL REACTION BETWEEN A NICKEL BASE SUPERALLOY AND A CERAMIC MATERIAL

The influence of Hf content on the wettability and interfacial reaction between a nickel base superalloy and a ceramic material was investigated by using a sessile drop method.The wetting angle was measured through the geometric parameters of the metal drop.The interfacial morphology and elements distribution were studied by SEM and EPMA,respectively.XPS was employed to study the phase formation at the metal-ceramic interface.The relationship between wettability and interfacial reaction was discussed.It was found that the wetting angle of the metal drop on the ceramic substrate was decreased with increasing Hf content in the alloy and the wettability of the studied superalloy on the ceramic material was enhanced with increasing Hf content in the alloy.When the content of Hf increased from 0.1% to 2.0%(mass fraction),the wetting angle decreased from 132° to112°.The interfacial reaction led to a sharp decrease in wetting angle as Hf content was 1.5%.The product of interfacial reaction was HfO2 and the thermodynamic analysis indicated that Hf was able to substitute Si in SiO2 in the ceramic material.