Influences Of Orientations On Creep Behavior Of DD99 Nickel-based Single Crysral Superalloy

In this dissertation, the tensile creep property at different temperatures and stresses were investigated in DD99nickel-base single crystal superalloy with the [O1l] and [111] orientated specimen. The section morphology of specimen was observed by TEM and SEM. Then the tensile creep deformation mechanism of single crystal superalloy was discussed. The conclusions can be drawn as following:Nickel-base single crystal superalloy specimens were prepared in three orientations [001], [01l] and [111]. The dendrite morphology of all orientations was observed by Lecia optical microscope. The result showed that the dendrite was regular and secondary dendrite presented "+"shape in the cross-section of [001] orientated specimen. While the secondary dendrite also presented "+"shape and range along[011] orientation in the cross-section of [01l] orientated specimen. In the cross-section of [111] orientated specimen the angle between two secondary dendrites was 60°and presented dissymmetry growth. It was showed that the element segregation was obvious and the differences of the y' phase morphology and size were rather big in as-cast alloy. Suitable size of y' precipitates with cubic morphology was obtained by 4-step heat treatment.The tensile creep behavior of [01l] oriented specimen was tested and the corresponding microscopic deformation feature was observed. At low temperature and high stress, the y' precipitate kept cuboidal shape and the a/2<110>dislocation sheared into the cuboidal / precipitate which were dissociated into the Super-Shockley dislocation a/3 stacking faults in / precipitates were observed. At intermediate temperature and middle stress, the raft of / phase was obvious and formed similar rhombus-shape. Multi-glade systems were activated and dislocations shear into the / precipitate were the main deformation mechanism during creep stage. At high temperature and low stress the cuboidal y' precipitates turned into lamellar raft morphology along inclined 45°angle to the tensile axis during high temperature low stress tensile creep. The deformation mechanism was the dislocation climbing over the y' rafts and a large number of dislocations shearing into the y' rafts from the y/y' phase interfaces where the dislocation networks were damaged at the later creep stage. Compared with the [01l] orientated specimen the [111] orientated specimen had the similar creep time and the edge of y' precipitates dissolved, a/2dislocation deposited in the y/y' phase interfaces and caused stress focus which was the main deformation mechanism during creep rupture. At intermediate temperature and middle stress the morphology of y' precipitates turned to be round, the a/2dislocation sheared into the cuboidal y' precipitate and formed dislocation line. At high temperature and low stress / precipitates formed irregular rafts structures, <110>{lll} glide system was activated, the dislocation glided on {111} plate and formed dislocation networks at y/y' phase interfaces which also effectively impeded the slippage and climbing of dislocation at y/y' phase interfaces.