Microstructure And Properties Of A Containing 4%Ru Single Crystal Nickel-based Superalloy

In the dissertation, by the means of directional solidification technique, a single crystal nickel-based superalloy containing 4% Ru was designed and prepared. Different heat treatment regimes, creep-property tests, microstructure observations with SEM and TEM, composition analysis with EDS were conducted to investigate the influence of heat treatment on the extent of the elment segregation. Moreover, the deformation and damage characters of the alloy during creep at different conditions, the morphologies of microstructure defects in the alloy and their effect on the microstructure evolution and creep life were studied. The crack initiation and propagation characteristics of the alloy during creep were also studied. The main conclusions can be drawn as followings.There exists clear segregation in the as-cast alloy, of which the elements Ta, Co, Al, and Ru are mainly enriched in interdendritic regions, the elements W, Cr and Mo are mainly enriched in dendrite regions, the elements W and Mo have the biggest negative segregation factor and the elements Ru and Al have the biggest positive one. After full heat treatment, the segregation extent of the elements between inter-dendrite/dendrite regions of the alloy has been significantly reduced, and as the solution-treatment temperature rises, the segregation factor of the alloy is correspondingly reduced, indicating that proper heat treatment regimes may improve the homogenization extent of the elements and the creep resistance of the alloy.In the temperature range of 980~1010℃, the alloy containing 4%Ru has good creep resistance and long creep life. The creep activation energy of the alloy in the temperature ranges of 1070~1100 ℃, 980~1010 ℃ and 760~780 ℃ during steady-state creep are respectively calculated to be Q=409.9 kJ/mol, Q=439.2 kJ/mol and Q=467.3 kJ/mol. At the early stage of high temperature creep, the ?? phase in the alloy is transformed into rafted structure along the direction perpendicular to the applied stress, and during the steady-state creep, the deformation mechanism of the alloy is dislocations slipping, climbing in the matrix, and cutting rafted ?? phase. At the latter creep stage, the deformation mechanism of the alloy is dislocations slipping in matrix and shearing into ?? phase. During creep, significant amount of fine cubical ?? phase are precipitated in the ? matrix channels between the rafted ?? phase. And the fine cubical ?? phase is gradually grown up as the creep goes on, which may hinder the movement of dislocation in the matrix to improve effectively the creep resistance of alloy at high temperature.The cavity defects may obviously decrease the creep life of the alloy. During creep, the distribution of the von Mises stresses is different around the cavities with/without cracks. Compared to the stress distribution near the cavities without cracks, the bigger stress appears in the pole b region near the cavity with cracks. And as creep goes on, the maximum value of the stress in the pole b region further increases, which may promote the initiation and propagation of the cracks along the direction perpendicular to the stress axis, and this is thought to be the main reason for the alloy having a shorter creep life.