ALLOYING EFFECTS ON MICROSTRUCTURAL STABILITY AND γ′ PHASE NANO-HARDNESS IN Co-Al-W-Ta-Ti-BASE SUPERALLOYS

In previous work, Co-Al-W-Ta-Ti quinary alloys were found to show excellent creep resistance and microstructural stability at higher temperature as well as high gamma ' solvus temperature. In the current research, alloying effects on microstructural evolution, phase equilibria at 1000 similar to 1100 degrees C as well as gamma ' phase nano-hardness were investigated in Co-Al-W-Ta-Ti-X alloys (X = Cr, Mo, Nb, Ni and V). Phase diagram calculations were also performed using Pandat (TM)*. It is suggested that Ni and V additions increased gamma ' solvus temperature, while Cr, Mo and Nb additions presented a negative effect. The composition of the baseline alloy is located in the gamma+gamma ' two phase region at 1000 similar to 1050 degrees C but lies in the y+gamma '+chi+beta phase region at 1100 degrees C. Additions of Cr, Nb, V and Mo strongly promoted the precipitation of chi phase at 1100 degrees C, while beta phase co-existed in alloys containing Cr and V, respectively. Ni addition enlarged the gamma+gamma ' two phase region obviously in the investigated temperature range. The above experimental results generally suggested that gamma+gamma ' two phase region is smaller than the calculated phase equilibria. Nano indentation measurements showed that alloying additions increased gamma ' nano-hardness (gamma ' hardness) in the following order: V > Nb > Mo approximate to Ni, while Cr addition showed the negative effect distinctly. The current study is helpful to understand phase equilibria and microstructure in Co-Al-W-TaTi-base alloys and further develop multi-component y' strengthened Co-base superalloys.