Effect of Recrystallization Thickness on Creep Properties of Directionally Solidified DZ125 Alloy

The surface of as-cast columnar crystal DZ125 alloy was blew sand followed by standard heat treatment process to obtain the recrystallization layers with different thickness, and the creep life of the alloy was measured. The effect of recrystallization and its thickness on the creep properties of the alloy was analyzed. The microstructure of the alloy was observed by scanning electron microscope. The results show that after heat treatment the thickness of the recrystallization layer on the surface of the columnar crystal DZ125 alloy is different at varied sand blowing intensity; with the sand blowing intensity increasing, the recrystallization layer thickness increases. The carbide in the as cast DZ125 columnar crystal alloy is slice, strip or grain, and the carbide after the standard heat treatment is partly dissolved, the size of the slice is reduced, and holes appear on the surface of slice carbide. Strip carbides turn into grains, and the original grain size declines. A small amount of grain carbides are precipitated around the grain boundaries after recrystallization. The creep lives of 2 mm and 4 mm thick samples with different recrystallization thickness are roughly equal to each other under the condition of 980 degrees C and 235 MPa, while decreased by 30% compared to that of the columnar crystal alloy without recrystallization. During the creep, the carbides in the grains are accumulated and grow around the grain boundary, which is beneficial to improve the strength of grain boundary. During creep the crack initiation is mainly along with the recrystallization grain boundary which is vertical to the stress axis. Crack initiation points also exist in the internal of the columnar crystal alloy. The gamma' phase rafted in recrystallization grains indicates that the grain boundaries of recrystallization have certain strength, although they weaken the creep strength of the alloy.