Effect of Local Recrystallized Grains on the Low Cycle Fatigue Behavior of a Nickel-Based Single Crystal Superalloy

Xianfeng Ma,Hui-Ji Shi,Jishen Jiang,Jialin Gu,Wenjie Zhang

doi:10.3390/cryst9060312

Abstract

This paper aims to understand the effect of local recrystallization (RX) on the low cycle fatigue fracture of a turbine-blade single crystal nickel-based superalloy. The fatigue life of the single crystal superalloy was evidently decreased by local recrystallization. In single crystal specimens, casting porosity is the preferential fatigue crack initiation site, which is followed by crystallographic crack propagation along one or several octahedral slip planes. For all RX specimens, fatigue cracks preferred to initiate from local recrystallized grains and propagated through the recrystallized grains in a transgranular manner, followed by crystallographic crack propagation in the substrate single crystal superalloy. Moreover, fatigue tests indicated that locally recrystallized specimens exhibited temperature dependent fracture modes, i.e., transgranular cracking dominated at 550 °C, whereas intergranular cracking was preferred at 850 °C. Evident oxidation of fracture surfaces and strength degradation of grain boundaries at 850 °C was evidenced by scanning electronic microscopic observations. The present study emphasized the need to evaluate the effect of recrystallization according to the working conditions of turbine components, i.e., the local temperature.

Highlights

Due to its excellent mechanical properties under elevated temperatures, single crystal (SX) and directionally solidified (DS) superalloys, with the elimination of transverse grain boundaries, have been widely used as the primary materials for turbine blades [1]
It is seen that fatigue life decreases with increasing strain amplitude for both types of samples
Previous studies on directionally solidified superalloys, e.g., DZ4 and DZ40M, with uniform recrystallized layers exhibited a similar drop in fatigue life [10,11]

Summary

Introduction

Due to its excellent mechanical properties under elevated temperatures, single crystal (SX) and directionally solidified (DS) superalloys, with the elimination of transverse grain boundaries, have been widely used as the primary materials for turbine blades [1]. Grains can act as the preferred fatigue crack initiation sites of turbine blades [1,7]. Little research has been done regarding the effect of RX on the fatigue behavior of SX superalloys [8], aside from the limited studies on directionally solidified (DS) alloys [9,10,11]. Bürgel’s [8] early work on a recrystallized single crystal superalloy at 950 ◦ C showed no evident influence of recrystallization on the fatigue crack initiation life, except for a higher crack density.

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Conclusion