62 / 2021-06-30 10:15:49

A study on thermal fatigue behavior of additive manufactured CoCrWAlxNiy alloys

CoCrWAlxNiy alloys, Direct laser deposition, Thermal fatigue resistance

The Co-based alloys were widely applied to aerospace and auto industries due to their excellent elevated temperature strength and thermal fatigue resistance, but the mechanism of thermal fatigue behavior was not well understood, which limited the improvement of property and service life further. This work prepared CoCrWAl_xNi_y alloys by direct laser deposition, which were meant to provide an effective way to resist thermal fatigue. Microstructures of all alloys were characterized detailly and the effects of Al and Ni addition on properties were also investigated. All alloys consisted of γ-Co matrix and carbides that were identified to M₂₃C₆ and M₇C₃ phases, only CoCrW alloy contained ε-Co phase. The Al and Ni could improve stacking fault energy and stabilize γ-Co matrix. The mechanical properties were estimated by tensile test and the Al and Ni promoted ductility significantly. The 2Al15Ni alloy exhibited highest elongation to fracture (~14%) with a high ultimate strength (1353 MPa) among all alloys. The thermal fatigue test was conduced by dedicated device and the thermal cycle was from room temperature to 600 ℃. The thermal fatigue crack was investigated through in-situ observation and the oxidation had obvious function on the crack initiation and propagation besides thermal stress and phase transformation. The stress induced by oxidation was calculated based on Huntz theory and Stokes-Herring-Suo formalism. The internal and outer oxidation were discovered in alloy alloys respectively and former was more harmful in crack initiating. The outer oxide layer of 2Al15Ni alloy obtained lowest stress which was beneficial to inhibit cracks. Meanwhile, excellent ductility was helpful to slow crack propagation and 2Al15Ni also obtained lowest crack extension rate. The improved thermal fatigue model was established, which explained the effects of oxidation on thermal fatigue process and provided a better understanding of the failure in complex thermal-cooling fatigue condition.