Experimental Research On Microstructure Evolution Of Nickle-base Single-crystal Superalloy DD6 Under Thermal-mechanical Loadings

Nickel-base single-crystal superalloys are produced by the directional solidification technology. The microstructure is a single columnar crystal because of growth with the predetermined orientation, and the alloys completely eliminate the grain boundaries, as a result of that they have excellent comprehensive performance, such as high temperature strength and creep resistance, good resistance to oxidation and corrosion. Nickel-base single-crystal superalloys have been widely used in the fields of aerospace and ships and become the first choice of materials for turbine blade in advance aircraft engines. The excellent properties of nickel-base single-crystal superalloys are inseparable from microstructures and their evolution in service. The purpose of this paper is to study the special ?/?? microstructure, its evolution and strengthening mechanism at different temperatures, which has much value of scientific research and engineering applications.This paper takes the domestic second-generation DD6 nickle-base single-crystal superalloy as the research object to make experimental research. Stress-strain curves and mechanical properties of DD6 alloy at different temperatures have been got by the tensile tests at room temperature(RT), 650?, 850? and 1020?.The initial microstructure and its evolution at different temperatures have been observed by SEM and TEM, and the distribution of alloying elements was analyzed by EDS. The micro deformation mechanism and fracture modes of DD6 alloy at different temperatures and rafting of ?? phase at high temperature have also been studied.The stress-strain curves show that DD6 alloy presents obvious work hardening phenomenon at RT. With the temperature increase, the process of dynamic recovery accelerates, and dynamic recrystallization takes place at high temperature. It can be seen that DD6 alloy has the highest tensile strength and yield strength at 650?, which reaches 1020 MPa and 854 MPa respectively. But the elongation and reduction in area(RA) are quite low, especially RA is only 2.7%, which is about 1/6 of that at RT. When the temperature increases, the yield strength significantly reduces, and the ductility got restored. It indicates that the alloy has medium intermediate temperature brittleness(ITB), which is due to the cross-slip of dislocations and dynamic strain aging(serrated yielding) and glide plane decohesion.SEM and TEM observations show that the cubic ?? phases with 0.45?m side are distributed uniformly in the ? matrix along <100> direction after standard heat treatment. The volume fraction of ?? phase is about 70%. The lattice misfit between ?? phase and ? matrix has been calculated to be-0.45%. There is heterogeneous distribution of elements in two phases. Co, Cr, W and Mo mainly dissolve into ? matrix, while Al and Ti can form intermetallic compound Ni3(Al, Ti) with Ni, and Ta is also an important element of ?? phase. The directional diffusion of different elements lead to the type-N rafting of ?? phase of DD6 aolly with negative lattice misfit at 1020?.The micro deformation mechanism of DD6 alloy at room temperature and 650?is dislocations shearing ?? phase, and the alloy shows quasi-cleavage and cleavage fracture characteristics respectively. At high temperature, the alloy deforms in the way of by-pass ?? phase, and the alloy shows microvoid coalescence fracture characterized by dimple patterns with features of plastic fracture. At 850?,the deformation mechanism is that dislocations transforms from shearing ?? phase to by-pass ?? phase, and the fracture shows both cleavage and dimple patterns with features of plastic fracture.