Local Order and Mechanical Properties of the γ Matrix of Nickel-Base Superalloys

Abstract

For the first time Y single crvstals, closely matching with the chemical compositi& ofenena introduce high f&tion &ces larger-than the applied stress (2f = 2.4 zel) opposing dislocation movement. The MC2 matrix has a tensile strength 15 % higher than that of the AM3 matrix. This improvement in strength is partially due to the higher content in W. From deformation and neutron diffraction experiments, it appears that the hardening mechanism is not only a solid solution one, but also associated with a local order hardening which depends on the alloy. The superior mechanical properties of the MC2 y phase are attributed presumably to the presence of a DO22 short range order due to W. to 8.6 g cm-s. MC2 (d = 8.62 g.cm-3) is a very high strength single crystal superalloy showing a 5O’C operating temperature advantage over AM3. With a chemical composition quite similar to those or many conventional superalloys, -its creep behaviour is comparable to that of some recent superalloys which contain rhenium. The aim of this study is to elucidate the reasons for such a difference considering the microstructural aspects and the microscopic deformation processes occurring in the y phase . Here we have to deal with the question of order vs. disorder complicated by the fact that in the matrix of complex industrial alloys, not only short range order, but also long range order (for example, DO22 as well as L12 type) may be present. For Cr (20 30 at %) rich Ni-Cr alloys, neutron scattering, deformation experiments and in situ electron microscopy indicate the presence of short range order, of the type Ni2Cr [ 4, 6 ] or NisCr [3, 5,7, 8, 91. Short range order of the Ni-Cr pairs has also been found by X-ray scattering in NizCoCr, and the same was confirmed to exist in the MC2 matrix single crystals used in the present study [3, lo]. ExuerimentaJ nrocedure Introduction The microchemistries of the y MC2 and y AM3 phases in the two chase materials were determined by atom probe nanoanalysis [l 11. it must, however, be noted that-the compositions chosen and DreDared corresuond to the eouilibrium comnosition at 850°C (the iedperature oi the final agking treatmenf of the alloys). ?he composition of the corresponding y single crystals were checked to be verv similar to those of the matrices of the two-phase materials. They &e given in Table I. The W content is signif&ntly higher in the MC2 matrix. while the concentrations of Al. Ta and Ti are higher in the Ak3 matrix. As Al, Ti and Ta ark considered to favour precipitation of the y ‘phase and in order to check their influence on ordering, MC2 type matrix crystals without these y ’ formers were also grown. All these specimens were homogenized 3h at 1300°C and air cooled. In nickel base superalloys, most of the studies reported in the literature have, up to now, been devoted to the mechanical properties of the two phase y / y ’ material or the strengthening 1/ chase. Even though the creation and propagation of dislocations in _ Atiose materials, especially at high iemperature, occurs in the y matrix, the hardening contribution of the y matrix has been neglected or underestimated. With the exception of the paper by Beardmore et al. [l] very little information is available on the intrinsic characteristics of this f.c.c. phase, often considered to be disordered. Two main reasons can explain this : Direct microscopic observations within the very narrow y channels are not easy, The precise composition of the y phase is difficult to determine and due to solidification difficulties no matrix single crystals were available until now. In order to improve the mechanical properties of superalloys, it is necessary to understand the hardening mechanisms operating in the y phase itself, which, of course, are related to its structure and composition. For the first time, we succeeded in the fabrication of y single crvstals. almost exactlv of the chemical composition of the matrices of AMj and MC2 iidustrial nickel-base &peralloys [2]. These allovs were recentlv developed at ONERA for single crystal turbine blade applications. AM3&is a low density (d-= 8.25 g.cm-3) superalloy which exhibits a mechanical strength comparable to that of the first generation single crystal superalloys with densities close Table I : Composition (at.%) of the three different matrices investigated, as checked by chemical analysis.