Advanced Scale Bridging Microstructure Analysis of Single Crystal Ni‐Base Superalloys

Abstract

In the present work, we show how conventional and advanced mechanical, chemical, and microstructural methods can be used to characterize cast single crystal Ni‐base superalloy (SX) plates across multiple length scales. Two types of microstructural heterogeneities are important, associated with the cast microstructure (dendrites (D) and interdendritic (ID) regions – large scale heterogeneity) and with the well‐known γ/γ′ microstructure (small scale heterogeneity). Using electron probe micro‐analysis (EPMA), we can show that elements such as Re, Co, and Cr partition to the dendrites while ID regions contain more Al, Ta, and Ti. Analytical transmission electron microscopy and atom probe tomography (APT) show that Al, Ta, and Ti partition to the γ′ cubes while γ channels show higher concentrations of Co, Cr, Re, and W. We can combine large scale (EPMA) and small‐scale analytical methods (APT) to obtain reasonable estimates for γ′ volume fractions in the dendrites and in the ID regions. The chemical and mechanical properties of the SX plates studied in the present work are homogeneous, when they are determined from volumes with dimensions, which are significantly larger than the dendrite spacing. For the SX plates (140 mm × 100 mm × 20 mm) studied in the present work this holds for the average chemical composition as well as for elastic behavior and local creep properties. We highlight the potential of HRTEM and APT to contribute to a better understanding of the role of dislocations during coarsening of the γ′ phase and the effect of cooling rates after high temperature exposure on the microstructure.

On Shear Testing of Single Crystal Ni-Base Superalloys

“…Gao et al [57] have recently developed an anisotropic constitutive model for creep of single crystal Ni-base superalloys in the low-temperature and high-stress creep regime. They used uniaxial creep data for the Ni-base single crystal superalloy ERBO 1C, [3] published by Wollgramm et al [58] for h100i-, h110i-, and h111i-tensile directions, to tune the parameters of the coupled set of differential equations in their model. [57] For the present work, this model was implemented into a user-defined subroutine UMAT of the FEM software package ABAQUS 6.11.…”

“…[5,21] This result appears reasonable because the stress relaxation/redistribution during the viscoplastic deformation rapidly homogenizes the stress field heterogeneities inherited at the end of the initial elastic loading. [3] from the low-temperature high-stress creep regime. The shear stress of 300 MPa was chosen in order to have similar loading conditions on the activated slip systems as during h100i tensile creep testing under 800 MPa (resolved shear stress: 326 MPa).…”

“…Finally, an attempt is made to investigate the Ni-base superalloy single crystal ERBO-1 [3] using in-situ h1À10i{111} SEM micro-shear testing. The objective is to explore whether the small differences in c¢-volume fractions between dendritic (D, lower: 72 pct) and interdendritic regions (ID, higher: 77 pct) reported earlier [3,4] manifests itself in a difference in critical resolved shear stresses (CRSS) and whether large-scale microstructural heterogeneities manifest themselves during microshear testing.…”

“…The objective is to explore whether the small differences in c¢-volume fractions between dendritic (D, lower: 72 pct) and interdendritic regions (ID, higher: 77 pct) reported earlier [3,4] manifests itself in a difference in critical resolved shear stresses (CRSS) and whether large-scale microstructural heterogeneities manifest themselves during microshear testing. For this purpose, the micro double shear specimen shown in Figure 3(b) was derived from the macro double shear creep specimen shown in Figure 3(a).…”

“…In addition to this small-scale heterogeneity (c/c¢-microstructure), one can distinguish between former dendritic (D) and interdendritic regions (ID), even after extended homogenization heat treatments (dendrite spacing: % 400 lm). [3,4] Most mechanical and microstructural results on yield strength and on creep of single crystal superalloys are obtained using uniaxial testing. Much less emphasis has been placed on shear loading.…”