Mechanical processing and microstructural control in hot working of hot isostatically pressed P/M IN-100 superalloy

Abstract

The hot deformation behavior of hot isostatically pressed (HIPd) P/M IN-100 superalloy has been studied in the temperature range 1000–1200°C and strain rate range 0.0003–10 s⁻¹, using hot compression testing. A processing map has been developed on the basis of these data and using the principles of dynamic materials modelling. The map exhibited three domains: one at 1050°C and 0.01 s⁻¹, with a peak efficiency of power dissipation of ≈32%, the second at 1150°C and 10 s⁻¹, with a peak efficiency of ≈36% and the third at 1200°C and 0.1 s⁻¹, with a similar efficiency. On the basis of optical and electron microscopic observations, the first domain was interpreted to represent dynamic recovery of the γ phase, the second domain represents dynamic recrystallization (DRX) of γ in the presence of softer γ′, while the third domain represents DRX of the γ phase only. The γ′ phase is stable upto 1150°C, gets deformed below this temperature and the chunky γ′ accumulates dislocations, which at larger strains cause cracking of this phase. At temperatures lower than 1080°C and strain rates higher than 0.1 s⁻¹, the material exhibits flow instability, manifested in the form of adiabatic shear bands. The material may be subjected to mechanical processing without cracking or instabilities at 1200°C and 0.1 s⁻¹, which are the conditions for DRX of the γ phase.

Introduction

IN-100 is a γ–γ′ superalloy with a high volume content of strengthening γ′ phase. A high carbon variety (0.18 wt.%C) of this superalloy is essentially used in an as-cast condition for applications like turbine blades, whereas a low carbon modified version (0.07 wt.%C) is used for forged components. The latter alloy, when produced by the powder metallurgy (P/M) route, offers superior mechanical properties [1]. In the P/M processing, several compaction variables have been tried, e.g. hot isostatic pressing (HIP) and forging below or above the γ′ dissolution temperature have a significant influence on the stress rupture life and creep ductility of the material [1]. The γ′ dissolution in this alloy occurs over a temperature range (1145–1190°C) [2]and hence processing of this material requires detailed understanding of the effect of process variables. The most important commercial development in the mechanical processing of IN-100 has been the Gatorizing process 3, 4developed by Pratt and Whitney, USA, which involves heavy hot working to cause recrystallization and the development of a fine grained structure which is amenable to superplastic deformation. The superplastic behavior and the associated microstructural changes in P/M IN-100 have been studied extensively by a number of investigators 4, 5, 6, 7, 8, 9, 10. Superplasticity was observed [6]in the temperature range 920–1038°C and at lower strain rates (≈0.001 s⁻¹). The microstructures of superplastically deformed specimens showed that both γ and γ′ phases recrystallized [8]and their morphology changed significantly [9]. Dynamic recrystallization (DRX) resulting in grain refinement is associated with superplasticity [10]causing flow softening of the stress–strain behavior.

The aim of the present investigation is to study the high temperature deformation mechanisms in HIPd P/M IN-100 in a wide temperature and strain rate range, beyond that normally covered by superplasticity studies. The study will help in the understanding of the mechanism of DRX in this complex two phase material and in achieving microstructural control in hot working. For this study the approach of developing processing maps based on the Dynamic Materials Model 11, 12has been adopted. In this model, the workpiece deforming under hot working conditions is considered to be a dissipator of power. At any instant, the power dissipation occurs through a temperature rise (G content) and a microstructural change (J co-content) and the power partitioning between these two is decided by the strain rate sensitivity (m) of flow stress (σ). At a given temperature and strain, the J co-content is given by 11, 12:

$$\text{J=(σ∗}\text{ε}\text{̇}\text{∗m)/(m+1),}$$

where $\text{ε}\text{̇}$ is the strain rate. The J co-content of the workpiece, which is a non-linear dissipator, is normalized with that of an ideal linear dissipator (m=1) to obtain a dimensionless parameter called efficiency of power dissipation (η):

$$\text{η=J/J}{}_{\mathrm{<mtext>max</mtext>}}\text{=2m/(m+1).}$$

The variation of η with temperature and strain rate represents the constitutive behavior of the material and constitutes a power dissipation map. The various domains in the map may be correlated with specific microstructural processes and applied for microstructural control. The Dynamic Materials Model has, as its basis, the extremum principles of irreversible thermodynamics as applied to large plastic flow described by Ziegler [13]. Kumar [14]and Prasad [12]developed a continuum criterion combining these principles with those of separability of power dissipation and have shown that flow instability will occur during hot deformation if

$$\text{ξ(}\text{ε}\text{̇}\text{)=[δ}\text{ln}\text{{m/(m+1)}/(δ}\text{ln}\text{ε}\text{̇}\text{)]+m<0.}$$

The variation of the instability parameter, $\text{ξ(}\text{ε}\text{̇}\text{)}$ with temperature and strain rate constitutes an instability map which may be superimposed on the power dissipation map for delineating the regions of flow instability.

The technique has been used to optimize hot workability in a number of commercial alloys like Nimonic AP-1 [15], IN 718 [16], AISI 304L stainless steel 17, 18and Zircaloy [19]. The approach has also been used to understand the hot deformation behavior of two-phase alloys like α–β brass [20]and Ti–24Al–11Nb titanium aluminide [21]. The importance of the constitutive behavior of the individual phases constituting the two-phase material has been brought out in these studies.