Elsevier

Acta Materialia

Volume 50, Issue 10, 12 June 2002, Pages 2599-2612
Acta Materialia

Microstructural evolution during grain boundary engineering of low to medium stacking fault energy fcc materials

https://doi.org/10.1016/S1359-6454(02)00090-3Get rights and content

Abstract

Grain boundary engineering comprises processes by which the relative fractions of so-called special and random grain boundaries in microstructures are manipulated with the objective of improving materials properties such as corrosion, creep resistance, and weldability. One such process also referred to as sequential thermomechanical processing (TMP), consists of moderate strains followed by annealing at relatively high temperatures for short periods of time. These thermomechanical treatments on fcc metals and alloys with low to medium stacking fault energies result in microstructures with high fractions of Σ3n and other special boundaries, as defined by the coincidence site lattice (CSL) model. More importantly, the interconnected networks of random boundaries are significantly modified as a consequence of the processing. The modifications in the grain boundary network have been correlated with post-mortem electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) observations of the deformed and annealed states of the material. The evolution of the microstructure to a high fraction of Σ3n boundaries is correlated with the decomposition or dissociation of immobile boundaries during annealing. This is evidenced by TEM observations of the decomposition of relatively immobile boundaries into two components, one with very low energy and thus immobile, and the other a highly mobile boundary that migrates into neighboring areas of higher strain levels. The formation of low-energy grain boundaries through this mechanism and its effect on boundary network topology is discussed within the context of grain boundary engineering and linked to known microstructural evolution mechanisms.

Introduction

Efforts to engineer the grain boundary topology have resulted in increasing the fraction of boundaries in the microstructure that exhibit specific disorientations characterized by the coincident site lattice (CSL) model [1], [2]. Two different thermomechanical processing (TMP) approaches have been reported. One approach has been to deform the material to strains that are on the order of 6–8% followed by annealing at temperatures low enough to prevent recrystallization yet cause reorientation of grain boundaries toward lower energy configurations, i.e., low Σ boundaries [3], [4]. The second approach employs a multi-cycle treatment of moderate strain levels (5–30%) with annealing treatments at relatively high temperatures but for very short times, which are typically on the order of 5–30 min [5]. In the latter, the total forming reduction is broken up into several cycles of strain and annealing. The observed modifications in the microstructure result from the development of a high fraction of special grain boundaries (1<Σ≤29) whose spatial distribution interrupts the connectivity of the random boundary network [6], [7]. It is the modification of the network that is believed to be responsible for the property improvements.

In contrast, the annealed microstructure resulting from single-step deformation of magnitude on the order of the cumulative strain from the multi-step treatment does not show such dramatic differences in random grain boundary connectivity and fraction of Σ3n boundaries when compared to the as-received condition [6]. It is well known that a metallic alloy after undergoing deformations on the order of 60–70% (or higher) undergoes primary recrystallization within a short period of time for thermal treatments above half the homologous temperature (0.5 Tm).

There is a significant body of work in the literature on recrystallization and grain growth but the current understanding of the evolution of the microstructure and the mechanism(s) responsible for the increase in the grain boundary character distribution (GBCD) after several moderate strain and annealing cycles remains rather empirical in nature. For instance, the conventional view of grain boundary engineering links property improvements to an increase in the fraction of special boundaries. This description ignores the fundamental nature of polycrystalline microstructures that grain boundaries exist in a network in the microstructure and the topology is constrained by crystallography at the triple junctions.

The challenge of introducing low energy (low Σ) boundaries into a microstructure is significant. For instance, even the introduction of a single low Σ boundary into a simulation of 2D array of grains requires the rotation of an entire grain, and as a consequence, re-establishment of the five other boundaries, and the energetics of the system have also to decrease in the process. Based on this simulation, Miodownik [8] came to the conclusion that the severe constraints imposed by the network can be overcome by the nucleation of new grains, i.e., recrystallization, or the formation of low energy Σ3 boundaries during the migration of existing boundaries as originally described by Fullman and Fisher [9] for the case of grain growth. Introduction of low energy Σ3 boundaries into the boundary network conserves the crystallographic constraints at triple junctions and results in an increase in those triple junctions whose constituents are Σ3 and its variants (Σ3n) such as Σ9 and Σ27. Such triple junctions are observed [7] to be instrumental in breaking up the connectivity of the high-angle, high-energy random grain boundary networks during the process of grain boundary engineering.

In this communication, transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) techniques are employed to establish a case for the arguments given above. Although new mechanisms for recrystallization and grain growth are not revealed, the role of microstructural evolution mechanisms both during deformation and annealing toward the development of a grain boundary network in the microstructure is established. Hence, the objective is to characterize the evolutionary states of the microstructure as the sequential processing methodology is applied.

Section snippets

Materials processing and characterization

The effects of sequential TMP on the grain boundary character and triple junction distributions have been examined in several low to medium stacking fault energy (SFE) fcc metals and alloys such as oxygen-free electronic (ofe) Cu and the commercial Ni-base alloys Inconel 600, Inconel 690, and Hastelloy C-22 that are used in the fcc-based solid solution condition. Results from ofe-Cu and Inconel 600 have been reported by the present authors elsewhere [6], [7], [10], and are similar to the

Microstructures during grain boundary engineering

The importance of annealing twinning in the ‘optimization’ of the microstructure in low to medium SFE fcc materials is underscored by the influence of twin boundaries on the spatial distribution of triple junctions [10]. As can be seen from Fig. 1, there is a rather dramatic difference in the microstructures that developed in ofe-Cu as a consequence of the sequential processing when compared with conventional processing. The effect of using different processing routes on the microstructure,

Summary

Grain boundary engineering is the manipulation of relative fractions of special boundaries to improve materials properties. There have been many reports in the literature linking processing with the improvements in properties, but none as yet on the details of how these special boundaries appear in the microstructure. The main objective of the current paper is thus to contrast the evolution of the microstructure during the sequential TMP followed in grain boundary engineering with that observed

Acknowledgements

The assistance of Ms Lan Nguyen in sample preparation and metallography and the discussions with Dr Christopher Schuh are gratefully acknowledged. This work was performed under the auspices of the US Department of Energy by the University of California, Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48.

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