Five-parameter grain boundary distribution of commercially grain boundary engineered nickel and copper
Introduction
Grain boundary engineering (GBE) is the deliberate manipulation of grain boundary structure in order to improve material properties such as corrosion resistance, intergranular cracking or ductility. GBE exploits prolific annealing twinning in low stacking-fault energy metals and alloys. Recently GBE in this class of materials has been reviewed [1] and discussed [2] in detail. With reference to the coincidence site lattice (CSL) nomenclature, twins are one type of Σ3 boundary. A commercial grain boundary engineered material is required to have more than 50% Σ3 boundaries (as a fraction of the total number of boundaries) in the interface population [3].
The profuse annealing twinning associated with GBE gives rise to “multiple twinning”, whereby two Σ3 boundaries meet at a triple junction and produce a Σ9 boundary, which can be expressed as Σ3 + Σ3 → Σ9. Higher order Σ3n boundaries are similarly generated, e.g., Σ3 + Σ9 → Σ27. Because so many Σ3s exist in the microstructure, these reactions also happen in reverse, e.g., Σ3 + Σ9 → Σ3. The generalized expression of this concept is the “Σ3 regeneration mechanism”, which states that Σ3n + Σ3n+1 → Σ3, is observed more frequently at triple junctions than Σ3n + Σ3n+1 → Σ3n+2 [4]. The Σ3 + Σ9 reaction can of course also result in a Σ27 [5]. Increasingly, cluster statistics derived from percolation theory are also being used to add value to descriptions of GBE processing (e.g., [6]).
The central feature of the regeneration mechanism model is that the new Σ3s are essentially not annealing twins and are therefore not on {1 1 1} interface planes. Rather, they could occupy a variety of boundary planes. Σ3 boundary planes other than {1 1 1} are highly mobile and would therefore promote further encounters with other Σ3s, hence perpetuating Σ3 regeneration. The mobile Σ3s in the grain boundary engineered specimen would become directly incorporated in the grain boundary network and break up the remaining random boundaries, hence leading to the improved intergranular properties associated with grain boundary engineered metals. To date, the Σ3 regeneration model has not been fully validated by measurement of interface planes in an appropriately grain boundary engineered specimen. Although there are many reports of misorientation statistics associated with grain boundary and GBE investigations, measurements of boundary planes are far more sparse. Some previous investigations on the distribution of Σ3 boundary planes in copper and nickel have shown that the population comprises a high proportion of planes vicinal to {1 1 1} on the 〈0 1 1〉 zone (such as {23, 17, 17}/{7 7 5}) and other asymmetric tilt boundaries on the 〈1 1 0〉 zone, as well as coherent twins on {1 1 1} [7]. These results accord well with data to show the energy of Σ3 misorientations as a function of deviation from {1 1 1} [8].
Recently, a new stereological technique for measuring the five-parameter grain boundary distribution (FPGBD), which is described in detail elsewhere [9], has been developed to provide statistical data on the distribution of boundary planes in addition to misorientations. Results from analyses using the five-parameter technique, for example on magnesium oxide and on aluminium, have contributed to a growing body of evidence which suggests that special behaviour at grain boundaries is related to low-index planes rather than particular misorientations (e.g., [10], [11]). A selection of other studies on the relationship between the boundary plane and boundary properties is summarized elsewhere [12].
In the present study, we measure the FPGBD of commercially grain boundary engineered copper and nickel [13]. We also assess the intergranular corrosion of both grain boundary engineered and control specimens. Elsewhere there is an increasing number of cases in the recent open literature of property benefits brought about by GBE processing. For example, a direct correlation has been shown between intergranular stress corrosion cracking and proportion of CSL boundaries in austenitic alloys [14], and GBE processing has been used to improve the thermal stability of Inconel alloy 617 [15]. The data here will be interpreted with particular emphasis on the distribution of boundary planes present and its consequences for our understanding of GBE.
Section snippets
Experimental methods
All the specimens were obtained from Integran Technologies, Inc. The nickel samples were provided in a reference state (before GBE) and in a final state following the GBE process. The two samples will be referred to as non-GBE nickel and GBE nickel, respectively. The samples were prepared by first grinding the surface with SiC and diamond abrasives, then polishing using a vibratory chemomechanical process with a 0.05 μm SiO2 slurry, and finally electropolishing in a chilled 9:1
Results
The proportions of Σ3, Σ9 and Σ27 boundaries in the non-GBE and the GBE nickel are shown in Fig. 1. For ease of reference throughout this section, we will describe first the data from nickel (non-GBE and GBE), and afterwards compare it with that from copper. There is a large increase in the Σ3 and Σ9 proportion as a consequence of GBE processing: 63% of interface length is Σ3, compared to 43% before GBE. A substantial increase in Σ9s from 1% to 6.9% by length has accompanied the Σ3 increase,
Discussion
In both copper and nickel the most common plane present is the plane with lowest surface energy, {1 1 1} (Fig. 3). This tendency for boundaries to terminate on low surface energy planes, which assumes that frequency of occurrence is inversely related to energy, has been previously observed in materials having a variety of crystal structures [10], [11]. Such surfaces have low indices. For face-centred cubic (fcc) materials, the dominance of {1 1 1} boundary planes has been previously recorded in
Conclusions
An analysis of the FPGBD in GBE nickel and copper has been carried out. The main conclusions of the work are:
- 1.
The fractional area of {1 1 1} planes after GBE was roughly constant when the entire population was considered, but decreased in the population of non-Σ3 boundaries. This decrease shows that most of the newly generated Σ3 population was not coherent annealing twins, because these would have been on {1 1 1}. Rather, they have been formed by regeneration events.
- 2.
There was a larger proportion of
Acknowledgements
The work at Carnegie Mellon University was supported primarily by the MRSEC program of the National Science Foundation under Award Number DMR-0520425. The work at Swansea was partially supported by the Engineering and Physical Sciences Research Council.
References (20)
Acta Mater
(2004)Acta Mater
(1999)- et al.
Scripta Mater
(2006) - et al.
Acta Mater
(2003) - et al.
Acta Mater
(2004) - et al.
J Nucl Mater
(2007) Acta Metall Mater
(1990)- Viewpoint set no. 40, Scripta Mater Grain Boundary Engineering. In: Kumar M, Schuh CA, editors. vol. 54; 2006. p....
- Palumbo P. International Patent Classification C21D 8/00 8/10 C22F 1/10 1/08, no. WO 94/14986;...
- et al.
Scripta Mater
(2006)