Grain Cluster Microstructure and Grain Boundary Character Distribution in Alloy 690

Abstract

The effects of thermal-mechanical processing (TMP) on microstructure evolution during recrystallization and grain boundary character distribution (GBCD) in aged Alloy 690 were investigated by the electron backscatter diffraction (EBSD) technique and optical microscopy. The original grain boundaries of the deformed microstructure did not play an important role in the manipulation of the proportion of the R3 n (n = 1, 2, 3…) type boundaries. Instead, the grain cluster formed by multiple twinning starting from a single nucleus during recrystallization was the key microstructural feature affecting the GBCD. All of the grains in this kind of cluster had R3 n mutual misorientations regardless of whether they were adjacent. A large grain cluster containing 91 grains was found in the sample after a small-strain (5 pct) and a high-temperature (1100°C) recrystallization anneal, and twin relationships up to the ninth generation (R3 9 ) were found in this cluster. The ratio of cluster size over grain size (including all types of boundaries as defining individual grains) dictated the proportion of R3 n boundaries.

Grain orientation statistics of grain-clusters and the propensity of multiple-twinning during grain boundary engineering

“…the iterative process of twinning operations starting from a single nucleus during recrystallization as discussed in Ref. [10-12, 29, 30], so all the inner grains of a cluster are twins or twins of twins and so on of the nucleus (parent grain), and they could be reconstructed as a tree-shape twin-chain [11,24]. On the other hand, the process of multiple-twinning can be demonstrated as a twin-chain of orientations [11,[24][25][26], as shown in Fig.…”

“…4a. It was reconstructed on the basis of the rule that one orientation has four twinning-orientations at most [11,25]. In the kth generation there are N k =4×3 k-1 orientations at most [23,25].…”

“…The proportion of twin-boundaries could be increased to more than 60% from about 40% by the treatment of GB-engineering [11,12,[17][18][19][20][21], because the face-centered-cubic law stacking-fault-energy metals, such as Ni-based alloys, austenitic stainless steel and brass, are prone to twinning [1,2] in the wake of migrating of the recrystallization front boundaries during annealing. The iterative process of twinning operations starting from a single nucleus is referred to as multiple-twinning [21][22][23][24][25], and the generated annealing twins can be reconstructed as a twinning tree, which is referred to as twin-chain [8,11,22,26,27]. The assembly of all grains in the tree is named grain-cluster or twin-related-domain [8,11,12,24,26].…”

“…The iterative process of twinning operations starting from a single nucleus is referred to as multiple-twinning [21][22][23][24][25], and the generated annealing twins can be reconstructed as a twinning tree, which is referred to as twin-chain [8,11,22,26,27]. The assembly of all grains in the tree is named grain-cluster or twin-related-domain [8,11,12,24,26]. All inner grains of the grain-cluster have ∑3 n (n=1, 2, 3…) mutual misorientations, and all inner boundaries are types of ∑3 n but outer boundaries are crystallographically random.…”

The dependence of carbide morphology on grain boundary character in the highly twinned Alloy 690

“…This TMP not only increased the proportion of twin related low RCSL grain boundaries to more than 70%, but also resulted in a solution annealed state. The mechanism involved in the GBE was discussed by Xia et al elsewhere [26,27]. The aging treatments were carried out at 715°C for different time to cause carbide precipitation at grain boundaries.…”