Abstract
The dependence of the grain boundary character distribution for a Cu-4 at. % Ti polycrystal alloy (average grain size: 100 µm) on the nucleation of cellular discontinuous precipitates was systematically investigated. In an alloy over-aged at 723 K, cellular discontinuous precipitates consisted of a terminal Cu solid solution and a stable β-Cu4Ti lamellae nucleated at grain boundaries. Electron backscatter diffraction analysis revealed that the discontinuous precipitation reaction preferentially occurred at random grain boundaries with a Σ value of more than 21 according to the coincidence site lattice theory. On the other hand, few cellular discontinuous precipitates nucleated at low-angle and low-Σ boundaries, particularly twin (Σ 3) boundaries. These findings suggest that the nucleation of discontinuous precipitates is closely correlated with grain boundary character and structure, and hence energy and/or diffusibility. It should therefore be possible to suppress the discontinuous precipitation reaction through control of the alloy’s grain boundary energy, by means of texture control and third elemental addition.
Highlights
Age-hardened Cu-Ti alloys have attracted interest for their application in miniature electrical components, such as connectors and lead frames, because of their excellent mechanical properties and electrical conductivity [1,2,3,4,5,6]
grain boundaries (GBs) with misorientation angles greater than 15◦, we found suppressed at low-energy GBs
Quantitative evaluation of the GB energy at the misorientation angles and Σ values in the Cu-Ti alloy system is required in order to exactly comprehend the influence of GB energy on discontinuous precipitates (DPs) nucleation probability, which will be reported in future studies
Summary
Age-hardened Cu-Ti alloys have attracted interest for their application in miniature electrical components, such as connectors and lead frames, because of their excellent mechanical properties and electrical conductivity [1,2,3,4,5,6]. Such coarsened DPs eventually lead to serious deterioration of the mechanical and physical properties of the alloys, as seen in various commercial alloys based on Al, Cu, Co, Fe, Ni, and Pb [20,21,22,23,24,25,26] Many investigations on this phenomenon have been previously conducted, and the nucleation and growth of DPs has been examined [27,28,29,30]. The influence of the GB character on the nucleation of DPs in a commercial age-hardenable Cu-Ti alloy was experimentally investigated. For this purpose, we utilized electron backscatter diffraction (EBSD) to characterize the GB structure of specimens before aging. By integrating a number of ‘same region observations’ by EBSD and FESEM on specimens both before and after nucleation of the DPs, the relationship between the DP nucleation sites and the character of the GBs was statistically determined and discussed in terms of the GB energy and diffusivity
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