Quantitative analysis on boundary sliding and its accommodation mode during superplastic deformation of two-phase Ti-6Al
- PDF / 491,217 Bytes
- 10 Pages / 612 x 792 pts (letter) Page_size
- 49 Downloads / 215 Views
I.
INTRODUCTION
SINCE manufacturing costs and the weight of airframe structures can be reduced significantly through superplastic forming technology,[1] considerable efforts have been devoted during the past three decades to the investigation of the superplastic deformation behavior of various metallic alloys, especially the Ti-6Al-4V alloy. As a result, the role of several microstructural features such as grain size,[2] grain size distribution,[3] grain growth,[4] texture,[5] and volume fraction of the component phase[6–9] have been well established in relation to the actual forming process of the Ti-6Al-4V alloy. Nowadays, many components of airframe structures are being produced by superplastic forming of the Ti-6Al-4V alloy, and its commercial applications are expected to increase.[10] In contrast to the commercial success of the superplasticity of Ti-6Al-4V alloys, a satisfactory explanation of the mechanism for this deformation phenomenon is still not available. Although many investigations have been carried out on the mechanisms of superplastic deformation of two-phase alloys,[11,12,13] there remain two major, vague points. First, in the case of two-phase alloys like Ti-6Al4V, boundary slidings take place at the grain boundary and the phase boundary concurrently. While several efforts have been made to understand the resistance of the boundary slidings either by measuring the sliding displacement of the surface[14,15] or the flow stress of the bicrystal,[16,17] relative sliding resistance between the grain boundary and the phase boundary have not been quantified yet. Second, there has been an evident controversy over the deformation modes[7,8,9] of accommodation, which can be classified as
the isostress mode and the isostrain-rate mode. It is important to determine the deformation modes in relation to the a/b volume ratio and the grain size. In order to clarify the aforementioned points, more fundamental analysis considering the physical characteristics of superplastic deformation has to be made. The present article, therefore, attempts to analyze the superplastic deformation behavior of the Ti-6Al-4V alloy based on the inelastic deformation theory[18] in two aspects: one is to investigate the relative sliding characteristics at each type of boundary and the other is to verify the deformation mode of accommodation. For this purpose, the flow characteristics of the Ti-6Al-4V alloy with respect to the variation of a/b volume fraction are studied by load relaxation tests. In the following section, an inelastic deformation theory,[18] which consists of two mechanisms, i.e., grain matrix deformation (GMD) and grain boundary sliding (GBS), is briefly described. II.
INELASTIC DEFORMATION THEORY
A simple rheological and topological model for GMD and phase/GBS (P/GBS) has been proposed by Chang and Aifantis[18] and is schematically shown in Figure 1. The model represents that P/GBS is mainly accommodated by a dislocation process giving rise to an internal strain rate (a˚) and plastic strain rate (az ). A more de
Data Loading...
