Microstructural aspects of titanium metal matrix composites in consolidation processing
Introduction
Titanium-based metal matrix composites (MMCs) are increasingly attractive for a range of applications such as aerospace, transportation, and many others also due to their high strength, stiffness, and creep resistance especially at high temperatures [1], [2]. A number of fabricating techniques for MMCs have been developed including liquid and solid state processes. The Ti-based composites, however, are generally consolidated in the solid state since the nature of titanium plays an important role during the process [3].
In the present work, vacuum hot pressing (VHP) has been used for the development of Ti-MMCs using foil–fiber–foil (FFF) method, and in particular heterogeneous microstructures together with their evolution during the process are investigated by means of mechanism-based analysis. As detailed in Refs. [2], [3], the FFF process is a common solid state adhesive technique under an adequate pressure used to join a matrix foil and fiber arrays at an elevated temperature. During the consolidation, densification occurs by several mechanisms including plastic flow, diffusion, and power-law creep, and further finished products depend on many processing conditions such as applied pressure, temperature, together with geometrical factors (i.e., foil thickness, fiber arrays and volume fractions of fiber and matrix) [4], [5]. It is known that either increasing pressure or temperature leads to increasing densification rate, but the application of pressure contributes to the densification rate much more than temperature [4]. In addition, microstructural changes occur inevitably during the process result from either recrystallization or grain growth. Microstructural features in Ti–6Al–4V such as grain size, distribution of grain size, and crystallographic texture have been investigated by several authors [6], [7], and it has been found that increasing grain size increases the flow stress, and tends to reduce the maximum strain rate sensitivity. In two-phase Ti–6Al–4V alloy, both α and β phases undergo grain growth at an elevated temperature, in which α grains are generally isolated within the β matrix. Under certain conditions of hot working, recrystallization of microstructure also may occur, and it is usually found to be an important grain size control mechanism [8].
Knowledge of both fiber arrangements and microstructural evolution are essential in the design and optimization of composites since materials failure may occur by the applied conditions and geometrical factors coupled with subsequent non-uniform microstructures. The motivation for the present paper is therefore that heterogeneous microstructures are important in controlling densification behavior, and hence the achievable high quality products. The evolutions of heterogeneous microstructures of the materials have been investigated, and two types of possible mechanisms including grain growth kinetics and dynamic recrystallization have been analyzed according to the levels of consolidation.
Section snippets
Experimental procedure
The composites developed in this work consist of Ti–6Al–4V foil and SiC fiber as the matrix alloy, and the reinforcement, respectively. The matrix alloy produced by chemical milling processes, supplied by RMI Titanium. The fiber used in this work was Textron SCS-6 type with an average diameter of 140 μm. The specifications of SiC fiber and Ti–6Al–4V foil used for the tests are shown in Table 1. In order to investigate the densification behavior with heterogeneous microstructures of matrix
Results and discussion
Experiments have been conducted on the SiC/Ti–6Al–4V preforms using the foil–fiber–foil method and subsequent vacuum hot pressing. The effects of processing conditions on the consolidation have been investigated, together with the microstructural evolutions of the matrix materials. The microstructural features have been analyzed using optical and electron microscopy, and particularly digital processing techniques used to determine distributions of α and β grain sizes [9]. The initial
Summary
Experiments have been conducted on the SiC/Ti–6Al–4V preforms using the foil–fiber–foil method and subsequent vacuum hot pressing. The evolutions of heterogeneous microstructures of the materials have been investigated, and mechanism based microstructural changes have been analyzed through the levels of consolidation. The main results and conclusions are summarized as follows.
As the consolidation proceeds, microstructural evolutions are incorporated in the changes with heterogeneous grain size
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