Mechanical properties of Ti-6Al-4V/TiB composites with randomly oriented and aligned TiB reinforcements
Introduction
A great deal of progress has been made in the past decade in the technology and application of discontinuously reinforced titanium (DRTi). While a number of ceramic reinforcements have been considered, TiB has been identified as possessing the most appropriate balance of thermochemical stability, good mechanical properties, and thermal expansion [1]. Studies of solidification pathways [2], reaction kinetics [3], [4], [5], microstructures [6], [7], [8], [9], [10], [11], [12] and mechanical properties [1], [6], [7], [9], [10], [13] have been reported for Ti-alloy matrices reinforced with TiB. Processing approaches have included self-propagating synthesis [14], [15], solidification [2], [14], mechanical alloying [9], blended elemental powder metallurgy [1], [12], and prealloyed powder metallurgy [16]. The successful implementation of DRTi in automotive intake and exhaust valve applications [10], [12] emphasizes the progress in this class of discontinuously reinforced metals. However, while the ability to process and produce cost-competitive DRTi automotive valves is truly a significant accomplishment, it is not likely that the material currently used in the automotive market is appropriate for fracture-critical aerospace applications. A more stringent balance of critical properties requires an expanded understanding of constituent properties and the relationships between microstructure and properties. This understanding can provide the foundation for an effective materials development activity, and will enable prediction of material response in complex thermo-mechanical loading histories.
There are a number of important deficiencies in the current understanding of DRTi reinforced with TiB. Single-phase bulk samples of TiB are difficult to produce, and so the properties of TiB are available only by analysis of two-phase materials. The limited reports available [11], [13], [17], [18] show significant scatter, and the origins of this scatter are not understood. Among the possible explanations, anisotropic elastic properties have not been addressed to date. TiB crystallizes with an FeB-type structure with an orthorhombic structure, characterized by zigzag chains of boron atoms parallel to the [010] direction [19]. This strong anisotropy in crystal structure may be responsible for the observed preferred growth directions and low energy crystal faces [20]. This crystallographic anisotropy may also produce elastic properties that are significantly different along different crystallographic directions.
Anisotropic elastic properties would lead to a strong influence of particle orientation, which is typically neither controlled nor measured in DRTi. While isotropic properties are usually desired in an engineering structural material, several critical applications place a premium on obtaining the highest specific properties in only one direction. A random whisker orientation can be produced by a number of processing routes to provide isotropic properties, while strong whisker alignment through processes such as rolling or extrusion offers the possibility of controlled anisotropic properties. Controlled degrees of isotropy of elastic and plastic composite properties through control of the microstructure have not previously been explored.
Increasing the volume fraction, Vf, of ceramic reinforcements in metal matrix composites (MMCs) increases both the specific strength and specific stiffness, providing a more structurally efficient material. However, fracture properties, such as ductility and toughness, generally decrease with increasing Vf. In discontinuously-reinforced Al (DRA), a current upper limit of Vf ~ 0.25 exists for fracture-critical materials. However, it is not clear that the same limit will exist for DRTi materials. DRA typically employs age-hardenable matrices that generally have fracture toughness values of 25–45 MPa√m and tensile ductilities from 11–13% in the peak aged condition. The tensile ductilities of candidate Ti matrix alloys are similar to those of age-hardened Al alloys, but the fracture toughness values are much higher, typically 50–110 MPa√m. Thus, TiB volume fractions of >25% may be possible, and the upper limit of Vf achievable in DRTi, while retaining adequate fracture properties, warrants investigation.
In this paper we explore a range of responses possible in the DRTi system. The matrix selected is Ti-6Al-4V (all compositions are provided in wt %), and TiB reinforcement volume fraction. Vf.TiB, of 20 and 40% are used. Whisker orientation is varied to compare the properties in aligned, one-dimensional arrays (1D) and an isotropic, random three-dimensional array (3D) of TiB whiskers. Longitudinal and transverse properties are measured for the 1D material to explore the possibility of anisotropy of the TiB whiskers. The tensile mechanical properties are measured at room temperature (RT) and at 300 °C. The elastic properties are also measured at RT by resonant ultrasound spectroscopy (RUS). The effect of the heat treatment on the microstructure of the composite and the morphology of the TiB discontinuous reinforcements will be established, and the influence on mechanical properties will be provided.
Section snippets
Experimental
Ti-6Al-4V/TiB composites were produced by a powder metallurgy route. Commercially available atomized Ti powder (average particle size 30 μm), TiB2 powder (average particle size 14 μm) and Al-40V master alloy powder (maximum particle size 44 μm) were used. The chemical compositions of these powders are shown in Table 1, and the size distributions of Ti and TiB2 powders are shown in Fig. 1. The powder was blended in a planetary blender for 24 h, in the presence of Al2O3 balls to break down powder
Microstructure:
All blind die compacted and extruded composites exhibited oxygen and nitrogen contamination levels, respectively, of 0.25 and 0.013 wt.%, similar to the contamination levels of the source materials (Table 1, Table 2). Thus, the increase in interstitial contamination after processing was minimal, despite the high temperature processing used.
The microstructure of the HIP’ed unreinforced Ti-6Al-4V matrix consists of acicular α colonies with intergranular β-phase (Fig. 2a), which is typical since
Conclusions
Ti-6Al-4V/TiB composites were fabricated using a powder metallurgy route. Annealing studies of HIP’ed materials showed that increasing the anneal temperature accelerated the kinetics of the transformation of TiB2 particles to the stable TiB phase. Increasing the anneal temperature or duration led to more complete transformation, the formation of larger TiB whiskers, and a nearly complete elimination of densely-packed, fine TiB whisker aggregates which occurred in the vicinity of previous TiB2
Acknowledgements
The authors would like to thank C. Riviello for advice and comments during the course of this work. Gratitude is also expressed to D. McEldowney and B.V.R. Bhat for help with the resonant ultrasound spectroscopy. S.G. would like to acknowledge support of this research by the Air Force Research Laboratory under Contract No. F33615-96-C-5258 and by the French Minister of foreign affairs under the Lavoisier fellowship.
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