Ultrahigh Aspect Ratio TiB Nanowhisker-Reinforced Titanium Matrix Composites as Lightweight and Low-Cost Replacements for Superalloys

Abstract

With the increase in demand for high speed and efficient transport, there is a growing call for lightweight materials with high specific properties. Titanium matrix composites (TMCs) are one material of growing interest, whose properties make them a competitive replacement for many superalloys used in the aerospace industry. However, high cost, poor ductility, and the lack of a reliable manufacturing process have limited their application. In this study, we demonstrate the fabrication of high-performance titanium matrix composites reinforced with a network of high aspect ratio TiB nanowhiskers to overcome these challenges. A unique combination of fine BN nanopowder and a low-cost sintering method was employed. Ultralong, 40–50 μm TiB nanowhiskers with an aspect ratio of 500 have been produced from the in situ reaction between Ti and BN during slow heating/cooling, with temperatures at and below 1100 °C. As-sintered composites reached a hardness of 8.52 GPa, a reduced Young’s modulus of 152 GPa, and a yield strength of 1236.5 MPa, a 250% increase when compared to pure titanium as measured by nanoindentation. The improved mechanical properties were attributed to the ultrahigh aspect ratio of TiB, the low titanium grain size, and the near continuous network of TiB reinforcement. Slow heating/cooling allows for control of TiB formation to achieve the maximum aspect ratio as well as complete conversion of BN to TiB. In addition, the TiB network in the grain boundary acted to restrict Ti grain growth during manufacturing and improve the overall TMC properties. The proposed manufacturing method has great potential for the low-cost production of high performance TMCs.

TiB Nanowhisker Reinforced Titanium Matrix Composite with Improved Hardness for Biomedical Applications

“…This particular nanowhisker had a length and diameter of approximately 18.4 µm and 47 nm, respectively, which was measured along with several others from each set of sintering conditions to obtain the average aspect ratios of the nanowhiskers in each sample. As shown in our previous study these whiskers are TiB [ 15 ]. However, to analyse the effect of BN concentration on the chemical nature of the samples, XRD was conducted on a CP-Ti + 10% BN powder sample and then the same powder after sintering at 1150 °C for 6 h with subsequent surface etching ( Figure 2 b).…”

“…Figure 1 b is a low magnification SEM image showing the spherical CP-Ti powders used in this work. Figure 1 c,d are secondary electron (SE) and back-scatter electron (BSE) images, respectively, showing the powders after mixing for 4 h in a low energy Turbula shaker mixer with a 3:1 ratio of steel balls [ 15 ]. Figure 1 c shows the fine BN powders are well distributed across the surface of the CP-Ti and similarly in Figure 1 d, where dark patches are indicative of BN.…”

“…One batch of samples was produced with 5 vol% BN and another with 10 vol% BN. Powders were then mixed, compacted and vacuum sintered using the same method as previously reported [ 15 ]. Samples were sintered at peak temperatures ranging from 1050 to 1200 °C with the holding times ranging from 2 to 6 h to facilitate the in situ reaction between Ti and BN for the formation of TiB.…”

“…Nanoindentation was chosen in favour of tensile tests because local property measurement facilitates the study of TiB size and microstructural effects, without being affected by other sample variables present in sintered samples, such as porosity. A 12 mN load was chosen as tests conducted with this load in a previous work gave mechanical properties on CP-Ti comparable to that obtained from tensile tests [ 15 ]. Oliver–Pharr (OP) analysis was used to determine hardness (H) and reduced elastic modulus ( E r ) from the load (P)—depth (h) curve.…”

“…Titanium matrix composites (TMCs) have many of the desirable properties for bone implants, with significantly improved hardness and wear properties when compared to CP-Ti. By reinforcing with stiff and strong ceramic particles or fibres, Ti has shown significantly improved strength, hardness and wear resistance [ 13 , 14 , 15 , 16 , 17 , 18 , 19 ]. In addition, many of the most promising reinforcements (Ti x B, TiC and Ti x N) can be manufactured ‘in situ’ meaning that the reinforcement phase is formed during processing (typically facilitated by a reaction at high temperature), which reduces overall process steps and leads to strong bonding at the reinforcement interface [ 20 , 21 , 22 ].…”