Formation of TiBwreinforcement in in-situ titanium matrix composites

Abstract

Titanium matrix composites (TMCs) are being used widely in aerospace industries because of their excellent mechanical properties at room temperature and elevated temperature [1–5]. But their mechanical properties vary with the species and shape as well as size of reinforcement. Traditionally, TMCs are prepared by powder technology [6, 7] or liquid metallurgy [8], where ceramic particles are directly incorporated into solid or liquid matrixes. In recent years, TMCs are fabricated by in-situ techniques [9–11], where reinforcements form through liquid reactions. TMCs fabricated by in-situ techniques have better properties because they overcome the shortcomings of traditional techniques. Reactive sintering [12, 13] is one of in-situ techniques fabricating titanium matrix composites. In this process, exothermic reactions between powders facilitate in-situ formation of reinforcements. Advantages of reactive sintering of composites are able to obtain small-sized reinforcements over a wide range of volume fractions, together with clean and thermodynamic stable reinforcement-matrix interfaces. Ceramic particles, such as carbide, boride and nitride are used to reinforcements of titanium matrix composites. TiB is particularly suitable for as reinforcing phases in Ti-based reactive sintered composites because of its high exothermicity of formation and thermodynamic stability in Ti. Although there have been much efforts [14–16] directed now to the study on in-situ formation of reinforcements in titanium matrix composites, there still is a lack of understanding of reaction mechanism, which hinders the optimization of Ti-TiB composite fabrication, including controlling the composition and the microstructure. Therefore the study on the formation of reinforcement is essentially for the attainment of excellent mechanical properties of composites. It is found [17] that in-situ synthesized TiB whiskers by adding of Al content into Ti-B become finer than pure Ti-B. Subsequently, the microstructure evolution [18] during in-situ forming TiB whiskers reinforced Ti matrix composite is studied. But the formation mechanism of TiB whiskers in Ti-B-Al system has not been clearly understood. In the present paper, the reinforcement’s formation mechanisms and their phase formation sequence of the reactive sintering of Ti-TiB composites from elemental powders are studied. DTA is used to investigate thermal effects of the process and a combination of DTA and XRD is used to investigate the formation of in-situ TiB phase. The material used in the present investigation is the mixture of Ti, B, and Al powders, and its composition is Ti-45 vol%TiB, which corresponds to a mixture of 88.3 m%Ti, 7.5 m%B, and 4.2 m%Al. These powders were blended and compressed into pellets. Samples were taken from the interior of the pellets. In the DTA experiments, samples were heated up to 1500 ◦C at a linear heating rate of 10 ◦C/min in a protective argon atmosphere. The in-situ TiB phase formation sequence was investigated with XRD analysis of intermediate reaction products. The intermediate stages in the reactive sintering process were identified in the DTA experiments. Samples for XRD analysis of the intermediate stages in the reactive process were prepared in the DSC equipment, so as to ensure that the reaction was halted at the desired stage. Then samples were polished and etched, the microstructures corresponding to intermediate stages were observed with SEM. The DTA curve for Ti-B-Al system is shown in Fig. 1. It can be seen clearly that multiple reactions happen while this system is being heated. These reactions include one endothermic reaction and two exothermic reactions. These reactions are indicated by “A”, “B” and “C” in Fig. 1, respectively. In Fig. 1, the temperature corresponding to the endothermic reaction, indicated by “B”, is about 665 ◦C, which is almost melting point of Al. Before Al is melted, there is a small exothermic reaction, marked with “A”, which is attributed to solid-solid surface diffusion reaction between Al and Ti powders. After Al is melted, the temperature corresponding to another exothermic reaction, “C” is 710 ◦C. It closely follows melting point. From the shape of reaction peak in DTA curve, it is known that this reaction releases a great deal of heat. Considering possible reactions in Ti-B-Al, it is considered as TiB formation, because free enthalpy ( H ) of the reaction between Ti and B is up to 231– 252 kJ/mol [19]. The next step in the investigation is to find out the influence of intermediate reactions on TiB formation and the reaction sequence in the Ti-B-Al system. On the basis of the DTA results of Fig. 1, samples were taken at intermediate stages of the reactive sintering process, that is, at 560 ◦C, reaction “A”; at 680 ◦C, after “B”; at 710 ◦C, reaction “C”; 750 ◦C, after “C”; and at 1500 ◦C. They are marked with “ ” in Fig. 1. Fig. 2a–e show the corresponding XRD patterns. It is seen from Fig. 2a that there exists TiAl3, TiAl and Ti phases at 560 ◦C, this temperature is lower than the melting point of Al, so Ti, B and Al powders are in pure solid state. At this temperature, the solid solubility