Synthesis and Characterization of TiAl‐Based Nanocomposites by a Melt Electromagnetic Stirring Process

Abstract

Ceramic nanoparticles were considered difficult to be dispersed well in liquid metal, especially in nonreactive systems, by stir mixing methods due to their high specific surface energy. In this paper, an electromagnetic stirring process was exploited to disperse ceramic nanoparticles in an active TiAl alloy melt. The results indicate that finely dispersed nanoparticles can be obtained by stir mixing with the aid of reactive wetting. The presence of nanoparticles during solidification can refine the primary dendrites dramatically, which was primarily determined by the extent of particle pushing at the liquid-solid interface. Among the studied particles, TiC can refine the (g þ a 2 ) lamellar colonies, which is attributed to the dissolution of carbon and subsequent precipitation of Ti 2 AlC during L þ b ! b þ a. The Vickers hardness of the nanocomposites as a function of stirring time was tested, and the Ti-45Al/TiC nanocomposite produced by 10 min stir mixing has the highest value, measuring 450 HV.g-TiAl-based intermetallic alloys have attracted a wide range of research interest because of their low density, high specific strength, and good oxidation resistance, and these alloys have been regarded as a potential substitute for superalloys in the aerospace industry. [1][2][3] Synthesis of metal matrix nanocomposites (MMNCs) is promising for the development of this class of alloys because they combine the desirable properties of the matrix and the reinforcement. [4,5] The general methods to synthesize nanocomposites are always solid-state processing methods such as mechanical alloying and cold or hot pressing sintering. [6][7][8] However, its application has been impeded by its difficulty in preparing parts or products with complicated shapes and controlling grain growth in consideration of full compactness. [4] Liquid-state processing methods have the advantage of near-net shape in the following casting process, but large-scale agglomerates have been thought very hard to be dispersed well due to the material's high surface energy and poor wettability in the liquid state, especially in non-reactive systems. [9] However, there have been successful cases in which fine dispersions of nanoparticles were obtained, such as by enhancing the force field through ultrasonic dispersion (USD), [10,11] improving the wetting between the particles and the melt through adding active agents and combing with mechanical stir mixing. [12] Both methods have been verified feasible in aluminum alloys, but fabrication of g-TiAl-based nanocomposites using these methods seems to be a double-edged sword because, on one hand, the highly active TiAl melt may improve the wettability of nanoparticles without the addition of other active agents but, on the other hand, contamination from the USD probe or the mechanical blade can hardly be avoided. Thus, the electromagnetic stirring method becomes a desirable option that can make reasonable use of the reactive wetting of the TiAl-ceramic system. In this paper, an electromagnetic stirring process w...