Design and microstructures of Ti/TiAl/Al system functionally graded material

Abstract

Since the concept of functionally graded materials (FGM) was proposed, a great deal of research work on such a kind of advanced material has been done worldwide for different applications. Recently, FGMs have come to show high potential for the application in dynamic high-pressure technology [1]. Shock wave techniques offer unique capabilities for the experimental characterization of material properties at very high pressures and strain rates [2]. But the usefulness of the shock wave experiments can be expanded by using a kind of material with a density gradient [1], from which extremely-high pressure can be offered at a much cooler temperature, and one can deduce some of the properties of different materials at the pressure and temperature. Recently, the application of graded-density layered materials in high-pressure technology has been reported [3]. However, they were only fabricated by bonding a series of thin plates such as Ta, Cu, Ti, Al, Mg and TPX-plastic, thus the density of the layered materials rises with great steps in the thickness direction. It is expected to achieve a better effect [1] by using a graded material with a continuous or quasi-continuous density change. Moreover, the graded material with a wide density range is desirable, namely, its density of one surface should be kept very low, while that of another surface should be kept as high as possible [1]. So far, many kinds of metal/metal [4, 5], metal/ ceramic [6, 7] or metal/polymer [8] FGMs have been reported. However, the density range is not wide enough to meet the demand in shock wave experiments. The authors designed the material system of W/Mo/Ti/Al/plastic to compose such a graded material. Only when high relative density of every transient layer of such a FGM is ensured can good performance be obtained in factual application. So, the densification of every part of the above graded material should be studied. According to the previous research results [9, 10], the W/Mo and Mo/Ti system FGMs can be densified by powder metallurgy method under the same sintering conditions of 1473 K—30 MPa—1 h. But, until now, the Ti/Al system FGM has not been specially studied. The purpose of this work is to design and fabricate this system graded material. It will be very difficult to directly densify the Ti/Al graded material because there exists a great difference in melting point between the metal Ti and Al. To solve this problem and to ensure that the density of Ti/Al system FGM changes quasi-continuously in the thickness direction, a transient phase of TiAl was introduced (Ti3Al and TiAl3 were not chosen, because the density of Ti3Al was too close to that of Ti, and because of great brittleness of TiAl3, respectively). That is, Ti/TiAl/Al system FGM is designed, in which Ti/TiAl part would be densified under the above sintering condition, while the TiAl side needs to be joined to metal Al with brazing method. The microstructures of this FGM are presented in this paper, and the formation mechanism of the graded microstructures as well as joining interface is also discussed. The TiAl was synthesized by thermal explosion reaction: First, high-purity Ti powders (−400 mesh) and Al powders (−300 mesh) were mixed mechanically in the atom ratio of 1 : 1. Then, the mixture was heated to 973 K at a heating rate of 25–30 K/min in a furnace with a flowing argon. Under this condition the thermal explosion reaction of Ti+Al=TiAl was occurred. Afterwards the reaction product was heated to 1473 K and held at that temperature for 30 min to improve its homogeneity. Finally, the cooled reaction product was smashed, ground and sieved through −400 mesh. Xray diffraction analysis (XRDA) method was applied to identify phases of the product, and the results show