Z.J. Wei

Microstructure and mechanical properties of TiC/Ti–6Al–4V composites processed by in situ casting route

Abstract TiC/Ti–6Al–4V composites containing various volume fractions of TiC were produced by induction skull melting and common casting utilising in situ reaction between titanium and carbon powder. The microstructure and room tensile properties of as cast and heat treated TiC/Ti–6Al–4V composites were investigated. Bar-like or small globular eutectic TiC were found in 5 vol.-%TiC/Ti–6Al–4V composite, whereas the equiaxed or dendritic primary TiC particles were found to be the main reinforcements in 10 and 15 vol.-%TiC/Ti–6Al–4V composites. The as cast TiC/Ti–6Al–4V composites have shown higher strength but lower ductility than those of monolithic Ti–6Al–4V alloy. The shape and fracture of TiC particles can strongly influence the fracture and failure of the composites, and so the ultimate tensile strengths and elongations of as cast composites reduce with the increase in volume fraction of TiC. TiC particles appear to be spheroidised, and titanium precipitation can be found within large TiC particles after heat treatment at 1050°C for 8 h, which can promote the resistance to fracture of composites. Therefore, the elongations of the composites increase significantly, and the ultimate tensile strengths also have marginal increase especially for the 10 and 15 vol.-%TiC/Ti–6Al–4V composites after heat treatment.

Microstructure evolution of as-cast Nb-Ti-C alloys

Abstract Nb-Ti-C alloys with different Ti contents (4%–16%, mole fraction) were fabricated by vacuum non-consumable arc-melting method. The results indicate that the alloys contain niobium solid solution (Nb ss ), carbides (Nb 2 C, (Nb, Ti)C) and eutectic of Nb ss /MC. Carbides in the Nb-Ti-C alloys change from Nb 2 C to (Nb, Ti)C with the increase of titanium content. Microstructures in the as-cast state include primary plate-like Nb 2 C, eutectic of Nb ss /Nb 2 C, Nb ss and secondary needle-like Nb 2 C in Nb-5Ti-4C alloy, and other microstructures are primary Nb ss , eutectic of Nb ss /(Nb, Ti)C. Morphology of carbides are changed apparently through adding different Ti contents, and morphology of Nb ss changes from the cellular structure to dendrite.

Modification and control of TiC morphology by various ways in arc melted TiC/Ti–6Al–4V composites

Abstract The effects of carbon sources, cooling rate, alloying elements and heat treatment on the morphology of TiC were investigated in arc melted TiC/Ti–6Al–4V composites. It was observed that different carbon sources of the composites led to different morphologies of TiC, and the carbon source of carbon powder was found to be more useful in restraining the dendritic growth of TiC. Increasing the cooling rate can reduce the size of TiC, and the refinement effect is more obvious in the composite with carbon powder as the carbon source. The addition of 0·3 wt-%Ni promotes the dendritic growth of TiC; however, the TiC particles become fine and dispersed when 0·3 wt-%Sn is added in the composites. TiC particles in the composites can be dissolved and gradually spheroidised with increasing holding time when heat treated at 1050°C.

Effect of microstructural characteristics on room temperature tensile properties of in situ synthesised TiC/TA15 composite

Abstract In the present paper, a 10 vol.-%TiC particulate reinforced TA15 composite with wall thicknesses of 6, 12 and 18 mm was fabricated using in situ casting route. An effort was made to investigate the room temperature tensile properties as a function of the microstructures of the as cast and heat treated composites. The refinement of α phase with decreasing wall thickness leads to the enhancement in the yield strength of the composite, whereas the elongation shows an opposite trend with yield strength in the as cast condition. After β heat treatment, the yield strength as well as tensile strength shows similar value as a result of similar size of α phase. Compared with the as cast and heat treated tensile properties, heat treatment improves the yield and tensile strengths at the expense of elongation as the wall thicknesses are 12 and 18 mm.

Effect of Cr addition on microstructures and nanohardness of rapidly solidified Ti–48Al alloy

Abstract In this present work, the microstructure and nanohardness of rapidly solidified Ti–48 at-%Al alloy with various Cr additions were experimentally investigated using the single roller melt spinning technique. Ti–48Al alloy with various Cr additions were prepared by arc melting for comparison. In the arc melted alloy, the volume fraction of the interdendritic γ phase decreases, and the lamellar structure and the B2 phase increase with the increase in Cr addition. After rapid solidification, the Ti–48Al alloy consists of the γ phase and α2 the phase, with the γ phase as the matrix. The α2 phase exists as particles or in lamellar structure, which embed in the matrix. With 2 at-%Cr addition, the alloy ribbons mainly consist of equiaxial α2 grains and small particles of the B2 phase, with few lamellar structures occasionally found at the triple grain boundary. Increasing Cr content to 4 at-%, the grain size of the B2 phase increases, and lamellar structures disappear. The change in nanohardness was discussed based on the microstructural observations. It shows a certain increase in the nanohardness as Cr content increases to 4 at-%. This can be attributed to the changes in the microstructures.

Structural evolutions and mechanical behaviours of carbides in Nb–Ti–C alloys

Abstract The structural evolution of the (Nb, Ti)C and its effect on the mechanical behaviours of Nb–(21, 35)Ti–4C and Nb–25Ti–8C (at-%) alloys have been comprehensively investigated. The alloys consist of Nb solid solution (Nbss) and (Nb, Ti)C both for the as cast and the heat treated samples; however, the secondary Nbss precipitates within the (Nb, Ti)C after heat treatment and the composition of the secondary Nbss is mainly the Nb. The driving force of Nbss precipitation from (Nb, Ti)C is summarised as two aspects: the lack of C atoms in (Nb, Ti)C induced by the fast cooling rate; and the difference of thermodynamic stability of TiC and NbC. The small sized long strip secondary Nbss, which adopts a well defined orientation relationship between the (Nb, Ti)C, is obtained in the case of low driving force. On the condition of high driving force, the microstructure of the Nbss precipitations becomes large sized massive and layered. The brittleness of (Nb, Ti)C is reduced by the secondary Nbss precipitations proved by nanoindentation tests.

High pressure solidification process and effect on nanohardness of Ti–48 at-%Al alloy

Abstract In this work a tungsten carbide six-anvil apparatus was introduced to investigate the high pressure solidification effect on Ti–48 at-%Al alloy. The experimental processes of Ti–48 at-%Al alloy solidified under different pressures were presented in detail. The mechanical properties of Ti–48 at-%Al alloy solidified under different pressures were examined by a Nanoindentor XP tester. It shows a certain increase in the nanohardness of the lamellar structure as the pressure increases from normal pressure to 4 GPa. When the pressure is 4 GPa, the nanohardness increases by 50·2% compared with that of normal pressure.

Mechanical properties of Nb based multiphase alloy studied by nanoindentation and atomic force microscopy

Abstract The mechanical properties and deformation behaviours of as cast and heat treated Nb–21Ti–4C–xAl (x: 0, 5, 10 and 15 at-%) alloys are comprehensively investigated using nanoindentation and atomic force microscopy. For the Nb–21Ti–4C alloy, nanoindentation tests are performed for the Nb solid solution (Nbss) matrix, carbide and the interface between them. The results show that the hard carbide, which has a strong bonding with the Nbss matrix, can enhance the alloy before and after heat treatment, and the eutectoid transformation of the large sized carbide after heat treatment leads to less possibility for the forming of cracks on the carbide surface, which in turn improves the toughness. For the Nb–21Ti–4C–(0, 5, 10, 15)Al alloys, the hardness of the Nbss matrix increases significantly with increasing Al fraction for both as cast and heat treated alloys. However, deviations of the elastic modulus are inconspicuous with the Al fraction for the as cast and heat treated alloys.

Optimization of processing parameters for preparation of Al-3Ti-1B grain refiner

Abstract Al-3Ti-1B master alloys were prepared at different processing parameters by the reaction of halide salts, and the grain refining response of Al-7Si alloy was investigated with Al-3Ti-B master alloy. The microstructure of master alloy and its grain refining effect on Al-7Si alloy were investigated by means of OM, XRD and SEM. Experimental results show that, the size of Al3Ti particles presented in Al-3Ti-1B master alloys increases with the increase of reaction temperature and decreases with the increase of cooling rate. The grain refining efficiency of Al-3Ti-1B master alloy on Al-7Si alloy is mainly attributed to heterogeneous nucleation of Al3Ti particles, and the morphology of α(Al) changes from coarse dendritic to fine equiaxed. As a result, Al-3Ti-1B master alloy is prepared by permanent mold, and holding at 800 °C for 30 min, which has better grain refining performance on Al-7Si alloy.