Chunming Zou

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.

Effect of sintering neck on compressive mechanical properties of porous titanium

In order to study the role of sintering neck in porous titanium with helical pores, the effect of size and position of sintering neck on compressive mechanical properties of porous titanium single cell was studied by using numerical simulation method. The results show that the compressive mechanical properties of the porous titanium unit cell are determined by the helical pore structure and sintering neck. Contribution coefficient of sintering neck is approximately 3.5 times larger than that of helical pore structure. With the increase of the relative diameter of sintering neck, compressive yield stress and elastic modulus of the cell are constantly increased. The sintering point of C1 is the most important sintering position. Under the same condition, increasing the size of sintering neck at C1 is much effective to the increasing of compressive properties.

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.

Microstructure and nanohardness of Ti-48%Al alloy prepared by rapid solidification under different cooling rates

The influence of cooling rate on the microstructure and nanohardness of the rapidly solidified Ti-48%Al alloys prepared by melt spinning method was studied. The results show that the microstructure of the rapidly solidified ribbons is refined and homogenized compared with that of the conventionally cast mother alloy. With increasing the cooling rate, the grain size decreases greatly. The relationship between the ribbon thickness and the wheel speed was examined, and the effect of the wheel speed on the cooling rate was also studied. It was found that the ribbon thickness decreases inversely with the wheel speed. The nanohardness increases with increasing cooling rate due to the grain size strength.

Interfacial reactions between Ti-1100 alloy and CaO crucible during casting process

Abstract Ti-1100 alloys were melted in a controlled atmospheric induction furnace equipped with a CaO crucible. The microstructure, chemical composition, microhardness and metal–crucible interfacial reactions were systematically investigated. The results demonstrate that the primary solidification microstructure in the as-cast alloys was the typical Widmanstatten structure. The interactions between crucible and molten alloys are attributed to slight chemical dissolution and weak physical erosion. According to the line scanning analysis, the interfacial layer (α-case) thicknesses of Ti-1100 samples in the bottom and side wall are about 18 and 17 μm, respectively, which are slightly lower than those presented from microhardness tests (25 and 20 μm). The formation of α-case was caused by interstitial oxygen atoms. The standard Gibbs energy of reaction CaO(s)=Ca+O for Ti-1100 alloy was also determined. The equilibrium constant and the interaction parameter between calcium and oxygen were obtained as lg K=-3.14 and eCaO = 3.54.

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.

Microstructural characterization of in situ synthesized TiB in cast Ti-1100–0.10B alloy

Abstract The as-cast Ti-1100–0.10B alloy was prepared by a vacuum induction-melting technology. The microstructural characterization of in situ synthesized TiB and the interfacial structure of TiB/Ti matrix were systematically investigated. The results demonstrate that the TiB phase precipitates preferably at the prior β grain boundaries. The TiB phase grows along [010] direction with a typical needle-like morphology. The transversal section of TiB is hexagonal, and the interfaces between TiB and the Ti matrix are clean without precipitation phase. The stacking faults are observed on the (100) plane of the TiB phase, which are ascribed to the locations of boron (B) atoms in the crystal structure of TiB and the lattice mismatch energy between TiB and Ti matrix.

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.