Narrow Results

Two biomedical alloys, Ti-39Nb-5.1Ta-7.1Zr (TNTZ1) and Ti-35.3Nb-5.1Ta-7.1Zr (TNTZ2) were melted using LZ5 type centrifugal equipment for casting titanium. The solidification microstructures of TNTZ1 and TNTZ2 alloys cast in graphite crucible without centrifugal force are mostly big arborescent crystal with dispersed porosity in it. After pouring into a graphite mould under centrifugal force, the lower part of ingot showed mostly arborescent crystals with few equiaxed crystals, while the upper part of ingot is basically spherical equiaxed crystals. The solidification defects, such as dispersed pores were very difficult to find. Nb is supposed to refine the arborescent crystals. X-ray diffraction (XRD) analysis showed Ti-phase and Nb-phase existing in the above two alloys. The content of Ti was found relatively low in crystal interior and relatively high in crystal boundary. The content of Nb/Zr was uniform in whole microstructure.

The effects of carbon and oxygen additions on the microstructure and mechanical properties of a new type low-cost beta titanium alloy Ti-4.3Fe-7.1Cr-3Al have been studied. The results showed that with the carbon addition the grain size was refined significantly, followed by the tensile strength increase and ductility improvement. At 0.2 wt.% carbon addition the 0.2%YS increased by 22% and elongation increased to a peak value of 23%. Also with the carbon addition the oxygen content in the alloy could be allowed to reach 0.2wt.% without tensile property decrease. After ageing treatment, it was found that ageing temperature other than carbon content played a key role on the α-phase precipitation and ageing hardness. The hot-rolling deformation accelerated the ageing response. The optimal ageing process was decided to be 500°C for 1h under the pretreatment of 850°C hot-rolling followed by immediately water quenching. The reasons for these improvements were discussed in terms of the carbide pinning and α-phase precipitation.

In this study a nanometer-sized metallic glass (nano-MG) Ti₅₀Cu₅₀ was generated with constrained atoms at both ends and was extended until fracture under a tensile load by molecular dynamics simulation using the general embedded-atom model (GEAM) potential. Totally different mechanical behavior was observed in the nano-MG, such as strain hardening and necking, both of which have been discovered in a few real and simulated MGs and can be related to the generation of shear transformation zones (STZs). A dramatic drop in Young's modulus was found due to the surface effect. Such effect results from the large fraction of surface atoms which have a different surrounding configuration from bulk atoms. At fracture the nano-MG breaks by atomic separation as reported in metal nanowires. The fracture strain is as large as about 120%, indicating that nano-MGs are intrinsically ductile.

This work focuses on developing near-alpha Ti alloys via the selective addition of small concentrations of low-cost eutectoid $\beta$-stabilizers like Cu and Mn. In particular, these newly designed near-alpha Ti alloys are manufactured via the cheapest powder metallurgy route of cold pressing plus sintering. Moreover, thermomechanical deformation of the sintered alloys via hot forging in the $\alpha +\beta$ and $\beta$ field was also investigated aiming to enhance the mechanical properties through reduction of the residual porosity and microstructural control. It is found that the initial addition of a small amount of eutectoid $\beta$-stabilizers leads to higher tensile properties with comprision to pure Ti produced by powder metallurgy, and this is due to the formation of a coarse lamellar structure due to the presence of $\beta$-stabilizers. Further enhancement of the strength is achieved by means of hot thermomechanical processing thanks to sealing of the residual pores, texturing, and refinement of the microstructural features.

TiC/TiN-reinforced composite coatings were fabricated on the substrate of Ti–6Al–4V alloy using laser remelting. X-ray diffraction (XRD) was used to identify the phases in the laser-clad composite coating; the interface characterization of the dilution zone-clad zone (IDC) and the dilution zone-heat-affected zone (IDH) was observed with a scanning electron microscope (SEM). The results show that the microstructure of a cross-section has stratification characterization, and consists of the clad zone (CZ), the dilution zone (DZ), the diffusion layer (DL) and the heat-affected zone (HAZ). The layer-by-layer microstructure results from the boundary layer phenomenon of viscous melt-fluid and diffusion. The kind of reinforced particle has an effect on the interface morphology, microstructure and flow characterization of the melt-fluid. The phase constitution in the clad zone consists of (Cr–Ni–Fe), TiC, Ni₄B₃, Ti₂Ni, Cr₂B and M₂₃C₆ for TiC+NiCrBSi coating, and (Cr–Ni–Fe), TiN, NiB, Cr₂Ti and Ti₂Ni for TiN+NiCrBSi coating. The interfaces of the IDC in the NiCrBSi-clad layer is clear and clean; those of TiC+NiCrBSi and TiN+NiCrBSi are illegible. Ti–Ni phases with acicular microstructure link dilution zone and clad zone, and two kinds of phase with acicular microstructure, are similar in composition and shape.

The microscopic morphology and microstructure of Ti-N and Ti-Ni phase between the dilution zone and the clad zone in laser remelting NiCrBSi/TiN layer on a Ti-6Al-4V alloy were characterized using TEM and SEM. The experimental results showed that during laser irradiation heating, TiN particles were partially dissolved into the melted Ni-base alloy, and the dissolved Ti and N atoms were precipitated in the form of TiN, TiN_0.3. Ti exhibits height activity, it combines with Ni forming Ti₂Ni, TiNi matrix intermetallic during laser remelting, faults exist in the Ti₂Ni and TiNi phase, and crystal lattice of TiNi phase is superlattice. Lastly, the cause of the formation of the Ti-N and Ti-Ni phase is discussed.

Recent developments of laser surface modification of titanium alloys for increasing their corrosion, wear and oxidation resistance are introduced. The effects of laser processing parameters on the resulting surface properties of titanium alloys are reviewed. The problems to be solved and the prospects in the field of laser modification of Ti alloys are discussed. Due to the intrinsic properties, a laser beam can be focused onto the metallic surface to produce a broad range of treatments depending on the input energy. Thus, composite strengthening coatings can be fabricated by the methods of laser alloying, cladding, pulse laser deposition (PLD), etc., which are promising techniques of producing a layer of new materials on the surface of titanium alloys.

Laser cladding is an advanced surface modification technology with broad prospect in making wear-resistant coating on titanium alloys. In this paper, the influences of laser cladding processing parameters on the quality of coating are generalized as well as the selection of cladding materials on titanium alloys. The microstructure characteristics and strengthening mechanism of coating are also analyzed. In addition, the problems and precaution measures in the laser cladding are pointed out.

Laser cladding of the Fe₃Al + B₄C/TiN + Al₂O₃ pre-placed powders on the Ti-6Al-4V alloy can form the Ti₃Al/Fe₃Al + TiN/TiB₂ composite coating, which improved the wear resistance of the Ti-6Al-4V alloy surface. In this study, the Ti₃Al/Fe₃Al + TiN/TiB₂ composite coating has been researched by means of X-ray diffraction and scanning electron microscope. It was found that during the laser cladding process, Al₂O₃ can react with TiB₂, leading to the formations of Ti₃Al and B. This principle can be used to improve the Fe₃Al + B₄C/TiN laser-cladded coating on the Ti-6Al-4V alloy. Furthermore, during the cladding process, C consumed the oxygen in Fe₃Al + B₄C /TiN + Al₂O₃ molten pool, which retarded the productions of the redundant metal oxides.

This work is based on micro-structural performance of the Ti–B₄C–C laser alloying coatings on Ti–6Al–4V titanium alloy. The test results indicated that laser alloying of the Ti–B₄C–C pre-placed powders on the Ti–6Al–4V alloy substrate can form the ceramics reinforced hard alloying coatings, which increased the micro-hardness and wear resistance of substrate. The test result also indicated that the TiB phase was produced in alloying coating, which corresponded to its (101) crystal plane. In addition, yttria has a refining effect on micro-structures of the laser alloying coating, and its refinement mechanism was analyzed. This research provided essential experimental and theoretical basis to promote the applications of the laser alloying technique in manufacturing and repairing of the aerospace parts.

The study of chip morphology during high-speed milling is crucial in determining processing quality, but the relationship between chip formation mechanisms and process parameters in titanium alloy high-speed machining has been a difficult issue. This paper investigates milling force and temperature variation, as well as chip morphology and surface quality after milling to improve workpiece surface quality and machining efficiency and provide guidance on selecting milling parameters. Factorial analysis confirms that milling speed worsens the serrated layer of chips. In addition, the experiment confirms the influence of tool rake angle on shear strain in the milling zone, the impact of rotational speed on tool wear, and the effect of tool wear on chip morphology. The results show that increasing milling speed worsens chip serration and increases tool wear.

Niobium can replace vanadium in Ti-Al-V alloys in order to avoid the slow accumulation of potentially harmful vanadium ions. Niobium is known to be a biocompatible material; however the role of its addition on bone-implant adhesion is not well known. In order to investigate this problem, a detailed thermodynamic analysis concerning the nucleation of a calcium phosphate coating on the niobium surface immersed in a body fluid solution (SBF) was carried out. With this purpose, Eh-pH and pi-pH diagrams of the Na-Nb-H₂O and Ca-Nb-H₂O systems were constructed. It was verified that a sodium niobate layer will form during chemical pre-treatment of the alloy in NaOH solution. This layer is replaced by calcium niobate during the subsequent exposure of the alloy to the SBF solution. From the pCa-pH diagram of the Nb-Ca-P-H₂O system, the thermodynamic stability of the niobate-apatite, Ca₁₀(PO₄)₆(NbO₃)₂, over a wide range of pH values was characterised. This is a good indication that niobium do not alter the mechanism of hydroxyapatite precipitation, guaranteeing in this way the strong chemical adhesion provided by the biomimetic process.

Titanium alloys are expected to be much more widely used for hard tissue implant materials due to their superior biocompatibility and high corrosion. However, the larger Young's modulus of present titanium alloys always leads to stress shielding and harmful effects on human bones and results in premature failure of the implant. Recently a d-electron alloy design method has been proposed to design low elastic modulus titanium alloys and achieve some actual results. In this paper, a series of the Ti-Nb-Zr alloys has been designed with the d-electron alloy design method, and the phase structure, lattice parameter and elastic modulus have been investigated. The results show that with the increase of and , the lattice parameters of these alloys increase monotonously with the single phase structure of bcc β phase. The maximum increment reaches 4.5% compared with that of matrix β phase. As the and increase up further, the phase structure begins to change from bcc β phase to hcp α phase. Correspondingly the elastic modulus decreases first and then increase, from 96.8 GPa to 67.4 GPa, and then to 83.2GPa with the increase of and . The lattice parameter and phase structure both exhibit significant influences on the elastic modulus of these alloys.

With a novel high-power electrospark deposition (ESD) processing unit, Ti-Cu coatings were prepared on TC11 titanium alloy substrate using optimized processing parameters. The Ti-Cu coatings consisted of many secondary layers and metallurgically bonded to the substrate. X-ray diffraction (XRD) as well as optical microscope (OM) and scanning electron microscopy (SEM) were performed to determine the composition and microstructure of the resulting deposition. The superficial layer of the Ti-Cu coating was composed of copper-titanium and aluminum-titanium intermetallics. Microhardness tests showed that the microhardness of Ti-Cu coating was more twice than that of the substrate. The flame-retardant properties of TC11 substrates with Ti-Cu coatings were evaluated through the laser ablation testing on the surface using a laser power of 1KW and 1.5KW with an ablation time of 10s and 5s respectively. The Ti-Cu coatings exhibited the flame-retardant properties under (1KW, 10s) ablation parameter conditions on the TC11 titanium alloys which were characterized by the phenomena of spontaneous combustion under some certain environmental conditions.