H.S. Liu

Corrosion behavior of Ti–Nb–Ta–Zr–Fe alloy for biomedical applications in Ringer's solution

Abstract The corrosion resistance of Ti–25Nb–10Ta–1Zr–0.2Fe (mass fraction, %) (TNTZF) alloy in Ringers solution at 37 °C was investigated by potentiodynamic polarization measurement. Ti–6Al–4V ELI (Extra low interstitial) alloy was also investigated to make a comparison. The results show that TNTZF alloy has higher corrosion potential, lower corrosion current density, more stable passive current density and wider passive region compared with Ti–6Al–4V ELI alloy, which indicates that TNTZF alloy has better corrosion resistance. In addition, pitting corrosion is observed on the surface passive film of Ti–6Al–4V ELI alloy but is not found on that of TNTZF alloy. The XPS analysis results reveal that the passive film formed on TNTZF alloy is composed of Nb 2 O 5 , NbO 2 , Ta 2 O 5 , ZrO 2 , TiO and Ti 2 O 3 oxides in the matrix of TiO 2 , which makes the passive film more stable and protective than that formed on Ti–6Al–4V ELI alloy and contributes much to its superior corrosion resistance.

Internal oxidation thermodynamics and microstructures of Ag-Y alloy

The thermodynamic data of pure Ag and Y were calculated. The phase constitution, composition of micro-region and microstructures of Ag-Y alloy after internal oxidation were investigated by X-ray diffractometry(XRD), energy dispersion spectrometry(EDS) and scanning electron microscopy(SEM). The results show that the internal oxidation behavior of Ag-Y alloy is feasible from the view of thermodynamics. The upper limit of oxygen partial pressure of Ag-Y alloy oxidation is a function of temperature. Two phases (Ag and Y2O3) appear in Ag-Y alloy after the internal oxidation. The surface of Ag-Y alloy is convex because of the volume expansion of oxide in the alloy and the composition of the convex part is Ag. In Ag-Y2O3 sintered bulk Y2O3 particles are distributed inhomogeneously and conglomerated seriously, but they are dispersed uniformly in the Ag matrix after severe plastic deformation.

Influence of Sc on high temperature strengthening behavior of Ti-6Al-4V alloy

Abstract Vacuum arc melting technique was used to prepare Ti-6Al-4V alloy containing Sc (0.3% and 0.5%, mass fraction). The ingots were melted twice by vacuum self-consumable electrode arc furnace. Forging of ingots was started in β-phase region and finished in high (α+β)-phase region. Annealing after forging was performed in low (α+β)-phase region for 30 min. Isothermal high temperature compression tests were conducted using thermal simulation machine under Ar atmosphere at 850 °C and 1 000 °C, and the strain rate were 0.001, 0.01, 0.1 and 1.0 s−1. Optical microscope(OM), scanning electron microscopy(SEM), energy dispersive spectrum(EDS) and transmission electron microscope(TEM) were used to study the microstructure evolution during high temperature deformation. The results show that, the peak stress value of alloys increases with increasing Sc content after deformation at 850 °C, however, there is no obvious strengthening of Sc when the alloys are deformed at 1 000 °C. Sc exists as Sc2O3 forms by internal oxidation during forging procedure, only minor Sc solutes in matrix. At 850 °C, the interaction between dislocation and participated particles and twinning mechanism controls the deformation procedure accompanied recrystallization. At 1 000 °C, the deformation of alloys containing Sc is mainly controlled by twinning, while the deformation of alloy without Sc is not only controlled by twinning, but also the interaction between dislocation and precipitated particles inside the twinning lamellar.

Precipitation hardening and microstructure evolution of the Ti–7Nb–10Mo alloy during aging

A biomedical β titanium alloy (Ti-7Nb-10Mo) was designed and prepared by vacuum arc self-consumable melting. The ingot was forged and rolled to plates, followed by quenching and aging. Age-hardening behavior, microstructure evolution and its influence on mechanical properties of the alloy during aging were investigated, using X-ray diffraction, transmission electron microscopy, tensile and hardness measurements. The electrochemical behavior of the alloy was investigated in Ringers solution. The microstructure of solution-treated (ST) alloy consists of the supersaturated solid solution β phase and the ωath formed during athermal process. The ST alloy exhibits Youngs modulus of 80 GPa, tensile strength of 774 MPa and elongation of 20%. The precipitation sequences during isothermal aging at different temperatures were determined as β+ωath→β+ωiso (144 h) at Taging=350-400 °C, β+ωath→β+ωiso+α→β+α at Taging=500°C, and β+ωath→β+α at Taging=600-650 °C, where ωiso forms during isothermal process. The mechanical properties of the alloy can be tailored easily through controlling the phase transition during aging. Comparing with the conventional Ti-6Al-4V alloy, the Ti-7Nb-10Mo alloy is more resistant to corrosion in Ringers solution. Results show that the Ti-7Nb-10Mo alloy is promising for biomedical applications.

Effect of multicomponent modifier on microstructure and mechanical properties of high Ni–Cr–Mo cast iron

Abstract The microstructure and mechanical properties of high Ni–Cr–Mo indefinite chilled cast iron with the addition of a newly developed multicomponent modifier consisting of mixed rare earths, Si–Ca alloy and Bi–Sb alloy have been investigated through optical microscopy, X-ray diffraction and scanning electron microscopy, along with hardness, impact toughness and wear resistance measurements. After the addition of the modifier, the grain sizes of the primary austenite and eutectic carbides are found to be greatly refined, and the typically highly continuous net-like carbides become less interconnected but rather appear more blocky shaped. Such microstructure changes lead to mechanical property improvement in the cast specimen, with its hardness increased from 43 to 50 HRC, impact toughness from 6·3 to 7·8 J cm−2 and ⩾20% increase in abrasive wear resistance.

Microstructure and mechanical properties of high strength Al–Zn–Mg–Cu alloys used for oil drill pipes

Abstract Three Al–Zn–Mg–Cu alloys used for oil drill pipes (Alloy A: Al–6.9Zn–2.3Mg–1.7Cu–0.3Mn–0.17Cr; Alloy B: Al–8.0Zn–2.3Mg–2.6Cu–0.2Zr, Alloy C: Al–8.0Zn–2.3Mg–1.8Cu–0.18Zr) were studied by hardness tests, tensile tests and transmission electron microscopy (TEM). The results show that the ultimate tensile strength, yield strength and elongation for Alloys A, B and C are 736 MPa, 695.5 MPa and 7%; 711 MPa, 674 MPa and 12.5%; 740.5 MPa, 707.5 MPa and 13%, respectively after solid solution treatment ((450 °C, 2 h)+(470 °C, 1 h)) followed by aging at 120 °C for 12 h. The dominant strengthening phases in Alloy A are GPII zone and η′ phase, the main precipitate in Alloy B is η′ phase, and the main precipitates in Alloy C are GPI zone, GPII zone and η′ phase, which are the reason for better comprehensive properties of Alloy C. The increase of zinc content leads to the improvement of the strength. The increase of copper content improves the elongation but slightly decreases the strength. Large second-phase particles formed by the increase in the manganese content induce a decrease in the elongation of alloys.

Effects of external stress aging on morphology and precipitation behavior of θ″ phase in Al-Cu alloy

Abstract The exposure of Al–5Cu alloy to an external stress with normal aging was carried out. The effects of external stress-aging on the morphology and precipitation behavior of θ″ phase were investigated by transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and first principle calculation. The size of the θ″ phase precipitated plates in stress-aging (453 K, 6 h, 50 MPa) is 19.83 nm, which is smaller than that of those present (28.79 nm) in stress-free aging (453 K, 6 h). The precipitation process of θ″ phase is accelerated by loading external stress aging according to the analysis of DSC results. The apparent activation energy for the external stress-aging is 10% lower than the stress-free one. The first principle calculation results show that the external stress makes a decrease of 6% in the interface energy. The effects of the stress on aging process of the alloy are discussed on the basis of the classical theory. The external stress changes the morphology and precipitation behavior of θ″ phase because the critical nucleation energy is decreased by 19% under stress aging.

Determination of Isothermal Section of Fe-Ni-Zr Ternary System at 1198K

Phase relations in the Fe-Ti-Zr ternary system at 1 173 K were investigated by means of diffusion-triple approach together with electron probe microanalysis(EPMA) technique. A series of tie lines and tie-triangles were determined and the isothermal section at 1 173 K was established, which consists of four three-phase fields: β(Ti, Zr)+FeZr2+FeTi, FeZr2+FeTi+Fe2Zr, FeTi +Fe2Zr +Fe2Ti and Fe2Zr +Fe2Ti +Fe. The results show that the largest solubility of Ti in Fe2Zr is about 11.3%(mole fraction) and the solid solubility of Ti in FeZr2 is about 26.9%, the solid solubility of Zr in Fe2Ti is about 8.1% and the solid solubility of Zr in FeTi is 7.2%. The binary compound FeZr2 is nearly a linear compound. No ternary compound is found.

Observation and characterization of isothermal α″ in a new β-type titanium alloy

Abstract Orthorhombic α″ phase often forms in Ti alloys through martensite transformation during quenching. Here, we report the formation of orthorhombic α″ phase in a low modulus β-Ti alloy during isothermal ageing. The coexists with α and ω phases, and remains stable for 120 h during ageing at 673 K. The phase is highly coherent with the β matrix, sharing the same basic orientation relationship as phase: //[0 0 2]α″, [1 0 0]β//[1 0 0]α″, [0 1 1]β//[0 1 0]α″. But differing from orthorhombic phase, the lattice constants of phase are determined as a = 0.352 nm, b = 0.494 nm, and c = 0.464 nm.

First-principles Calculation Assisted Thermodynamic Modeling of Ti-Co-Cu Ternary System

Thermodynamic assessment of Ti-Co-Cu ternary system has been carried out by combining flrst-principle calculation and CALPHAD method. Firstly, formation enthalpies of stable and hypothesized compounds were calculated by flrst-principles method. Then, based on reported experimental information, a thermodynamic description of the Ti-Co-Cu ternary system was performed. Solution phases were treated as substitutional solutions of which excess Gibbs energies were formulated by Redlich-Kister polynomial, and the intermediate phases were described with sublattice models. All measured isothermal sections were reasonably reproduced. In addition, liquidus projection of this ternary system was further calculated, which may be useful for relevant materials processing.