Effect of prior cold work on age hardening of Cu–3Ti–1Cr alloy

The influence of 50%, 75% and 90% cold work on the age hardening behavior of Cu–3Ti–1Cr alloy has been investigated by hardness and tensile tests, and light optical and transmission electron microscopy. Hardness increased from 118 Hv in the solution-treated condition to 373 Hv after 90% cold work and peak aging. Cold deformation reduced the peak aging time and temperature of the alloy. The yield strength and ultimate tensile strength reached a maximum of 1090 and 1110 MPa, respectively, following 90% deformation and peak aging. The microstructure of the deformed alloy exhibited elongated grains and deformation twins. The maximum strength on peak aging was obtained due to precipitation of the ordered, metastable and coherent β′-Cu₄Ti phase, in addition to high dislocation density and deformation twins. Over-aging resulted in decreases in hardness and strength due to the formation of incoherent and equilibrium β-Cu₃Ti phase in the form of a cellular structure. However, the morphology of the discontinuous precipitation changed to a globular form on high deformation. The mechanical properties of Cu–3Ti–1Cr alloy are superior to those of Cu–2.7Ti, Cu–3Ti–1Cd and the commercial Cu–0.5Be–2.5Co alloys in the cold-worked and peak-aged condition.     

Thermal stability and mechanical properties of nanostructured Ti–24Nb–4Zr–7.9Sn alloy

Thermal stability of the nanostructured grains of cold-rolled Ti–24Nb–4Zr–7.9Sn alloy and corresponding variations in mechanical properties were investigated. The activation energy for grain growth was found distinct below and above the ( + β)/β transus of 950 K, with values of 47 and 206 kJ/mol, respectively. Due to the pinning effect of the precipitates at β grain boundaries, grains sizes can be maintained at less than 100 nm during prolonged annealing at temperatures up to 773 K, and are less than 1 μm for annealing temperature up to 923 K and time up to 2 h. Annealing above the β transus resulted in coarse grains with sizes of tens of micrometers in less than 2 h. Tensile and hardness tests showed rapid strengthening with the increase of annealing time below 773 K, which was attributed to both the rapid formation of nano-sized precipitates and the slow growth rate of β grains. By adjusting the grain size of the cold-rolled material the high strength/low Young's modulus match desirable for implant applications can be improved over the hot-rolled bars with coarse grains.     

Surface hardening of biomedical Ti–29Nb–13Ta–4.6Zr and Ti–6Al–4V ELI by gas nitriding

The microstructure and hardness near the surface of a biomedical titanium alloy, Ti–29Nb–13Ta–4.6Zr (TNTZ), subjected to gas nitriding at 1023–1223 K was investigated in comparison with the conventional biomedical Ti–6Al–4V ELI (Ti64). After gas nitriding, the microstructure near the specimen surface was observed by optical microscopy, X-ray diffraction (XRD), Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS). In both alloys, two types of titanium nitrides (TiN and Ti₂N) are formed and the phase is precipitated by gas nitriding. Furthermore, the oxygen impurity in the gas nitriding atmosphere reacts with the titanium nitrides; thus, TiO₂ is formed at the outermost titanium nitride layer. The surface hardening was also evaluated by Vickers hardness measurement. The Vickers hardness near the surface of TNTZ and Ti64 increases significantly by gas nitriding.     

Characterization of prior cold worked and age hardened Cu–3Ti–1Cd alloy

The influence of cold deformation by 50%, 75% and 90% on the age-hardening behavior of a Cu–3Ti–1Cd alloy has been investigated by hardness, tensile tests and light optical as well as transmission electron microscopy. The hardness of Cu–3Ti–1Cd alloy increased from 111 Hv in the solution-treated condition to 355 Hv in 90% cold worked and peak aged condition. The yield and ultimate tensile strengths of Cu–3Ti–1Cd alloy reached maxima of 922 MPa and 1035 MPa, respectively, on 90% deformation and peak aging. The microstructure of the deformed alloy exhibited elongated grains and deformation bands. The maximum strength on peak aging was brought about by the precipitation of ordered, metastable, coherent β′ Cu₄Ti phase, in addition to high dislocation density and deformation twins. Both the hardness and the strength of the alloy decreased on overaging due to the development of the incoherent equilibrium phase β Cu₃Ti in a cellular structure form. However, the morphology of the discontinuous precipitation was changed to globular form at high deformation levels.     

The effect of lamellar separation on the properties of a Ti–46Al–2Nb–2Cr intermetallic alloy

The relationships between the γ and ₂ lamellae apparent separation and hardnesses as well as the peak flow stresses estimated in hot compression tests obtained for specimens made of Ti–46Al–2Nb–2Cr alloy are presented. The lamellae separations were estimated using image analysis on the microstructure of the specimens. The procedure employing a fast Fourier transformation of secondary electron microstructural images for fully automatic lamellae separation measurements is described. It was found that the effect of the apparent lamellae separations of γ and ₂ on the peak flow stress is significant. For the microstructure with thick lamellae of γ and ₂ phases, the peak flow stress decreases. The hardness of the specimens decreases with an increase of the lamellae apparent separation as well.     

The influence of impurity level and tin addition on the ageing heat treatment of the 356 class alloy

The influence of the addition of 0.5 wt.% Sn to Al–7Si–0.3 Mg alloys (356 and A356) on their ageing behaviour and mechanical properties was evaluated. Adding Sn led to a reduction of the iron rich intermetallics volume fraction, and of hardness. During solution heat treatment, Mg went into the solid solution, and Sn particles grew by competitive growth, concentrating at phase boundaries and interfaces. During aging β″ and Si precipitated. In the alloys with Sn, the β″ precipitation was accelerated and its hardening effect was greater, whereas the Si precipitation did not changed significantly. The mechanical properties of the A356 alloy were compatible with the hardening achieved during the heat treatment and to the amount of defects (pores) present in the microstructure. The yield strength and elongation of the A356 + 0.5% Sn alloy decreased after solution heat treatment and with increasing ageing temperature. These detrimental effects were minimized by treating this alloy in the T5 condition at 150 °C.     

Study of transformation behavior in a Ti–4.4 Ta–1.9 Nb alloy

An alloy of composition Ti–4.4 wt.% Ta–1.9 wt.% Nb is being developed as a structural material for corrosion applications, as titanium and its alloys possess excellent corrosion resistance in many oxidizing media. The primary physical metallurgy database for the Ti–4.4 wt.% Ta–1.9 wt.% Nb alloy is being presented for the first time. Determination of the β transus, M_s temperature and classification of the alloy have been carried out, employing a variety of microscopy techniques, X-ray diffraction (XRD), micro-hardness and differential scanning calorimetry (DSC). The β transition temperature or β transus determined using different experimental techniques was found to agree very well with evaluations based on empirical calculations. Based on chemistry and observed room temperature microstructure, the alloy has been classified as an + β titanium alloy. The high temperature β transforms to either ′ or + β by a martensitic or Widmanstatten transformation. The mechanisms of transformation of β under different conditions and characteristics of different types of have been studied and discussed in this paper.     

Effects of heat treatments on the microstructures and mechanical properties of Mg–3Nd–0.2Zn–0.4Zr (wt.%) alloy

Microstructure and mechanical properties of as-cast and different heat treated Mg–3Nd–0.2Zn–0.4Zr (wt.%) (NZ30K) alloys were investigated. The as-cast alloy was comprised of magnesium matrix and Mg₁₂Nd eutectic compounds. After solution treatment at 540 °C for 6 h, the eutectic compounds dissolved into the matrix and small Zr-containing particles precipitated at grain interiors. Further aging at low temperatures led to plate-shaped metastable precipitates, which strengthened the alloy. Peak-aged at 200 °C for 10–16 h, fine β″ particles with DO₁₉ structure was the dominant strengthening phase. The alloy had ultimate tensile strength (UTS) and elongation of 300–305 MPa and 11%, respectively. Aged at 250 °C for 10 h, coarse β′ particles with fcc structure was the dominant strengthening phase. The alloy showed UTS and elongation of 265 MPa and 20%, respectively. Yield strengths (YS) of these two aged conditions were in the same level, about 140 MPa. Precipitation strengthening was the largest contributor (about 60%) to the strength in these two aged conditions. The hardness of aged NZ30K alloy seemed to correspond to UTS not YS.     

Infrared brazing of high-strength titanium alloys by Ti–15Cu–15Ni and Ti–15Cu–25Ni filler foils

Microstructures and fracture behaviors of infrared heated, vacuum brazed Ti–6Al–4V and Ti-15-3 alloys using two Ti–Cu–Ni braze fillers have been characterized to establish the effects of brazing process parameter and chemical composition on the strength of brazed joints. The brazed joint initially contains two prominent phases; a Ti alloy matrix alloyed with V, Cr, Ni, Cu and Al and a Cu–Ni-rich Ti phase. Brazing temperature and soak time control the amount of Cu–Ni-rich Ti phase in the brazed joints. The fracture mode changes from brittle cleavage to quasi-cleavage to ductile dimple as the amount of Cu–Ni-rich Ti phase is reduced in the brazed joint. Both brazing temperature and soak time are critical to eliminate the Cu–Ni-rich Ti phase for optimal shear strength and ductile fracture of brazed joints. A post-brazing annealing at lower temperature is also shown to be an effective way to homogenize the microstructure of brazed joint for improved joint strength.     

Spinodal decomposition existence of the β Ti–Cr binary alloy: computer simulation of the real alloy system and experimental investigations

The spinodal decomposition of β Ti–Cr binary alloys system still questionable, since there are rare experimental data moreover simulation results for the real alloy system have not been found. Transmission electron microscopy (TEM) and quantitative computer simulations results based on Khachaturyan's diffusion equation have been employed to study the microstructure evolution occurring in the β Ti–Cr alloys. Our study results reveal that the metastable β undergoes a phase separation reaction through a spinodal decomposition. The coherent two phase fields show extremely fine plate-like precipitates lying parallel to {1 0 0} plane. Those precipitates are high elastically-induced from the first step of phase separation.     

Formation of nanocrystallized and amorphous structures in the cavitation-eroded surface for Ti3Al–Nb alloy

Cavitation erosion of Ti₃Al–Nb alloy was investigated by using rotating disc equipment. It is showed that Ti–24Al–15Nb–1Mo alloy has excellent cavitation erosion resistance. The cavitation-eroded surface was observed by TEM and nanocrystallized and amorphous structures were found in some zones. On the basis of experimental results and analyses, the impacts of microjets can cause plastic deformation in the specimen surface layer. Because of the transient high temperature produced by the collapse of bubbles, some deformed zones are recrystallized and a nanocrystallized structure is formed. The nanocrystallized structure with lower melting point is then molten by transient high temperature and quickly solidified in water, which produces an amorphous structure.     

Mechanical behavior and shape memory effect of an aged NiAl–Fe alloy

The mechanical behavior and shape memory effect of an aged NiAl–Fe alloy has been investigated. It was found that the first yielding stress of NiAl–Fe alloy in a compression test was decreased with the precipitation of a Ni₅Al₃ phase after aging at 473–673 K, and increased as the aging temperature increased higher. The one-way shape recovery of NiAl–Fe alloy increased as the aging temperature increased from 473 to 673 K, and decreased as the aging temperature increased higher than 673 K. The morphology of precipitates in the NiAl–Fe alloy aged at 473–873 K was investigated, and the mechanism of its effect on the mechanical properties is discussed.     

Effects of ZnO on piezoelectric properties of 0.01PMW–0.41PNN–0.35PT–0.23PZ ceramics

The microstructure and piezoelectric properties of the 0.01Pb(Mg_1/2W_1/2)O₃–0.41Pb(Ni_1/3Nb_2/3)O₃–0.35PbTiO₃–0.23PbZrO₃ + 0.1 and 0.3 wt.% Y₂O₃ + x ZnO ceramics were investigated. The crystal structure changed from psudocubic to tetragonal when ZnO added. The average grain size increased from 4 μm to 8 μm with the addition of ZnO by oxygen diffusion, even if the growth rate was low. When ZnO added until 0.5 wt.%, the , k_p and d₃₃ values of specimens were slightly increased regardless Y₂O₃ contents. The curie point of PMW–PNN–PT–PZ ceramics were increased from 162 °C to 232 °C, as increasing the ZnO contents. When ZnO added, the k_p of specimens slightly was increased regardless Y₂O₃ contents. The mechanical quality factors were abruptly decreased regardless Y₂O₃ contents, when ZnO added until 0.75 wt.%. The optimized piezoelectric properties were obtained; d₃₃ = 730 (pC/N), k_p = 60, Q_m = 50, and  = 4750, when PMW–PNN–PT–PZ + 0.3 wt.% Y₂O₃ + 0.5 wt.% ZnO sintered at 1200 °C for 1 h.     

The effect of grain size and composition on the fracture toughness of Ti–Al–Nb alloys undergoing stress-induced martensitic transformation

The effect of grain size and composition on the fracture toughness of Ti–Al–Nb alloys in β solution-treated condition was investigated. The fracture toughness of the alloys was found to increase with an increase in grain size initially, reach a maximum and subsequently decrease with further increase in grain size. This trend was attributed primarily to the effect of grain size on the enhancement of fracture toughness due to stress-induced martensitic transformation (SIMT) at the crack tip, which in turn can be related to the effect of grain size on trigger stress for SIMT. Alloys containing higher Al and Nb showed a higher toughness for the same grain size, which was also explained in terms of effect of composition on the trigger stress.     

Creep behaviour and creep microstructures of a high-temperature titanium alloy Ti–5.8Al–4.0Sn–3.5Zr–0.7Nb–0.35Si–0.06C (Timetal 834): Part I. Primary and steady-state creep

The tensile creep behaviour of the high-temperature near -Ti alloy Ti–5.8Al–4.0Sn–3.5Zr–0.7Nb–0.35Si–0.06C (Timetal 834) with a duplex microstructure has been extensively investigated in the temperature range from 500°C to 625°C and the stress range from 100 to 550 MPa. Both primary and secondary creep are being considered. The results of the primary creep are analysed in terms of the dependencies of stress on strain (strain hardening) and on strain rate (strain rate sensitivity). It is shown that the strain-hardening exponent depends on temperature, and takes values between 0.5 for 500°C and 0.33 for higher temperatures; this would give a dependence of the primary creep strain of σ² and σ³. The strain rate exponents obtained in both primary and secondary creep have been found to be similar; this is also the case for the activation energies. It is thought that, in the stress and temperature range investigated, creep is controlled by bow-out and climb of dislocation segments pinned at lath boundaries and second-phase particle. Analysis of the dislocation substructure is presented to give some support for this mechanism.     

Laser boronising of Ti–6Al–4V as a result of laser alloying with pre-placed BN

This paper discusses the effect of CO₂ laser alloying of pre-placed BN coating with Ti–6Al–4V alloy. The formation of titanium boride and titanium nitride investigated using energy dispersive X-ray diffraction (EDXRD) result were related to the microhardness and microstructure. The nitrogen and boron diffusion during the laser boronising process identified using secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectrometry (XPS) analysis was compared with the EDXRD results. The surface hardness HV1500–1700 observed at the boronised layer was five to six times higher than that of untreated Ti–6Al–4V alloy. This was compared with needle platelet and dendrite type microstructures. Theoretically estimated surface temperature values were used to interpret the compound formation in the laser alloyed layer.     

Effect of small γ-precipitates on the two-way shape memory effect in Cu–Zn–Al alloys

The γ-precipitates in Cu–Zn–Al alloys, trained by the stabilization of the stress induced martensite (SSIM) method, have been studied. After the SSIM treatment, it was found that small γ-precipitates in the β-austenite are ellipsoidal, with a large strain field oriented in the same direction; while in the martensite the γ-precipitates changed their shape from ellipsoid to spheroid, and relaxed their strain fields. In order to check whether the strain field of the γ-precipitates is capable of producing a thermoelastic martensitic transformation, an in-situ observation, by heating a sample holder in TEM, was performed. It was found that during heating over a temperature A_s, the γ-precipitates with a spherical shape in the martensite recovered their strain field and elliptical shape. During cooling, the strain field of the γ-precipitates disappeared again. It was proposed that the strain field of the γ-precipitates, trained by the SSIM method, plays an important part in the thermoelastic martensitic transformation, and presents two-way shape memory effects.     

Microstructure and mechanical properties of two-phase Cr–Cr2Nb, Cr–Cr2Zr and Cr–Cr2(Nb,Zr) alloys

The microstructures and mechanical properties of binary and ternary Cr-based alloys containing Nb, Zr, or both Nb and Zr, have been studied in both the as-cast and annealed conditions. The level of alloying in each instance was targeted to lie below, or approximately at, the maximum solubility in chromium. The as-cast microstructures of these alloys consisted of Cr-rich solid solution surrounded by small amounts of interdendritic Cr–Cr₂X eutectic structure. Annealing at 1473 K resulted in solid-state precipitation of the Cr₂X Laves phase in the Cr–Nb and Cr–Nb–Zr alloys, but not in the Cr–Zr alloys. The binary Cr₂Nb phase consisted of an extensively twinned ({111}<112> twins) C15 structure whereas the presence of Zr modifies its appearance substantially; the twinned C15 structure persists. Oxides were occasionally present and their compositions were qualitatively determined. Vickers hardness primarily depended upon the volume fraction of the Cr₂X Laves phase present. Age hardening due to solid-state precipitation of Cr₂X Laves phase within the Cr-rich matrix was observed in the Nb-containing alloys. The room temperature bend strength of the alloys was strongly affected by the presence of grain-boundary Cr₂X phase. It is considered that porosity as well as oxides in the alloys also lowers their bend strength.     

Hard amorphous Ti–Al–N coatings deposited by sputtering

Ti–Al–N coatings were deposited by direct current reactive magnetron sputtering using two titanium and two aluminum targets. Two series of films with Al/(Al + Ti) atomic ratios of ≈ 23.5 and ≈ 34.5% were studied. The amount of nitrogen in the films was varied from 0 to 44at.%. The incorporation of N atoms led to a change of the -Ti lattice preferential orientation from <100> to <001>, a decrease in the degree of crystallinity, and subsequently to the collapse of the crystalline structure. Annealing at 975K promotes the formation of the Ti₃Al compound. The hardness increases smoothly with the nitrogen content. The high hardness values (31 and 41GPa) measured for the films with the highest N contents may be explained by the deposition of a nanocomposite phase. For the Ti–Al–N film deposited with Al/(Al + Ti) atomic ratio of 34.5% the -Ti structure was completely transformed to TiO₂ upon oxidation. The high oxidation resistance of the film deposited with 44at.% N at 1075K is characteristic of Ti–Al–N films.     

Al–Si coating fused by Al + Si powders formed on Ti–6Al–4V alloy and its oxidation resistance

An Al–Si coating was successfully produced by means of the low oxygen pressure fusing technology for improving the oxidation resistance of Ti–6Al–4V alloy. The Al and Si concentration in coating and coating thickness could be controlled by adjusting powder mixing ratio and changing the technical parameters (fusing temperature and time), respectively. At 1273 K, the weight gain of the Al–20Si coating increased with prolonging fusing time and its equation could be described as Δm² = 3.62t. After 105 h oxidation, the oxidation rate of the Al–20Si coated specimen with fusing time 100 min was about two to four times than that of the Al–10Si coated specimen with fusing time 60 min.