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.     

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.     

Effect of various surface conditions on fretting fatigue behavior of Ti–6Al–4V

An experimental investigation was conducted to explore the fretting fatigue behavior of Ti–6Al–4V specimens in contact with varying pad surface conditions. Four conditions were selected: bare Ti–6Al–4V with a highly polished finish, bare Ti–6Al–4V that was low-stress ground and polished to RMS #8 (designated as ‘as-received’), bare Ti–6Al–4V that was grit blasted to RMS #64 (designated as ‘roughened’) and stress relieved, and Cu–Ni plasma spray coated Ti–6Al–4V. Behavior against the Cu–Ni coated and as-received pads were characterized through determination of a fretting fatigue limit stress for a 10⁷ cycle fatigue life. In addition, the behavior against all four-pad conditions was evaluated with S-N fatigue testing, and the integrity of the Cu–Ni coating over repeated testing was assessed and compared with behavior of specimens tested against the as-received and roughened pads. The coefficient of friction, μ, was evaluated to help identify possible crack nucleation mechanisms and the contact pad surfaces were characterized through hardness and surface profile measurements.

An increase in fretting fatigue strength of 20–25% was observed for specimens tested against Cu–Ni coated pads as compared to those tested against as-received pads. The experimental results from the S-N tests indicate that surface roughness of the coated pad was primarily responsible for the increased fretting fatigue capability. Another factor was determined to be the coefficient of friction, μ, which was identified as ˜0.3 for the Cu–Ni coated pad against an as-received specimen and ˜0.7 for the bare as-received Ti–6Al–4V. Specimens tested against the polished Ti–6Al–4V pads also performed better than the specimens tested against as-received pads. Fretting wear was minimal for all cases, and the Cu–Ni coating remained intact throughout repeated tests. The rougher surfaces got smoother during cycling, while the smoother surfaces got rougher.     

Effect of prior cold work on age hardening of Cu-4Ti-1Cd alloy

This research is part of a project whose scope was to develop high strength ternary alloys based on Cu-Ti system with the primary aim of substituting them for toxic and expensive Cu-Be alloys. In this pursuit, age hardening behaviour of Cu-4Ti-1Cd alloy has already been investigated and the present paper reports the investigations on the influence of prior cold work by rolling of 50, 75 and 90% on the age hardening of a Cu-4Ti-1Cd alloy using hardness and tensile tests and optical as well as transmission electron microscopy. As a result of cold work followed by aging, hardness of the alloy increased from 237 Hv in solution treated condition to 425 Hv on 90% cold work and peak aging. Similarly, yield and tensile strengths of the alloy reached maxima of 1037 and 1252 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 obtained due to precipitation of ordered, metastable and coherent β^l, Cu₄Ti phase in addition to high dislocation density and deformation twins. Both hardness and strength of the alloy decreased on overaging due to the formation of incoherent and equilibrium β, Cu₃Ti phase. However, the morphology of the discontinuous precipitation was changed to globular shape due to large deformations and overaging.     

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.     

Effects of Sn content on the microstructure, phase constitution and shape memory effect of Ti–Nb–Sn alloys

The effect of Sn content on the microstructure, phase constitution and shape memory effect of Ti–16Nb–xSn (x = 4.0, 4.5, 5.0 at%) alloys were investigated by means of optical microscopy, X-ray diffraction, transmission electron microscopy and bending test. With the increase of Sn content, the β phase becomes stable. The solution-treated Ti–16Nb–4Sn alloy is composed of ″ and β phases at room temperature, whereas the solution-treated Ti–16Nb–5Sn alloy is only composed of β phase at room temperature. TEM observation shows that there is parallel lamellar ″ martensite with the substructure of () type I twin in the Ti–16Nb–4Sn alloy. There exists the dislocation wall inside the single β phase in the Ti–16Nb–5Sn alloy. The shape recovery ratio decreases with increasing the bending strain and the bending temperature, which is in correspondence with the different deformation mechanisms at different temperature ranges. The shape recovery ratio shows a decreasing trend with the increase of Sn content at the same bending strain and temperature. The maximum completely recovery strain is around 4%.     

Internal stress and adhesion of amorphous Ni–Cu–P alloy on aluminum

This study investigated the effect of saccharin on the internal stress and the adhesion of amorphous Ni–Cu–P deposited on aluminum. An amorphous Ni–Cu–P deposit with slight compressive stress can be produced when one adds 8–10 g/l saccharin into the Ni–Cu–P deposition solution. The stress relief mechanism was investigated. The addition of saccharin restrains the coalescence of the islands within Ni–Cu–P nodules and reverses the internal stress of the electroless Ni–Cu–P deposit from tensile to compressive. The adhesion strength of the Si/Ti/Al/Ni–Cu–P multilayer specimen obtained with 10 g/l saccharin is around 35 to 45 MPa, and the fracture occurs at the silicon substrate after the pull test. The shear strength of the Ti/Al/Ni–Cu–P bump (100×100 μm) on Si is 132.9±12.7 g, and the fracture occurs at the Ni–Cu–P deposit after the shear test. Moreover, the inhibition of coalescence of the fine islands within Ni–Cu–P nodules increases the brightness and the hardness of the deposit.     

On the sequence of clustering and ordering in a meltspun Cu–Ti alloy

Although it is well established that dilute Cu–Ti alloys with titanium in the range of 2.5–5 wt.% decompose by a spinodal mechanism, the sequence of ordering and clustering processes in the early stages has been a matter of controversy. An attempt has been made in this work to resolve some of the issues by carrying out transmission electron microscopy (TEM) investigations on a dilute meltspun Cu–Ti alloy. Decomposition was studied as a function of ageing over a temperature range of 573–723 K. The results seem to suggest that clustering precedes the formation of long range ordered (LRO) phases in this alloy. Formation of a transitory special point N₃M phase was observed for the first time in this work. This disappeared on prolonged ageing giving rise to the metastable Cu₄Ti (D1_a).     

Preparation and mechanical properties of a new Zr–Al–Ti–Cu–Ni–Be bulk metallic glass

In this paper, a new Zr₃₇Al₁₀Ti_12.5Cu_11.25Ni₉Be_20.25 bulk metallic glass is reported. The present alloy was prepared by water quenching in a silica tube of φ10×85 mm. The amorphicity of the quenched bulk samples was examined using X-ray diffraction analysis and optical microscopy. The thermal stability was evaluated by differential scanning calorimetry (DSC) at a heating rate of 10 K/min. The characteristic data of the bulk metallic glass are presented, including glass transition temperature (T_g) and crystallization temperature (T_x). Results show that the present alloy exhibits large glass forming ability. For comparison with the well-known Zr–Ti–Cu–Ni–Be metallic glass, it was found that aluminum has a little effect on the vitrification of the present alloy but influences physical properties. Specifically, Al enhances the Young's modulus by 21.4% and Vickers hardness by 20% and reduces density by 7.2%.     

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.     

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.     

Consolidation of mechanically alloyed powder mixture of Cu–Zn alloy and graphite

A powder mixture consisting of Cu–29.7at.% Zn alloy and graphite was mechanically alloyed in a planetary ball mill. The supersaturated solid solubility of carbon in the Cu–29.7at.% Zn alloy was determined to be 38.5at.% C (alloy composition: Cu–18.3at.% Zn–38.5at.% C) by the change in lattice parameter of the alloy. Supersaturated Cu–24.2at.% Zn–18.5at.% C alloy powder consolidated by a static compression stress of 1.4 GPa was found to have a relative density of 89.7%, a Vickers hardness of 147.2, and a compressive strength of 1.4 GPa which is equal to the statically consolidated compression stress. Moreover, the supersaturated solid-soluble carbon did not precipitate. When dynamically consolidated by a 93 g projectile at a speed of 38.1 m s⁻¹ (estimated impact compression stress of 2.3 GPa) after static precompression of 0.4 GPa, the alloy powder was found to have a relative density of 93%, a Vickers hardness of 177, and a compressive strength of 2.3 GPa which is equal to the impact compression stress. Supersaturated solid solubility of 18.5at.% C decreased to 15at.% C after impact consolidation. The mechanically alloyed powders can maintain supersaturated solid solubility when consolidated by impact pressure, and especially when consolidated by static pressure.     

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.     

Fracture behavior and mechanical properties of Mg–4Y–2Nd–1Gd–0.4Zr (wt.%) alloy at room temperature

The microstructure, mechanical properties and fracture behavior of gravity die cast Mg–4Y–2Nd–1Gd–0.4Zr (wt.%) (WNG421) alloy are studied at room temperature in different thermal conditions, including as-cast, solution-treated and different aging-treated (both isothermal and two-step aging) conditions. The results indicate that WNG421 alloy shows different behaviors of crack initiation and propagation in different thermal conditions during tensile test at room temperature. After pre-aged at 200 °C for 5 h, the hardness of WNG421 alloy first reduces and then increases when secondary aged at 250 °C (two-step aging). The peak hardness and corresponding tensile strength of the two-step aged alloy both increases compared with those in 250 °C isothermal peak-aged condition. Tensile strength of WNG421 alloy at room temperature in low temperature (200 °C) isothermal peak-aged condition is much higher than that in high temperature (250 °C) isothermal peak-aged condition.     

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.     

Aging behaviour of Al–Cu–Mg alloy–SiC composites

The age-hardening kinetics of powder metallurgy processed Al–Cu–Mg alloy and composites with 5, 15 or 25 vol.% SiC reinforcements, subjected to solution treatment at 495 °C for 0.5 h or at 504 °C for 4 h followed by aging at 191 °C, have been studied. The Al–SiC interfaces in composites show undissolved, coarse intermetallic precipitates rich in Cu, Fe, and Mg, with its extent varying with processing conditions. Examination of aging kinetics indicates that the peak-age hardness values are higher, and the time taken for peak aging is an hour longer on solutionizing at 504 °C for 4 h, due to greater solute dissolution. Contrary to the accepted view, the composites have taken longer time to peak-age than the alloy, probably due to lower vacancy concentration, large-scale interfacial segregation of alloying elements, and inadequate density of dislocations in matrix. The composite with 5 vol.% SiC with the lowest inter-particle spacing has shown the highest hardness.     

Synthesis of in situ Al–TiB2 composites using stir cast route

When elemental Ti and B powders were added to molten Al at above 1000°C, fine in situ TiB₂ particulates were formed through Al–Ti–B exothermic reaction. By optimising the nucleation of TiB₂, the tensile and yield strengths of a synthesised Al–15V_f%TiB_s composite were twice that of matrix material. Modification of Al-matrix with 4.5 wt%Cu tripled the tensile and yield strengths at peak-aged condition. Owing to the co-presence of brittle Al₃Ti flakes with TiB₂ particles in the composites synthesised by the Al–Ti–B system, ductility was reduced to 68% and 84% in composites with Al- and Al–Cu matrices, respectively. When the (Ti + B) mixture was incorporated with 3 wt%C, TiB₂ and TiC reinforcing phases were simultaneously produced in the composite with Al–Cu matrix. Such an approach reduced Al₃Ti compound in the composite considerably. Although the presence of Cu in the composite was found to promote the formation of Al₃Ti, its effect on the fluidity caused the melt recovery to increase from 33% to 52%.     

Effect of oxidation on the creep behaviour of copper–chromium in situ composite

The creep deformation and fracture behaviours of a Cu–Cr in situ composite were investigated in air and in vacuum over a temperature range of 400–650 °C to study the effect of environment. The similarities of the activation energy and the stress exponent in air and in vacuum strongly suggest that the oxygen and/or the oxide have no direct effect on the deformation mechanism of Cu–Cr in situ composite. The higher creep rate of the composite in air than in vacuum is due to the gradual decrease of the cross-sectional area of the matrix due to increasing thickness of the oxide layer. The mechanism of damage was found to be similar for all the creep tests performed.     

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.     

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.