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.     

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 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%.     

High-temperature internal friction on TiAl intermetallics

In order to study the microscopic mechanisms controlling the plastic deformation at high temperature of two γ-TiAl alloys with nominal compositions: Ti–46.5Al–4(Cr, Nb, Ta, B) and Ti–45Al–(5–10)Nb (in at.%), internal friction (IF) measurements have been performed. A subresonant torsion pendulum has been used working in two different modes; changing the temperature between 300 and 1200 K at a constant frequency, and varying the frequency between 0.001 and 10 Hz at a fixed temperature. The resulting mechanical spectra show an anelastic relaxation peak at about 1060 K at 1 Hz in one of the alloys and a high-temperature background in both of them. Activation parameters of the loss peak have been calculated and an activation enthalpy of about 3.8 eV is obtained. The characteristics and a possible responsible mechanism for this relaxation peak are discussed.     

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.     

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.     

Investigations of MX and γ′/γ″ precipitates in the nickel-based superalloy 718 produced by electron beam melting

Samples with a composition similar to the nickel-based superalloy Inconel alloy 718 were produced by electron beam melting of prealloyed powder and investigated with respect to type and composition of the strengthening precipitates. The matrix consists of γ grains orientated in nearly the same direction, almost like a single crystal. Coarse precipitates (<2 μm), mostly of the (Ti,Nb)(C,N,B) type with B1 structure, are aligned along the growth direction. TEM and APFIM investigations of the γ matrix revealed very fine γ″ precipitates of around 5–10 nm in size. Additionally, at small angle grain boundaries, coarser γ″ precipitates of 50–100 nm in size have been observed. The 0 01 γ//0 0 1 γ″ and {1 0 0} γ//{1 0 0} γ″ orientation relationship between γ and γ″, known from literature [M. Sundararaman, P. Mukhopadhyay, Mater. Charact. 31 (1993) 191–196], was confirmed. Some γ′ precipitates of 2–5 nm in size were observed by means of FIM.     

Research and Improvement on structure stability and corrosion resistance of nickel-base superalloy INCONEL alloy 740

INCONEL alloy 740 is a newly developed Ni–Cr–Co–Mo–Nb–Ti–Al superalloy in the application to ultra-supercritical boilers with steam temperatures up to 700 °C. By means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), micro-chemical phase analyses, and corrosion-resisting test, this paper investigates the structure stability of the alloy at elevated temperature and concentrates on coal ash corrosion performance of the alloy under the simulated coal ash/flue gas condition. Experimental results show that the most important structure instabilities of the alloy during prolonged aging are γ′ coarsening, γ′ to η transformation and G phase formation at grain boundary. The performance of corrosion resistance of the alloy would meet the requirement of ultra-supercritical boiler tubes. The phase computation by means of Thermo-Calc has been adopted in chemical composition modification for structure stability improvement. Two suggested new modified alloys in adjustment of the Al and Ti contents and in control of Si level, and also in maintenance of Cr content of the alloy were designed and melted for experimental investigation. These two modified alloys exhibit more stable microstructure during 760 °C long time aging.     

On orientation relationship of the Ti5Si3 precipitates in a TiAl alloy

The orientation relationship (OR) and interface structure between ζ-Ti₅Si₃ precipitates and γ-TiAl phase in a Ti–Al based alloy composed of γ-TiAl and ₂-Ti₃Al lamellae have been investigated using transmission electron microscopy. Various orientation relationships defined by a pair of parallel directions and planes are discussed with the method of basic vector transformation matrix in the reciprocal space from γ-TiAl to ζ-Ti₅Si₃ precipitate phase and two new kinds of orientation relationships between ζ-Ti₅Si₃ and γ-TiAl phases have been found. Periodical interface fringes at γ-TiAl/ζ-Ti₅Si₃ interface are analyzed according to the Moiré fringes and interface misfit dislocations.     

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.     

The effect of copper addition on the structure and strength of an Al–Li alloy

In this study the effect of copper addition on the structure, precipitation kinetics and hardness in the Al–Li and Al–Li–Cu alloys aged at 200°C was investigated. The structures of precipitates were studied using X-ray-small-angle-scattering (XSAS) and transmission electron microscopy (TEM) methods. The changes in the structure parameter (Rg) of both alloys was calculated using two methods, the Guinier approximation and correlation function γ(r). By use of a plot of r γ(r) the distribution law of the T₁ disc thickness was obtained and the coexisting spherical particles of δ′ were estimated. Two types of δ′ precipitates of approximately 2 nm size and above 8 nm and the T₁ precipitates of thickness between 3 and 4 nm were observed.     

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.     

Microstructure and grain refining performance of Al–5Ti–1B master alloy prepared under high-intensity ultrasound

The microstructure and grain refining performance of an Al–5Ti–1B master alloy prepared under high-intensity ultrasound were investigated. With applying continuous high-intensity ultrasound vibrations in the reaction, the Al–5Ti–1B master alloy is successfully manufactured in 4 min. Compared with conventional Al–5Ti–1B master alloys, the mean size and the size spread of TiB₂ particles in the prepared master alloy are evidently decreased. The narrower particle size spread significantly improves the grain refining performance of the master alloy, which proves the calculation predictions by Greer. Consequently, the limiting grain size of commercial purity aluminium refined by the new master alloy can reach 45 μm.     

Effect of β-spodumene on the phase development in an alumina/aluminium-titanate system

The effect of β-spodumene additions on the in situ phase formation and abundances in an Al₂O₃–Al₂TiO₅ system in the temperature range 1000–1400 °C has been studied by neutron diffraction and differential thermal analysis. Results show that β-spodumene began to decompose by phase separation and partial melting at 1290 °C, followed by complete melting at 1330 °C. Formation of Al₂TiO₅ was observed to occur at 1310 °C and its abundance increased with temperature. The addition of β-spodumene as a sintering aid did not cause its reaction with alumina or rutile to form additional phases. Addition of β-spodumene in excess of 5 wt% resulted in pronounced vitrification, which partly recrystallised when cooled to room temperature. The temperatures of Al₂TiO₅ formation and melting of β-spodumene are consistent with the results of differential thermal analysis.     

A new way to produce Al + Cr coating on Ti alloy by vacuum fusing method and its oxidation resistance

An Al + Cr coating successfully produced by the vacuum fusing method was proposed for improving the oxidation resistance of Ti–6Al–4V alloy specimens. Unlike powder pack cementation, this technique was much more effective and cheap without the need for long time diffusion, and the Al concentration and the thickness of vacuum fusing coating layer could be controlled by adjusting powder mixing ratio and adding the immersed times, respectively. Compared with the specimen with an Cr-modified aluminide coating, the oxidation resistance of the specimens with vacuum fused Al + Cr coating was about two times than that of the specimens with Cr-modified aluminide coating. During oxidation, Cr additions suppressed metastable θ-alumina formation. It is only one phase -alumina scale that developed on the vacuum fused Al + Cr coating surface, while a single metastable θ-alumina scale formed on the Cr-modified aluminide coating surface.     

Internal friction during martensitic transformation in high manganese Mn–Cu alloys

Low-frequency internal friction and elastic modulus were studied for manganese-rich Mn–Cu alloys in the temperature range of martensitic transformation (20–300 °C). It is shown that the some special features of the transformation peak and its temperature are caused by the degree of the spinodal decomposition. The phenomenological model connecting an-elastic effects with the stages of evolution of the structure during martensitic transformation in manganese-rich Mn–Cu alloys (tweed structure–“parquet” structure–classical twinning martensite) is presented.     

Effect of forging conditions and annealing temperature on fatigue strength of two-phase titanium alloys

This paper reports on the results of studies on the influence of deformation degree and temperature in the die forging process and annealing temperature on fatigue strength of forgings made of two-phase, martensitic titanium alloys (Ti–6Al–4V and Ti–6Al–2Mo–2Cr). Dilatometric and metallographic studies have been carried out along with fatigue tests. The influence of phase composition and microstructure obtained after cooling at different rates on fatigue strength has been determined.     

Microstructural changes inside the lamellar structures of alloy ZA27

The microstructural changes inside the lamellar structure of the cast and aged alloy ZA27 were studied using TEM, XRD and SEM techniques. Using TEM, the network of transitional phase η_m^′ was determined to be of an fcc crystal structure inside the lamellae η phase during ageing at 150 °C. The mechanism of the decomposition of the η phase lamellae can be summarized as follows: η → η_m^′ + η′ →  + η. The adjacent co-existence of the ε phase and the T′ phase inside the phase lamellae confirmed that a four phase transformation,  + ε → T′ + η, had occurred during the prolonged ageing.     

Electrochemical corrosion properties of Ti–6Al–4V implant alloy in the biological environment

The electrochemical corrosion behavior of Ti–6Al–4V implant alloy was investigated in three biological solutions, i.e. urine, serum and joint fluid. The corrosion properties of Ti–6Al–4V implant alloys were examined by using electrochemical techniques, such as the potentiodynamic method, cyclic voltammetry, electrochemical impedance spectroscopy (EIS). The electrochemical corrosion characteristics of Ti–6Al–4V implant alloys in three biological solutions were measured in terms of the corrosion potential (E_corr), the corrosion current density (i_corr), and ac polarization resistance (R_p). The corrosion kinetic parameters were calculated from both the Tafel plot analyses and EIS analyses. The dependence of impedance versus potentials was studied at 37 °C at various offset potentials in three biological solutions. The ac circuit model for Ti–6Al–4V implant alloy at corrosion interface in biological solution was proposed, which was based on a simple Randles equivalent circuit. It was found that the Ti–6Al–4V implant alloy in three biological solutions showed a characteristic of a capacitive behavior. The experimental results of Tafel plot analyses were found in good agreement with that of EIS analyses.     

Formation of nanocrystalline structure of Ti–6Al–4V alloy by cyclic hydrogenation–dehydrogenation treatment

Ti–6Al–4V (Ti64) sheet specimens were cathodically hydrogenated in sulfuric acid solution at ambient conditions. The hydrogenated specimens were then sent to go through the designed thermohydrogen processing (THP) twice to obtain a nano-sized grain structure. The average grain size of resulted microstructure was found to be 10–20 nm obtained by TEM. Qualitative and quantitative analyses performed by employing X-ray diffractometry (XRD) and elemental analysis (EA) showed that the addition of As₂O₃ as hydrogenation promoter in electrolyte significantly increased the hydrogen uptake. The high concentration of hydrogen arising from promoter action is the key factor in grain refinement. The optimal processing parameter found for grain-refining Ti64 was: (1) electrolytic hydrogenation at 100 mA cm⁻² for 3 h in 1 N H₂SO_4(aq) by adding 0.1 g L⁻¹ As₂O₃; (2) β transformation carried out at 850 °C for 1 h in air furnace, followed by a furnace cooling to 590 °C and held for 6 h; (3) oxide film removed and then dehydrogenated at 650 °C and 1.0 × 10⁻⁶ Torr for 10 h; (4) repeated the same processes once more.