Work-hardening effect and strain-rate sensitivity behavior during hot deformation of Ti–5Al–5Mo–5V–1Cr–1Fe alloy

The work-hardening effect and strain-rate sensitivity behavior during hot deformation have been quantitatively investigated in this present paper. Isothermal compression experiment of Ti–5Al–5Mo–5V–1Cr–1Fe titanium alloy has been conducted for verification. Linear relationship between work-hardening rate and true strain/stress has been derived from Kocks–Mecking dislocation relation. The work-hardening effect shows two obvious stages with strain: steady fluctuations and linear decreasing. Obvious work-hardening effect could be demonstrated under lower temperatures and higher strain rates. The work-hardening decrease at linear-decreasing regime becomes more stronger with temperature elevated and rate lowered, reverse-proportional to Zener–Hollomon parameters. Strain-rate sensitivity coefficient for hot deformation was decomposed into three parts from JMAK recrystallization kinetics. The influence of strain rate on DRX evolution has been termed as the major factor determining strain-rate sensitivity. Strain-rate sensitivity coefficients for steady-state deformation (ɛ = 0.7) of Ti–5Al–5Mo–5V–1Cr–1Fe alloy have been characterized as a function of deformation parameters and strain-rate sensitivity has been identified more obvious with temperature elevated and rate lowered.     

Low density steel 1·2C–1·5Cr–5Al designed for bearings

Abstract

A novel alloy design, designated as 1·2C–1·5Cr–5Al, has been proposed with high aluminium(～5 wt-%) and more carbon(～1·2 wt-%) addition into the classical 1C–1·5Cr bearing steel for lowering density and improving performance simultaneously, which is approximate 8 wt-% lighter than convention. In order to understand preliminarily the suitability of the novel alloy for bearing application, the martensite starting temperature and hardness, related to microstructure evolution and mechanical properties, respectively, after partial austenitisation treatment with undissolved carbides have been investigated carefully. The martensite starting temperature is comparable with conventional 1C–1·5Cr alloy. The hardness of 860±3 HV20 achieved is much higher than convention.     

On mechanism of flow softening in Ti–5Al–5Mo–5V–3Cr

Abstract

The flow behaviour of Ti–5Al–5Mo–5V–3Cr with an initial microstructure containing acicular α platelets has been characterised during isothermal subtransus forging. Flow softening was observed, following yielding and limited hardening, for all investigated temperatures and strain rates, before a steady state flow regime being reached at a strain of ～0·5. The acicular α plates were found to have been fragmented by the forging process, which is concurrent with previous findings. The flow behaviour of the fully retained β phase below the β transus temperature has been established and found to be similar to that of the steady state flow of platelet α in a β matrix. Forging the acicular α microstructure to low strains resulted in higher dislocation concentrations in the β matrix than could be observed in the α precipitates, supporting the hypothesis of hardening through dislocation impedance. Evidence of fragmentation via a pinch-off mechanism was found where slip was observed to have been transmitted across an α plate. Thus following yield the flow behaviour is dominated by the pile-up of dislocations at the α/β interface before the transmission of slip leads to plate fragmentation resulting in flow softening to a steady state regime governed entirely by the β matrix.     

Mechanical performance of extruded Ti–46Al–5Nb–1W titanium aluminide

Abstract

Titanium aluminide alloys offer considerable promise for use in high temperature applications, such as gas turbines. In this study an extruded Ti–46Al–5Nb–1W alloy has been examined, in terms of its tensile and creep behaviour. A reasonably fine and uniform microstructure was found in this bar product. This gave excellent properties, with tensile strengths up to ～950 MPa at room temperature, along with 1% elongation. These properties were accompanied by a very good creep behaviour, with low primary strains at the lower stresses and very low secondary creep rates. Comparison of the creep properties of this titanium aluminide alloy with other similar compositions and some typical nickel alloys shows that it is significantly superior to first generation titanium aluminides but also nickel alloys, such as IN718 and Udimet 720Li. However, the strain controlled fatigue performance of the titanium aluminide alloy was significantly poorer than these same wrought nickel alloys.     

Consolidation of Cr3C2 doping WC–1TiN–5MgO micro–nano composites by two-step sintering

The effect of Cr₃C₂ additions on WC–1TiN–5MgO composites by two-step hot-pressing sintering (heated to 1750°C and then immediately cooled to 1575°C with a soaking time of 60?min under a sintering pressure of 50?MPa) was comprehensively investigated. The microstructure was characterized by means of scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), and X-ray diffractometry (XRD). Mechanical properties, such as hardness, transverse rupture strength, and fracture toughness, were measured. The experimental results show that no η-phase or brittle phases such as W₂C were formed, and excellent mechanical properties were achieved for 0.6?wt.% Cr₃C₂ additions with a hardness of 24.76?GPa, a flexural strength of 1257.1?MPa, and a fracture toughness of 10.08?MPa?·?mm^1/2. Cr₃C₂ addition brought about an improvement in the sinterability and contributed to the homogeneous distribution of second phase nanosized MgO. Crack deflection and crack bridging are the major mechanisms contributing to the drastically enhanced flexural strength and fracture toughness.     

Effect of corrosion attack on the fatigue behavior of an as-cast Mg–7%Gd–5%Y–1%Nd–0.5%Zr alloy

Through investigating and comparing the fatigue behavior of an as-cast Mg–7%Gd–5%Y–1%Nd–0.5%Zr alloy in both laboratory air and 3.5 wt.% NaCl solution, the effect of corrosion attack on fatigue crack initiation has been disclosed. The S–N curves showed that the fatigue strength in air was 120 MPa and not sensitive to the loading frequency, whereas the fatigue strength in NaCl solution decreased from 80 MPa to 60 MPa with the loading frequency decreasing from 20 Hz to 5 Hz. Observations to fracture surfaces demonstrated that in air, fatigue cracks preferentially initiated at the oxide inclusions. However, the fatigue crack initiation in NaCl solution was associated with corrosion pits. Moreover, multiple fatigue cracks initiated at pits on fracture surfaces of corrosion fatigue failed samples when the loading frequency decreased to 5 Hz. Based on the measured “defect area” of oxide inclusions, the predicted fatigue strength in air could be well fitted with the experimental data. However, due to the occurrence of hydrogen embrittlement and crack initiation at multiple sites, the fatigue strength of samples tested in NaCl solution cannot be predicted.     

Welding of rapidly solidified Alloy 8009 (Al–8·5Fe–1·7Si–1·3V): preliminary study

Abstract

Alloy 8009 is a rapidly solidified, dispersion strengthened Al–8·5Fe (wt-%) alloy designed for high temperature (up to 400°C) aerospace applications. Both fusion and solid state joining techniques were shown to produce bonds. Fusion techniques destroyed the base metal microstructure with primary Fe₃Al, loss of solute, formation of larger aluminium grains, and the formation of grain boundary FeAl₃ and intermetallics enriched with silicon and vanadium. Solid state friction stir welding did not cause a significant modification to the dispersoid population but there was a loss of solute to dispersoid/matrix interfaces.

MST/3500     

Effects of Sn on microstructure and mechanical properties of as-rolled Mg–5Zn–1Mn alloy

Effects of Sn on microstructure and mechanical properties of Mg–5Zn–1Mn alloy subjected to high strain rate rolling (9.1?s^-1), 300°C and 80% pass reduction are investigated. With higher Sn content, the dynamic recrystallisation (DRX) grain size gradually decreases due to the stronger pinning of nano-scale precipitates at grain boundaries and the DRX fraction first increases due to the enhanced effect on DRX by decreasing stacking fault energy and then decreases due to more precipitates at grain boundaries. Ultimate tensile strength (UTS) and elongation to rupture (Er) of as-rolled alloys increase and then decrease. Alloy with 0.9 mass% Sn exhibits the highest DRX fraction (95?vol.-%), the finer DRX grain size (1.22?µm), UTS of 358?MPa and Er of 20.4%.     

Subgrain growth in presence of nanosized dispersoids in Al–4·5Zn–1Mg alloy

Abstract

In this paper, an analytical model for subgrain growth in the presence of nanosized dispersoids is presented. The growth rate of subgrains is correlated to the mobility of low angle grain boundaries (LAGBs) and the net driving force for growth. The driving force is considered as the difference between stored energy, being inversely proportional to the average subgrain size, and the Zener drag pressure. A material dependent constant necessary for the determination of the mobility of LAGBs is estimated by fitting the model predictions into the experimental results. Model predictions of the evolution of subgrain sizes with annealing time at different temperatures show that subgrain growth intensifies with increasing annealing temperature. The magnitude of the Zener drag pressure has a predefined effect on the subgrain growth rate. The model predicts that when the P_Z/γ_s ratio is smaller than 1 μm^?1, the Zener drag pressure has an effect on subgrain size and the subgrain growth rate tends to decrease. However, when the P_Z/γ_s ratio is larger than 1 μm^?1, there is a limit beyond which the subgrain size does not increase with increasing annealing time. The limiting subgrain size is a function of the surface boundary energy and Zener drag pressure.     

Relationship between microstructure and deformation behaviour during dynamic compression in Ti–3Al–5Mo–5V alloy

Abstract

The relationship between microstructure and deformation and damage behaviour during dynamic compression in Ti–3Al–5Mo–5V alloy has been studied using several experimental techniques, including optical microscopy, scanning electron microscopy and microhardness measurements. It was found that the deformation behaviour during dynamic compression was closely related to deformation parameters. After dynamic deformation, the deformation shear band that formed in the titanium alloy had microhardness similar to that of the matrix. However, the microhardness of the white shear band was much higher than the matrix microhardness. The effects of deformation parameters, including deformation rate and deformation degree, on deformation localisation were investigated. Based on the results from the present work, the microstructure and deformation processing parameters can be optimised. In addition, treatment methods after dynamic compression were explored to restore alloy properties. Using post-deformation heat treatment, the microstructure and property inhomogeneity caused by shear bands could be largely removed.     

Temperature and strain rate effect on tensile properties of 9Cr–1·8W–0·5Mo–VNb steel

Abstract

Tensile tests have been carried out on 9Cr–1·8W–0·5Mo–VNb steel (grade 92) over wide ranges of temperature (300–923 K) and strain rate (3×10^?3–3×10^?5 s^?1). The tensile strength of the steel decreased slowly with temperature at relatively lower temperature range, whereas rapidly in the higher temperature range with a plateau in the intermediate temperature range. The decrease in strain rate decreased the tensile strength of the steel both at lower and higher temperature ranges. Elongation to fracture and reduction in area increased with increase in temperatures and decrease in strain rate at higher temperature regime with a plateau in the intermediate temperature regime. The ductile mode of tensile failure has been observed in the investigated temperatures and strain rates. The plateau in the variation of tensile strength with temperature, the negative strain rate sensitivity of tensile strength and minimum in ductility of the steel in the intermediate temperature range are considered as a consequence of dynamic strain ageing. The rapid decrease in tensile strengths and increase in ductility at high temperatures have been attributed to the dynamic recovery.     

Corrosion behavior of Ti–8Al–1Mo–1V alloy compared to Ti–6Al–4V

The corrosion behavior of Ti–8Al–1Mo–1V alloy was investigated in 3.5% NaCl and 5% HCl solutions. Corrosion properties of Ti–6Al–4V alloy were also evaluated under the same conditions for comparison. It was found that both Ti–8Al–1Mo–1V and Ti–6Al–4V alloys exhibited spontaneous passivity and low corrosion current densities in 3.5% NaCl solution. The potentiodynamic polarization curves obtained in 5% HCl solution revealed an active–passive transition behavior and similar corrosion rates for the examined alloys. However, the results of the weight loss experiments under accelerated immersion conditions (5 M HCl at 35 °C) indicated that Ti–8Al–1Mo–1V alloy exhibited inferior corrosion behavior compared to Ti–6Al–4V alloy. These results were confirmed by scanning electron microscopy (SEM) analysis of the samples after immersion tests which revealed that the β phase was corroded preferentially for both alloys, but to a larger extent in the case of Ti–8Al–1Mo–1V alloy.     

Mechanical properties and defective effects of bcc V–4Cr–4Ti and V–5Cr–5Ti alloys by first-principles simulations

V–(4–5) wt.% Cr–(4–5) wt.% Ti alloys are important candidate structural materials for the first-wall and blanket in future fusion reactor. Thus it is necessary to study the fundamental mechanical properties and the irradiation effects of the V-based alloys. Within a random solid solution model, the elastic constants and ideal strength of the V–4Cr–4Ti and the V–5Cr–5Ti alloys were calculated and compared with those of pure V solid. According to the theoretical Cauchy pressure and the ratio of bulk modulus and shear modulus, both alloys exhibit good ductility. Within the 250-atom supercell, inclusion of one vacancy defect or one interstitial H (He) atom will further enhance the ductility of these alloys.     

Carrier Mobility in GaSb–V2Ga5and GaSb–GaV3Sb5 Eutectic Alloys

Experimental evidence is presented that semiconductor–metal eutectics with a low content of the metallic phase (4 vol %) are similar in electronic structure to inhomogeneous semiconductors. The microstructure of undoped and Te-doped GaSb–V₂Ga₅ and GaSb–GaV₃Sb₅ eutectic alloys is examined, and the Hall mobility of carriers in these alloys is determined. The anomalous temperature variation of Hall mobility in GaSb–V₂Ga₅ ( T ²) and GaSb–GaV₃Sb₅ ( T ⁵) is interpreted in terms of infinite clusters ofn-type metallic inclusions embedded in a p-type semiconductor matrix and interconnected through overlapping inhomogeneous interfaces. It seems likely that the difference in conductivity type between the semiconductor matrix and the infinite clusters gives rise to a random large-scale potential relief. Te compensation of GaSb in the eutectic alloys causes the Hall mobility to vary more rapidly with temperature, T ³ to T ¹⁰, which is interpreted as due to an increase in the amplitude of the random large-scale potential relief, the formation of infinite clusters, and partial compensation of unintentional acceptor doping in the semiconductor matrix.     

EPR study of V2O5–P2O5–Li2O glass system

glass system, with 0 < x 50 mol%, was prepared and investigated by EPR method. For low content of V₂O₅ all the spectra present a hyperfine structure typical for isolated V⁴⁺ ions. With the increasing of V₂O₅ content, the EPR absorption signal showing hyperfine structure is superposed by a broad line without hyperfine structure characteristic for clustered ions. At high V₂O₅ content, the vanadium hyperfine structure disappears and only the broad line can be observed in the spectra. Spin Hamiltonian parameters g , g , A , A , dipolar hyperfine coupling parameters, P, and Fermi contact interaction parameters, K, have been calculated.The composition dependence of line widths of the first two absorptions from the parallel band and of the broad line characteristic to the cluster formations was also discussed.     

Intergranular cavity damage and creep fracture of 1Cr–0·5Mo steels

Abstract

The factors controlling the intergranular fracture of three 1Cr–0·5Mo steels, tested at 550°C, have been examined. Failure results from the nucleation and growth of grain-boundary cavities. It is shown that creep life is dependent on the maximum principal stress, and that variations in the rupture properties of the steels are controlled by their susceptibility to nucleate intergranular cavities. Increasing the metalloid element content and, in particular, increasing the austenitizing temperature from 930 to 1300°C resulted in an increase in the cavity nucleation rate and a concomitant decrease in the rupture life. The cavity nucleation rate was found to be dependent on the maximum principal stress and when this dependence is used in conjunction with a simple cavity diffusion growth model the stress-state dependence of rupture life and the effect of residuals and austenitizing temperature on fracture properties could be predicted. These results are discussed in terms of the material and fabrication factors and service conditions that designers and operators of high-temperature plants must consider so that the plant may be operated safely and efficiently.

MST/81     

Dynamic compressive behaviour and microstructural evolution of extrusion-shear Mg–6Zn–1Cu–1Y–0.6Zr alloy

In this work, the alloy Mg–6Zn–1Cu–1Y–0.6Zr was prepared using an extrusion-shear method, which combines traditional extrusion with the equal channel angular pressing. Dynamic compressive behaviour and microstructural evolution were studied along the extrusion direction with strain rates in the range of 695–1995?s^?1 using a Split-Hopkinson pressure bar. The dynamic compression properties have a distinct positive strain rate strengthening effect. A texture transition from ?10–10? into ?0001? is found in the Mg–6Zn–1Cu–1Y–0.6Zr alloy. Analysis of the microstructural evolution shows that {10–12} extension twinning and (0002) basal-type slip are major deformation mechanisms. The absorption energy density dramatically increases as the strain rate increases, results indicate that dynamic recrystallisation and high yield strength are mainly responsible for the high energy absorption capacity.     

Microstructure and mechanical properties of as aged Mg–3Sn–1Al and Mg–3Sn–2Zn–1Al alloy

Effect of Zn on the microstructure, age hardening response and mechanical properties of Mg–3Sn–1Al alloy which is immediately aged at 180°C after extrusion process (T5) was investigated. It was found that the Zn can refine the microstructure, remarkably improve the aging response with the peak hardness increases to 75 HV and the time to peak hardness reduces from ～110 to ～60 h, which is attributed to the solid solution hardening of Al, Zn and an amount of finer Mg₂Sn precipitates. The as aged Mg–3Sn–2Zn–1Al alloy exhibits better mechanical property at room temperature or 150°C than that of Mg–3Sn–1Al alloy, which is ascribed to the fine grained microstructure and thermally stable Mg₂Sn particles dispersed at grain boundaries and in the matrix.     

Plastic flow and microstructural evolution in Ti–6Al–2Zr–1Mo–1V alloys during hot deformation

Abstract

The hot deformation behaviour and microstructural evolution in Ti–6Al–2Zr–1Mo–1V alloys have been studied using isothermal hot compression tests. The processing map was developed at a true strain of 0·7 in the temperature range 750–950°C and strain rate range 0·001–10 s^?1. The corresponding microstructures were characterised by means of a metallurgical microscope. Globularisation of lamellae occurring to a greater extent in the range 780–880°C and 0·001–0·01 s^?1 had a peak power dissipation efficiency of 58% at about 850°C and 0·001 s^?1. The specimens deformed in 750–880°C and 0·01–10 s^?1 showed an instability region of processing map, whereas the specimens deformed in 880–950°C and 1–10 s^?1 indicated three kinds of flow instabilities, i.e. macro shear cracks, prior beta boundary cracks and flow localisation bands.