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.     

Effect of aging on the tensile properties and microstructures of a near-alpha titanium alloy

The effect of aging temperature between 650 °C and 750 °C for different aging times on the tensile properties and microstructures of Ti60 alloy were studied. The results show that the strength of the alloy increases first and then decreases with the aging temperature increases from 650 °C to 750 °C. The reduction of area of the alloy is more sensitive to the aging time than elongation. With increasing aging temperature and time, the volume fracture and grain size of silicides and α₂ phase increase gradually. The silicides have the strengthen effect on the Ti60 alloy, but the effect weakens when the silicides grow up. The loss of ductility is mainly attributed to the precipitation of α₂ phase after aging treatment.     

Anisotropy of Young's modulus and tensile properties in cold rolled α′ martensite Ti–V–Sn alloys

Young's modulus and tensile properties of cold rolled Ti–8 mass% V and (Ti–8 mass% V)–4 mass% Sn alloy plates consisting of α′ martensite were investigated as a function of tensile axis orientation in this work. A single phase of α′ (hcp) martensite is obtained in Ti–8 mass% V and (Ti–8 mass% V)–4 mass% Sn alloys by quenching after solution treatment. By 86% cold rolling, acicular α′ martensite microstructures change into extremely refined dislocation cell-like structure with an average size of 60 nm, accompanied with the development of cold rolling texture in which the basal plane normal is tilted from the plate normal direction (ND) toward transverse direction (TD) at angles of ±49° for Ti–8% V alloy and ±46° for (Ti–8 mass% V)–4 mass% Sn alloy. No apparent anisotropy of Young's modulus (E) is observed for as-quenched Ti–8% V (E = 76–83 GPa) and (Ti–8% V)-4%Sn (E = 69–79 GPa). In contrast, Young's modulus increases with increasing angle from the rolling direction (RD) to TD for cold rolled Ti–8% V (E = 72–94 GPa) and (Ti–8% V)–4%Sn (E = 63–85 GPa). The observed anisotropy of Young's modulus can be reasonably explained in terms of the cold rolling α′ texture.0.2% proof stress and tensile strength are independent of tensile orientation for cold rolled Ti–8% V and (Ti–8% V)–4%Sn alloys. In contrast, larger elongation to fracture is obtained in specimens deviated by 30°, 45° and 60° from RD than by 0°, 75° and 90°. Scanning electron microscopy (SEM) fractographs reveal that quasi-cleavage-like fracture plane appears in 0° specimen of cold rolled Ti–8% V which shows brittle fracture and other specimens of cold rolled Ti–8% V and (Ti–8% V)–4%Sn alloys are fractured accompanied with necking and dimple formation. It is suggested from these results that brittle fracture is related to the activation of limited number of slip system and Sn addition leads to the activation of multiple slip systems.     

Tensile and fatigue properties of a cast aluminum alloy with Ti, Zr and V additions

The development of aluminum alloys for automotive powertrain applications is in high demand due to the required weight reduction and fuel efficiency. The aim of this study was to evaluate the microstructure and mechanical properties of a newly developed Al-7%Si-1%Cu-0.5%Mg cast alloy with further additions of Ti, Zr and V. The microstructure of the alloys consisted of Al dendrites surrounded by Al-Si eutectic structures with Mg/Cu/Fe-containing Si particles, and contained nano-sized trialuminide precipitates in the Ti/Zr/V added alloys. The alloys had a significantly (60-87%) higher yield strength but lower ductility than A356-T6 and 319-T6 alloys. With the addition of Ti/Zr/V both monotonic and cyclic yield strengths increased, but ductility and hardening capacity decreased due to reduced dislocation storage capacity caused by stronger interactions between dislocations and trialuminide precipitates. The Zr/V-modified alloy had a longer fatigue life, and all the alloys exhibited cyclic stabilization at low strain amplitudes and cyclic hardening at higher strain amplitudes. With increasing strain amplitude, the extent of cyclic hardening increased. Both cyclic yield strength and cyclic strain hardening exponent were higher than the corresponding monotonic yield strength and strain hardening exponent, indicating that a stronger cyclic hardening ability of the alloys developed. Fatigue cracks were observed to initiate at near-surface defects, and crack propagation was mainly characterized by the formation of fatigue striations together with secondary cracks.     

A study on the mechanical properties of cryorolled Al–Mg–Si alloy

The mechanical properties of a precipitation hardenable Al–Mg–Si alloy subjected to cryorolling and ageing treatments are reported in this present work. The severe strain induced during cryorolling of Al–Mg–Si alloys in the solid solutionised state produces ultrafine microstructures with improved mechanical properties such as strength and hardness. The improved strength and hardness of cryorolled alloys are due to the grain size effect and higher dislocation density. The ageing treatment of cryorolled Al–Mg–Si alloys has improved its strength and ductility significantly due to the precipitation hardening and grain coarsening mechanisms, respectively. The reduction in dimple size of cryorolled Al–Mg–Si alloy upon failure confirms the grain refinement and strain hardening mechanism operating in the severely deformed samples.     

Effect of equal channel angular pressing and heat treatment on the microstructure of Cu–Al–Be–B shape memory alloy

The combination of equal channel angular pressing (ECAP) and heat treatment was carried out to modify the microstructure of a Cu–Al–Be–B shape memory alloy. Microstructures of the alloy after ECAP and subsequent quenching were investigated by optical microscopy and X-ray diffraction (XRD). The alloy with 8 passes of ECAP at 743 K is characterized with ultra-fine grains (~ 2 μm), but with smaller fraction of martensites which implies the lower shape memory effect (SME). After reheated at 873 K and oil-quenched to room temperature, the grains become coarsen (~ 50 μm) but still finer than that of as-received (100–300 μm), and the fraction and order of martensites were increased simultaneously.     

Effect of Al2O3 particles on mechanical properties of directionally solidified intermetallic Ti–46Al–2W–0.5Si alloy

The effect of Al₂O₃ particles on microhardness and room-temperature compression properties of directionally solidified (DS) intermetallic Ti–46Al–2W–0.5Si (at.%) alloy was studied. The ingots with various volume fractions of Al₂O₃ particles and mean ₂–₂ interlamellar spacings were prepared by directional solidification at constant growth rates ranging from 2.78×10⁻⁶ to 1.18×10⁻⁴ ms⁻¹ in alumina moulds. The ingots with constant volume fraction of Al₂O₃ particles and various mean interlamellar spacings were prepared by directional solidification at a growth rate of 1.18×10⁻⁴ ms⁻¹ and subsequent solution annealing followed by cooling at constant rates varying between 0.078 and 1.889 K s⁻¹. The mean ₂–₂ interlamellar spacing λ for both DS and heat-treated (HT) ingots decreased with increasing cooling rate according to the relationship λ∝ ^−0.46. In DS ingots, microhardness, ultimate compression strength, yield strength and plastic deformation to fracture increased with increasing cooling rate. In HT ingots, microhardness and yield strength increased and ultimate compression strength and plastic deformation to fracture decreased with increasing cooling rate. The yield stress increased with decreasing interlamellar spacing and increasing volume fraction of Al₂O₃ particles. A linear relationship between the Vickers microhardness and yield stress was found for both DS and HT ingots. A simple model including the effect of interlamellar spacing and increasing volume fraction of Al₂O₃ particles was proposed for the prediction of the yield stress.     

Mechanical properties of the hot-rolled Mg–12Gd–3Y magnesium alloy

Tensile and high cycle fatigue (HCF) properties of the hot-rolled Mg–12Gd–3Y (wt.%) magnesium alloy have been investigated. The magnesium alloy exhibits a fatigue strength of about 150 MPa, which is much higher than that of the commercial Mg–8Al–Zn alloy AZ80. Aging heat-treatment (T5) improved the fatigue life of the Mg–12Gd–3Y alloy. Fatigue cracks nucleated at the intense slip bands in the as-rolled alloy. After T5 treatment, however, the fatigue crack nucleation site shifted to the phase boundaries between MgGdY particles and Mg matrix. T5 heat-treatment retarded the crack initiation and thus improved the fatigue life of the Mg–12Gd–3Y alloy.     

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.     

Effect of beta flecks on mechanical properties of Ti–10V–2Fe–3Al alloy

The effects of beta flecks on tensile properties and low-cycle fatigue life were investigated at room temperature for Ti–10V–2Fe–3Al alloy. It was found that beta flecks had a significant influence on tensile ductility and low-cycle fatigue life. The greater the volume fraction of beta flecks (P_A) or maximum area of beta flecks (S_max), the lower the tensile ductility and low-cycle fatigue life. Extensive scanning electron microscopy (SEM) and light microscopy (LM) observation showed that under tensile load, cracks preferentially nucleated at β grain boundaries of beta flecks, then grew, connected and propagated along grain boundaries to form characteristics of intergranular fracture and quasi-cleavage fracture. While under an alternating load, beta flecks acted as sites for low-cycle fatigue crack nucleation due to inhomogeneous alternating strains between soft GB and aged beta matrix. The presence of beta flecks accelerates both the crack nucleation and early crack propagation.     

Effect of microstructure on internal friction and Young's modulus of aged Cu–Be alloy

This study is intended to determine the effect of microstructure on internal friction and Young's modulus as a function of stress in the elastic region of an aged Cu–Be alloy and to investigate the influence of transformed phases caused by precipitation hardening on both properties. Results show that internal friction and Young's modulus were influenced by different precipitates. Transformed phases and micromechanical mechanisms could be responsible for the changes in internal friction and Young's modulus. The microstructure of the alloy having Guinier–Preston zones appears to have low internal friction and dependence on stress after aging at 315 °C for 2 h. The Young's moduli of the long-term aged samples increased by approximately 2 GPa compared to that for the short-term aged samples and elastic instability up to a stress of 20% of the yield strength was observed.     

Deformation behavior and microstructural evolution during the semi-solid compression of Al–4Cu–Mg alloy

The effects of the process parameters, including deformation temperature and strain rate, on the deformation behavior and microstructure of an Al–4Cu–Mg alloy, have been investigated through isothermal compression. Experiments were conducted at deformation temperatures of 540 °C, 560 °C, and 580 °C, strain rates of 1 s⁻¹, 1×10⁻¹ s⁻¹, 1×10⁻² s⁻¹, and 1×10⁻³ s⁻¹, and height reductions of 20%, 40%, and 60%. The experimental results show that deformation temperature and strain rate have significant effect on the peak flow stress. The flow stress decreases with an increase of deformation temperature and/or a decrease of the strain rate. Above a critical value of the deformation temperature, the flow stress quickly reaches a steady value. Experimental materials A and B have equiaxed and irregular grains, respectively, prior to deformation. The microstructures vary with the process parameters in the semi-solid state. For material B, the irregular grains transform to equiaxed grains in the process of semi-solid deformation, which improves the deformation behavior.     

The effect of microstructure on tensile properties, deformation mechanisms and fracture models of TG6 high temperature titanium alloy

The tensile properties at room temperature and 600 °C of TG6 titanium alloy with different microstructures {bi-modal microstructures with thick α lamella (BTL) and fine α lamella (BFL), and a mixed microstructure with different morphologies of α phase} were obtained. It was found that the BFL microstructure possessed the highest tensile strength, and the elongations of the BTL and BFL microstructures were almost the same of about 13% at room temperature and 17% at 600 °C, respectively. In addition, the mixed microstructure had the lowest plasticity. The tensile deformation mechanisms of α lamella (α_L), primary α phase (α_p), equiaxed α phase (α_e) and α colonies were researched by the analysis of respective dislocation morphologies. Notably, the accommodative deformations through grain/phase boundaries sliding determined the deformation models of α_L, α_p, and α_e. Compared to the thick α_L and α colony, the fine α_L and α colony activated more slip systems due to their excellent accommodative deformation capability. Furthermore the deformation mechanisms at room temperature and 600 °C were different from each other. Scanning electron microscope (SEM), energy-dispersive spectrometer (EDS) and transmission electron microscopy (TEM) were used to research the crack propagation paths and fracture models. Crack propagation path crossing α colonies and α_p were discussed, respectively. The colonies boundaries, α_p/colonies boundaries, α_e/α_e boundaries and silicide were found to be the stress concentration locations. The micro-plasticity of tensile specimens determined the fracture morphologies and fracture models.     

Effect of duplex aging on microstructure and mechanical behavior of beta titanium alloy Ti–15V–3Cr–3Al–3Sn under unidirectional and cyclic loading conditions

The microstructure and mechanical behavior of beta titanium alloy Ti–15V–3Cr–3Al–3Sn (Ti15-3) under unidirectional and high cycle fatigue (HCF) loading conditions were studied after single aging (SA) and duplex aging (DA). After SA, well-developed grain boundary α and micro precipitate free zones (micro PFZs) were present in the microstructure. Whereas after DA, grain boundary α was poorly developed and micro PFZs were absent. DA resulted in smaller size, higher density and volume fraction of alpha particles. DA led to higher work hardening rate, better strength-ductility combination and higher HCF life. Improved mechanical behavior after DA is explained based on microstructural observations.     

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.     

Hot compressive deformation behavior and microstructure evolution of Ti–6Al–2Zr–1Mo–1V alloy at 1073 K

Hot compressive behaviors of Ti–6Al–2Zr–1Mo–1V alloy at 1073 K, as well as the evolution of microstructure during deformation process, were investigated in this paper. The results shows that flow stress increases up to a peak stress, then decease with increasing strain, and forms a stable stage at last. The grain size also shows an decrease at first and increase after a minimum value. Dislocations are observed to produce at the interface of α/β phase, and the phase interface and dislocation circle play an important role in impeding the movement of dislocation. As strain increase, micro-deformation bands with high-density dislocation are founded, and dynamic recrystallization occurs.     

The variation of strain rate sensitivity exponent and strain hardening exponent in isothermal compression of Ti–6Al–4V alloy

The deformation behavior in isothermal compression of Ti–6Al–4V alloy is investigated in the deformation temperatures ranging from 1093 K to 1303 K, the strain rates ranging from 0.001 s⁻¹ to 10.0 s⁻¹ at an interval of an order magnitude and the height reductions ranging from 20% to 60% at an interval of 10%. Based on the experimental results in isothermal compression of Ti–6Al–4V alloy, the effect of processing parameters and grain size of primary α phase on the strain rate sensitivity exponent m and the strain hardening exponent n is in depth analyzed. The strain rate sensitivity exponent m at a strain of 0.7 and strain rate of 0.001 s⁻¹ firstly tends to increase with the increasing of deformation temperature, and maximum m value is obtained at deformation temperature close to the beta-transus temperature, while at higher deformation temperature it drops to the smaller values. Moreover, the strain rate sensitivity exponent m decreases with the increasing of strain rate at the deformation temperatures below 1253 K, but the m values become maximal at a strain rate of 0.01 s⁻¹ and the deformation temperature above 1253 K. The strain rate affects the variation of strain rate sensitivity exponent with strain. Those phenomena can be explained reasonably based on the microstructural evolution. On the other hand, the strain hardening exponent n depends strongly on the strain rate at the strains of 0.5 and 0.7. The strain affects significantly the strain hardening exponent n due to the variation of grain size of primary α phase with strain, and the competition between thermal softening and work hardening.     

Microstructure and mechanical properties of Mg–Zn–Ca alloy processed by equal channel angular pressing

An ultrafine-grained (UFG) Mg–5.12 wt.% Zn–0.32 wt.% Ca alloy with an average grain size of 0.7 μm was produced by subjecting the as-extruded alloy to equal channel angular pressing (ECAP) for 4 passes at 250 °C. The fine secondary phase restricted the dynamic recrystallized (DRXed) grain growth during the ECAP processing, resulting in a remarkable grain refinement. A new texture was formed in the ECAPed Mg alloy with the {0 0 0 2} plane inclined at an angle of 58° relative to the extrusion direction. The yield stress (YS) was decreased in the as-ECAPed alloy with finer grains, indicating that the texture softening effect was dominant over the strengthening from grain refinement. The ductility of the as-ECAPed alloy was increased to 18.2%. The grain refinement caused an obvious decrease in work hardening rate in the as-ECAPed alloy during tensile deformation at room temperature.     

Effects of age heat treatment and thermomechanical processing on microstructure and mechanical behavior of LAZ1010 Mg alloy

An Mg–Li–Al–Zn (designated as LAZ1010) alloy containing about 10 wt% of Li has been prepared by melting and solidification in a carbon steel crucible, and extruded at a billet preheating temperature of 200 °C with an extrusion ratio of approximately 29. Effects of age heat treatments and thermomechanical processing on microstructures and mechanical properties were performed in this study. Hardness, optical microscopy, X-ray diffraction studies, and tensile testes were carried out to explore the variations in microstructures and mechanical behaviors during processing. The results showed that LAZ1010 alloy presented age hardening effect at temperatures below 50 °C. Rapid decrease in hardness with aging temperature at intermediate temperatures should be resulted from the transformation of θ phase into the equilibrium phase AlLi. Kocks–Mecking type plots were used to illustrate different stages of work hardening of the cold rolled specimens. The results indicated that cold rolled LAZ1010 alloy showed stage III and stage IV work hardening behaviors.