The tensile deformation behavior of Ti–3Al–4.5V–5Mo titanium alloy

The tensile deformation behavior of Ti–3Al–4.5V–5Mo titanium alloy was studied. The results show that there are obvious yield points on true stress–true strain curves of annealing structures, then a stress drop occurs. The curves show linear work-softening after yielding at annealing temperature of 720–780 °C and linear work-hardening at annealing temperature of 800–840 °C. Elastic energy stored in the α-phase is dramatically released after plastic deformation of the β-phase, which leads to the stress drop.     

Creep behavior of Mg–2 wt.%Nd binary alloy

A binary magnesium alloy, Mg–2 wt.%Nd, has been prepared. Under the condition of temperature between 150 and 250 °C and applied stress between 30 and 110 MPa, the alloy exhibits good creep resistance due to both solution-hardening and especially precipitation-hardening. Tiny precipitates forming dynamically during creep have been observed, which play an important role in restricting dislocation movements. When the creep tests are carried out at the temperature range between 150 and 250 °C, the stress exponents lie in the range of 4.5–7.1 at low stresses, which is consistent with the “five-power-law”. The values of stress exponent increase up to 9.8–29.5 at high stresses indicate power-law breakdown. When the creep tests are carried out under the applied stress between 30 and 90 MPa, the apparent activation energy values vary from 70.0 to 96.0 kJ/mol at low temperatures, but increase to 199.9–246.1 kJ/mol at high temperature range. Dislocations in basal plane are activated in the primary creep stage, but as creep goes on, they are observed in non-basal plane. The creep is mainly controlled by both dislocation-climb and cross-slip.     

Microstructure and creep properties of a cast intermetallic Ti–46Al–2W–0.5Si alloy for gas turbine applications

The microstructure and creep properties including minimum creep rate, time to 1% creep deformation and creep fracture time of a cast TiAl-based alloy with nominal chemical composition Ti–46Al–2W–0.5Si (at.%) were investigated. The creep specimens were prepared from investment-cast plate and two large turbine blades. Constant load creep tests were performed in air at applied stresses ranging from 150 to 400 MPa in the temperature range 973–1073 K. The microstructure of the specimens is characterised by optical, scanning and transmission electron microscopy before and after creep deformation. The minimum creep rate is found to depend strongly on the applied stress and temperature. The power law stress exponent of minimum creep rate is n = 7.3 and the apparent activation energy for creep is Q_a = 427 ± 14 kJ/mol. The initial microstructure of the creep specimen is unstable. The ₂(Ti₃Al)-phase transforms to γ(TiAl)-phase and needle-like B2-precipitates during long-term creep testing at all testing temperatures. At lower applied stresses, the creep specimens fail by the growth and coalescence of cavities and small cracks formed along the γ/₂ interfaces. At the highest applied stresses, the specimens fail by nucleation and propagation of cracks.     

Steady state creep and creep recovery behaviours of pre-aging Al–Si alloys

The creep and creep recovery of pre-aging Al–1 wt.%Si and Al–1 wt.%Si–0.1 wt.%Zr–0.1 wt.%Ti alloys have been investigated at room temperature under different constant stresses. The aging temperature dependence of steady creep rate, _st, and the recovery strain rate, π, show that under the same test conditions first alloy yields creep or creep recovery rates much higher as compared with those of second alloy. The stress exponent n was found to change from 2.5 to 7.43 and 4.57 to 11.99 for two alloys, respectively, characterizing dislocation slipping mechanism. The activation energies of steady state creep of the two alloys were found to be 78.4 kJ/mol and 32.8 kJ/mol for Al–Si and Al–Si–Zr–Ti alloys, respectively. The microstructure of the samples studied was investigated by optical and transmission electron microscopy (TEM).     

Metallurgical assessment of an emerging Al–Zn–Mg–Cu P/M alloy

The objective of this work was to conduct a detailed assessment of the microstructure and mechanical properties of an emerging Al–Zn–Mg–Cu powder metallurgy (P/M) alloy known as Alumix 431D. A variety of techniques were considered including optical microscopy, X-ray diffraction, electron-probe micro-analysis, thermal dilatometry, and differential scanning calorimetry as well as apparent hardness, tensile testing, and bending fatigue. Alumix 431D exhibited many of the same attributes found in wrought counter parts such as 7075. A sintered density of approximately 99% of theoretical was achieved, indicating that the alloy was highly responsive to sintering. Once heat treated, a T6 hardness of 86 HRB and a room temperature ultimate tensile strength of 448 MPa were noted. Thermal analyses implied that the precipitation behaviour of Alumix 431D closely mimicked comparable 7XXX series wrought alloys and was largely premised on the precipitation of η-phase variants. Tensile properties of the alloy in a T1 temper were found to be relatively stable at temperatures up to 150 °C and 1000 h of exposure time. Those of T6 specimens degraded under the same exposure conditions to the point where equivalency with the T1 product was noted.     

Compressive creep behavior of as-cast and aging-treated Mg–5 wt% Sn alloys

The microstructure and compressive creep behaviors of as-cast and aging-treated Mg–5 wt% Sn alloys are investigated in this paper. The compressive creep resistance of aging-treated Mg–5 wt% Sn alloy is much better than that of as-cast alloy at the applied stresses from 25 MPa to 35 MPa and the temperatures from 423 K to 473 K, which is mainly due to the dispersive distribution of Mg₂Sn phase in the aging-treated Mg–5 wt% Sn alloy. The calculated average values of stress exponent n and activation energy Q_c suggest that dislocation cross slip and dislocation climb happen respectively in as-cast and aging-treated Mg–5 wt% Sn alloys during creep.     

Microstructure and high temperature deformation characteristics of sol–gel derived aluminium titanate–mullite composites

Aluminium titanate (AT)–mullite composites with varying compositions were processed by sol–gel technique. The influence of mullite on the microstructure and creep deformation of AT–mullite composites was investigated. In the composites mullite addition was varied from 0 to 100 vol.%. The AT-80 vol.% mullite composite sintered at 1600 °C resulted in fine-grained microstructure with an average grain size of 2.5 μm. From the steady-state creep analysis of the different AT–mullite composites, the activation energies for the creep deformation and stress exponents were determined. The activation energies in the range 655–874 kJ mol⁻¹ were obtained for various the sol–gel derived AT–mullite composites. Similarly stress exponent values were found in the range 1.5–1.9.     

Dislocation analysis of die-cast Mg–Al–Ca alloy after creep deformation

Tensile creep tests were combined with detailed transmission electron microscopy in order to characterize the dislocation movements during creep and to explain the creep properties of the Mg–Al–Ca AX52 die-cast alloy at 473 K and stresses from 15 to 70 MPa. TEM observations indicate that dislocations are generated within the primary α-Mg grain in the die-casting process, which consist of both the basal and non-basal segments. The basal segments of dislocations are able to bow out and glide on the basal planes under the influence of a stress, and the jogs follow the basal segments with the help of climb during creep. The creep mechanism for the alloy is deduced as dislocation climb due to the formation of sub-boundaries during creep, while the easy glide of the basal segments of dislocations is controlling the creep rates immediately after the stress application of creep tests.     

Thermomechanical processing of Ti–5Al–5Mo–5V–3Cr

The constitutive behaviour and microstructural evolution of the near-β alloy Ti–5Al–5Mo–5V–3Cr in the α + β condition has been characterised during isothermal subtransus forging at a range of temperatures and strain rates. The results indicate that Ti–5Al–5Mo–5V–3Cr has a shallower approach curve, and therefore, offers a more controllable microstructure than the near-β alloy Ti–10V–2Fe–3Al. Flow softening is small in magnitude in both alloys in the α + β condition. The steady state flow stresses obey a Norton–Hoff constitutive law with an activation energy of Q = 183 kJ mol⁻¹, which is similar to the activation energy for self-diffusion in the β phase, suggesting deformation is dominated by dynamic recovery in the β matrix. Good evidence is found for the existence of ω phase after both air cooling and water quenching from above the β transus. In addition, dissolution of the α phase is found to be slow at near-transus temperatures.     

Constitutive equations in creep of Mg–Al alloys

The creep response of commercial Mg–Al alloys for die-casting was described by taking into account the distinctive microstructure consisting of α-Mg primary grains and a divorced eutectic formed by supersaturated α-Mg and β-Mg₁₇Al₁₂ typical of these materials. The α-Mg grains were assimilated to soft zones in a composite reinforced by hard zones rich in precipitates, the latter being the grain boundary regions of supersaturated α-Mg and β-Mg₁₇Al₁₂. Constitutive equations correlating minimum creep rate, applied stress and temperature were derived for dilute Mg–Al solid solutions and used to calculate the forward stress acting in soft and hard regions, respectively, on the assumption that both creep with a similar strain rate. The role of the particles was then expressed by a threshold stress; as a consequence, Si addition resulted in a substantial increase in the threshold stress in the hard zones, a mechanism that explains the reduction in minimum creep rate observed in the high-temperature low-stress regime in Mg–Al–Si compared with Mg–Al–Zn alloys.     

Structure and mechanical properties of extruded Mg–Gd based alloy sheet

The Mg–8Gd–2Y–1Nd–0.3Zn–0.6Zr (wt.%) alloy sheet was prepared by hot extrusion technique, and the structure and mechanical properties of the extruded alloy were investigated. The results show that the alloy in different states is mainly composed of α-Mg solid solution and secondary phases of Mg₅RE and Mg₂₄RE₅ (RE = Gd, Y and Nd). At aging temperatures from 200 °C to 300 °C the alloy exhibits obvious age-hardening response. Great improvement of mechanical properties is observed in the peak-aged state alloy (aged at 200 °C for 60 h), the ultimate tensile strength (σ_b), tensile yield strength (σ_0.2) and elongation () are 376 MPa, 270 MPa and 14.2% at room temperature (RT), and 206 MPa, 153 MPa and 25.4% at 300 °C, respectively, the alloy exhibits high thermal stability.     

Creep behaviour of Cu-30% Zn at intermediate temaperatures

The creep properties of single-phase Cu-30% Zn alpha brass were investigated in the intermediate temperature range 573–823 K (0.48–0.70T _m, whereT _m is the absolute melting point). Inverse, linear, and sigmoidal primary creep transients were usually observed above 573 K under stresses resulting in minimum creep rates between 10^–7 and 2×10^–4s^–1, while normal primary creep occurred under all other conditions. The creep stress exponent decreased from about 5.4 at 573 K to about 4.1 above 623 K, and the activation energy for creep varied between 170 and 180 kJ mol^–1. A detailed analysis of the data, as well as a review of the literature, suggests that no clearly defined class M to class A to class M transition exists in this alloy, although the characteristics of both class A and class M behaviour are observed under nominally similar stresses and temperatures. It is concluded that Cu-30% Zn does not conform to the normally accepted characterization of class A or class M solid solution alloys.     

The elevated-temperature fatigue behavior of boron-modified Ti–6Al–4V(wt.%) castings

This work investigated the effect of nominal boron additions of 0.1 and 1.0 wt.% on the elevated-temperature (455 °C) fatigue deformation behavior of Ti–6Al–4V(wt.%) castings for maximum applied stresses between 250 and 450 MPa (R = 0.1 and 5 Hz). Boron additions resulted in a dramatic refinement of the as-cast grain size, and larger boron additions resulted in larger titanium-boride (TiB) phase volume percents. The boron-containing alloys exhibited longer average fatigue lives than those for Ti–6Al–4V, which was suggested to be related to the reduced as-cast grain size and the addition of strong and stiff TiB phase. The Ti–6Al–4V–0.1B alloy exhibited the longest average fatigue lives. The TiB phase cracked during the fatigue experiments and this resulted in a decreasing Young's modulus with increased cycle number. Each alloy exhibited α-phase cracking and environmentally assisted surface edge cracking.     

Steady-state creep behaviour of Ti3Al-base intermetallics

The steady-state creep behaviour of Ti₃Al and Ti₃Al+10 wt% Nb was studied in the temperature range 550 to 825° C and in the stress range 69 to 312 MN m^–2. The temperature and stress dependences of the steady-state creep rates were determined for both intermetallics, and the activation energy and stress-exponent were measured. At temperatures above 700° C, the stress dependence of the steady-state creep rate indicated two distinct creep regimes: at stresses above 138 MN m^–2, the creep was controlled most probably by dislocation climb; at stresses below 138 MN m^–2, a transition regime with a lower stress-exponent value was obtained.     

Characterization of microstructural changes in an Al-6.8 wt.% Mg alloy by electrical resistivity measurements

Microstructural changes in an AlMg6.8 alloy after different thermo mechanical (TMT) and sensitization treatments were investigated by electrical resistivity measurements. The electrical resistivity was most affected by the content of Mg solute atoms in the α-Al matrix, due to β-phase precipitation, while contribution of the dislocation density to the resistivity of the AlMg6.8 alloy was less pronounced. The amount and distribution of the β-phase precipitated during sensitization were found to be strongly affected by the microstructure developed under the previously applied TMTs, i.e. by the dislocation density and the primary β-phase particles in the dual (α + β) phase structure. During sensitization of the specimens with a recovered/recrystallized dual (α + β) phase structure, precipitation of randomly distributed, globular β-phase particles occurred. Sensitization of cold deformed and recrystallized single α-Al structures induced β-phase precipitation in the form of a continuous layer along the shear bands/grain boundaries.     

A study of tensile properties in Al–Si–Cu and Al–Si–Mg alloys: Effect of β-iron intermetallics and porosity

The effect of β-iron intermetallics and porosity on the tensile properties in cast Al–Si–Cu and Al–Si–Mg alloys were investigated for this research study, using experimental and industrial 319.2 alloys, and industrial A356.2 alloys. The results showed that the alloy ductility and ultimate tensile strength (UTS) were subject to deterioration as a result of an increase in the size of β-iron intermetallics, most noticeable up to β-iron intermetallic lengths of 100 μm in 319.2 alloys, or 70 μm in A356.2 alloys. An increase in the size of the porosity was also deleterious to alloy ductility and UTS. Although tensile properties are interpreted by means of UTS vs. log elongation plots in the present study, the properties for all sample conditions were best interpreted by means of log UTS vs. log elongation plots, where the properties increased linearly between conditions of low cooling rate–high Fe and high cooling rate–low Fe. The results are explained in terms of the β-Al₅FeSi platelet size and porosity values obtained.     

Stress corrosion cracking of Al-Zn-Mg alloy AA-7039 by slow strain-rate method

The stress corrosion behaviour of Al-Zn-Mg alloy AA-7039 in an aqueous 3.5 wt % Nad solution (pH=1) was studied with the specimens under constant strain rate as function of ageing state and cold working. The tests were carried out at temperatures of 30 and 45°C and strain rates between 7.6×10^–7 and 7.6×10^–6 sec^–1 and the apparent activation energy for mechanical deformation in oil and stress corrosion cracking (SCC) process in NaCl solution were determined. The fracture energy in NaCl solution under constant strain rate, as compared with experiments in oil, was lowered in the overaged specimens and markedly lowered in sequence of the peak-aged and the underaged specimens. The values of fracture energy for peak-aged and overaged specimens were nearly similar to those in oil at the higher strain rate of 7.6×10^–6 sec^–1. The relative fracture energy was comparatively lowered in the fine-grained specimens, as compared to coarsegrained specimens. The apparent activation energy for mechanical processes in oil was found to be 103 kJ mol^–1 in the peak-aged and 96.5 kJ mol^–1 in the overaged specimens. Those for SCC processes in NaCl solution were 47.5 kJ mol^–1 in the peak-aged and 51.5 kJ mol^–1 in the overaged specimens. The results suggest that stress corrosion (SC) cracks are initiated by electrochemical dissolution of grain boundaries (gbs) and propagated by mechanical processes such as creep.     

Effect of precipitates on damping capacity and mechanical properties of Ti–6Al–4V alloy

The present study was concerned with the effects of over-aging on damping property and fracture toughness in Ti–6Al–4V alloy. Damping property and toughness become important factors for titanium implants, which have big modulus difference between bone and implant, and need high damping capacity for bone-implant compatability. Widmanstätten, equiaxed, and bimodal microstructures containing fine α₂ (Ti₃Al) particles were obtained by over-aging a Ti–6Al–4V alloy. Over-aging heat treatment was conducted for 200 h at 545 °C. Fracture toughness, Charpy impact, and bending vibration tests were conducted on the unaged and the over-aged six microstructures, respectively. Charpy absorption energy and apparent fracture toughness decreased as over-aging was done, even if the materials were strengthened by precipitation of very fine and strong α₂-Ti₃Al particles. On the other hand, damping properties were enhanced by over-aging in Widmanstätten and equiaxed microstructures, but was weakened in bimodal microstructure due to the softening of tempered martensite and the decreasing of elastic difference between tempered martensite and α phase contained α₂ particles, etc. These data can provide effective information to future work about internal damping and fracture properties of Ti–6Al–4V alloy.     

Microstructure and superplasticity of laser welded Ti–6Al–4V alloy

In this paper laser beam welding (LBW) was used to join Ti–6Al–4V alloy as a pre-forming operation before superplastic deformation (SPF) process. Superplastic deformation behavior of laser welded Ti–6Al–4V alloy was investigated. The results indicated that the welded Ti–6Al–4V alloy had good superplasticity when deformed at temperature range of 870–920 °C and strain rate range of 10⁻³–10⁻² s⁻¹, and the elongation was 233–397%. The microstructure observation indicated that dynamic recrystallization happened in the weld bead, and the acicular structure of weld bead was transforming into equiaxed grains during tensile process.