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.     

Characterization of tensile properties and microstructures in directionally solidified Al–Si alloys using linear roughness index

The tensile properties in directionally solidified Al–Si alloys and in gravity die casting of the same composition are presented. Examples of relating both the tensile and the microstructural properties of these alloys with the fracture roughness index are indicated. The roughness index was measured on vertical sections cut through the tensile fracture surface. The tensile properties examined were the yield and ultimate tensile strengths, strain at fracture, and Young's modulus. The analyzed microstructural features were porosity, dendrite area fraction, secondary dendrite arm spacing, and Si particle spacing. In almost all cases an unambiguous correlation was found between the roughness index and the tensile or the microstructural properties. A marked improvement in ductility was observed for directionally solidified samples over their gravity die casting processed counterparts. The roughness index diminished going from die cast to direction solidification, and this is likely accompanied by change in the fracture mechanisms.     

Microstructural investigations on as-cast and annealed Al–Sc and Al–Sc–Zr alloys

Al–Sc and Al–Sc–Zr alloys containing 0.05, 0.1 and 0.5 wt.% Sc and 0.15 wt.% Zr were investigated using optical microscopy, electron microscopy and X-ray diffraction. The phase composition of the alloys and the morphology of precipitates that developed during solidification in the sand casting process and subsequent thermal treatment of the samples were studied. XRD analysis shows that the weight percentage of the Al₃Sc/Al₃(Sc, Zr) precipitates was significantly below 1% in all alloys except for the virgin Al0.5Sc0.15Zr alloy. In this alloy the precipitates were observed as primary dendritic particles. In the binary Al–Sc alloys, ageing at 470 °C for 24 h produced precipitates associated with dislocation networks, whereas the precipitates in the annealed Al–Sc–Zr alloys were free of interfacial dislocations except at the lowest content of Sc. Development of large incoherent precipitates during precipitation heat treatment reduced hardness of all the alloys studied. Growth of the Al₃Sc/Al₃(Sc, Zr) precipitates after heat treatment was less at low Sc content and in the presence of Zr. Increase in hardness was observed after heat treatment at 300 °C in all alloys. There is a small difference in hardness between binary and ternary alloys slow cooled after sand casting.     

Effects of temper condition and corrosion on the fatigue performance of a laser-welded Al–Cu–Mg–Ag (2139) alloy

The effects of temper condition and corrosion on the fatigue behavior of a laser beam welded Al–Cu–Mg–Ag alloy (2139) have been investigated. Natural aging (T3 temper) and artificial aging (T8 temper) have been applied prior to welding. Corrosion testing has been performed by exposing the welded specimens to a salt spray medium for 720 h. Aging influences the corrosion behavior of laser welds. In the T3 temper, corrosion attack is in the form of pitting in the weld area, while in the T8 temper corrosion is in the form of pitting and intergranular corrosion in the base metal. In the latter case corrosion is attributed to the presence of grain boundary precipitates. Corrosion degrades the fatigue behavior of 2139 welds. The degradation is equal for both the T3 and T8 tempers and for the corrosion exposure selected in this study corresponds to a 52% reduction in fatigue limit. In both cases fatigue crack initiation is associated with corrosion pits, which act as stress raisers. In the T3 temper, the fatigue crack initiation site is at the weld metal/heat affected zone interface, while for the T8 temper the initiation site is at the base metal. Fatigue crack initiation in uncorroded 2139 welds occurs at the weld toe at the root side, the weld reinforcement playing a principal role as stress concentration site. The fatigue crack propagates through the partially melted zone and the weld metal in all cases. The findings in this paper present useful information for the selection of appropriate heat treatment conditions, to facilitate control of the corrosion behavior in aluminium welds, which is of great significance for their fatigue performance.     

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.     

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.     

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.     

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.     

Performance of AlTi5B1, AlTi3B3 and AlB3 master alloys in refining grain structure of aluminium foundry alloys

Abstract

The capacity of AlTi5B1, AlTi3B3 and AlB3 grain refiners to refine the grain structures of AlSi7Mg and AlSi11Cu2 foundry alloys was investigated. The performance of AlTi5B1, well established to be the best grain refiner for wrought aluminium alloys, is not nearly as good with the AlSi7Mg and AlSi11Cu2 alloys. Relatively smaller grains are obtained with the AlTi3B3 grain refiner in both alloys. The AlB3 grain refiner, on the other hand, improves the grain structure only as much as the AlTi5B1 grain refiner does. With as much as 0·04–0·1 wt-%Ti, the commercial alloys cannot enjoy the outstanding potency of the AlB₂ particles since the B supply is readily transformed to TiB₂ particles. However, the grains of the Ti free AlSi7Mg and AlSi11Cu2 alloys (～0·005 wt-%Ti) are very small and nearly globular for the entire range of holding times when inoculated with AlB3, implying not only a remarkable grain refining capacity but also a strong resistance to fading of the grain refinement effect. The lack of Ti in the melt allows the entire B to form AlB₂ particles, the perfect substrates to promote the nucleation of α-Al crystals. Aluminium castings can enjoy grain sizes well below 200 μm, with an addition of 0·02 wt-%B, provided that they are Ti free. That the potent substrates are made available just before the nucleation of α-Al crystals avoids fading effects and is a further advantage of the AlB3 grain refiner in recycling operations.     

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.     

The microstructure,hardness and tensile properties of a new super high strength aluminum alloy with Zr addition

This research is based on the addition of zirconium to a new super high strength Al–12.24Zn–3.25 Mg–2.46Cu alloy. The results showed that Zr addition decreases average grain size (approximately 20%), forms rosette-like microstructure and introduces proper distribution of the second phases. In addition, due to the presence of Al₃Zr particles in Zr-refined specimen, the area fraction of recrystallized regions decreased by 65% as compared with unrefined specimen. Furthermore, tensile strength, yield strength and particularly elongation values of the extruded refined sample were enhanced nearly 34%, 25% and 1850%, respectively, when compared with not-extruded unrefined one. Microstructural observations and fractographic examinations of the fractured surfaces of several extruded specimens indicated dimple-like mechanism while the presence of coarse eutectic constituent was responsible for brittle mode of fracture in not-extruded specimens.     

Influence of additives on the microstructure and tensile properties of near-eutectic Al–10.8%Si cast alloy

The continuing quest for aluminum castings with enhanced mechanical properties for applications in the automotive industries has intensified the interest in aluminum–silicon alloys. In Al–Si alloys, the properties are influenced by the shape and distribution of the eutectic silicon particles in the matrix, as also by the iron intermetallics and copper phases that occur upon solidification. The detailed microstructure and tensile properties of as-cast and heat-treated new experimental alloy belonging to cast Al–Si near-eutectic alloys have been investigated as a function of Fe, Mn, Cu, and Mg content. Microstructural examination was carried out using optical microscopy, image analysis, and electron probe microanalysis (EPMA), wavelength dispersive spectroscopic (WDS) analysis facilities. Tensile properties upon artificial aging in the temperature range of 155–240 °C for 5 h were also investigated. The results show that the volume fraction of Fe-intermetallics increases as the iron or manganese contents increase. Compact polygonal or star-like particles form when the sludge factor is greater than 2.1. The Al₂Cu phase was observed to dissolve almost completely during solution heat treatment of all the alloys studied, especially those containing high levels of Mg and Fe, while Al₅Cu₂Mg₈Si₆, sludge, and α-Fe phases were found to persist after solution heat treatment. The β-Al₅(Fe,Mn)Si phase dissolved partially in Sr-modified alloys, and its dissolution became more pronounced after solution heat treatment. At 0.5% Mn, the β-Fe phase forms when the Fe content is above 0.75%, causing the tensile properties to decrease drastically. The same results are obtained when the levels of both Fe and Mn are increased beyond 0.75%, because of sludge formation. On the other hand, the tensile properties of the Cu-containing alloys are affected slightly at high levels of Mg as a result of the formation of Al₅Cu₂Mg₈Si₆ which decreases the amount of free Mg available to form the Al₂CuMg phase. The results also show that, for the heat-treated alloys, peak aging is achieved at 180 °C, although the highest quality index corresponds to 155 °C aging temperature, for all the alloys investigated. Accordingly, 155 °C may be considered as the optimal aging treatment. It is also consistent with this observation that quality index is more sensitive to variations in tensile ductility than in tensile strength.     

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.     

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.     

Influence of matrix structure on the fatigue properties of an alloyed ductile iron

Rotary bending fatigue tests were conducted on ductile iron containing 1.25 wt% nickel, 1.03 wt% copper and 0.18 wt% molybdenum with various matrix structures. Several heat treatments were applied to obtain ferritic, pearlitic/ferritic, pearlitic, tempered martensitic, lower and upper ausferritic structures in the matrix of a pearlitic as-cast alloyed ductile iron. The tensile properties (ultimate tensile strength, 0.2% yield strength and percent elongation), the hardness and the microstructures of the matrixes were also investigated in addition to fatigue properties. Fractured surfaces of the fatigue specimens were examined by the scanning electron microscope. The results showed that the lowest hardness, tensile and fatigue properties were obtained for the ferritic structure and the values of these properties seemed to increase with rising pearlite content in the matrix. While the lower ausferritic structure had the highest fatigue strength, the upper ausferritic one showed low fatigue and tensile properties due to the formation of the second reaction during the austempering process.     

Microstructure and Texture Formation During Near Conventional Forging of an Intermetallic Ti–45Al–5Nb Alloy

Texture formation was studied in an intermetallic Ti‐45at%Al‐5at%Nb alloy after uniaxial compression and near conventional forging. Depending on the deformation conditions the texture of the γ‐TiAl phase is formed by pure deformation components, components related to dynamic recrystallization, or transformation components. This changing corresponds with microstructural observations. The α₂‐Ti₃Al and the α‐Ti(Al) phase show a similar texture as it is known for Ti and Ti‐base alloys after compressive deformation at elevated temperatures. In contrast to the γ texture, no significant change of the α/α₂ texture was observed in the temperature range between 800 °C and just below the α‐transus temperature (T_α = 1295 °C).     

Effect of rapid solidification on the microstructure and mechanical properties of hot-pressed Al–20Si–5Fe alloys

The aim of this work is to study the effect of cooling rate and subsequent hot consolidation on the microstructural features and mechanical strength of Al–20Si–5Fe–2X (X = Cu, Ni and Cr) alloys. Powder and ribbons were produced by gas atomization and melt spinning processes at two different cooling rates of 1 × 10⁵ K/s and 5 × 10⁷ K/s. The microstructure of the products was examined using optical microscopy, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The particles were consolidated by hot pressing at 400 °C/250 MPa/1 h under a high purity argon atmosphere and the microstructure, hardness and compressive strength of the compacts were evaluated. Results showed a profound effect of the cooling rate, consolidation stage, and transition metals on the microstructure and mechanical strength of Al–20Si–5Fe alloys. While microstructural refining was obtained at both cooling rates, the microstructure of the atomized powder exhibited the formation of fine primary silicon (~ 1 μm), eutectic Al–Si phase with eutectic spacing of ~ 300 nm, and δ-iron intermetallic. Supersaturated Al matrix containing 5–7 at.% silicon and nanometric Si precipitates (20–40 nm) were determined in the microstructure of the melt-spun ribbons. The hot consolidation resulted in coarsening of Si particles in the atomized particles, and precipitation of Si and Fe-containing intermetallics from the supersaturated Al matrix in the ribbons. The consolidated ribbons exhibited higher mechanical strength compared to the atomized powders, particularly at elevated temperatures. The positive influence of the transition metals on the thermal stability of the Al–20Si–5Fe alloy was noticed, particularly in the Ni-containing alloy.     

Effect of matrix structure on the impact properties of an alloyed ductile iron

An investigation was performed to examine the influence of the matrix structure on the impact properties of a 1.03% Cu, 1.25% Ni and 0.18% Mo pearlitic ductile iron. Specimens were first homogenized at 925 °C for 7 h and a fully ferritic structure was obtained in all ductile iron samples. Then, various heat treatments were applied to the homogenized specimens in order to obtain pearlitic/ferritic, pearlitic, tempered martensitic, lower and upper ausferritic matrix structures. The unnotched charpy impact specimens were tested at temperatures between − 80 °C and + 100 °C; the tensile properties (ultimate tensile strength, 0.2% yield strength and elongation) and the hardnesses of the matrix structures were investigated at room temperature. The microstructures and the fracture surfaces of the impact specimens tested at room temperature were also investigated by optical and scanning electron microscope. The results showed that the best impact properties were obtained for the ferritic matrix structure that had the lowest hardness, yield and tensile strength. Ductile iron with a lower ausferritic matrix had the best combination of ultimate tensile strength, percent elongation and impact energies of all structures.     

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.     

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.