An analysis of hot deformation of an Al–Cu–Mg alloy produced by powder metallurgy

The high-temperature plasticity of a 2014 aluminium alloy produced by powder metallurgy was investigated in a wide range of temperatures and strain rates. When the strain rate was plotted as a function of stress (either peak flow stress in torsion, or applied stress in tensile creep), the alloy exhibited the same threshold-like behaviour observed in similar materials. The microstructure of representative torsioned samples was analysed in a transmission electron microscope (TEM) and the characteristics of particles and precipitate distribution were estimated. The dependence on stress and temperature was analysed by means of the conventional constitutive equations used for describing the hot-working behaviour and by means of a modified form of the sinh-equation, where the stress was substituted by an effective stress i.e. by the difference between the actual stress and a threshold stress. This temperature-dependent threshold stress was found to be a constant fraction (15%) of the Orowan stress generated by the dispersion of alumina particles and of precipitated intermetallic phases.     

Tensile behaviour of powder metallurgy processed (Al–Cu–Mg)/SiCp composites

Abstract

The tensile behaviour of Al–Cu–Mg alloy matrix composites produced by a powder metallurgy process was investigated as a function of particle size in the as extruded, homogenised, and peak aged conditions. The tensile behaviour of the corresponding matrix alloy which was produced in a similar manner, designated as Control, was also studied. There was a significant increase in the 0.2% yield strength of Control and all the metal matrix composites (MMCs) after homogenisation treatment (53–68%) and peak aging (93–109%), as compared to their values in the as extruded condition. The ultimate tensile strength (UTS) of Control as well as the MMCs also increases considerably after homogenisation treatment (39–70%), however, subsequent peak aging did not result in any further increase in UTS in case of any of the MMCs. It was found that the finer the reinforcement size, the higher the 0.2% yield strength and UTS in all the conditions. On the other hand, ductility decreased considerably after homogenisation treatment and subsequent peak aging. The results are discussed in the light of dislocation strengthening as well as reinforcement damage.     

Synthesis and characterisation of nanostructured Al–Al3V and Al–(Al3V–Al2O3) composites by powder metallurgy

In this study, the formation and characterisation of Aluminium (Al)-based composites by mechanical alloying and hot extrusion were investigated. Initially, the vanadium trialuminide (Al₃V) particles with nanosized structure were successfully produced by mechanical alloying and heat treatment. Al₃V–Al₂O₃ reinforcement was synthesised by mechanochemical reduction during milling of V₂O₅ and Al powder mixture. In order to produce composite powders, reinforcement powders were added to pure Al powders and milled for 5?h. The composite powders were consolidated in an extrusion process. The results showed that nanostructured Al-10?wt-% Al₃V and Al-10?wt-% (Al₃V–Al₂O₃) composites have tensile strengths of 209 and 226?MPa, respectively, at room temperature. In addition, mechanical properties did not drop drastically at temperatures of up to 300°C.     

Formation of Ti3AlC2 by mechanically induced self-propagating reaction in Ti–Al–C system at room temperature

Abstract

The formation of Ti₃AlC₂ was first investigated by mechanically induced self-propagating reaction (MSR) in Ti–Al–C system at room temperature. The effects of the milling parameters on the formation of Ti₃AlC₂ were discussed. The phase composition and microstructure were analysed and observed by using X-ray diffraction and scanning electron microscopy, respectively. The formation mechanism of Ti₃AlC₂ was analysed. An MSR was ignited during mechanical alloying of Ti, Al and C powders after a short time. An exothermic reaction between Ti and Al in the Ti–Al–C system first occurred after a certain milling time. Then, Ti–C reaction was induced at high temperature. All of the above reactions were exothermic that resulted in Ti–Al liquid formation. The previously formed TiC dissolved into and nucleated in the Ti–Al liquid. At last, Ti₃AlC₂ formed between the Ti–Al melt and the TiC. The final products consist of Ti₃AlC₂, TiC and Al₃Ti.     

Microstructure and mechanical properties of hot extruded Mg–Al–Mn–Ca alloy produced by rapid solidification powder metallurgy

The microstructure and mechanical properties of hot extruded Mg–Al–Mn–Ca alloy was investigated. Both rapid solidified powders and cast billets were extruded at 573, 623 and 673 K to optimize the processing conditions for obtaining better mechanical response. Powder was consolidated to prepare the extrusion billets using both cold compaction and Spark Plasma Sintering at 473 K. The tensile properties of the extruded alloy were then evaluated and correlated to the observed microstructure. The results show that the use of rapid solidified powder could lead to effective grain refinement, which in turn resulted in the improved mechanical response, especially compared to the extruded conventional cast material.     

The ballistic performance of SiC–AA7075 functionally graded composite produced by powder metallurgy

The potential of silicon carbide reinforced Functionally Gradient Material (FGM) to be used as armor material was investigated under the impact of armor piercing projectile. For this purpose, the SiC–Aluminum Alloy (AA) 7075 functionally graded composite at different thicknesses was produced from the metallic and ceramic powders via powder metallurgy method. Before the ballistic testing, the precipitation hardening behavior of the samples was determined. And also, the microstructural characterizations of the samples were done with the aid of microscopy techniques. Next, the FGM samples were tested using armor piercing projectile to analyze their impact behavior. In the produced samples, some pore formation was detected. The ballistic experiments showed that the investigated FGMs (up to a thickness of 25 mm) did not withstand the impact of the projectile. At the tested samples, some major cracks and plug formation were detected at macrolevel while there were some microcracks, deformed and elongated grains in the regions near to the deformation zone of the samples.     

Wear behaviour of 85Al–10La–5Ni alloy prepared by powder metallurgy

Abstract

The dry wear behaviour of 85Al–10La–5Ni (at.-%) alloy hot pressed has been studied. The result shows that 85Al–10La–5Ni alloy possessed excellent wear resistance. The wear resistance of the alloy pressed at 773 K is three times as high as that of the A355 aluminium alloy. The fine high hardness intermetallic compounds contribute to the wear resistance of the alloy.     

The effect of production parameters on microstructure and wear resistance of powder metallurgy Al–Al2O3 composite

Aluminum matrix composite is one of the most conventional types of metal matrix composites. This paper deals with the effect of production parameters on wear resistance of Al–Al₂O₃ composites. Alumina powder with a particle size of 12, 3 and 48 μ and pure aluminum powder with particle size of 30 μ were used. The amount of added alumina powder was up to 20%. Ball milling was utilized to blend the powders. The range of sintering temperature and time were 500, 550 and 600 °C and 30, 45, 60 and 90 min respectively. It was found that increasing sintering temperature results in increasing density, hardness and wear resistance and homogenization of the microstructure. However at certain sintering temperatures and time, considerable grain growth and reduction of hardness value occurred, leading to the degradation of wear resistance. The results showed that at high alumina content, relative density of the composite increases. However, after raising the particle size of alumina, relative density initially increases and then drops to lower values. Increasing weight percent of alumina powder leads to higher hardness and consequently improves the wear resistance of Al–Al₂O₃ composite. The use of fine alumina particles has a similar effect on hardness and the wear resistance. Finally, a finer grain size was observed, at high amount and low size of the reinforcement particle.     

Constitution of Ti–Al–Ru system

Abstract

The constitution of the Ti–Al–Ru system has been studied in detail. Metallography, X-ray diffraction, electron microscopy, and X-ray spectroscopy have been used to establish the phase diagram between 17 and 37 at.-%Al and 1 and 29 at.-%Ru in the temperature range 1250–770°C. Ternary isothermal sections within the range of investigation and selected phase composition data are presented and phase relationships are discussed. Results show only a small solubility (< 1at.-%) of ruthenium in Ti₃Al and TiAl which are involved in equilibria with a ternary intermetallic compound.

MST/963     

Fabrication of Ti–Nb–Ag alloy via powder metallurgy for biomedical applications

Ti and some of its alloys are widely used as orthopedic implants. In the present study, Ti–26Nb–5Ag alloys were prepared by mechanical alloying followed by vacuum furnace sintering or spark plasma sintering (SPS). The microstructure and mechanical properties of the Ti–Nb–Ag alloys were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDX), compressive and micro-hardness tests. The effect of different sintering methods on the microstructure and properties of Ti–Nb–Ag alloy was discussed. The results showed that the titanium alloy sintered by vacuum furnace exhibited a microstructure consisting of α, β and a small amount of α″ martensite phase; whilst the SPS sintered alloy exhibited a microstructure consisting of α, β and a small amount of α″ martensite phase, as well as a nanostructured Ag homogeneously distributed at the boundaries of the β phases. The Ti–Nb–Ag alloy sintered by SPS possessed fracture strength nearly 3 times of the alloy sintered by vacuum furnace.     

Microstructure characteristic for high temperature deformation of powder metallurgy Ti–47Al–2Cr–0.2Mo alloy

Hot compression tests of a powder metallurgy (P/M) Ti–47Al–2Cr–0.2Mo (at. pct) alloy were carried out on a Gleeble-3500 simulator at the temperatures ranging from 1000 °C to 1150 °C with low strain rates ranging from 1 × 10⁻³ s⁻¹ to 1 s⁻¹. Electron back scattered diffraction (EBSD), scanning electron microscope (SEM) and transmission electron microscope (TEM) were employed to investigate the microstructure characteristic and nucleation mechanisms of dynamic recrystallization. The stress–strain curves show the typical characteristic of working hardening and flow softening. The working hardening is attributed to the dislocation movement. The flow softening is attributed to the dynamic recrystallization (DRX). The number of β phase decreases with increasing of deformation temperature and decreasing of strain rate. The ratio of dynamic recrystallization grain increases with the increasing of temperature and decreasing of strain rate. High temperature deformation mechanism of powder metallurgy Ti–47Al–2Cr–0.2Mo alloy mainly refers to twinning, dislocations motion, bending and reorientation of lamellae.     

Formation mechanism of ZrB2–Al2O3 nanocomposite powder by mechanically induced self-sustaining reaction

ZrB₂–Al₂O₃ nanocomposite powder was produced by aluminothermic reduction in Al/ZrO₂/B₂O₃ system. In this research, high energy ball milling was used to produce the necessary conditions to induce a mechanically induced self-sustaining reaction. The ignition time of the composite formation was found to be about 13 min. The synthesis mechanism in this system was investigated by examining the corresponding sub-reactions as well as changing the stoichiometry of reactants. Thermal behavior of the system was also studied.     

Precipitation in cold-rolled Al–Sc–Zr and Al–Mn–Sc–Zr alloys prepared by powder metallurgy

The effects of cold-rolling on thermal, mechanical and electrical properties, microstructure and recrystallization behaviour of the AlScZr and AlMnScZr alloys prepared by powder metallurgy were studied. The powder was produced by atomising in argon with 1% oxygen and then consolidated by hot extrusion at 350 °C. The electrical resistometry and microhardness together with differential scanning calorimetry measurements were compared with microstructure development observed by transmission and scanning electron microscopy, X-ray diffraction and electron backscatter diffraction. Fine (sub)grain structure developed and fine coherent Al₃Sc and/or Al₃(Sc,Zr) particles precipitated during extrusion at 350 °C in the alloys studied. Additional precipitation of the Al₃Sc and/or Al₃(Sc,Zr) particles and/or their coarsening was slightly facilitated by the previous cold rolling. The presence of Sc,Zr-containing particles has a significant antirecrystallization effect that prevents recrystallization at temperatures minimally up to 420 °C. The precipitation of the Al₆Mn- and/or Al₆(Mn,Fe) particles of a size ~ 1.0 μm at subgrain boundaries has also an essential antirecrystallization effect and totally suppresses recrystallization during 32 h long annealing at 550 °C. The texture development of the alloys seems to be affected by high solid solution strengthening by Mn. The precipitation of the Mn-containing alloy is highly enhanced by a cold rolling. The apparent activation energy of the Al₃Sc particles formation and/or coarsening and that of the Al₆Mn and/or Al₆(Mn,Fe) particle precipitation in the powder and in the compacted alloys were determined. The cold deformation has no effect on the apparent activation energy values of the Al₃Sc-phase and the Al₆Mn-phase precipitation.     

Hardness and wear resistance improvement of surface composite layer on Ti–6Al–4V substrate fabricated by powder sintering

A wear resistant surface composite layer on Ti–6Al–4V substrate was fabricated using powder sintering method. The surface composite layer consisted of Ti–6Al–4V matrix and different fractions of TiN particles as reinforcement phase. The surface layer and the substrate were directly bonded together while the powders were cold formed and then sintered at an elevated temperature. The two layers showed good metallurgical bond. In this study, 5%, 10% and 15% TiN weight fractions were adopted to fabricate the surface composite layer. Effects of TiN addition on the microstructure, hardness and wear resistance were investigated. It was found that the wear resistance of the surface composite layer was improved due to the addition of TiN compared to that of pure Ti–6Al–4V.     

Formation of Al3Ti and Al2O3 from an Al–TiO2 system for preparing in-situ aluminium matrix composites

In-situ processing offers significant advantages over conventional processing from both technical and economic standpoints. Al–TiO₂ is one of the interesting systems that has recently been receiving some attention. In this paper, the formation mechanism of Al₃Ti and Al₂O₃ from an Al–TiO₂ system is investigated by using thermal analysis, XRD and microstructural characterisation. It is found that the in-situ processing involves three intermediate steps. In addition, TiO and γ-Al₂O₃ are transitional phases, which form during the reactive process.     

Microstructure of SiCw reinforced Al–12Ti composites prepared by powder metallurgical technique

Abstract

Silicon carbide whisker reinforced Al–12Ti composites were fabricated by a powder metallurgical technique, and the microstructures were characterised by the means of X-ray diffraction, SEM, TEM, and energy dispersion X-ray analysis. It has been shown that secondary phase particles, Al₃ Ti, form in situ during hot pressing after premechanically ball milling, and a small amount of α-Ti is left because the in situ reaction between α-Ti and Al is not complete. High density dislocations including dislocation lines and dislocation loops exist in the coarse Al₃ Ti grains, while, hardly any dislocations can be found using TEM in the very fine (～150 nm) Al₃ Ti grains. In addition, nanometer equiaxed γ-Al₂O₃ and stick shaped Al₄ C₃ dispersoids form in the Al matrix as a result of the addition of a processing control agent. There is no fixed orientation relationships between γ-Al₂O₃ , Al₄ C₃ , and the Al matrix. Dislocations in the Al matrix are too sparse to be found even in the zones around SiC whiskers. Silicon carbide whiskers uniformly scatter in the Al matrix, and no reaction products are formed. A few microzones with nanosized (～20 nm) Al grains exist in the Al matrix, and an amorphous phase is usually found in the zones adjacent to SiC whiskers. The formation mechanism of the amorphous phase is discussed.     

On the role of matrix grain size and particulate reinforcement on the hardness of powder metallurgy Al–Mg–Si/MoSi2 composites

Six Al–Mg–Si composites reinforced with 15 vol.% of MoSi₂ intermetallic particles, together with three unreinforced monolith Al–Mg–Si (AA6061) alloys have been processed by powder metallurgy to quantify the roles of alloy matrix grain size and reinforcement particle on their solutionized hardness and ageing response. In the range studied, hardness of solutionized composites follows a Hall–Petch mechanism. Moreover, it can be rationalised as the sum of the hardness of the alloy matrix with the same matrix grain size (d) and a term H_R, that accounts for 17–27% of total hardness, is roughly constant and independent of reinforcing size and distribution. Matrix grain size is responsible for 50–65% of hardness, whereas the contributions of solid solution and Orowan strengthenings account for 17–26%. Upon heat treatment at 170 °C, hardening ability decreases linearly with d^?1/2, fitting all data points to a single equation independently of whether they correspond to the composites or to the monolith alloys.     

Microstructure and mechanical properties of in situ synthesised (TiC+TiB)/Ti–6Al–4V composites prepared by powder metallurgy

Abstract

Titanium matrix composites (TMCs) reinforced with hybrid reinforcements were synthesised by blending Ti–6Al–4V, Ti, B₄C and C powders followed by reactive hot pressing. The phases were identified by X-ray diffraction, and the microstructures were examined by optical microscopy and scanning electron microscopy (SEM). Mechanical properties were tested at room temperature (RT), 400, 450 and 500°C respectively. The results show that Ti–6Al–4V produced by hot pressing has higher strength and better plasticity than by casting; there are four kinds of reinforcements in TMCs, and the TMCs’ strength increases significantly with the addition of reinforcements both at RT and elevated temperature; the TMCs with 5 vol.-% of reinforcements have higher strength than that with 10 vol.-% at high temperature. The fracture surfaces were examined by SEM. It shows that the bond between the reinforcements and matrix is not so well that reinforcements’ debonding occurs even at RT.     

Calculated ternary diagram of Ti–Al–Si system

Abstract

The phase equilibria between β (body centred cubic, bcc), α (hexagonal closed packed, hcp), Ti₃ Al–α ₂ (hcp), and Ti₅ Si₃ (hcp) in the Ti–Al–Si system have been investigated in the temperature range 700–1200°C. Isothermal sections of the ternary phase diagram have been assessed employing thermodynamic software, which uses the compound energy model to describe the phases mathematically. Available experimental phase equilibria results on the Ti–Al–Si system were used to calculate missing binary and ternary interaction parameters and assess isothermal phase diagrams. Extrapolations in the resulting tie triangles indicate the existence of three eutectoid reactions in the Ti rich corner of the ternary diagram: β → α + Ti₅ Si₃ , α → α ₂ + Ti₅ Si₃ , and β → α ₂ + Ti₅ Si₃ . Additionally, extrapolations in the β + α₂ + α tie triangle observed at 1100°C indicate that two possibilities arise to represent a peritectoid reaction involving α, β, and α ₂phases: β + α₂ → α and β + α → α₂ , depending on the alloy composition and the effect of temperature on the solubitlity of Si in the α phase.     

Nanocrystalline matrix TiC–Al3Ti and TiC–Al3Ti–Al composites produced by reactive hot-pressing of milled powders

Mechanically alloyed nanocrystalline TiC powder was short-term milled with 40 vol.% of Al powder. The powders mixture was consolidated at 1200 °C under the pressure of 4.8 GPa for 15 s and at 1000 °C under the pressure of 7.7 GPa for 180 s. The bulk materials were characterised by X-ray diffraction, light and scanning electron microscopy, energy dispersive spectroscopy, hardness, density and open porosity measurements. During the consolidation a reaction between TiC and Al occurred, yielding an Al₃Ti intermetallic. The microstructure of the produced composites consists of TiC areas surrounded by lamellae-like regions of Al₃Ti intermetallic (after consolidation at 1200 °C) or Al₃Ti and Al (after consolidation at 1000 °C). The mean crystallite size of TiC is 38 nm. The hardness of the TiC–Al₃Ti and TiC–Al₃Ti–Al composites is 13.28 GPa (1354 HV1) and 10.22 GPa (1041 HV1) respectively. The produced composites possess relatively high hardness and low density. The results obtained confirmed satisfactory quality of the consolidation with keeping a nanocrystalline structure of TiC.