Thermodynamic characterization of Al–Cr,Al–Zr,and Al–Cr–Zr alloy systems

Abstract

The equilibrium phase diagrams of Al–Cr, Al–Zr, and Al–Cr-Zr, with particular reference to aluminium-rich alloys, have been critically reviewed. On the basis of these, and consistent with measured thermodynamic values, the binary systems have been thermodynamically characterized. Using these characterizations, phase equilibria have been extrapolated in the ternary, with the intention of augmenting the sparse experimental information concerning the equilibrium liquidus (0–10 at.%Cr, Zr) and solid solution range of aluminium in Al–Cr–Zr. Using the same parameters that define the equilibrium phase relationships, metastable phase relationships can also be extrapolated into the ternary.

MST/418     

Al–Ti–C phase diagram

Abstract

Equilibrium experiments have been performed at 1373, 1173, and 973 K, with alloys of compositions within the aluminium rich corner of the Al–Ti–C phase diagram. The samples have been metallographically investigated using light optical microscopy and a scanning electron microscope equipped with a system for energy dispersive spectrometry. Equilibrium phases, as well as effects of cooling, have been identified. Dynamic effects originating from cooling are discussed and a tentative phase diagram is proposed. It was predicted theoretically and confirmed experimentally that a class II reaction involving four phases occurs, i.e. Al(l) + TiC(s)?Al₃Ti(s) + Al₄C₃(s), below 1100 K.

MST/1807     

Solid state phase transformations in Ti–Al–C and Ti–Al–Nb–C alloys

Abstract

Results are reported of an investigation of solid state transformations in a series of α₂ based alloys having an aluminium content of 26 at.-% with carbon up to 3 at.-%; two α₂ basedquaternary Ti–Al–Nb–C alloys with 5 and 12 at.-%Nb and 3 at.-%C were also studied. Ordering occurs in the ternary Ti–Al–C alloys and also in the 23Al–5Nb–3C alloy on quenchingfrom 1250°C. Additional carbide precipitation was not observed in the ternary Ti–Al–C alloys on reheating to 750°C. Additions of niobium resulted in the presence of the β phase at 1050°C in the 5%Nb alloy and at 1050 and 750°C in the 12%Nb alloy. In the quaternary Ti–Al–Nb–C alloys, (Ti, Nb)₃AlC was found to be the primary phase and was present in the microstructure over the temperature range studied. In the 21Al–12Nb–3C alloy, the ordered β phase transformed to α″₂ martensite on quenching from 1250;amp;#x00B0;C.

MST/1306     

Selective oxidation of Al rich Fe–Mn–Al–C low density steels

Abstract

The surface quality of steels containing solutes that have a strong affinity for oxygen can be markedly compromised during processing at elevated temperatures. Here, we examine a class of steels that are being developed for low density applications and hence, have relatively large concentrations of aluminium and manganese. These two elements compete for what little oxygen is available in the predominantly hydrogen–nitrogen mixture used to protect the steel during heat treatment. It is found that although the general sequence of oxidation does not seem to vary as the aluminium concentration is increased, the stoichiometry of the oxides changes. Manganese rich oxide is always the first to form for all the dew points examined, but is rapidly overtaken by aluminium oxide when the dew point is kept below ?30°C. In the latter case, the eventual formation of an alumina film also halts the internal oxidation of aluminium. The results obtained using a variety of high resolution microscopy and analytical techniques have been analysed by calculating phase diagrams and by creating a finite difference model to reveal aspects of the internal oxidation problem.     

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.     

Liquid–Liquid Interfacial Tension in Ternary Monotectic Alloys Al–Bi–Cu and Al–Bi–Si

The liquid–liquid interfacial tension in the ternary monotectic alloys Al_34.5-x Bi_65.5Cu _x and (Al_0.345Bi_0.655)_100-x Si _x (mass%) has been determined as a function of its Cu (Si) content by a tensiometric technique. It is established that the interfacial tension gradually increases when either Cu or Si is added to Al–Bi alloys. The increase of can be related to the increase of the miscibility gap (both width and height) when Cu (Si) is added to the Al–Bi binary. The temperature dependences of the interfacial tension in binary Al_34.5Bi_65.5 and ternary Al_23.25Bi_65.5Cu_11.25 and (Al_0.345Bi_0.655)₉₅Si₅ monotectic alloys are well described by the power function with the critical-point exponent .     

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     

Grain refinement of Mg–Al alloys with Al4C3–SiC/Al master alloy

A novel Al₄C₃–SiC/Al master alloy for grain refinement of Mg–Al–Zn alloys has been developed in the present work. X-ray diffraction (XRD) and electron probe microanalysis (EPMA) results show the existence of Al₄C₃ and SiC particles in this master alloy. The master alloy presents good grain refining efficiency on both AZ31 and AZ63 alloys, but little effect on AZ91 alloy. After addition of 0.5 wt.% Al₄C₃–SiC/Al master alloy, the average grain size of AZ31 and AZ63 decreased dramatically from 1300 to 225 μm, and from 300 to 200 μm, respectively. However, no further refinement of grain size was achieved with additional amount of Al₄C₃–SiC/Al master alloy exceeding 0.5 wt.% for both AZ31 and AZ63 alloys in the present investigation. Duplex phase of Al₄C₃ and SiC was found to be located at the grain center of α-Mg and is proposed to be the nucleating agent during solidification of α-Mg.     

Dynamics of a Charged Fluctuator in an Al–AlOx–Al Single-Electron Transistor

We report detailed observations of random-telegraph charge fluctuations in a two-junction Al–AlO_x–Al single-electron transistor (SET). We measured the fluctuations from 85 mK to 3 K and observed that the SET switched between two states, causing charge shifts of Q_o=0.1±0.025 e on the SET's island. The transition rate out of each state was periodic in the gate voltage, varied non-monotonically with the device bias voltage, and was independent of the temperature below about 0.3 K. We discuss two effects which could contribute to the behavior of the transition rates, including heating of the defect by the island conduction electrons and inelastic scattering between the defect and electrons flowing through the SET.     

Extrusion processing of Al–Li–Mg–Zr alloy

Abstract

The hot deformation behaviour of an Al–Li–Mg–Zr alloy was characterised in hot torsion and extrusion. The alloy was found to have similar hot ductility to existing high strength aluminium alloys, but this could be maintained at higher temperatures. Billets were extruded over a range of process conditions and a limit diagram was constructed for surface cracking. All the extrusions were found to be partially recrystallised after deformation, but the volume fraction of recrystallisation was a strong function of billet temperature and extrusion ratio. In addition, the unrecrystallised areas contained a recovered substructure where the subgrain size was inversely proportional to the temperature compensated strain rate. The as extruded structure was retained during solution treatment and as a result final mechanical properties were strongly dependent on the extrusion conditions. The use of high billet temperatures and low extrusion ratios gave the best combination of strength and toughness.

MST/839     

Constitution of Ni3Al–Ni3Mo–Ni3W section of Ni–Al–Mo–W system

Abstract

The isothermal section of the Ni–Al–Mo–W system has been studied at 75 at.-%Ni at temperatures of 1523 and 1273 K. Constitutional data have been determined using electron probe microanalysis, X-ray diffraction, and microscopical examination. The alloys studied lay in the range 12·5–15 at.-%Al, 2·5–7·5 at.-%Mo, and 2·5–7·5 at.-% W. The phases present at 1523 K were γ, γ′, and α (based on the Mo–W continuous series of solid solutions); at 1273 K, NiMo(δ′) was also encountered. The γ/γ′ mismatch values lay in the range ?0·03 to ?0·75%. In the as-solidified state, the alloys consisted predominantly of γ-phase containing γ′-precipitates formed in the solid state.

MST/462     

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.     

Microstructure and Mechanical Properties of Rapidly Solidified Hypereutectic Al–Si and Al–Si–Fe Alloys

The microstructure and mechanical properties of rapidly solidified Al–18 wt% Si and Al–18 wt% Si–5 wt% Fe alloys were investigated by a combination of optical microscopy, scanning electron microscopy, transmission electron microscopy, x-ray diffraction, tensile testing, and wear testing. The centrifugally atomized binary alloy powder consisted of the -Al (slightly supersaturated with Si) and Si phases and the ternary alloy powder consisted of the -Al (slightly supersaturated with Si), silicon, and needle-like metastable Al–Fe–Si intermetallic phases. During extrusion the metastable -Al₄FeSi₂ phase in the as-solidified ternary alloy transformed to the equilibrium -Al₅FeSi phase. The tensile strength of both the binary and the ternary alloys decreased with a high-temperature exposure, but a significant fraction of the strength was retained up to 573 K. The specific wear gradually increased with increasing sliding speed but decreased with the addition of 5 wt% Fe to the Al–18 wt% Si alloy. The wear resistance improved with annealing due to coarsening of the silicon particles.     

Damping behaviour and mechanical properties of rapidly solidified Al–Fe–Mo–Si/Al alloys

In order to develop a new high damping aluminium alloy with strength and toughness for advanced aircraft structure application, rapidly solidified (RS) Al–Fe–Mo–Si/Al alloys were synthesized. The damping behaviour, mechanical properties and microstructures of the alloys were studied. Results showed that the damping capacities of RS Al–Fe–Mo–Si/10–15% Al alloys are stable between 7.0–10.0×10-3 at room temperature, which almost reach the high damping threshold, 10.0×10-3. At lower frequency (0.1–10 Hz) the damping capacity is decidely frequency and temperture dependent above 50°C, with lowest frequency and highest temperature resulting in the highest less factor. It was noted that mechanical properties of the Al–Fe–Mo–Si/10–15% Al alloys are both excellent at room temperature (b=536–564 MPa, =7.2–11.4%) and at elevated temperature (250°C: b=295–324 MPa). Analysis of microstructures reveal that the damping capacity arises from deformation of the pure Al areas, and strength at elevated temperature from the dispersion strengthening of intermetallic phase. © 1998 Chapman & Hall     

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.     

Fabrication of Al–Si coating on Ti–6Al–4V substrate by mechanical alloying

Al–Si coatings were synthesized on Ti–6Al–4V alloy substrate by mechanical alloying with Al–Si powder mixture. The as-prepared coatings had composite structures. The effects of Al–Si ratio, milling duration and rotational speed on the microstructure and oxidation behavior of coating were investigated. The results showed that the continuity and the anti-oxidation properties of the coating were enhanced with the increase of Al–Si weight ratio. The thickness of the coating largely increased in the initial 5-hour milling process and decreased with further milling. A rather long-time ball milling could result in the generation of microdefects in coating, which had an adverse effect on the oxidation resistance of coating. Both the thickness and the roughness of the coating increased with the raise of rotational speed. The low rotational speed would lead to the formation of discontinuous coating. The rotational speed had a limited effect on the coating oxidation behavior. Dense, continuous and high-temperature protective Al–Si coatings could be obtained by mechanical alloying with Al–33.3?wt.%Si powder at the rotational speed ranging from 250 to 350?rpm for 5?h.     

Precipitation of iron-rich intermetallics and mechanical properties of Al–Si–Mg–Fe alloys with Al–5Ti–B

Effects of added Al–5Ti–B master alloys on precipitation of iron-rich intermetallics and mechanical properties of A356 cast alloys with high Fe content (1.5?wt-%) were investigated using image analysis, scanning electron microscopy, and tensile testing. Results show that added Al–5Ti–B has apparent refinement on α (Al) grain size of A356 alloys that have high Fe content. 12?wt-% Al–5Ti–B is beneficial for improving mechanical properties of A356 cast alloys with high Fe content. Improved mechanical properties can be attributed to refined microstructure, the proper amounts of TiB₂ and Ti(AlSi)₃, and decreased porosity. An excessive amount of Al–5Ti–B deteriorates mechanical properties of alloys because it leads to the formation of large secondary intermetallics and increased porosity.     

Effect of Al content on stacking fault energy in austenitic Fe–Mn–Al–C alloys

Effect of Al content on the stacking fault energy (SFE) was investigated in the austenitic Fe–25Mn–(1.16–9.77)Al–0.68C (at%) alloys by X-ray diffraction line profile analysis and thermodynamic estimation, and was discussed on the basis of anomaly in shear modulus caused by the antiferromagnetic transition. The experimental results show that the stacking fault probability decreases with increasing Al content, the observed SFE increases linearly when Al content is lower than 6.27 at%, and markedly as it is more than 6.27 at%. The thermodynamic estimation indicates that the non-magnetic component of SFE increases faster than the observed one with increasing Al content in the antiferromagnetic state, and both are almost equal in the paramagnetic state. The magnetic order increases SFE in the antiferromagnetic state, and the magnetic component of SFE depends on the average magnetic moment and Néel temperature. The increases in the localized magnetic moment and the decrease in the Néel temperature are caused by the addition of Al atoms into the austenitic Fe–Mn alloys and are accompanied by the anomaly in shear modulus, which affects SFE in the antiferromagnetic state. The anomalous drop in shear modulus leads to the inconsistency for the variations of the observed SFE and non-magnetic component with Al content in the antiferromagnetic state.     

Mechanical properties of Ir–Nb–Pt–Al quaternary alloys

Ir–Nb–Pt–Al quaternary alloys had exhibited suitable microstructure for high-temperature usage in the previous study. In this work, investigations were made of the mechanical properties of these quaternary alloys. Compression tests at 1200 °C were carried out for five samples, and compression creep test at 1400 °C under 100 MPa was conducted for one sample. These alloys showed high strength and good creep resistance.     

Production of Fe3Al–TiC nanocomposite by mechanical alloying of FeTi2–Al–C powders

Abstract

The aim of the present work was to produce Fe₃Al/TiC nanocomposite by mechanical alloying of the FeTi₂30Al10C60 (in at-%) powder mixture. The morphology and the phase transformations in the powder during milling were examined as a function of milling time. The phase constituents of the product were evaluated by X-ray diffraction (XRD). The morphological evolution during mechanical alloying was analysed using scanning electron microscopy (SEM). The results obtained show that high energy ball milling, as performed in the present work, leads to the formation of a bcc phase identified as Fe(Al) solid solution and an fcc phase identified as TiC and that both phases are nanocrystalline. Subsequently, the milled powders were sintered at 873 K. The XRD investigations of the powders revealed that after sintering, the material remained nanocrystalline and that there were no phase changes, except for the ordering of Fe(Al), i.e. formation of Fe₃Al intermetallic compound, during the sintering process.