Calculations of electrical resistivity for Al–Cu and Al–Mg–Si alloys

Abstract

A system for thermodynamical calculations (Thermo-Calc) was used to derive the solid solubility of the alloying elements in commercial Al–Cu and Al–Mg–Si alloys. The electrical resistivity was then calculated using a model developed by the authors based on the Matthiessen's rule. The calculated resistivity agreed with the observed resistivity within ±2.5 nΩ m for the Al–Mg–Si alloys and ±2 nΩ m for the Al–Cu alloys, except for Al–Mg–Si alloys containing boron or chromium and Al–Cu alloys with special compositions.     

Partitioning of titanium during solidification of aluminium alloys

Abstract

The purpose of the present work is to compare the segregation behaviour of Ti solute in low solute alloys, such as nominally pure Al, and high solute alloys typical of Al–Si casting alloys. Microprobe measurements of Ti segregation within grains show that, under the solidification conditions employed, the measured partition coefficient of Ti in pure Al is 6.7 compared with the phase diagram prediction of ～7.5. Similar measurements in an Al–7Si–0.3Mg alloy resulted in a partition coefficient of 3.2. Based on the measured partition coefficients, this translates into a reduction in the growth restriction factor (a measure of the segregating power of solute elements) from 8.75 K in pure Al to 3.36 K in Al–7Si–0.3Mg with an addition of 0.05 wt-%Ti. This may explain why Ti in solution is a less effective grain refiner in casting alloys than in low solute content wrought alloys. In addition, the measured Ti segregation profiles were compared with predicted profiles based on the Scheil or similar solidification relationships. It was found that the predictions of the Scheil equation produced a good fit with the measured Ti segregation profiles once it was adapted for the geometrical nature of dendritic solidification.     

Synthesis by ball milling and characterization of Al-Zn alloys

Mechanical ball milling is a useful technique for systems with positive enthalpy of mixing. With this technique solubility of a solute in a solid solution can be enhanced. Al-Zn system has positive heat of mixing. High energy ball milling has been employed to produce four alloys of Al with 2.5 to 10 wt% Zn. Powders of Al (1–125 m) and Zn (0.7–5.0 m) were mixed together in the desired proportion and milled with a powder to ball weight ratio of 1:20. The size and shape of the particles of as-received and alloy powders were examined in a scanning electron microscope (SEM) while their microanalysis was performed by energy dispersive system (EDS) attached with SEM. It has been observed that 120 h of milling of the powders produced homogeneous alloys. X-ray diffraction (XRD) patterns confirm complete solubility up to 10 wt% Zn in Al. Using the quasi-chemical theory of binary solid solutions, the enthalpy of mixing of 10 wt% Zn in Al has been determined to be 276 cals/mol. It is shown that stress exerted by very high density of dislocations, generated by mechanical milling, plays a major role in the enhancement of solubility. Hardness has been measured and it increases with increasing solute content.     

Low cycle fatigue resistance of Al–Li alloys

Abstract

Low cycle fatigue (LCF) resistance data from binary Al–Li, ternary Al–Li–Cu, and quaternary Al–Li–Cu–Mg alloys have been compiled and discussed. The LCF resistance is measured in terms of the variation of the number of reversals to failure 2N _fwith the plastic strain amplitude Δ? _p /2 as well as a modified average plastic strain energy per cycle (ΔW _p )_modified , obtained at different applied total strain amplitudes (Δ? _t /2). The data show the effects of microstructural features, namely dominant strengthening precipitates and the degree of recrystallisation as well as crystallographic texture. Lithium content, when in excess of 2·5 wt-%in aluminium decreases the low cycle fatigue resistance the most. The degree of aging, the degree of recrystallisation, and the degree of texture also influence the LCF resistance; among which the effect of the degree of aging is the most pronounced. The effects of lithium content in aluminium solid solution and strengthening precipitates obtainable by the change in the Li/Cu ratio are found to be marginal. Based on a modified total cyclic plastic strain energy till fracture, it is shown that most of the Al–Li alloys exhibit degradation in their LCF resistance in both hypotransition (higher fatigue lives) and hypertransition (lower fatigue lives) regions. Such degradation is attributed to the combined effects of mechanical fatigue, strain localisation through dislocation–precipitate interaction, environmental effects, and finally strain localisation through the high angle grain boundaries. In comparison with the currently used 2XXX and 7XXX series aluminium alloys, Al–Li alloys require substantial improvement before they can be considered for fatigue critical applications.     

Microstructure and tensile properties of Ni3 (Si,Ti) alloys containing second phase dispersions

Abstract

The alloying behaviour, microstructure, and high temperature mechanical properties of quaternary polycrystalline Ni₃ (Si,Ti), which was alloyed with transition elements V, Nb, Zr, and Hf beyond their maximum solubility limits, were investigated. The solubility limits of the quaternary elements in the L1₂ Ni₃ (Si,Ti) phase were determined to be ranked in the sequence of Nb > V > Hf > Zr, and correlated with the size misfit parameter between Si and the quaternary element X, and with the difference in formation enthalpy between Ni₃ Si and Ni₃ X. The second phases (dispersions) formed beyond the solubility limit were identified as a face centred cubic type Ni solid solution for the V containing Ni₃ (Si,Ti) alloy and Ni₃ X type compounds of the Nb, Zr, and Hf containing Ni₃ (Si,Ti) alloys. The second phase dispersions in the L1₂ phase matrix resulted in strengthening over a wide range of temperatures. The high temperature tensile elongation was improved by the introduction of the second phase dispersions. Among the quaternary Ni₃ (Si,Ti) alloys observed in the present study, the Nb containing Ni₃ (Si,Ti) alloy with the Nb containing second phase dispersion was shown to have the most favourable mechanical properties.     

Characteristics of TiC dendrites in as solidified Ti–Al–C alloys

Abstract

Results are reported on the dendrite secondary arm spacing of a series of as cast Ti–C and Ti–Al–C alloys in the composition range up to 10 at.-%C and 15 at.-%Al. The presence of Al leads to a significant decrease in the dendrite spacing, an effect of potential interest for improving mechanical properties. The structural refinement is attributed mainly to the slower diffusion of Al as compared with carbon, in the solute partitioning required for coarsening of dendrite arms.     

Development of corrosion resistant magnesium alloys Part 1 Characterisation of splat quenched Mg–10Al and Mg–16Al alloys

Abstract

The surfaces and bulk of splat quenched Mg–10Al and Mg–16Al (wt-%) alloys were investigated. The surfaces were studied using X-ray photoelectron spectroscopy (XPS) , X-ray excited Auger electron spectroscopy (X-AES), Rutherford backscattering spectrometry (RBS) , scanning electron microscopy (SEM), and electron probe microanalysis (EPMA) and the bulk using X-ray diffraction (XRD) and transmission electron microscopy (TEM), respectively. The solid solubility of Al in Mg was extended to at least 16 wt-%. The quantity of Al ions present on the surfaces of the alloys increased with Al content. Contrary to the suggested layered MgO/Mg–Al–oxide/alloy structure, the surfaces of the alloys consisted of an admixture of magnesium spinel (MgAl₂O₄) in periclase (MgO) and brucite (Mg(OH)₂) of non-uniform thickness varying between 10 and 50 nm, determined via RBS, X-AES, and depth profiling using XPS.

MST/1714a     

Investigation of ordering phenomenon in Me–Pt (Me=Fe,Ni) liquid alloys

Abstract

The phase diagrams of Fe–Pt and Ni–Pt liquid alloy systems show the existence of FePt and NiPt intermetallic compounds, respectively, in their solid intermediate states, and the associative tendency between unlike atoms in these liquid alloys has been analysed using the self-association model. The concentration dependences of mixing properties such as the free energy of mixing, G_M; the concentration fluctuations, S_cc(0), in the long-wavelength limits; the chemical short-range order (CSRO) parameter, α₁; as well as the chemical diffusion, enthalpy and entropy of the mixing of Fe–Pt and Ni–Pt liquid alloys have been investigated to determine the nature of ordering in the liquid alloys. The results show that heterocoordination occurs in the alloys at all concentrations. The effect of CSRO on S_cc(0), chemical diffusion, D, and the order parameter, α₁, has been considered. The ordering phenomenon in the liquid alloys is also related to the effect of the atomic size mismatch volume on S_cc(0).     

Evaluation of properties and microstructure of non-heat treatable Al–Mg–Li–C–O alloys with variable Li concentration

Abstract

Fine grained Al–Mg–Li–C alloys, with lithium concentrations from 0.7 to 1.5 wt-%, have been produced by a mechanical alloying–powder metallurgy route. An initial range of compositions was chosen for manufacture into 10 kg billets which were uniaxially forged into plate; subsequently two compositions, alloy A (Al–3.7Mg–0.7Li–1.0C (wt-%) and alloy B (Al–4.4Mg–1.4Li–1.0C), were down-selected for a 20 kg scale-up exercise. Billets were forged at 300°C, using an 8:1 reduction ratio, which provided a sufficient level of work to develop properties, while avoiding excessive grain growth. Alloy B exhibited tensile properties (0.2% proof stress 450 MPa; ultimate tensile strength 510 MPa; strain to failure 6%) that exceeded the AECMA specification for AA 5091. Both alloys were confirmed as non-heat treatable and therefore exploitable in the as forged T1 condition. Microstructural analysis has confirmed that a fine grain size (<1 µm) and nanoscale Al₂O₃/Al₄C₃ and MgO dispersoids provided significant Hall–Petch and Orowan strengthening, respectively, capable of increasing the 0.2% proof stress to 450 MPa. Although optimisation of thermomechanical practice is still required, these Al–Mg–Li–C alloys show considerable potential for aerospace, land, and space applications.     

Microstructure and strength modelling of Al–Cu–Mg alloys during non-isothermal treatments: Part 1 – Controlled heating and cooling

Abstract

A model is developed to predict the precipitation kinetics and strengthening in Al–Cu–Mg alloys during non-isothermal treatments consisting of controlled heating and cooling. The prediction of the precipitation kinetics is based on the Kampmann and Wagner model. The precipitation strengthening by the shearable Cu–Mg co-clusters is modelled on the basis of the modulus strengthening mechanism and the strengthening by the non-shearable S phase precipitates is based on the Orowan looping mechanism. The model predictions are verified by comparing with hardness, transmission electron microscopy and differential scanning calorimetry data on 2024-T351 aluminium alloys. The microstructural development and strength predictions of the model are generally in close agreement with the experimental data.     

Factors affecting dynamic recrystallization of metals and alloys

Abstract

The high-temperature mechanical behaviour of copper, Cu–Al alloys, and nickel has been examined using torsional testing with hollow testpieces in conjunction with microstructural observations on deformed and quenched specimens using both optical and electron microscopy. Dynamic recrystallization occurred in these materials as the restoration process during high-temperature deformation. The factors influencing dynamic recrystallization have been considered, including materials of high stacking fault energy. It was found that the regime of dynamic recrystallization and the transition in flow stress behaviour could be reasonably represented in terms of the Zener–Hollomon parameter. In Cu–Al solid solution alloys, although the addition of the solute aluminium into copper lowered the stacking fault energy, dynamic recrystallization was retarded to higher strains due to the reduced mobility of the grain boundary. By mechanical and microstructural analysis of the behaviour of various single phase metals and alloys during dynamic recrystallization, the factors influencing the behaviour (i.e. stacking fault energy (solute elements), Zener–Hollomon parameter (deformation condition), and strain) can be summarized on a three dimensional schematic.

MST/587     

On the glass forming ability of liquid alloys

Abstract

By using the concepts of the short-range order (SRO) and middle-range order (MRO) characterizing structures, an attempt has been made to describe the glass forming ability (GFA) of liquid alloys. This includes the effect of more than two kinds of SRO in the liquid caused by the addition of second and third elements to a metallic solvent. The minimum solute concentration is related to the atomic volume mismatch estimated from the cube of the atomic radius. The optimum solute concentration for good glass formability in several binary and some ternary alloys is discussed on the basis of the empirical guideline. A new approach to obtaining good GFA of liquid alloys is based on four main factors: (i) formation of new SRO and coexistence of two or more kinds of SRO, (ii) stabilization of dense random packing structure by restraining the atomic redistribution for initiating the nucleation and growth process, (iii) formation of a stable cluster (SC) or the MRO by the harmonious coupling of SROs, and (iv) difference between SRO characterizing the liquid structure and the near-neighbor environment in the corresponding equilibrium crystalline phases. The use of interaction parameters, widely used in the thermodynamics of multicomponent metallic solutions, is proposed for effectively selecting the third solute element (X₃) for enhancing the GFA of a metallic liquid (M) containing the second solute (X₂). Fe₇₀-B₂₀-(X₃)₁₀ alloys (X₃=Cr, W, Nb, Zr and Hf) are used for illustration. Two typical model structures denoted by the Bernal and chemical-order types are used in describing the new glass structure as a function of solute concentration.     

Effect of iron on the solubility of hydrogen in tantalum

Pressure-composition-isotherms for Ta–Fe–H systems have been investigated in the temperature range 673–873 K. Tantalum–iron alloys (Ta–xFe, x = 0, 1.6 and 3.2 atom  % Fe) were prepared by arc melting using high purity elements. The equilibrium solid solubility of hydrogen in the alloys decreases with an increase of iron content. Thermodynamic parameters of the solution process—the Gibb’s free energy, enthalpy, and entropy, for each of the solutions have been calculated. The relative partial molar enthalpy becomes less negative with increase in iron content, whereas the entropy values are nearly constant for these alloys. The solubility changes were explained on the basis of change in lattice strain energy of tantalum due to iron addition.     

Finite element implementation of a kinetic model of dynamic strain aging in aluminum–magnesium alloys

Soare and Curtin (Acta Mater. 2008; 56 :4091–4101, 4046–4061) have recently developed a model of dynamic strain aging in solute‐strengthened alloys. Their constitutive law describes time‐dependent solute strengthening using rate equations that can be calibrated using atomistic simulations. In this paper, their material model is incorporated into a continuum finite element simulation, with a view to completing a multi‐scale method for predicting the formability of solute‐strengthened alloys. The Soare–Curtin model is first re‐formulated as a state‐variable constitutive law, which is suitable for finite element computations. An efficient numerical procedure is then developed to track the strength distribution of aging mobile and forest dislocations in the solid during deformation. The method is tested by simulating the behavior of a 3D aluminum–magnesium alloy tensile specimen subjected to uniaxial loading at constant nominal strain rate. The model predicts the influence of strain rate on the steady‐state flow stress of Al–Mg alloys, but no Portevin–Le Châtelier bands or serrated flow were observed in any of our simulations, and the influence of strain rate on tensile ductility is not predicted correctly. The reasons for this behavior and possible resolutions are discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

Microstructure,mechanical properties and shape memory effect of Cu–Hf–Al–Ni alloys

The influence of hafnium element’s incorporation on a Cu–xHf–13.0Al–4.0Ni (wt-%) (x?=?0.5, 1.0 and 2.0) high-temperature shape memory alloy was investigated systematically. The results show that the matrix of Cu–xHf–13.0Al–4.0Ni (x?=?0.5, 1.0 and 2.0) alloys is 18R martensite, and an orthorhombic-structured Cu₈Hf₃ phase is formed and distributed at the grain boundaries. The grain size is significantly reduced with increasing Hf content. The mechanical properties of Cu–xHf–13.0Al–4.0Ni (x?=?0.5, 1.0 and 2.0) alloys are improved by Hf doping due to the combination of refinement strengthening, solid solution strengthening and second phase strengthening. After heating under pre-strain of 10%, the shape memory effect of the Cu–1.0Hf–13.0Al–4.0Ni alloy reaches 5.6%, which is obviously higher than that of the Cu–13.0Al–4.0Ni alloy.     

Correlation of alloy microstructure with oxidation behavior in chromia-forming intermetallic-reinforced Cr alloys

Abstract

The oxidation behavior of the Cr(X) solid solution and intermetallic Cr₂X Laves phases (X = Nb, Ta) was studied individually and in combination at 1,100°C in humid air. The two-phase Cr(Ta) + Cr₂Ta alloys exhibited superior oxidation resistance to either the single-phase Cr(Ta) or the single-phase Cr₂Ta alloys. However, the two-phase Cr(Nb) + Cr₂Nb alloys exhibited lower oxidation resistance than either the single-phase Cr(Nb) or Cr₂Nb alloys. The two-phase Cr(Ta) + Cr₂Ta alloys were also found to exhibit superior oxidation resistance to the two-phase Cr(Nb) + Cr₂Nb alloys. This was hypothesized to be related to the lower amount of the Cr₂X phase in the Cr(Ta)–Cr₂Ta eutectic and to the lower solubility of Ta in the Cr(X) phase, relative to the Cr–Nb system. Both of these factors resulted in significantly less oxidation and nitridation in the Cr(Ta) + Cr₂Ta alloys.     

Formation of metastable phases and nanocomposite structures in rapidly solidified Al-Fe alloys

In the present work the structure and morphology of the phases of nanocomposites formed in rapidly solidified Al-Fe alloys were investigated in details using analytical transmission electron microscopy and X-ray diffraction. Nanoquasicrystalline phases, amorphous phase and intermetallics like Al₅Fe₂, Al₁₃F₄ coexisted with α-Al in nanocomposites of the melt spun alloys. It was seen that the Fe supersaturation in α-Al diminished with the increase in Fe content and wheel speed indicating the dominant role of the thermodynamic driving force in the precipitation of Fe-rich phases. Nanoquasicrystalline phases were observed for the first time in the dilute Al alloys like Al-2.5Fe and Al-5Fe as confirmed by high resolution TEM. High hardness (3.57 GPa) was measured in nanocomposite of Al-10Fe alloy, which was attributed to synergistic effect of solid solution strengthening due to high solute content (9.17 at.% Fe), dispersion strengthening by high volume fraction of nanoquasicrystalline phase; and Hall-Petch strengthening from finer cell size (20-30 nm) of α-Al matrix.     

Characterization of the breakaway al content in alumina-forming alloys

Abstract

Residual Al contents were measured in Fe- and Ni-base alloys in order to improve lifetime prediction models based on the consumption of the Al reservoir. Measurements were made using electron probe microanalysis (EPMA) on both foil (50–125 μm thickness) and plate (0.5–2mm) material. For FeCrAl, NiCrAl and Fe₃Al plate specimens, significant Al concentration gradients were measured. An Al gradient is not considered in current reservoir type models. Residual Al contents at the onset of breakaway oxidation were somewhat lower than previously reported for several FeCrAl alloys and significantly higher (»10at.%) for Fe₃Al and Fe–Al alloys. The implications of these results for performance and lifetime predictions are discussed.     

Fe–AI–Zn ternary phase diagram at 450°C

Abstract

Several Fe–Al and Fe–Al–Zn alloys were vacuum melted, annealed at 450°C under atmospheric pressure, and then quenched in iced water. The structure and composition of phases were determined using microstructural, X-ray diffraction, and X-ray energy dispersive analyses. The maximum solubility of zinc was found to be 5·3 wt-% in FeAl₃, 20·9 wt-% in Fe₂Al₅, and 2·0 wt-% in FeAl at 450°C. Based on these results, an isothermal section for the aluminium rich corner of the Fe–Al–Zn ternary phase diagram is proposed.

MST/1196     

Relationships of diboride phases in Al–Ti(Zr)–B alloys

Abstract

The relationships of diboride phases in Al–Ti(Zr)–B alloys with a variable Ti/B ratio close to the stoichiometry of TiB₂ were studied. The formation of diboride solid solutions was confirmed. A grain refinement mechanism is proposed as that diboride particles in the Al–Ti–B master alloys reacting with aluminium upon adding into an aluminium melt and release titanium into the melt through forming a (Ti,Al)B₂ solid solution and maintain a thin dynamic Ti rich layer on the surfaces of the (Ti,Al)B₂ particles, which nucleates α-Al grains in solidification. The poisoning effect of zirconium on grain refinement of aluminium by Al–Ti–B master alloys is also discussed.