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     

Complete mechanical characterization of nanocrystalline Al–Mg alloy using nanoindentation

In an effort to explore alternate means of mechanical characterization of small material volumes, a nanocrystalline Al–Mg alloy synthesized via cryomilling and consolidated by cold isostatic pressing with subsequent extrusion was subjected to nanoindentation testing. The data collected from these tests was subjected to two different data analysis techniques (one proposed by Dao et al. (2001) and one proposed by Ogasawara et al. (2006)) in an effort to investigate the capabilities of such techniques in full, accurate elastoplastic characterization. A commercially available, coarse-grained sample of this same Al–Mg alloy was also tested to investigate these models’ capabilities of distinguishing between the two types of material. Nanoindentation, as expected, proved to accurately predict the elastic modulus of a tested material. Also, these methods provided evidence that through determination of strain-hardening exponent and yield stress, they could reasonably estimate the plastic properties of a tested material. Both models seemed to slightly overestimate the strength of the nanocrystalline material (according to previously reported values for similar material). In terms of the coarse-grained material, Ogasawara’s model appeared to overestimate the strength while Dao’s model provided estimations closer to values reported in literature. Finite element analysis was used as a verification mechanism for the property values extracted from the nanocrystalline material, and initial results show signs of good accuracy of characterization.     

Diffraction characterization of microstructure scale fatigue crack growth in a modern Al–Zn–Mg–Cu alloy

SEM-based electron backscattered diffraction (EBSD) measurements characterize constituent-particle nucleated fatigue crack path relative to local grain orientation and crack wake defect distribution for Al–Zn–Mg–Cu alloy 7050-T7451 stressed in moist air. Crack propagation is primarily transgranular; consisting of facets parallel to {1 0 0}, {1 1 0} and high-index planes with no evidence of {1 1 1} slip-based cracking; and is also inter-subgranular involving pre-existing or fatigue process zone generated subgrain boundaries. Dislocation substructure develops close to the fatigue crack surface due to dynamic recovery of crack tip cyclic plasticity. Crack growth through subgrain structure explains the broad occurrence of crack features without a low-index orientation and is justified based on trapped-hydrogen embrittlement. A failure criterion for environmental fatigue modeling must capture a failure mechanism based on: (a) formation of localized defect structure from cumulative cyclic plasticity (perhaps H sensitive), and (b) subsequent embrittlement due to interaction of H trapped at this defect structure with microstructure-sensitive local tensile stresses normal to this weakened interface. Crack interaction with subgrain (and grain) boundaries produces local deflections and branches that arrest over a short distance. Such features should cause a distribution of microstructure-sensitive growth rates.     

Ageing response of a Al–Cu–Li 2198 alloy

The effects of different ageing treatments on microstructure evolution, properties and fracture are investigated in the present study. 2198 alloy exhibits strong ageing response during ageing. It is found that tensile properties, hardness and conductivity of 2198 alloy are very sensitive to ageing temperatures, which corresponds to different microstructures. In the naturally-aged condition (T3), only δ′ (Al₃Li) was detected. After artificial ageing (T8), large amounts of precipitates emerged and major precipitates that were detected turned to be δ′, θ′ (Al₂Cu) and T₁ (Al₂CuLi) phase. Exposure to higher temperature caused greater amounts of the precipitation. The constitution and morphology of precipitates varies with different ageing temperature; the major precipitates are δ′, θ′ when ageing below 160 °C, while above 160 °C, T₁ phase comes out in large numbers, becoming dominate strengthening phases gradually. Fracture transforms from a typical dimple type to a dimple-intergranular mixed type with the rise of ageing temperature.     

Experimental characterization of a Si–Mo–Cr ductile cast iron

High temperature-resistant ductile cast irons behavior is highly interesting for the manufacture of components, such as exhaust manifolds for automotive applications. In the present paper the temperature-dependent static, high cycle and low cycle fatigue behavior of a heat-resistant Si–Mo–Cr ductile cast iron (Fe–2.4C–4.6Si–0.7Mo–1.2Cr) is investigated. Tensile and high cycle fatigue properties, in terms of elastic modulus, yield stress, elongation at break, fatigue limits, and the stress-life Basquin’s curve parameters have been determined at room temperature, 160 °C, 500 °C and 800 °C, thus covering the usual temperature range to which actual components, obtained with this kind of material, are subjected. The alloy showed good monotonic properties at low temperature, but showed to be fragile during fatigue tests, due to the high Silicon content in the alloy. At 500 °C mechanical properties are still good, with a 40% decrease with respect to 160 °C, and ductility is increased. The last temperature level of 800 °C has caused a noticeable drop of the cast iron strength, due to softening and oxidation effects.     

Effect of thermomechanical processing on superplasticity in Ti–Al–Mo–Zr alloy

Abstract

Superplasticity in terms of total tensile elongation was studied in a titanium alloy of nominal composition Ti–6·5Al–3·3Mo–1·6Zr (wt-%) for three strain rates (1·04 × 10^?3, 2·1 × 10^?3, and 4·2 × 10^?3s^?1) and in the temperature range 1123–1223 K for microstructures obtained by different processing schedules. Fine equiaxed microstructure with a low aspect ratio of 1·15 was accomplished in this alloy by combining two types of deformation. While the first step consists of heavy deformations for refining and intermixing the phases, a second step, consisting of light homogeneous reductions in several stages, was necessary to remove the banding that developed during the first step. The resulting microstructure underwent enormous tensile elongation (1700–1725%), even under relatively high strain rates (1·04 × 10^?3 and 2·1 × 10^?3s^?1), making this alloy most suitable for commercial superplastic forming. The present investigation also revealed that the usual sheet rolling practice of heavy reductions to refine the microstructure leads to localised banding which could not be removed by annealing; therefore, the tensile elongation was limited to 770% only. The reason for this may be attributed to the resistance in grain boundary sliding and rotation encountered in microstructures with shear bands and grains with high aspect ratio. Strain enhanced grain growth was also greater in these microstructures.

MST/555     

Design and mechanical characterization of a Zr–Nb–O–P alloy

In this study, Sn-free Zr–1.5Nb–O–P alloys were manufactured and their mechanical properties were characterized. The ultimate tensile strength (UTS) of cold rolled Zr–1.5Nb–O–P alloy with 160 ppm phosphorous (680 MPa) were close to that of a commercially available Zr–1Nb–1Sn–0.1Fe alloy (720 MPa), achieving a good mechanical strength without the addition of Sn, an effective solution strengthening element. The UTS of recrystallized Zr–1.5Nb–O–P alloy with 160 ppm phosphorous (533 MPa) was far greater than that of a commercially available Zr–1Nb–O (323 MPa) because of the strengthening due to higher Nb and oxygen content combined with phosphorous strengthening. The activation volumes for the cold rolled Zr–1.5Nb–P alloys were not much different from those of annealed Zr–1.5Nb–P alloys despite the higher dislocation density in the cold rolled alloys. Insensitivity of the activation volume to the dislocation density and the decrease of the activation volume with the addition of phosphorous support the suggestion linking the activation volume with the activated bulge of dislocations limited by segregation of oxygen and phosphorous atoms.     

Fabrication and characterization of porous Ti–7.5Mo alloy scaffolds for biomedical applications

Porous titanium and titanium alloys are promising scaffolds for bone tissue engineering, since they have the potential to provide new bone tissue ingrowth abilities and low elastic modulus to match that of natural bone. In the present study, porous Ti–7.5Mo alloy scaffolds with various porosities from 30 to 75 % were successfully prepared through a space-holder sintering method. The yield strength and elastic modulus of a Ti–7.5Mo scaffold with a porosity of 50 % are 127 MPa and 4.2 GPa, respectively, being relatively comparable to the reported mechanical properties of natural bone. In addition, the porous Ti–7.5Mo alloy exhibited improved apatite-forming abilities after pretreatment (with NaOH or NaOH + water) and subsequent immersion in simulated body fluid (SBF) at 37 °C. After soaking in an SBF solution for 21 days, a dense apatite layer covered the inner and outer surfaces of the pretreated porous Ti–7.5Mo substrates, thereby providing favorable bioactive conditions for bone bonding and growth. The preliminary cell culturing result revealed that the porous Ti–7.5Mo alloy supported cell attachment.     

Structure and mechanical properties of Ti–5Cr based alloy with Mo addition

The effects of molybdenum (Mo) on the structure and mechanical properties of a Ti–5Cr-based alloy were studied with an emphasis on improving its strength/modulus ratio. Commercially pure titanium (c.p. Ti) was used as a control. As-cast Ti–5Cr and a series of Ti–5Cr–xMo (x = 1, 3, 5, 7, 9 and 11 wt.%) alloys were prepared by using a commercial arc-melting vacuum-pressure casting system, and investigated with X-ray diffraction (XRD) for phase analysis. Three-point bending tests were performed to evaluate the mechanical properties of all specimens and their fractured surfaces were observed by using scanning electron microscopy (SEM). The experimental results indicated that Ti–5Cr–7Mo, Ti–5Cr–9Mo and Ti–5Cr–11Mo alloys exhibited ductile properties, and the β-phase Ti–5Cr–9Mo alloy exhibited the lowest bending modulus. However, the Ti–5Cr–3Mo and Ti–5Cr–5Mo alloys had much higher bending moduli due to the formation of the ω phase during quenching. It is noteworthy that the Ti–5Cr–9Mo alloy exhibited the highest bending strength/modulus ratios at 26.0, which is significantly higher than those of c.p. Ti (8.5) and Ti–5Cr (13.3). Furthermore, the elastically recoverable angle of the Ti–5Cr–9Mo alloy (30°) was greater than that of c.p. Ti (2.7°). The reasonably high strength (or high strength/modulus ratio) β-phase Ti–5Cr–9Mo alloy exhibited a low modulus, ductile property, and excellent elastic recovery capability, which qualifies it as a novel implant materials.     

Phase characterization in as-cast F-75 Co–Cr–Mo–C alloy

Microstructural characterization of a cast acetabulum of ASTM F-75 alloy has been carried out in order to clarify conflicting reports from the literature. The present investigation revealed that although sigma (σ) and M₂₃C₆ carbide were the only secondary phases formed in the face centered cubic cobalt-base alpha matrix (Co-α), as identified by X-ray diffraction, the observed microstructure was quite complex. Scanning and transmission electron microscopy indicated the presence of coarse and fine lamellar cellular colonies, grain boundary film carbide, and different types of coarse blocky particles, including single-phase σ, dual-phase σ/M₂₃C₆, a binary eutectic comprised of σ and Co-α phases, and a three-phase feature comprising the binary eutectic and solid state formed M₂₃C₆. The carbide has probably formed during cooling from casting due to σ metastability. While it is proposed that the some lamellar cellular colonies were formed by discontinuous precipitation, it is not clear whether all lamellar structures present in the as-cast alloy occurred due to the same mechanism. The results obtained for the tensile properties are discussed in view of the observed microstructure.     

Formability of a high-strength Al–Mg6.8 type alloy sheet

Room temperature formability testing was performed on an AlMg6.8 type alloy sheet with a fully recrystallized structure (average grain diameter 18 m) and after partial annealing with a retained deformed structure. The yield strengths attained after full recrystallization and after partial annealing, were 175 and 283 MPa respectively. Such an increase in strength is followed by formability degradation, maximized around the plain strain state to either 42%, as obtained using the limiting dome height test (LDH), or 35% after using forming limit curves (FLC). A comparison with known high-strength formable alloys has shown that the tested alloy in the recrystallized condition has a better stretch formability (at the same or even higher yield stress level), while in the unrecrystallized-partially annealed condition it has a lower formability, limiting its application to moderate forming requirements for very high-strength parts. © 1998 Chapman & Hall     

Orientation of silicon particles in a binary Al–Si alloy

The orientations Si-crystals take in aluminium, in an alloy with composition Al–1.3at%Si, were investigated by transmission electron microscopy. Hardness was measured for isothermal heat-treatments at 175 °C and 260 °C. Conditions analysed by TEM were 17 h at 175 °C and an additional 3 h at 260 °C, both containing a high density of small Si-crystals, the finest corresponding to 175 °C. Two main orientation relationships were found: The first accounted for approximately 60% of Si precipitates in condition 17 h_175 °C. Despite a high number density and well-aligned interfaces, the Si precipitates have negligible influence on hardness. Findings are consistent with Ge particles in Al–Ge alloys.     

Development of a high strain rate superplastic Al–Mg–Zr alloy

Abstract

For superplastic forming of aluminium to break out of the niche market that it currently occupies, alloys will be required to possess a higher strain rate capability, appropriate in service properties, and a significantly lower price and to be capable of volume production. This paper describes an approach that has been developed in an attempt to address these fundamental requirements. A series of Al–Mg–Zr alloys with increasing levels of zirconium (0–1 wt-%)has been prepared via extrusion consolidation of cast particulate (solidification rate ～10³ K s^-1). The superplastic properties of the resultant cold rolled sheet have been evaluated as a function of thermomechanical treatment and zirconium addition. It has been found that increasing the level of zirconium has the twofold effect of improving the superplastic properties of the alloy while significantly decreasing the concomitant flow stress. At present the optimum superplastic behaviour has been obtained at strain rates of 10^-2 s^-1, with the 1%Zr material exhibiting ductilities in excess of 600%. The manufacturing route produces a bimodal distribution of Al₃Zr comprising >1 µm primary particles in combination with nanoscale solid state precipitates. The current postulation is that this high strain rate superplasticity is conferred by a combination of particle stimulated and strain induced recrystallisation.     

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.     

Nitriding of Co–Cr–Mo alloy in nitrogen

Using the results of a thermodynamic analysis, a Co–Cr–Mo alloy was successfully nitrided in nitrogen at temperatures of 1073–1473 K. The near-surface microstructure of the treated Co–Cr–Mo alloy was characterized using X-ray diffraction, field-emission scanning electron microscopy, electron probe micro-analyzer, and transmission electron microscopy equipped with energy-dispersive X-ray spectroscopy. The results indicated that the highest nitriding efficiency was achieved at the treatment temperature of 1273 K, with the size and coverage of the nitride particles on sample's surface increasing with an increase in the treatment duration. After nitriding at 1273 K for 2 h, numerous nitride particles, consisting of an outer Cr₂N layer and an inner π phase layer, were formed on top of the nitrogen-containing γ phase, and some π phase also precipitated in the alloy matrix at the sub-surface level.     

Structure and properties of Al–Li–Cu–Mg–Zr alloy AA 2091 in sheet form

Abstract

The structure and properties of Al–Li–Cu–Mg–Zr alloy AA 2091 in sheet form have been studied. Static and dynamic behaviour of material aged to the ‘damage tolerant’ category has been found to be at least equivalent to current BS L109 (AA 2024-T3) sheet while re-solution heat treatment did not appear to degrade properties. In contrast, although static strength parameters of stretched material aged to the ‘medium strength’ category were comparable with those of BS L157 (AA 2014-T6), re-solution heat treatment effected a noticeable decrease, with a failure to achieve 0·2%PS levels. Mechanical properties and subsequent fracture behaviour were correlated with submicrostructure throughout the investigation.

MST/958     

Micromechanism of improvement in crack initiation and propagation toughness of a Ti–Al–Sn–Zr–Mo alloy by coarsening prior β-grains

Abstract

Instrumented Charpy V impact tests and static and dynamic fracture toughness tests were carried out on Ti–6Al–2Sn–4Zr–6Mo alloys in which the prior β-grain size was varied by heat treatment. The effect of microstructure on the toughness was then examined. With increasing prior β-grain size, the elongation, crack initiation, and particularly propagation toughness increased and the strength decreased slightly. The increase in crack initiation toughness was caused mainly by the increase in Widmanstätten α-lath size or spacing, while the increase in crack propagation toughness was caused by the deflection of the crack propagation path, which was brought about by the decrease in intersubcolony spacing. The intersubcolony spacing decreased with increasing number of ‘diffusion controlled’ Widmanstätten α nucleating sites, which were introduced by the deformation strain.

MST/786     

Microstructural characterization of Mg–Al–Sr alloys

The microstructural details of fourteen Mg–Al–Sr alloys were investigated in the as-cast form by a combination of scanning electron microscopy/energy dispersive spectrometer (SEM/EDS) analysis and quantitative electron probe microanalysis (EPMA). The heat transfer method coupled with the DSC measurement has been utilized to determine the solidification curves of the alloys. The morphology and the chemical composition of the phases were characterized. The microstructure of the alloys is primarily dominated by (Mg) and (Al₄Sr). In the present investigation, ternary solid solubility of three binary compounds extended into the ternary system has been reported and denoted as: (Al₄Sr), (Mg₁₇Sr₂) and (Mg₃₈Sr₉). The (Al₄Sr) phase is a substitutional solid solution represented by Mg_xAl_4–xSr and has a plate-like structure. The maximum solubility of Al in Mg₁₇Sr₂ was found to be 21.3 at%. It was also observed that Mg₃₈Sr₉ dissolved 12.5 at% Al.     

Electron microscopy characterization of an as-fabricated research reactor fuel plate comprised of U–7Mo particles dispersed in an Al–2Si alloy matrix

To understand the microstructural development of nuclear fuel plates during irradiation, it is imperative to know the microstructure of a fuel plate after all the fabrication steps have been completed and before it is inserted into the reactor. To this end, a U–7 wt.% Mo alloy research reactor dispersion fuel plate with Al–2 wt.% Si matrix was destructively examined using scanning and transmission electron microscopy to characterize the developed microstructure after fabrication. Of particular interest for this study was how the Si that was added to the fuel matrix partitioned between the various fuel plate phases during fabrication. Si was added to the matrix so that the microstructure that developed during fuel fabrication would exhibit good irradiation behavior. SEM analysis was used to identify the representative microstructure, the compositions of the various phases, and the partitioning behavior of the fuel and matrix constituents. TEM analysis was employed to definitively identify the phases in the U–7Mo alloy and the phases that formed due to diffusional interactions between the fuel particles and matrix during fuel plate fabrication. The TEM results are the first reported for an as-fabricated U–7 wt.% Mo dispersion fuel plate with an Al alloy matrix. SEM results showed that a significant portion of the original γ-(U–Mo) fuel particles had transformed to a lamellar microstructure, comprised of α-U and either γ or γ' phases, and the fuel/matrix interaction layers were enriched in Si. TEM analysis identified an ordered fcc (U–Mo)(Al–Si)₃ type of phase, which formed at the decomposed U–7Mo/matrix interface and extended into the lamellar microstructure. Some regions of the U–7Mo particles retained the single-phase γ-(U–Mo). Small precipitate phases were observed in the fuel meat matrix that contained Fe, Al, and Si. The Si that is added to the matrix of a U–Mo dispersion fuel plate to improve irradiation performance appears to result in the creation of a Si-rich (U–Mo)(Al–Si)₃ type of fuel/matrix interaction layer during fabrication that appears to exhibit favorable behavior during irradiation compared to the behavior of the layers that form in U–Mo dispersion fuel plates without Si in the matrix.