Development of high-performance quaternary LPSO Mg–Y–Zn–Al alloys by Disintegrated Melt Deposition technique

In the present study, new quaternary MgY_1.65Zn_0.74Al_0.53 and MgY_3.72Zn_1.96Al_0.45 alloys (wt.%) were synthesized employing the Disintegrated Melt Deposition (DMD) casting technique followed by hot extrusion. Microstructural characterization revealed the presence of 14H long-period stacking ordered structure (LPSO) and Mg₄Y₂ZnAl₃ phases aligned along the direction of extrusion in both alloys. Refined grains (⩽5 μm) due to the effect of dynamic recrystallization (DRX) were also observed to co-exist with larger worked grains (⩾20 μm) in the extruded microstructures. Compared to monolithic Mg, significant increase in the microhardness (∼67–88%), tensile yield strength (∼245–290%) and ultimate tensile strength (∼113–144%) were observed in the Mg–Y–Zn–Al alloys. Despite the significant increase in strength of materials, failure strains of both Mg–Y–Zn–Al alloys were comparable to monolithic Mg. Ignition temperatures of both Mg–Y–Zn–Al alloys were found to outperform commercially available AZ31, AZ80 and WE43 (high-temperature) Mg alloys, and the highest ignition temperature of 770 °C was achieved in the MgY_3.72Zn_1.96Al_0.45 alloy.     

A new 1.9 GPa maraging stainless steel strengthened by multiple precipitating species

A new ultra-high strength maraging stainless steel with composition of 13Cr–13Co–4.5Ni–3.5Mo–0.5Ti (at.%) has been developed. It was demonstrated that the ultimate tensile strength of the steel could reach 1.9 GPa with reasonable ductility. This breakthrough was achieved by a combined strengthening of three different species of precipitates. The evolution of precipitates with respect of size, morphology and chemical composition during aging at 500 °C was characterized using atom probe tomography (APT) and transmission electron microscopy (TEM). The precipitates were identified to be η-phase Ni₃(Ti, Al) phase, Mo-rich R′ phase and Cr-rich α′ phase, developing out of the precursor clusters, Ni–Ti–Al-rich cluster, Mo-rich cluster and Cr-rich cluster, separately. The segregation of Mo and Cr atoms at the precipitate/matrix interfaces was detected and is considered to impede the coarsening of η-phase. Based on the characterizations, the precipitation process of these phases and their effect on mechanical properties were analyzed.     

Effect of heat-treatment on phase stability and grain coarsening in a powder-processed Al–Ni–Co–Zr–Y alloy

Processing of Al alloys via metastable amorphous intermediates can give much higher volume fractions of dispersed strengthening phases than in conventional precipitation- or dispersion-hardened systems. Here, we report a study on an Al–Ni–Co–Zr–Y alloy processed by gas atomization and consolidated/devitrified by warm extrusion. X-ray diffraction and electron microscopy are used to reveal the effects of heat-treatments at 300–500 °C for up to 96 h on the phase stability and coarsening behavior of the alloy. In all samples, the microstructure contains 22 % by volume of Al₁₉(Ni,Co)₅Y₃ plates surrounded by grains of FCC Al. Samples heat-treated at 350 °C and above also contain fine Al₃Y and Al₃Zr particles as minority phases. The softening of the alloy is limited for heat-treatment temperatures of up to 400 °C, and the Al₁₉(Ni,Co)₅Y₃ plates coarsen slowly. At higher temperatures, abnormal coarsening is observed with the development of a secondary population of much larger Al₁₉(Ni,Co)₅Y₃ plates. An analysis of the coarsening kinetics gives a constant coarsening exponent of 3, but a distinct transition in the activation energies. These values suggest that the normal coarsening at lower temperatures occurs by short-circuit diffusion, whereas the abnormal coarsening at higher temperatures involves lattice diffusion. The Al grain size is dictated by the Al₁₉(Ni,Co)₅Y₃ inter-plate separation, and grain growth is limited by the extent of plate coarsening. Such systems could form the basis of new high-strength high-temperature Al alloys for structural applications.     

Phase transformations in Al–Mg–Zn alloys during high pressure torsion and subsequent heating

The structure, phase composition, and their thermal evolution were studied in case of ternary Al–Zn–Mg alloys before and after high-pressure torsion (HPT) in Bridgman anvils. The as-cast non-deformed alloys contained the fine particles of Mg₃₂(Al,Zn)₄₉ (τ phase), MgZn₂ (η phase), AlMg₄Zn₁₁ (η′ phase), and Mg₇Zn₃ phases embedded in the matrix of Al-based solid solution. During heating in differential scanning calorimeter (DSC), all these phases dissolved around 148 °C. The τ nanoparticles coherent with (Al) matrix-formed instead around 222 °C. HPT of the as-cast alloys strongly refined the grains of (Al) solid solution from 500 μm to 120–150 nm. The particles of τ, η, η′, and Mg₇Zn₃ phases fully dissolved in the (Al) matrix. During the following DSC-heating, particles of η phase appeared and grew. Their amount became maximal around 166 °C. The growth of η phase in the fine-grained HPT-treated alloys instead of τ phase in the coarse-grained ones is explained by the shift of the (Al) + η/(Al) + η + τ/(Al) + τ lines in the Al–Zn–Mg ternary phase diagram due to the grain boundary (GB) adsorption. At 166 °C the η phase formed the continuous flat layers in numerous (Al)/(Al) GBs. This corresponds to the complete GB wetting by the η phase. Other (Al)/(Al) GBs contain separated lenticular η particles (incomplete GB wetting). Increasing the temperature from 166 to 320 °C led to the disappearance of the completely wetted (Al)/(Al) GBs. In other words, the transition from complete to the incomplete wetting of (Al)/(Al) GBs by the η phase proceeds between 166 °C and 320 °C.     

Effects of solute content on microstructure of nano precipitate-fine grain synergistically reinforced copper alloys

ABSTRACT

Alloying of Fe, Co was reported to tailor microstructure of copper alloys into a nanoprecipitate-fine grain (NPFG) structure, i.e. nano-sized iron-rich precipitates dispersed inside refined grains. Here, we investigate the solute effect of Sn, Zn on NPFG structure in as-cast copper samples. Mechanisms are proposed to account for the solute effect on precipitate and grain features. Solutes restrict coarsening but facilitate undesirable morphology transition from spherical to angular of iron-rich precipitates. Meantime, solutes allow more precipitates to be active in the nucleation of copper and consequently induce finer grains. Minor Sn is added to optimise NPFG structure and leads to an excellent strength–ductility combination in Cu–1.5Fe–0.1Sn (wt-%) alloy. This work provides a solute-alloying strategy to achieve desired mechanical properties in metals.     

Microstructural homogeneity and mechanical behavior of a selective laser melted Ti-35Nb alloy produced from an elemental powder mixture

Although using elemental powder mixtures may provide broad alloy selection at low cost for selective laser melting(SLM), there is still a concern on the resultant microstructural and chemical homogeneity of the produced parts. Hence, this work investigates the microstructure and mechanical properties of a SLM-produced Ti-35 Nb composite(in wt%) using elemental powder. The microstructural characteristics including ? phase, undissolved Nb particles and chemical homogeneity were detailed investigated.Nanoindentation revealed the presence of relatively soft undissolved Nb particles and weak interface bonding around Nb-rich regions in as-SLMed samples. Solid-solution treatment can not only improve chemical homogeneity but also enhance bonding through grain boundary strengthening, resulting in43 % increase in tensile elongation for the heat-treated Ti-35 Nb compared to the as-SLMed counterpart. The analyses of tensile fractures and shear bands further confirmed the correlation between the different phases and the ductility of Ti-35 Nb. In particular, the weak bonding between undissolved Nb and the matrix in the as-SLMed sample reduces its ductility while the ? grains in solid-solution treated Ti-Nb alloy can induce a relatively stable plastic flow therefore better ductility. This work sheds insight into the understanding of homogenization of microstructure and phases of SLM-produced alloys from an elemental powder mixture.     

Wrapping effect of secondary phases on the grains: increased corrosion resistance of Mg–Al alloys

The discrete secondary phases usually cause severe galvanic corrosion, thereby resulting in rapid degradation for Mg–Al alloys in orthopaedics application. In this study, CaO was introduced into Mg–Al–Zn (AZ61) alloy via selective laser melting (SLM) to ameliorate the characterisations of the secondary phases, with an aim to improve its corrosion behaviour. Results revealed that CaO reacted with Mg and Al in Mg–Al alloys during SLM, suppressing the formation of coarse Mg₁₇Al₁₂ phase and promoting the formation of (Mg, Al)₂Ca phase. Meanwhile, the rapid solidification during SLM promoted the homogeneous precipitation of the second phase. As a result, inert (Mg, Al)₂Ca phase homogeneously wrapped the Mg grains, which effectively protected them from the invasion of corrosion solution. Thus, the degradation rate was remarkably reduced from 0.073 to 0.031?mg?cm^–2?h^–1. Furthermore, AZ61-9CaO exhibited good cytocompatibility. This work suggested that AZ61-9CaO was promising candidates for orthopaedics implants.     

Elastic properties of random L1<Subscript>2</Subscript>–Al<Subscript>3</Subscript>(Sc<Subscript>0.5</Subscript>TM<Subscript>0.5</Subscript>) alloys from first-principle SQSs calculations

Special quasi-random structures (SQSs) with 32 atoms have been generated to model appropriate supercell structure of pseudo-binary random L1₂–Al₃(Sc_0.5TM_0.5) (TM = Y, Ti, Zr, Hf, V, Nb and Ta) alloys. The optimized lattice parameters were in good agreement with the experimental data, and the obtained formation energies showed that all L1₂–Al₃(Sc_0.5TM_0.5) alloys were stable from energetic point of view. As the atomic radius of substitution elements TM in the same Period decreased, the values of C ₁₂ and C ₄₄ for L1₂–Al₃(Sc_0.5TM_0.5) alloys exhibited an overall tendency of increase, implying an enhanced Poisson effect and larger resistance to {100} 〈001〉 shear. The elastic isotropy of L1₂–Al₃(Sc_0.5TM_0.5) alloys was overall lowered and the ductility could be improved. The calculated electronic structure demonstrated that below the Fermi level the hybridization of transition-metal d states with Al p states was reduced with decreasing of atomic radius of substitution elements TM in the same Period, which uncovered underlying mechanism for stability and elastic properties of L1₂–Al₃(Sc_0.5TM_0.5) alloys.     

The influences of rare earth content on the microstructure and mechanical properties of Mg–7Zn–5Al alloy

The influences of rare earth (RE) on the microstructure and mechanical properties of Mg–7Zn–5Al alloy were studied. The results indicate that both the dendrite and grain size of the alloy can be refined by low RE addition. The Al₂REZn₂ phase will be formed with increasing the RE content, however the high RE addition results in the grain coarsening in the alloy due to the decrease of the contribution of Al and Zn solutes on the grain refinement. The strengthening and weakening mechanisms caused by RE addition only lead to the obviously improve on the room temperature ultimate tensile strength. The mechanical properties of the studied alloys can be improved by aging treatment, and the aged Mg–7Zn–5Al–2RE alloy exhibits optimal mechanical properties at room temperature.     

Effect of Al on the microstructure and mechanical properties of Mg–6Zn–2Sn–0.5Mn alloy

The microstructure and mechanical properties of Mg–6Zn–2Sn–0.5Mn–xAl (x?=?0, 1, 2, 3) alloy are investigated. The addition of Al leads to the refinement of grain size and the formation of Al₆Mn, Mg₃₂(Al,Zn)₄₉ also forms when the amount of Al is higher than 2?wt-%. Because of the addition of Al, the precipitates in the alloy after ageing treatment are refined. The alloy containing 1?wt-% Al shows good mechanical properties in the as-cast state which is attributed to the refined grains and low volume fraction of large second phases, it also shows high strength after ageing treatment resulted mainly from the homogeneously distributed fine precipitates, the yield strength, ultimate tensile strength and elongation are 183, 310?MPa and 11%, respectively.     

Non-isothermal aging of a high-Zn-containing Al–Zn–Mg–Cu alloy: microstructure and mechanical properties

The non-isothermal aging behaviour of a newly developed Al–Zn–Mg–Cu alloy containing 17?wt-% Zn was investigated. Hardness and shear punch tests demonstrated that during non-isothermal aging, the mechanical properties of the alloy first increased and then decreased. The best properties were obtained in a sample which was non-isothermally aged upto 250°C with heating rate of 20°C?min^?1, due to the presence of η′/η (MgZn₂) phases. This was confirmed by differential scanning calorimetery. After homogenisation, residual eutectic phases remained at triple junctions or in a spherical form. During aging, these phases transformed into rodlike S (Al₂CuMg)-phase at 400°C, with sizes ranging from 50 to 250?nm. The precipitation sequence in this high-Zn alloy was similar to that for conventional Al–Zn–Mg–Cu alloys.     

Selective laser melting of Zn–Ag alloys for bone repair: microstructure,mechanical properties and degradation behaviour

Zn possesses good biodegradability and biocompatibility, but its strength and hardness are insufficient for bone implants. In this study, Ag was introduced into Zn to improve the mechanical properties by selective laser melting. The results showed that Ag was dissolved in Zn, which generated constitutional undercooling in front of the advancing solid/liquid interface during solidification, making more nucleation events occur and thus refining the grains. When Ag content exceeded its solid solubility in Zn, AgZn₃ phase is formed, which acted as active nucleation sites for Zn grains, further refining the grains. The refinement of the grains effectively hindered the plastic deformation and dislocation. As a result, the compressive strength and hardness were improved by about 100% and 116%, respectively. When Ag content continued increasing and became excessive, AgZn₃ phase grew rapidly, coarsening the grains. Accordingly, the mechanical properties slightly decreased. These results demonstrated that the Zn–Ag alloys are potential implant biomaterials.     

A study of the microstructure, phase composition, and mechanical properties of extruded Mg–9Er–6Y–xZn–0.6Zr magnesium alloys

The microstructure, phase composition, and mechanical properties of Mg–9Er–6Y–xZn–0.6Zr (x = 1, 3, 5 wt%; nominal chemical composition) series alloys were investigated through optical microscopy, X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectrometry, transmission electron microscopy, and tensile tests. Numerous granular Mg₂₄(Er, Y, Zn)₅ phases were distributed in a discontinuous network mainly along the grain boundaries in the alloy with 1 wt% Zn. With increasing Zn content, the Mg₂₄(Er, Y, Zn)₅ phases in the alloys gradually disappeared, the amount of block Mg₁₂Zn(Y, Er) phases increased, and the block size became larger. In addition, a few lamellar phases grew parallel with one another from the grain boundaries to the grain interior in the alloys. The crystallographic structures of the Mg₁₂Zn(Y, Er) and Mg₂₄(Er, Y, Zn)₅ phases were confirmed as 18R-type long-period stacking ordered structures and body-centered cubic structures, respectively. The Mg₁₂Zn(Y, Er) phases with long-period stacking ordered structures increased the strength and toughness of the alloys more than the Mg₂₄(Er, Y, Zn)₅ phases with body-centered cubic structures.     

Strengthening mechanisms in solution treated Mg–yNd–zZn–xZr alloy

A number of solution treated Mg–0.52Nd–0.08Zn–xZr (x = 0, 0.01, 0.03, 0.07, 0.12, and 0.14), Mg–yNd–0.08Zn–0.12Zr (y = 0, 0.17, 0.34, and 0.52) and Mg–0.52Nd–zZn–0.12Zr (z = 0, 0.08, 0.19, and 0.38) (at.%) cast alloys were investigated in terms of grain boundary and solid solution strengthening in this study. The hardness and yield strength of these alloys are determined by the average grain size (Zr content) and the concentration of Nd and Zn elements. The hardness can be predicted as HV₅ ≈ 23 + 3.07 d ^?0.5 (m^?0.5) + 26.5 C _Nd (at.%) + 10.5 C _Zn (at.%), with the average error of about 1.3 %. When the interactions among different solution atoms were not considered, the yield strength can be expressed as σ_0.2 (MPa) = 21 + 0.42 d ^?0.5 (m^?0.5) + (813^3/2 C _Nd (at. %) + 964^3/2 C _Zn (at. %))^2/3, with the average error of about 2.0 %. When the interactions among different solution atoms were considered, more exact yield strength prediction could be obtained with the average error of about 1.6 %.     

Selective laser melting of high-performance pure tungsten: parameter design,densification behavior and mechanical properties

Selective laser melting (SLM) additive manufacturing of pure tungsten encounters nearly all intractable difficulties of SLM metals fields due to its intrinsic properties. The key factors, including powder characteristics, layer thickness, and laser parameters of SLM high density tungsten are elucidated and discussed in detail. The main parameters were designed from theoretical calculations prior to the SLM process and experimentally optimized. Pure tungsten products with a density of 19.01 g/cm³ (98.50% theoretical density) were produced using SLM with the optimized processing parameters. A high density microstructure is formed without significant balling or macrocracks. The formation mechanisms for pores and the densification behaviors are systematically elucidated. Electron backscattered diffraction analysis confirms that the columnar grains stretch across several layers and parallel to the maximum temperature gradient, which can ensure good bonding between the layers. The mechanical properties of the SLM-produced tungsten are comparable to that produced by the conventional fabrication methods, with hardness values exceeding 460 HV_0.05 and an ultimate compressive strength of about 1 GPa. This finding offers new potential applications of refractory metals in additive manufacturing.     

Investigation and improvement on structural stability and stress rupture properties of a Ni–Fe based alloy

The structural stability and stress rupture properties of a Ni–Fe based alloy, considered as boiler materials in 700 °C advanced ultra-supercritical (A-USC) coal-fired power plants, was studied. Investigation on the structural stability of the existing alloy GH984 shows that the most important changes in the alloys are γʹ coarsening, the γʹ to η transformation and the coarsening and agglomeration of grain boundary M₂₃C₆ during thermal exposure. The stress rupture strength was found to be slightly lower than the requirement of 700 °C A-USC. The fracture mode of creep tested specimens was intergranular fracture. Detailed analysis revealed that η phase precipitation is sensitive to Ti/Al ratio and can be suppressed by decreasing Ti/Al ratio. The coarsening behavior of γʹ phase is related to Fe content. Adding B and P was suggested to stabilize M₂₃C₆ and increase grain boundary strength. Based on the research presented and analysis of the data, a modified alloy was developed through changes in composition. For the modified alloy, η phase is not observed and M₂₃C₆ is still blocky and discretely distributes along grain boundary after thermal exposure at 700 °C for 20,000 h. Moreover, the creep strength is comparable to the levels of Ni-based candidate alloys for 700 °C A-USC.     

Development of Ni–Fe–Al-based alloys precipitating cubic γ′ for fabrication of nanoporous membranes

During this study, new Ni-base alloys precipitating cubic γ′ were developed which shall be used for the production of polycrystalline nanoporous membranes. The polycrystalline nanoporous membranes are produced through a combination of cold rolling and heat treatment in order to get directional coarsening of the γ′-phase which is selectively dissolved afterwards. Conventional Ni-based superalloys have a γ/γ′ -microstructure with cubic γ′-precipitates and show the needed etching behaviour but their high strength and limited ductility at room temperature do not allow to produce the polycrystalline nanoporous membranes by means of the before mentioned method. Thus, the new alloys with simpler composition were developed which have a γ/γ′ microstructure. The alloy Ni–13Fe–8Al–4Ti (composition in atomic percent) which was produced by Schmitz (Cullivier, ISBN 978-3-86955-523-2, 16) served as basis and showed the promising characteristics. To obtain cubic γ′-precipitates, the misfit was estimated to values of at least |0.2| % by a method presented by Mishima (Acta Metall 33:1161–1169, 23). Further, the phase compositions as well as phase volume fractions of γ-matrix and γ′-phase were calculated by means of Thermocalc^® simulations (database: TTNi7). The etching behaviour of the new alloys was adjusted by adding chromium and molybdenum which passivate the γ-matrix so that the γ′-precipitates dissolve during the leaching process. The well-aligned cubic γ′-precipitates were obtained by partially replacing titanium by niobium. Furthermore, the hardness could be significantly lowered compared to conventional superalloys by reduction of alloying elements. Hence, the promising alloys were found to get directional coarsening of the γ′-precipitates in a combined process of cold rolling and heat treatment.     

Sc and Sc–Zr effects on microstructure and mechanical properties of Be–Al alloy

Herein, we investigated the effects of Sc and Sc–Zr on the microstructure and mechanical properties of Be–Al alloy, showing that Sc alloying resulted in Be grain refinement and reduced the secondary dendritic arm spacing (SDAS) of these grains by 1/3, whereas Sc–Zr alloying further decreased the SDAS to 7.5?µm and afforded equiaxed/cellular-like morphology with further refined Be grains. The above alloying resulted in the formation of intermetallic compounds (Be₁₃Sc, Be₁₃Zr, and Al₃(Sc_1–xZr_x)), increasing the macrohardness of the Be–Al alloy, with the microhardness and elastic modulus of the Be phase increasing to a larger extent than those of Al. Importantly, Sc–Zr alloying resulted in better microstructure modification and mechanical reinforcement than Sc alloying.     

Influence of Heat Treatments on the Microstructure and Mechanical Properties of Two Fine Mg–Li–Y Alloy Wires for Bioresorbable Applications

Two Mg–4Li–xY (x = 0.5 and 2.0 wt%) alloy wires are investigated for application in bioresorbable medical devices that experience high levels of plastic deformation. The two wires are supplied cold drawn to a diameter of 125 μm, and a series of thermal treatments are applied to maximize ductility. The ductility of the alloys is maximized soon after complete recrystallization. Prolonged annealing causes grain coarsening in the Mg–4Li–0.5Y alloy and precipitation of a Mg₂₄Y₅ phase in both alloys. Both wires are shown to achieve ≈20% elongation to failure in tension and survive high idealized bending strains (>40%). When heat treated for optimum mechanical properties for the intended application, the Mg–4Li–0.5Y alloy develops an intense transverse basal texture; however, this is shown to weaken with increased Y content in the Mg–4Li–2Y alloy wire. The increased Y content also prevents grain coarsening, indicating that the increased Y content restricts grain boundary mobility during annealing. Both alloys have relatively high ductility, meaning both are identified as promising new materials for application in bioresorbable medical devices that require to achieve and support high levels of plastic deformation during their life cycle.     

Precipitates,zones and transitions during aging of Al-Zn-Mg-Zr 7000 series alloy

Abstract

Age hardening of an industrial 7000 series alloy in the temperature range 70-150° C has been followed by mechanical testing, electrical conductivity measurement, differential scanning calorimetry and extensive electron microscopy (TEM). The property changes during aging are interpreted in terms of structural transformations that involve two types of Guinier-Preston (GP) zones (I and II), the metastable hardening precipitate η′ and the stable phase η-MgZn₂, as well as coarsening, changes of composition and internal order within zones and precipitates. Time-temperature ranges of the transformations during aging, and its dependence on quenching temperature, are estimated from TEM observations. The role of the GP(II) zones in the aging of alloys quenched from temperatures above 450°C is emphasized.