Stability of metastable phases and microstructures in the ageing process of Al–Mg–Si ternary alloys

Precipitation behaviour of Al–Mg–Si alloys, with balanced (Mg/Si=2), excess silicon (Mg/Si<2) and excess magnesium (Mg/Si>2) compositions, were studied by differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and Vickers hardness tests. Four significant exothermic peaks were observed in DSC curves which were attributed to metastable clusters, , and stable phases. The peaks corresponding to and were formed closely in the DSC curves but showed different behaviour in isothermal annealing. The additional peak verifying the precipitation of phases, which has recently been proposed by some workers, was not detected. Transmission electron microscope observations and Vickers microhardness tests showed that precipitates played a major role in improving the hardness, but not precipitates. © 1998 Chapman & Hall     

What is going on in magnesium alloys?

China has been developed into one of the most active regions in terms of both fundamental and applied research on magnesium (Mg) and its alloys in the world from a solid base laid by its prominent metallurgist and materials scientists over the past decades. Nowadays, a large number of young-generation researchers have been inspired by their predecessors and become the key participants in the fields of Mg alloys, which consequently led to the establishment of China Youth Scholar Society for Magnesium Alloys Research in 2015. Since then, the first two China Youth Scholars Symposiums on Mg Alloys Research had been held at Harbin (2015) and Chongqing (2016) China, respectively. A number of crucial research interests related to fundamental and applied Mg research were discussed at the conferences and summarized in this short perspective, aiming to boost far-reaching initiatives for development of new Mg-based materials to satisfy the requirements for a broad range of industrial employments. Herein, four main aspects are included as follows: i) Plastic deformation mechanism and strengthening strategy, ii) Design and development of new Mg-based materials, iii) Key service properties, and iv) New processing technologies.     

Thermodynamic assessment of Co–Al–W system and solidification of Co-enriched ternary alloys

Based on the assessed three constituent binary systems and reported phase diagram data, the Co–Al–W ternary system was thermodynamically optimized by using CALculation PHAse Diagram (CALPHAD) method. The newly reported ternary phase γ′ was described with a two sublattice model, (Co,Al,W)_0.75:(Co,Al,W)_0.25, while liquid Fcc, Hcp(Co), and Bcc(W) were treated as substitutional solution phases. In order to describe the ternary solubility in the binary phases Co₇W₆ and CoAl, the models (Al,Co,W)₇W₂(Al,Co,W)₄ and (Al,Co,W)_0.5(Al,Co,W)_0.5(Va)₃ were, respectively, adopted. The rest 11 phases in the ternary systems were simply treated as stoichiometric compounds because their homogeneous ranges are small and can be neglected. A set of self-consistent parameters were obtained, which reasonably reproduced the phase relationship in Co-enriched corner. Finally, to confirm the reliability of the present assessment,the so-called Scheil–Gulliver model was used to simulate the solidification processes of three typical Co-based alloys. It was shown that the as-cast microstructures of the three ingots were well explained or predicted.     

Precipitation and aging in high-conductivity Cu–Cr alloys with additions of zirconium and magnesium

Abstract

The precipitation reactions responsible for age hardening in a high-conductivity Cu–Cr–Zr–Mg alloy have been investigated by analytical transmission electron microscopy and compared briefly with the processes that occur in simpler Cu–Cr and Cu–Cr–Mg alloys. Aging at low temperatures (400°C) results in the formation of Guinier–Preston zones. Peak hardness, obtained by aging for 24 h at 450°C, is found to be a result of the fine scale precipitation of an ordered compound, possibly of the Heusler type, with the suggested composition CrCu₂(Zr, Mg). Overaging results in the formation of coarse precipitates of Cr and CU₄Zr. The intergranular precipitate which forms in the Cu–Cr–Zr–Mg alloy is Cu₄Zr. This phase precipitates both as discrete particles on the grain boundaries and as thin ( ~ 5 nm) continuous intergranular films.

MST/89     

TEM observation of icosahedral,new crystalline and glassy phases in rapidly quenched CdCu alloys

TEM study of microstructures observed in rapidly solidified CuCd alloys in the compositional range between 37.2 to 60.5 at% Cd demonstrates for the first time:

• 1.1. The formation of an icosahedral CuCd quasicrystalline phase;
• 2.2. The existence of a tetragonal CdCu phase first identified by x-ray diffraction by Dey and Quader as the Cd₃Cu₄ phase.
• 3.3. The existence of a new CuCd cubic fcc phase with a giant unit cell of a ≅ 3.25 nm, close to the Cd₃Cu₄ composition.
• 4.4. Formation of Cu-37.2 at% Cd metallic glass.
• 5.5. The possibility of forming giant unit cell crystalline structures from the melt and by solid-state precipitation during the limited times of rapid quenching.

     

Hot deformation and processing map of an as-extruded Mg–Zn–Mn–Y alloy containing I and W phases

The hot deformation characteristics of an as-extruded ZM31 (Mg–Zn–Mn) magnesium alloy with an addition of 3.2 wt.% Y, namely ZM31 + 3.2Y, have been studied via isothermal compression testing in a temperature range of 300–400 °C and a strain rate range of 0.001–1 s^− 1. A constitutive model based on hyperbolic-sine equation along with processing maps was used to describe the dependence of flow stress on the strain, strain rate, and deformation temperature. The flow stress was observed to decrease with increasing deformation temperature and decreasing strain rate. The deformation activation energy of this alloy was obtained to be 241 kJ/mol. The processing maps at true strains of 0.1, 0.2, 0.3 and 0.4 were generated to determine the region of hot workability of the alloy, with the optimum hot working parameters being identified as deformation temperatures of 340–500 °C and strain rates of 0.001–0.03 s^− 1. EBSD examinations revealed that the dynamic recrystallization occurred more extensively and the volume fraction of dynamic recrystallization increased with increasing deformation temperature. The role of element Y and second-phase particles (I- and W-phases) during hot compressive deformation was discussed.     

Deformation-driven formation of equilibrium phases in the Cu–Ni alloys

The homogeneous coarse-grained (CG) Cu–Ni alloys with nickel concentrations of 9, 26, 42, and 77 wt% were produced from as-cast ingots by homogenization at 850 °C followed by quenching. The subsequent high-pressure torsion (5 torsions at 5 GPa) leads to the grain refinement (grain size about 100 nm) and to the decomposition of the supersaturated solid solution in the alloys containing 42 and 77 wt% Ni. The lattice spacing of the fine Cu-rich regions in the Cu–77 wt% Ni alloy was measured by the X-ray diffraction (XRD). They contain 28 ± 5 wt% Ni. The amount of the fine Ni-rich ferromagnetic regions in the paramagnetic Cu–42 wt% Ni alloy was estimated by comparing its magnetization with that of fully ferromagnetic Cu–77 wt% Ni alloy. According to the lever rule, these Ni-rich ferromagnetic regions contain about 88 wt% Ni. It means that the high-pressure torsion of the supersaturated Cu–Ni solid solutions produces phases which correspond to the equilibrium solubility limit at 200 ± 40 °C (Cu–77 wt% Ni alloy) and 270 ± 20 °C (Cu–42 wt% Ni alloy). To explain this phenomenon, the concept of the effective temperature proposed by Martin (Phys Rev B 30:1424, 1984) for the irradiation-driven decomposition of supersaturated solid solutions was employed. It follows from this concept that the deformation-driven decomposition of supersaturated Cu–Ni solid solutions proceeds at the mean effective temperature T _eff = 235 ± 30 °C. The elevated effective temperature for the high-pressure torsion-driven decomposition of a supersaturated solid solution has been observed for the first time. Previously, only the T _eff equal to the room temperature was observed in the Al–Zn alloys.     

PHB,crystalline and amorphous magnesium alloys: Promising candidates for bioresorbable osteosynthesis implants?

In this study various biodegradable materials were tested for their suitability for use in osteosynthesis implants, in particular as elastically stable intramedullary nails for fracture treatment in paediatric orthopaedics. The materials investigated comprise polyhydroxybutyrate (PHB), which belongs to the polyester family and is produced by microorganisms, with additions of ZrO₂ and a bone graft substitute; two crystalline magnesium alloys with significantly different degradation rates ZX50 (MgZnCa, fast) and WZ21 (MgYZnCa, slow); and MgZnCa bulk metallic glasses (BMG). Push-out tests were conducted after various implantation times in rat femur meta-diaphysis to evaluate the shear forces between the implant material and the bone. The most promising materials are WZ21 and BMG, which exhibit high shear forces and push-out energies. The degradation rate of ZX50 is too fast and thus the alloy does not maintain its mechanical stability long enough during the fracture-healing period. PHB exhibits insufficient mechanical properties: it degrades very slowly and the respective low shear forces and push-out energy levels are unsatisfactory.     

Interaction between sulfate-reducing bacteria and aluminum alloys——Corrosion mechanisms of 5052 and Al-Zn-In-Cd aluminum alloys

Microbiologically influenced corrosion caused by sulfate-reducing bacteria(SRB) poses a serious threat to marine engineering facilities.This study focused on the interaction between the corrosion behavior of two aluminum alloys and SRB metabolic activity.SRB growth curve and sulfate variation with and with aluminum were performed to find the effect of two aluminum alloys on SRB metabolic activity.Corrosion of 5052 aluminum alloy and Al-Zn-In-Cd aluminum alloy with and without SRB were performed.The results showed that both the presence of 5052 and Al-Zn-In-Cd aluminum alloy promoted SRB metabolic activity,with the Al-Zn-In-Cd aluminum alloy having a smaller promotion effect compared with 5052 aluminum alloy.The electrochemical results suggested that the corrosion of the Al-Zn-In-Cd aluminum alloy was accelerated substantially by SRB.Moreover,SRB led to the transformation of Al-Zn-In-Cd aluminum alloy corrosion product from Al(OH)3 to Al2 S3 and NaAlO2.     

Thermodynamics and kinetics of phase transformation in rare earth–magnesium alloys: A critical review

Magnesium and its alloys are significant superior metallic materials for structural components in automobile and aerospace industries due to their excellent physicomechanical properties. The Mg–rare earth(RE) systems have attracted great interests because RE additions can improve both the deformability and the strength of Mg alloys through solid solution strengthening and precipitation hardening mechanisms.This paper focuses on the interface stability, together with thermodynamics and kinetics of nucleation and growth of the key phases and matrix phases in Mg–RE alloys. In this paper, the theory and recent advances on Mg–RE alloys, especially for the interface stability, thermodynamics and kinetics of nucleation and growth of the key phases and matrix phases, together with their relationships with micro-structures,and macroscopic properties, are reviewed. By combining the thermodynamics/kinetics integrated simulations with various advanced experimental techniques, reverse design of Mg–RE alloys starting from the target service performance is put forward as a kind of scientific paradigm with rational design.     

Characterization of silicon phases in spray-formed and extruded hypereutectic Al–Si alloys by image analysis

The silicon phases in the spray-formed and extruded hypereutectic Al–Si alloys (AlSi18, AlSi25 and AlSi35) have been quantitatively evaluated by means of image analysis technique. The influence of silicon content in the alloys, thermal conditions during spray forming of the alloys and hot extrusion of the spray-formed alloys on the size, shape, dispersion and orientation of the silicon phases have been studied and discussed. In general, the silicon phases are greatly refined and uniformly distributed in the spray-formed Al–Si alloys. This improvement in the silicon phases is further facilitated by low thermal input as well as fast cooling conditions during spray forming. The silicon particles in the as-extruded Al–Si alloys appear more homogeneous and regular than those in the as-deposited Al–Si alloys but exhibit a certain amount of anisotropy and a tendency to preferred orientation. The silicon particles, depending on the particle size and shape, may fracture or coarsen during extrusion.     

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 %.     

Density and viscosity of ternary Al–Cu–Si liquid alloys

Densities and viscosities of ternary Al–Cu–Si liquid alloys have been investigated over a wide temperature and composition range. Density was measured using electromagnetic levitation as a container-less technique, while viscosity was measured by means of a high-temperature oscillating cup viscometer. In this ternary system, binary interaction parameters as well as a third (ternary) interaction parameter need to be taken into account for the excess volume to describe the liquid densities. The temperature dependences of the viscosities are well described by the Arrhenius law. A maximum of the activation energy of viscous flow is found in that compositional range in which intermetallic phases exist in the solid state.     

Microstructure and mechanical properties of as-extruded AZ80–xSn magnesium alloys

The effect of 0–2?wt-% Sn addition on AZ80 magnesium alloys after 350°C extrusion has been studied by analysing microstructure and mechanical properties. The results indicated that dynamically recrystallised grains were fine and homogeneous with less than 1?wt-% Sn addition. In AZ80–0.5Sn alloy, a large number of Mg₁₇Al₁₂ precipitated phases formed in grains and at grain boundaries during extrusion process. With more than 1?wt-% Sn addition, the size of dynamically recrystallised grains increased and the number of Mg₁₇Al₁₂ phases decreased. The strength of as-extruded AZ80–0.5Sn alloy enhanced largely as compared with that of the as-extruded AZ80 alloy. AZ80–0.5Sn alloy had the outstanding tensile and compressive properties.     

Density and viscosity of ternary Cr–Fe–Ni liquid alloys

The density and viscosity of ternary Cr–Fe–Ni liquid alloys have been investigated over a wide temperature range. The density was measured using electromagnetic levitation as a container-less technique, while viscosity was measured by means of a high-temperature oscillating cup viscometer. Although, the concentration dependence of density shows the influence of the second order (binary) interaction parameter in excess volume, the influence of a third order (ternary) interaction parameter in excess volume can be neglected. The temperature dependences of the viscosities are well described by the Arrhenius law. The viscosity increases monotonically as Fe or Cr concentration increases. For constant temperature, the viscosity as a function of iron molar faction can be described by a thermodynamic model using the enthalpy of mixing as input parameter.     

Morphology and properties of Mg2Si and Mg2(SixSn1−x) reinforcements in magnesium alloys

In this paper, the effects of Sr, Sb, Sr+Sb and Sn on Mg₂Si reinforcement phases in an Mg–Al–Zn–Si alloy are studied, and the structures and characteristics of Mg₂(Si_xSn_1?x) phases are analysed with first-principle calculations. The results show that the coarse eutectic Mg₂Si can be refined by modifying processes with Sr, Sb, and their combination. When alloying with Sn, a new reinforcement phase Mg₂(Si_xSn_1?x) forms by a substitution reaction, instead of Mg₂Si. Calculations indicate that Mg₂(Si_xSn_1?x) has a certain percentage of covalent bonds, which ensure it has sufficient hardness to act as a reinforcement phase. Calculated results for physical parameters, such as the bulk modulus and shear modulus, indicate that an Mg₂(Si_xSn_1?x) intermetallic exhibits greater ductility than Mg₂Si.

Highlights

• The coarse eutectic Mg₂Si can be refined by modifying processes.

• A new phase Mg₂(Si_xSn_1?x) forms by substitution reaction during solidification.

• Mg₂(Si_xSn_1?x) has certain covalent bound percentage.

• Mg₂(Si_xSn_1?x) has better plasticity than that of Mg₂Si.

     

Strength evaluation of α and β phases by nanoindentation in Ti–15Mo alloys with Fe and Al addition

Abstract

The strengths of the α precipitate and the β matrix were evaluated by nanohardness in the Ti?15Mo?1Fe and Ti?15Mo?3Al alloys and compared to those of the Ti?15Mo alloy. The α phases with similar size (a long axis of a few micrometres and a short axis of a few hundred nanometres), distribution and volume fraction were obtained in three alloys by adjusting the aging temperature. Analyses by SEM-EDS confirmed that Fe and Mo were enriched in the β phase and depleted in the α phase, while Al was enriched in the α phase and depleted in the β phase. Tensile tests were carried out, and the tensile strength was shown to be higher in the Ti?15Mo?1Fe and Ti?15Mo?3Al alloys than in the Ti?15Mo alloy. The nanohardness measurements indicated that the α phase was softer than the β phase in both Ti?15Mo?1Fe and Ti?15Mo alloys, while it was harder in the Ti?15Mo?3Al alloy. The increased tensile strength was mainly caused by the strength of the Fe enriched β phase in the Ti?15Mo?1Fe alloy and by the strength of the Al enriched α phase in the Ti?15Mo?3Al alloy.