Jian Feng Nie

Characterisation of strengthening precipitate phases in a Mg–Y–Nd alloy

Abstract Strengthening precipitate phases in a Mg–Y–Nd based alloy (WE54), aged at 250°C, have been characterised using transmission electron microscopy. Precipitation at 250°C involves formation of three separate metastable phases, {11 2 0} α platelets of an as yet unidentified phase, and the phases designated β′ and β1, preceding formation of the equilibrium phase β. All three phases β′, β1 and β are present in significant fractions in peak-aged samples. The β′ phase has a base-centred orthorhombic structure, with a potential point group of mmm. The β1 phase has an f.c.c. structure (space group Fm 3 m , a=0.74±0.01 nm), which renders it isomorphous with a family of intermetallic compounds of the general form Mg3X, where X represents Nd, Ce, La, Pr, Dy and Sm. The equilibrium phase β has an f.c.c. structure (space group, F 4 3 m , a=2.2±0.1 nm), which makes it isomorphous with Mg5Gd. The formation of β1 phase is shown to generate significant shear strain energy, and a mechanism of shear strain energy accommodation is proposed, involving nucleation in association with β′ phase. With prolonged ageing at 250°C, the β1 phase transforms in situ to the equilibrium β phase.

Periodic Segregation of Solute Atoms in Fully Coherent Twin Boundaries

The Strength of Impurities The practical strength of a material (rather than its theoretical strength) is influenced by the presence of defects between crystalline domains and the inclusion of impurities. In some cases, synergistic effects may arise where the impurity atoms segregate to the domain boundaries, although kinetic barriers may limit the extent to which the impurity atoms can order. Nie et al. (p. 957) show the segregation of oversized and undersized solute atoms at coherent twin boundaries in a magnesium alloy. The minimization of strain energy drives the differently sized impurities to different twin boundaries, strengthening the material. Thermally driven ordering of solute atoms can lead to an unexpected strengthening of a metal alloy. The formability and mechanical properties of many engineering alloys are intimately related to the formation and growth of twins. Understanding the structure and chemistry of twin boundaries at the atomic scale is crucial if we are to properly tailor twins to achieve a new range of desired properties. We report an unusual phenomenon in magnesium alloys that until now was thought unlikely: the equilibrium segregation of solute atoms into patterns within fully coherent terraces of deformation twin boundaries. This ordered segregation provides a pinning effect for twin boundaries, leading to a concomitant but unusual situation in which annealing strengthens rather than weakens these alloys. The findings point to a platform for engineering nano-twinned structures through solute atoms. This may lead to new alloy compositions and thermomechanical processes.

Precipitation in magnesium alloy WE54 during isothermal ageing at 250 C

There has been a rapid growth in interest in the development of higher strength, creep-resistant magnesium alloys for the automotive and aerospace industries. All three precipitate phases have been described to form as plates on {l_brace}1{bar 1}00{r_brace}{sub {alpha}} planes of the magnesium matrix phase, and it has been proven difficult to distinguish them on the basis of morphology. These prismatic precipitate plates have been suggested to play an important role in strengthening such magnesium alloys. However, there has been little detailed characterization of these phases and there is a lack of experimental evidence to support the reported precipitation sequence. The role of the metastable intermediate precipitate phase(s) in the nucleation of successive intermediate or equilibrium precipitate phases remains to be established. It is thus the purpose of the present paper to report experimental observations of the precipitation sequence in a WE54 alloy during isothermal ageing at 250 C, with particular attention given to the role of intermediate precipitate phase(s) in nucleating successive metastable and equilibrium precipitation products.

Precipitation hardening of Mg-Ca(-Zn) alloys

Binary Mg-1Ca alloy exhibits a moderate precipitation-hardening response during isothermal ageing at 200 °C. The ternary addition of a small concentration of Zn (1wt%) to the binary alloy leads to a substantial increase in peak hardness and an accelerated rate of ageing. The maximum hardness achievable (63 HV) is comparable with that of ZC63, Mg-8Al-lRE and Mg-1.3RE alloys. This significant increase in maximum hardness appears to be associated with a refined distribution of precipitates. The primary intermetallic phase in as-cast Mg-1Ca-1Zn alloy is a ternary phase with an atomic composition of 69.4 ± 1.4 at.%Mg, 27.0 ± 0.3 at.%Ca, and 3.6 ± 1.0 at%Zn. This phase has a hexagonal crystal structure (point group 6/mmm), with lattice parameters of approximately a = 0.61 ± 0.01 nm, c = 1.02 ± 0.01 nm, which appears isomorphous with Mg2Ca (space group P63/mmc). Two solid state precipitate phases have been observed in as-cast microstructure of Mg-1Ca-1Zn alloy, a hcp phase with a = 0.623 nm, c = 1.012 nm, and a hcp phase with a = 0.556 nm, c = 1.042 nm. These two precipitate phases have a thin plate shape, and have identical orientation relationships with respect to the matrix phase: (0001)P // (0001)m, [2110]P // [1010]m.

On the Structure, Transformation and Deformation of Long-Period Stacking Ordered Phases in Mg-Y-Zn Alloys

Ternary Mg-Y-Zn alloys have attracted considerable attention due to their unique microstructures and promising mechanical properties. The microstructure of these alloys contains predominantly intermetallic phases of 18R and/or 14H long-period stacking ordered (LPSO) structures, depending on the alloy processing conditions. Such 18R and/or 14H structures or similar LPSO structures are also found in other magnesium alloys such as Mg-Gd-Zn, Mg-Gd-Y-Zn, Mg-Dy-Zn, Mg-Ho-Zn, Mg-Er-Zn, Mg-Y-Cu, Mg-Y-Ni, and Mg-Y-Co. In addition, LPSO structures such as 24R and 10H have also been reported, even though they are less frequently observed. This paper provides a review of the current status on the characterization of the structure, thermal stability, transformation, and deformation of the LPSO structures using high-angle annular dark-field scanning transmission electron microscopy and electron diffraction. Unsolved issues on the LPSO structures are also highlighted and discussed.

Zr-Containing Precipitates in Mg-3wt%Nd-0.2wt%Zn-0.4wt%Zr Alloy during Solution Treatment at 540°C

The microstructure of Mg-3wt%Nd-0.2wt%Zn-0.4wt%Zr (NZ30K) alloy after solution treatment was investigated and several kinds of interesting phases containing zirconium were found in this study. NZ30K was gravity cast using permanent die casting. After high tempering solution treatment at 540°C, cluster particles were observed inside the grains under optical microscopy. The detailed investigations were carried out on transmission electron microscopy (TEM). Four kinds of precipitates were visible inside grains with different shapes: block-like, globular, short rod-like, long rod-like. The block-like particle was identified as ZrH2 phase and the globular one was Zn2Zr3 phase. The other two were still Zr-containing phases, which could not be identified to any of existed compounds containing Zr. The formation of Zn-Zr compounds would probably be due to relative high ratio of Zr to Zn in the center of grains. ZrH2 would be the results of reaction of zirconium with H element during solution treatment, which probably came into the alloy during melting.

On the form of the age-hardening response in high strength aluminium alloys

Abstract The form of the age-hardening response in high strength aluminium alloys has been modelled using appropriate versions of the Orowan equation and precipitation-hardening equations developed for rationally-oriented precipitate plates. When the shape and orientation of precipitates are taken into account, the change in critical resolved shear stress (CRSS) produced following isothermal ageing of the Al–Cu–Li and Al–Cu alloys can be modelled exclusively using equations assuming interfacial strengthening and Orowan hardening, respectively. Combined with direct observations of the interaction between dislocations and the T 1 and θ′ precipitates, respectively, in fractured tensile samples, these results suggest that it is not necessary to invoke a transition from predominantly dislocation shearing to Orowan looping to account for the form of the ageing curves of these alloys.

Formation of bainite as a diffusional–displacive phase transformation

Abstract Formation of bainite in steels requires redistribution of carbon and is thus diffusional, but not necessarily diffusion controlled. Formation of bainitic ferrite involves an invariant plane strain change of shape and is thus unquestionably displacive. A conceptual framework is provided, within which these apparently conflicting descriptions of the transformation may be reconciled.

Comments on the “dislocation interaction with semicoherent precipitates (ω phase) in deformed Al-Cu-Mg-Ag alloy”

In a recent paper, Li and Wawner [1] reported results of a study of the interaction between dislocations and {111}a precipitate plates of V (Al2Cu) phase in a plastically deformed Al-Cu-Mg-Ag alloy using both conventional and high resolution transmission electron microscopy (HRTEM). Based on experimental observations that V phase was sheared in plastically-deformed samples, they propose a strengthening model for aluminium alloys containing {111} a precipitate plates. This model is based essentially on the order strengthening mechanism, currently accepted for spherical particles [2,3], and includes increments in strength arising from both order strengthening and interfacial (chemical) strengthening for sheared precipitates. However, it does not take into account the plate-like shape and orientation of theV precipitates. Despite this major deficiency, the model is claimed to be applicable not only toV precipitate plates in Al-Cu-Mg-Ag alloys, but also to other {111} a precipitate plates, such as T1 (Al2CuLi) phase in Al-Cu-Li alloys [4,5]. The aim of this comment is to demonstrate that this model is an oversimplification and inevitably incorrect.

On the role of Ag in enhanced age hardening kinetics of Mg–Gd–Ag–Zr alloys

Abstract The addition of Ag to the age hardenable Mg–Gd–Zr alloy system dramatically enhances early stage age hardening kinetics. Using atom probe tomography (APT), Ag-rich clusters were detected in a Ag-containing Mg–Gd–Zr alloy immediately after solution treatment and water quenching. During subsequent isothermal ageing at 200 °C, a high density of basal precipitates was observed during the early stages of ageing. These basal precipitates were enriched with Ag and Gd, as confirmed by APT. It is posited that Ag-rich clusters in the context of quenched-in vacancies can attract Gd atoms, increasing diffusion kinetics to facilitate the formation of the Ag + Gd-rich basal precipitates. The rapid formation of Ag + Gd-rich precipitates was responsible for accelerated ageing.