On the formation of faceted Al3Zr (β′) precipitates in Al–Li–Cu–Mg–Zr alloys

Trace additions of Zr to Al alloys inhibit recrystallization through the formation of spherical and coherent Al₃Zr (β′) precipitates. Recently, observations have been made of faceted β′ precipitates in several hot deformed Al alloys, although no systematic experimental study of either the causes of the formation of such precipitates or their orientation relationships with the Al matrix has so far been reported. A detailed examination of the orientation relationships shows that the cube-on-cube orientation relationship existing between spherical, coherent β′ precipitates and the Al matrix does not hold good for the faceted β′ particles and that the faceted β′ particles are twin-related with the matrix. It is shown that the twin-related β′ particles are not incoherent, but bound by large facets fully coherent with the matrix, and that such particles are associated with fairly significant coherency strains. A probable shape of the faceted β′ is also described.     

Shear induced chemical mixing in heterogeneous systems

Shear-induced mixing in heterogeneous Cu alloy systems was investigated by molecular dynamics simulation. Each system, which comprised a single spherical particle within a Cu matrix, was subjected to cyclical shearing events to high strains at 100 K. The particles investigated were Cu, Ag, Ni, Fe, Nb, and V. fcc particles were observed to undergo “superdiffusive” mixing with dislocations crossing the particle-matrix interface. The initial rate of mixing in these systems increased quadratically with particle radius. bcc particles showed different behaviors. For Nb and V dislocations did not cross into the particles and the initial rates of mixing increased linearly with particle radius. The Nb particles remained spherical but developed amorphous Cu-Nb shells at the particle-matrix interface. The V particle showed some faceting, but it too formed amorphous layers on non-facetted interfaces. The Fe particle behaved similarly to Nb and V particles initially, but more like fcc particles at high strains. The molecular dynamics results are compared with experimental observations.     

高强高导电CuCrZr合金时效过程中析出相的演化规律

对固溶态CuCrZr合金经不同温度时效后的析出相进行显微观察,并对其电导率进行了测试。结果表明:450 ℃时效30 min的析出相为5 nm以下的单质Cr相,并且与基体呈cube-on-cube取向关系。450 ℃峰值时效120 min时析出相为CrCu₂Zr相和Cr相,尺寸为10nm左右,且与基体共格;600 ℃和800 ℃过时效30 min后析出相主要演变为球状的Cr相和棒状的Cu₄Zr相。在600 ℃时效处理后部分棒状析出相已显著长大至50 μm左右,而800 ℃时效处理后几乎看不到细小的析出相,其中棒状Cu₄Zr析出相长大至200 μm以上,球状纯Cr析出相也接近50 μm。CuCrZr合金在450 ℃时效时导电率随时效时间的延长不断增高,在120 min后达到最大值且几乎不再变化。根据析出相转化率与导电率的线性关系,建立了合金在400、450、500和600 ℃下的析出动力学方程。     

Precipitate stability and morphology in irradiation environments

A solid under irradiation constitutes a nonequilibrium dynamical system, and therefore the modeling of phase stability in such systems requires a kinetic approach. This paper presents some recent advances in the atomistic modeling of precipitation under irradiation, using kinetic Monte Carlo simulations. It illustrates that vacancy-assisted and thermodynamically driven phase separation, in competition with forced atomic mixing promoted by nuclear collisions, can lead to three very different outcomes: dissolution of pre-existing precipitates, the formation of compositional patterns at the nanoscale, or macroscopic decomposition. Interstitial transport, on the other hand, when coupled with solute fluxes, can induce the precipitation of mushy precipitates, even in ideal alloy systems. These diverse phase evolutions are shown to be rationalized by continuum modeling.     

Decomposition processing and precipitation hardening of rapidly solidified Al-Cr-Y-Zr alloy

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Three-Dimensional Atom-Probe Tomographic Analyses of Lead-Telluride Based Thermoelectric Materials

Precipitates in bulk p-type thermoelectric materials, PbTe-SrTe and PbTe-PbS, are studied using three-dimensional (3-D) atom-probe tomography (APT). APT is capable of characterizing chemically materials in 3-D with subnano-scale spatial resolution on an atom-by-atom basis, which enables us to characterize secondary phases in the PbTe matrix as well as the dopant distributions at different imperfections. We demonstrate that APT provides accurate information about the compositions and morphologies of nanoprecipitates. In the PbTe-SrTe system, different morphology of precipitates is observed and the SrTe composition is confirmed. Also, segregation of Na dopants at mesoscale imperfections, dislocations and grain boundaries, and at matrix/precipitate interfaces is observed. In the PbTe-PbS system, PbS precipitates are observed. The PbS precipitates exhibit faceting, and have a morphology that depends on the bulk Na concentration. A predominance of {100} faceted precipitates is observed for 2 mol.% Na. Using 3-D APT, we demonstrate that Na segregation at matrix/precipitate interfaces is most likely responsible for the change in their morphologies, which occurs by reducing the interfacial free energy of {100} facets.     

Secondary precipitation in Al–Zn–Mg–(Ag) alloys

Secondary ageing of age-hardenable aluminium alloys occurs at temperatures below the solvus of GP zones after a preliminary ageing at a higher temperature. The phenomenon has technological interest, as it may be included in heat treatments giving a substantial benefit on the mechanical properties. In the present work, positron annihilation lifetime spectroscopy (PALS) is applied in combination with Vickers hardness measurements for an investigation on secondary ageing of Al–4wt.%Zn–3wt.%Mg–xAg, where x=0, 0.1, 0.2, 0.3, 0.5 wt.%. Ageing regimes have been characterised by the substantially different evolutions that are observed. The results shed light on the interplay between the formation of coherent solute aggregates (clusters or GP zones) and the precipitation of semi-coherent or incoherent precipitates, which are in competition to control the hardening effects. PALS data show that secondary ageing in the ternary Al–Zn–Mg alloys produces coherent aggregates even in the presence of a well-developed stage of semi-coherent or incoherent precipitation that is obtained if the alloys are first aged to peak hardness. In the presence of Ag, on the contrary, the effects of coherent aggregation during secondary ageing are observed only if the preliminary ageing is interrupted well before reaching peak hardness.     

Microstructure dependence of strength of Ir-base refractory superalloys

Precipitation hardening was investigated in Ir–Nb and Ir–Zr alloys with a two-phase structure consisting of the fcc matrix and L1₂ coherent precipitate phases, similar to that in Ni-base superalloys. Cuboidal L1₂ precipitates and plate-like L1₂ precipitates were formed with coherent interfaces in the fcc matrix in the Ir–Nb and Ir–Zr alloys, respectively. Effects of precipitate shape and coherency strains on precipitation hardening are discussed in terms of lattice misfit. Plate-like precipitates forming a 3-dimensional maze structure in the Ir–Zr alloys were profitable to precipitation hardening in both factors, that is precipitate shape and coherency strains.     

铜基引线框架材料C194合金的析出相研究

用透射电子显微镜对市售的两种编号为A、B的C194铜带的显微组织和析出相进行了观察分析,结果表明两种试样的析出相都是Fe3P,只是A试样的析出相有两种形态,一是直径为50nm～70nm的球形Fe3P,另一种为10nm左右的微小Fe3P粒子。B试样中只有分布均匀且密度较高的Fe3P微粒子。B试样的微观结构均匀性优于A试样,这是两者性能差异的主要原因之一。     

Aspects of interphase boundary structure in diffusional phase transformations

Distinctions among fully coherent, partly coherent and incoherent interphase boundaries are presented. Introduction of a single linear misfit compensating defect is concluded to represent the most useful definition of the transition between fully and partly coherent interfaces. The limit between partly and fully incoherent interphase boundaries is taken operationally as the absence of detectable misfit accommodating defects (i.e., localization) at the interface by high-resolution transmission electron microscopy. An answer to the question of whether or not a precipitate crystal can be fully or partly coherent at one or more boundary orientations and incoherent at others is developed.     

Simulation of shear banding in heterophase co-deformation: Example of plane strain compressed Cu–Ag and Cu–Nb metal matrix composites

The co-deformation and shear localization in heterophase alloys is studied using two-dimensional crystal plasticity finite element simulations on plane strain compressed Cu–Ag and Cu–Nb metal matrix composites. The aim is to study the fundamentals of micromechanics, co-deformation and shear banding in materials with heterophase interfaces. It is observed that, depending on the initial orientations of the crystals, co-deformation of the constituent heterophases often proceeds via collective mechanisms, i.e. by pronounced shear banding triggered by stress concentration at the interfaces. This phenomenon leads to highly localized strains within the bands, exceeding the average strain in part by two orders of magnitude. Shear band development is related to the inherent mechanical properties of each crystal and also to the properties of the abutting crystals. The predicted topology and nature of the cross-phase shear bands, i.e. the extreme local strains, significant bending of the interface regions, and sharp strain localization that propagates across the interfaces, agree well with experimental observations in cold-rolled composites. The simulations reveal that cross-phase shear banding leads to large and highly localized values of stress and strain at heterophase interfaces. Such information is essential for a better understanding of the micromechanical boundary conditions inside co-deformed composites and the associated shear-induced chemical mixing.     

相变中的界面

讨论了各类相变中的界面分类。重构型相变没有共格界面，相变产物形状也无系统性变化。位移型相变有完全共格或部分共格的界面，有可见的相变产物形状变化。完全共格界面出现在无成分变化的马氏体型相变以及有成分变化的扩散位移型相变中。这二种类型界面的迁移依靠台阶（马氏体相变中称之谓相变位错）的运动完成。部分共格界面也可能出现在马氏体型或较为少见的扩散型相变中。     

Modeling the effect of Al3Sc precipitates on the yield stress and work hardening of an Al–Mg–Sc alloy

The yield stress and work hardening behaviour of precipitation hardening alloys is directly related to the nature of the interaction between mobile dislocations and precipitates. In commercial systems such as aluminium alloys, the understanding of this problem is complicated by the overlap between various mechanisms and the interplay between the volume fraction and size of precipitates and the residual solid solution content. In this study a model Al–2.8 wt.% Mg–0.16 wt.% Sc alloy has been chosen for examination since precipitation involves simple spherical precipitates, the absence of metastable phases and the solid solution effect is dominated by the magnesium content. Using a previously development precipitation model, it has been possible to develop an integrated yield stress/work hardening model in which the shearable/non-shearable transition and the size distribution of precipitates are explicitly accounted for. The agreement between the model and the experiments is excellent and the shearable/non-shearable transition radius is consistent with experimental observations.     

Phase-field simulation of abnormal grain growth due to inverse pinning

The possibility of abnormal grain growth due to inverse pinning was verified using phase-field simulations. In bicrystalline systems with circular precipitates, the perfect wetting condition is required for the long-distance migration of the interface between the matrix grains. If the distance between precipitates that are perpendicular to the interface exceeds a critical value, the migration is not observed irrespective of the wetting condition. In polycrystalline systems, abnormal grain growth occurs with the aid of the driving force for grain growth even though l_lim exceeded the critical value, where l_lim is the minimum distance between precipitates. Furthermore, the perfect wetting condition is not required for the abnormal grain growth in the polycrystalline systems. These facts enlarge the possibility of inverse pinning in real alloy systems.     

Predicting equilibrium shape of precipitates as function of coherency state

A general approach is proposed to predict the equilibrium shapes of precipitates in crystalline solids as function of size and coherency state. The model incorporates effects of interfacial defects such as misfit dislocations and structural ledges on transformation strain and on interfacial energy. Using α precipitation in α/β titanium alloys as an example, various possible equilibrium shapes of precipitates having different defect contents at interfaces are obtained by phase-field simulations. The simulation results agree with experimental observations in terms of both precipitate habit plane orientation and defect content at the interface. In combination with crystallographic theories of interfaces and experimental characterization of habit plane of finite precipitates, this approach has the ability to predict the coherency state (i.e. defect structures at interfaces) and equilibrium shape of finite precipitates.     

Dose and dose rate effects on coherent-to-incoherent transition of precipitates upon irradiation

A typical precipitation hardened alloy, Cu-Co dilute alloy was selected to study the precipitation behavior and irradiation effect on precipitates. It is found that the principal effect of ion irradiation on the coherent precipitates is loss of coherency, and TEM cross-section observations show that the fraction of the incoherent precipitates is dependent on dose but not on dose rate during heavy ion irradiation.     

A simulation study of the shape of β′ precipitates in Mg–Y and Mg–Gd alloys

The metastable β′ phase is a key strengthening precipitate phase in a range of Mg–RE (RE: rare-earth elements) based alloys. The morphology of the β′ precipitates changes from a faceted and nearly equiaxed shape in Mg–Y alloys to a truncated lenticular shape in Mg–Gd alloys. In this work, we study effects of interfacial energy and coherency elastic strain energy on the morphology of β′ precipitates in binary Mg–Y and Mg–Gd alloys using a combination of first-principles calculations and phase-field simulations. Without any free-fitting parameters and using the first-principles calculations, CALPHAD databases and experimental characterizations as model inputs (lattice parameters of the β′ phase, elastic constants and chemical free energy of Mg matrix and interfacial energies of the coherent β′/Mg matrix interfaces), the phase-field simulations predict equilibrium shapes of β′ precipitates of different sizes that agree well with experimental observations. Factors causing the difference in the equilibrium shape of β′-Mg₇Y and β′-Mg₇Gd precipitates are identified, and possible approaches to increase the aspect ratio of the β′ precipitates and thus to enhance the strength of Mg–RE alloys are discussed.     

Atomic-scale study of nucleation of dislocations from fcc–bcc interfaces

Using atomistic simulations, we reveal the role of the atomic interface structure on the nucleation of glissile dislocations from a low-energy, atomically flat, incoherent face-centered cubic–body-centered cubic interface with a Kurdjumov–Sachs orientation relationship. Several loading states are simulated to systematically probe the selection of slip systems. Contrary to conventional expectation, the preferred nucleation sites are not always associated with pre-existing misfit dislocations, and the preferred slip systems are not determined solely by the Schmid factor. Amongst the two or more systems that may be geometrically favored, the activated slip system depends on the structure of the nucleation site. The system–site combination is such that the dislocation deposited in the interface after the nucleation event lowers the interfacial energy and has a relatively low self-energy. The fundamental correlations established here apply to interfaces of other orientation relationships that are also flat and have spatially non-uniform shear resistance.     

Maraging behavior in an Fe-19.5Ni-5Mn alloy I: Precipitation characteristics

The aging behavior of an Fe-19.5Ni-5Mn alloy has been studied in detail. A substantial maraging-hardening response was obtained upon aging at between 300–550°C, and it displayed classical hardening behavior. The pronounced hardness was attributed to strain hardening caused by coherent, fine spherical precipitates. The activation energy for precipitation, calculated from microhardness data, was 41 kcal/gmole. Ordered fct θ-NiMn precipitates were identified with two different shapes, depending on the aging temperature. Higher aging temperatures resulted in disk-shaped precipitates, while rod precipitates appeared at lower temperatures. Twin-related Widmanstätten austenite grains appeared in a lenticular shape that were coupled together by a twin boundary. Their orientation relationship with the parent martensite was found to be of the Kurdjumov-Sachs (K-S) type.