Effect of La on the crystallization behaviour of amorphous Al94 − xNi6Lax (x = 4–7) alloys

Melt spun and annealed Al-alloys containing 6 at.% Ni and 4–7 at.% La were investigated by means of differential scanning calorimetry (DSC), X-ray diffractometry (XRD), small angle neutron scattering (SANS), transmission electron microscopy and hardness measurements. XRD studies revealed that all the ribbons were fully amorphous, whereas SANS indicated concentration fluctuations. DSC showed that the amorphous alloys undergo a two-stage crystallization process upon heating. The details of the crystallization pathway depend on the La concentration. Upon annealing, SANS measurements revealed two different distributions of particles which can be explained by a core shell structure or two different types of particles. Moreover, significant changes in hardness occur which are attributed to changes in microstructure.     

Atomistic structure of the Cu(1 1 1)/α-Al2O3(0 0 0 1) interface in terms of interatomic potentials fitted to ab initio results

We propose an atomistic model to describe the copper/sapphire interface by means of simple interatomic potentials involving only a few fitting parameters. Successful results are achieved when the copper atoms in the monatomic layer closest to the interface have properties different from the bulk. This layer is to accommodate the ionic/covalent bonding in the ceramics to the metallic bonding in copper. For an oxygen terminated interface, we fit the parameters of the potentials to the results of a rigid tensile test (explained in the text) simulated from first principles. The results of atomic relaxation near the interface are shown to be consistent with ab initio and experimental results available in the literature. Calculations reveal highly interesting relaxation dynamics near the interface. In the early stage of relaxation, a periodic network of partial misfit dislocations is formed, which later transforms into an irregular network due to the instability of the layer of copper atoms atop the oxygen atoms. This explains the interface incoherency observed in high-resolution electron microscopy. Calculations based on the FK model reproduce this effect.     

Nucleation and growth mechanism of α-lamellae of Ti alloy TA15 cooling from an α + β phase field

The mechanism of nucleation and growth of α-lamellae when a TA15 Ti alloy with an equiaxed structure is cooled from an α + β phase field were studied by end quenching experiments and thermal simulation tests using a Gleeble-3500. The results showed that for the Ti alloy TA15 the nucleation and growth of α-lamellae involved four steps, including nucleation of α_GB, growth of α_GB, nucleation of α_WGB, and growth of α_WGB. Widmanstätten α, α_WGB, grew faster than grain boundary α, α_GB, and equiaxed α in common cases. It was found for the first time that the mode of nucleation of α_WGB for alloy TA15 was interface instability, i.e. α_WGB nucleated through surface instability and the protuberance of α_GB and equiaxed α, and the α_WGB nucleus did not have an independent and complete surface. A new model of the nucleation of α_WGB and phase transformation is proposed. The growth of α_WGB in the TA15 alloy started from a small protuberance and spread into a β grain with a sectorial morphology, to become lamellar instead of spiculate or oblate cuboid in shape. The nucleation rate of α_WGB determined the thickness of α_WG, with, to some extent, an inverse relation between the nucleation rate and thickness of α_WGB.     

Multiscale modeling of plastic deformation of molybdenum and tungsten: I. Atomistic studies of the core structure and glide of 1/2〈1 1 1〉 screw dislocations at 0 K

Owing to their non-planar cores, 1/2〈1 1 1〉 screw dislocations govern the plastic deformation of body-centered cubic (bcc) metals. Atomistic studies of the glide of these dislocations at 0 K have been performed using Bond Order Potentials for molybdenum and tungsten that account for the mixed metallic and covalent bonding in transition metals. When applying pure shear stress in the slip direction significant twinning–antitwinning asymmetry is displayed for molybdenum but not for tungsten. However, for tensile/compressive loading the Schmid law breaks down in both metals, principally due to the effect of shear stresses perpendicular to the slip direction that alter the dislocation core. Recognition of this phenomenon forms a basis for the development of physically based yield criteria that capture the breakdown of the Schmid law in bcc metals. Moreover, dislocation glide may be preferred on {1 1 0} planes other than the most highly stressed one, which is reminiscent of the anomalous slip observed in many bcc metals.     

Thermophysical properties of TiB2SiC ceramics from 300 °C to 1700 °C

In the present work, high-frequency induction heating is used to fabricate TiB₂SiC ceramics and the relative density was more than 97%, and then the thermophysical properties of TiB₂SiC ceramics were investigated in detail. The specific heat showed the weak dependence on the test temperature due to the presence of the interface gap because the relative density was not 100%. As the sintering temperature increased, the thermal diffusivity of TiB₂SiC ceramics increased, which was due to the increase of relative density and grain growth. The thermal conductivity of TiB₂SiC ceramics showed a marked increase with increasing grain size and relative density. This could be attributed to a reduction in the number of grain boundaries that interrupt the heat flow path, resulting in an increase in the mean free path of the phonons. Larger grains led to an increase of mean free path of the phonons and thus contributed to a further increase in thermal conductivity.     

Microband evolution during large plastic strains of stable {1 1 0}〈1 1 2〉 Al and Al–Mn crystals

The deformation microstructures of Al and Al–Mn {1 1 0}〈1 1 2〉 single crystals have been characterized after room temperature channel-die compression up to true strains of 2.1. The evolution of local misorientations and microband structures were quantified by high-resolution electron backscatter diffraction in a field emission gun scanning electron microscope and their alignments compared with the traces of active slip planes and macroscopic shear stress planes. During plane-strain compression these “Brass” oriented crystals remain stable in terms of the final, average, orientation, with a small orientation spread. However, the microband alignment varies with strain and also with solute content. There is a general tendency for the microbands to be both crystallographic and non-crystallographic at low strains, then crystallographic, and finally mixed again at high strains (with some lamellar banding).     

Effects of temperature on structure and mobility of the 〈1 0 0〉 edge dislocation in body-centred cubic iron

Dislocation segments with Burgers vector b = 〈1 0 0〉 are formed during deformation of body-centred-cubic (bcc) metals by the interaction between dislocations with b = 1/2〈1 1 1〉. Such segments are also created by reactions between dislocations and dislocation loops in irradiated bcc metals. The obstacle resistance produced by these segments on gliding dislocations is controlled by their mobility, which is determined in turn by the atomic structure of their cores. The core structure of a straight 〈1 0 0〉 edge dislocation is investigated here by atomic-scale computer simulation for α-iron using three different interatomic potentials. At low temperature the dislocation has a non-planar core consisting of two 1/2〈1 1 1〉 fractional dislocations with atomic disregistry spread on planes inclined to the main glide plane. Increasing temperature modifies this core structure and so reduces the critical applied shear stress for glide of the 〈1 0 0〉 dislocation. It is concluded that the response of the 〈1 0 0〉 edge dislocation to temperature or applied stress determines specific reaction pathways occurring between a moving dislocation and 1/2〈1 1 1〉 dislocation loops. The implications of this for plastic flow in unirradiated and irradiated ferritic materials are discussed and demonstrated by examples.     

The effect of zirconium addition on microstructure and properties of ball milled and hot compacted powder of Al–12 wt% Zn–3 wt% Mg–1.5 wt% Cu alloy

Elemental powders of the composition Al–12 wt% Zn–3 wt% Mg–1.5 wt% Cu with addition of 1 and 2 wt% Zr were ball milled in a planetary high-energy ball mill and then hot pressed in vacuum under 600 MPa pressure at 380 °C. The effect of ball milling and hot pressing on the microstructure was investigated by means of X-ray diffraction measurements (XRD), light microscopy, analytical and scanning transmission electron microscopy (TEM). Ball milling for 80 h leads to homogenous, highly deformed microstructure of aluminium solid solution with grain size below 100 nm. In the powder with zirconium addition, some part of the Zr atoms diffused in aluminium up to 0.3 wt% Zr. The remaining was found to form Zr-rich particles identified as face centered cubic (fcc) phase. Good quality samples without pores and cracks obtained by hot pressing composed of grains and subgrains of size below 200 nm. The particles of MgZn₂ phase were identified which were located mainly between compacted particles of milled powder. Hot pressed powder showed Vickers microhardness of about 195 HV (0.2 N) and ultimate compression strength in the range 611–658 MPa in the compression test. Addition of zirconium had no influence on the strength of the compacted powders.     

Stress dependence of the Peierls barrier of 1/2〈1 1 1〉 screw dislocations in bcc metals

The recently formulated constrained nudged elastic band method with atomic relaxations (NEB + r) (Gröger R, Vitek V. Model Simul Mater Sci Eng 2012;20:035019) is used to investigate the dependence of the Peierls barrier of 1/2〈1 1 1〉 screw dislocations in body-centered cubic metals on non-glide stresses. These are the shear stresses parallel to the slip direction acting in the planes of the 〈1 1 1〉 zone different from the slip plane, and the shear stresses perpendicular to the slip direction. Both these shear stresses modify the structure of the dislocation core and thus alter both the Peierls barrier and the related Peierls stress. Understanding of this effect of loading is crucial for the development of mesoscopic models of thermally activated dislocation motion via formation and propagation of pairs of kinks. The Peierls stresses and related choices of the glide planes determined from the Peierls barriers agree with the results of molecular statics calculations (Gröger R, Bailey AG, Vitek V. Acta Mater 2008;56:5401), which demonstrates that the NEB + r method is a reliable tool for determining the variation in the Peierls barrier with the applied stress. However, such calculations are very time consuming, and it is shown here that an approximate approach of determining the stress dependence of the Peierls barrier (proposed in Gröger R, Vitek V. Acta Mater 2008;56:5426) can be used, combined with test calculations employing the NEB + r method.     

Stress-driven migration of symmetrical 〈1 0 0〉 tilt grain boundaries in Al bicrystals

Stress-induced migration of planar grain boundaries in aluminum bicrystals was measured for both low- and high-angle symmetrical 〈1 0 0〉 tilt grain boundaries across the entire misorientation range (0–90°). Boundary migration under a shear stress was observed to be coupled to a lateral translation of the grains. Boundaries with misorientations smaller than 31° and larger than 36° moved in opposite directions under the same applied external stress. The measured ratios of the normal boundary motion to the lateral displacement of grains are in an excellent agreement with theoretical predictions. The coupled boundary motion was measured in the temperature range between 280 and 400 °C, and the corresponding activation parameters were determined. The results revealed that for mechanically induced grain-boundary motion there is a misorientation dependence of migration activation parameters. The obtained results are discussed with respect of the mechanism of grain-boundary motion.     

Deformation paths and atomistic mechanism of B4 → B1 phase transformation in aluminium nitride

Two homogeneous deformation paths under uniaxial tension and shear are modeled for the wurtzite (B4) to rocksalt (B1) phase transformation of AlN by ab initio density functional theory. The B4 → B1 transition is a two-stage process along the deformation paths: an anti-parallel vertical movement of Al and N atoms along the crystallographic 〈0 0 0 1〉 axis, followed by a horizontal rearrangement of the relative positions of each type of atom. Each of these steps appears as a discontinuity in the tensile stress–strain curve. The shear path shows no discontinuity of the stress–strain curve in the first stage, while the second stage proceeds in a similar manner to the first stage of the tension path. No stress-free intermediate state is observed along the tension path owing to the sudden discontinuous lattice reconstruction, whereas along the shear path, a stress-free intermediate state is found at the end of the first stage, but it is intrinsically unstable because it is located at an unstable point of the energy–strain curve.     

Interaction between martensitic structure and defects in β ↔ β′ + γ′ cycling in CuAlNi single crystals. Model for the inhibition of γ′ martensite

The authors have previously reported an estimate of the energy associated with the inhibition effect of γ′ martensite after β ↔ β′ + γ′ cycling in CuAlNi single crystals. In this paper, a microscopic model is proposed to explain the γ′ inhibition, related to the localized interaction between a dislocation array and the twinned γ′ structure. Dislocations with Burgers vector [1 0 0]_β and line direction [1 1 1]_β in an isotropic β matrix are considered. The model takes into account the interaction between the martensitic stress-free transformation strains and the stress field created by the dislocation arrays. It is shown that the interaction is different for each twin-related variant in the γ′ martensite. The energy necessary to maintain the right volume relationship of the twinned γ′ variants to produce an undistorted β/γ′ habit plane is defined as the inhibition energy. A value of around 12 J mol⁻¹ was obtained, which is in reasonable agreement with experimental results.     

Influence of hydrogen on the deformation morphology of vanadium (1 0 0) micropillars in the α-phase of the vanadium–hydrogen system

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The influence of rolling temperature on texture evolution and variant selection during α → β → α phase transformation in Ti–6Al–4V

The role of starting texture in variant selection has been studied during α → β → α transformation in Ti–6Al–4V. By hot rolling at different temperatures followed by recrystallization, material with either a strong basal texture or a strong transverse texture was generated. Subsequently, both conditions were heat-treated above the β transus followed by slow cooling. The degree of variant selection was assessed by comparing the strength of the measured and predicted α texture from high temperature β texture, assuming equal occurrence of all possible variants during β → α transformation. It was found that, even though the material rolled originally at 800 °C displayed a stronger α texture after β heat treatment, it was the material rolled originally at 950 °C that showed greater variant selection. The variant selection mechanism is discussed in terms of the generated β texture and common 〈1 1 0〉 poles in neighbouring β grains selecting a similar α variant on both sides of the prior β grain boundary. Predictions of possible 〈1 1 0〉 pole misorientation distributions for the two investigated β textures showed that the combination of texture components generated during rolling Ti–6Al–4V at 950 °C increases the likelihood of having β grain pairs with closely aligned (1 1 0) planes compared to rolling at 800 °C. Therefore, it can be proposed that avoiding the generation of certain combinations of β texture components during thermomechanical processing has the potential for reducing variant selection during subsequent β heat treatment.