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.     

Creep- and coarsening properties of Al–0.06 at.% Sc–0.06 at.% Ti at 300–450 °C

Upon aging at 300–450 °C, nanosize, coherent Al₃(Sc_1−xTi_x) precipitates are formed in pure aluminum micro-alloyed with 0.06 at.% Sc and 0.06 at.% Ti. The outstanding coarsening resistance of these precipitates at these elevated temperatures (61–77% of the melting temperature of aluminum) is explained by the significantly smaller diffusivity of Ti in Al when compared to that of Sc in Al. Furthermore, this coarse-grained alloy exhibits good compressive creep resistance for a castable, heat-treatable aluminum alloy: the creep threshold stress varies from 17 MPa at 300 °C to 7 MPa at 425 °C, as expected if the climb bypass by dislocations of the mismatching precipitates is hindered by their elastic stress fields.     

Epitaxial Ni–Mn–Ga/MgO(1 0 0) thin films ranging in thickness from 10 to 100 nm

Thin films of Ni–Mn–Ga alloy ranging in thickness from 10 to 100 nm have been epitaxially grown on MgO(1 0 0) substrate. Temperature-dependent X-ray diffraction measurements combined with room-temperature atomic force microscopy and transmission electron microscopy highlight the structural features of the martensitic structure from the atomic level to the microscopic scale, in particular the relationship between crystallographic orientations and twin formation. Depending on the film thickness, different crystallographic and microstructural behaviours have been observed: for thinner Ni–Mn–Ga films (10 and 20 nm), the L2₁ austenitic cubic phase is present throughout the temperature range being constrained to the substrate. When the thickness of the film exceeds the critical value of 40 nm, the austenite-to-martensite phase transition is allowed. The martensitic phase is present with the unique axis of the pseudo-orthorhombic 7M modulated martensitic structure perpendicular to the film plane. A second critical thickness has been identified at 100 nm where the unique axis has been found both perpendicular and parallel to the film plane. Magnetic force microscopy reveals the out-of-plane magnetic domain structure for thick films. For the film thickness below 40 nm, no magnetic contrast is observed, indicating an in-plane orientation of the magnetization.     

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).     

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.     

Dislocation–grain boundary interaction in 〈1 1 1〉 textured thin metal films

The interaction of lattice dislocations with symmetrical and asymmetrical tilt grain boundaries in 〈1 1 1〉 textured thin nickel films was investigated using atomistic simulation methods. It was found that the misorientation angle of the grain boundary, the sign of the Burgers vector of the incoming dislocation and the exact site where the dislocation meets the grain boundary are all important parameters determining the ability of the dislocation to penetrate the boundary. Inclination angle, however, does not make an important difference on the transmission scenario of full dislocations. Only limited partial dislocation nucleation was observed for the investigated high-angle grain boundary. The peculiarities of nucleation of embryonic dislocations and their emission from tilt grain boundaries are discussed.     

Ceramic and single-crystal (1 – x)PMN–xPT constitutive behavior under combined stress and electric field loading

Pb(Mg_1/3Nb_2/3)O₃–0.32(PbTiO₃), PMN–0.32PT, single crystals have been characterized under combined stress and electric field loading [McLaughlin EA, Liu T, Lynch CS. Relaxor ferroelectric PMN–32%PT crystals under stress and electric field loading: I-32 mode measurements. Acta Mater 2004;52:3849, McLaughlin EA, Liu T, Lynch CS. Relaxor ferroelectric PMN–32%PT crystals under stress, electric field and temperature loading: II-33-mode measurements. Acta Mater 2005;53:4001] [1], [2], [3]. This approach is extended to PMN–0.26PT single crystals to explore the effect of composition on field driven phase transformations and to PMN–0.32PT ceramic specimens to compare with polycrystalline behavior. Electric displacement and strain were measured as a function of combinations of stress and both unipolar and bipolar electric fields. The single-crystal results indicate that compositions further from the morphotropic phase boundary require higher driving forces for field induced phase transformations. Evidence of these transformations is not apparent in the results from the ceramic specimens.     

Ordered phases and texture in spray-formed Fe–5 wt%Si

Fe–Si alloys have excellent soft magnetic properties, especially around 12 at.% Si. However, their industrial applications are limited because they lack the ductility required in rolling operations for the fabrication of thin sheets, thus leading to cracking. The brittleness of high silicon alloys is caused by order–disorder reactions at low temperatures. This work involved an analysis of the effect of heat treatment on the crystalline structure of thin sheets of Fe–5 wt%Si alloy obtained in a two-step fabrication route: (1) spray forming of Fe–3.5%Si + 2.0%Sip composite and (2) rolling and heat treatment of the composite to dissolve the silicon and homogenize its content across the thickness of sheet samples. Structural and microstructural analyses indicated the success in fabricating thin sheets of Fe–5 wt%Si alloys with such strategy. The presence of the ordered B2 phase had an important effect on the texture development and therefore on the magnetic properties of these alloys.     

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.     

Dislocation interaction of layers in the Ge/Ge-seed/GexSi1−x/Si(0 0 1) (x ∼ 0.3–0.5) system: Trapping of misfit dislocations on the Ge-seed/GeSi interface

High-resolution electron microscopy has been applied to study the dislocation redistribution between Ge and GeSi layers at the atomic scale. Ge_0.3Si_0.7 (30 nm in thickness) and Ge_0.5Si_0.5 (10 nm) buffer layers buried between the Si(0 0 1) substrate and the plastically relaxed Ge layer 0.5 μm thick remain in a metastable (stressed) state during the growth of Ge/Ge-seed/Ge_xSi_1?x/Si(0 0 1) (x ～ 0.3–0.5) heterostructures, though the buffer layer thickness is several times greater than the critical value for insertion of misfit dislocations (MDs). An ordered grid of edge MDs is observed only on the Ge/GeSi interface; the mean distance between the MDs is ～10 nm (which is close to the equilibrium value for the non-stressed Ge/Si system). After 30 min of annealing at 700 °С, the Ge_0.3Si_0.7 buffer layer still remains in a metastable state, and the edge MDs are located only on the Ge/GeSi interface with the same dislocation spacing of ～10 nm. At the same time, approximately one-half of MDs in the structure with the Ge_0.5Si_0.5 buffer layer passes through the Ge/GeSi interface to the GeSi/Si(0 0 1) interface, and the buffer layer plastically relaxes by almost 100%. An assumption is put forward that there exists a barrier for the MD transition from the Ge layer to the GeSi layer, which results in MD trapping on this interface. The magnitude of this barrier depends on the difference in the compositions of the main Ge (x = 1) film and the Ge_xSi_1?x buffer layer, and increases with increasing this difference.     

Fine-grained Mg–1Mn–0.5Al–0.5Ca–0.5Zn alloy with high strength and good ductility fabricated by conventional extrusion

Mg–1Mn–0.5Al–0.5Ca–0.5Zn (wt.%) alloy was fabricated by conventional extrusion at 673 K with an extrusion ratio of 25:1, followed by aging at 473 K. The microstructure was characterized by scanning electron microscopy, electron back-scattered diffraction, and transmission electron microscopy. The mechanical properties were determined by the tensile test. The peak-aged sample shows fine recrystallized grains with an average grain size of 1.7 μm. Area fraction of Al–Ca particles in the alloy increases significantly after peak aging. Meanwhile, both 〈a〉 and 〈c+a〉 dislocations were observed to remain in the alloy after hot extrusion. Thus, the peak-aged sample exhibits simultaneously high strength and good ductility with the ultimate tensile stress, tensile yield stress, and tension fracture elongation of 320 MPa, 314 MPa, and 19.0%, respectively.     

The ϵ → η → θ transition in 100Cr6 and its effect on mechanical properties

Low-temperature precipitation reactions in 100Cr6 are characterized using transmission electron microscopy and X-ray diffraction, and modelled using thermokinetic methods. Martensitically transformed 100Cr6 is shown to display a complex microstructure composed of plate martensite, primary carbides, retained austenite and one or more of the ?-, η- and θ-phases. It is demonstrated that the maximum tensile strength (in excess of 2 GPa) and ductility is achieved by the θ-phase and the maximum yield strength is found during the α′ + η → α′ + θ transition. The interplay between the amount of carbon in solid solution, the martensite tetragonality and its morphology are related to the precipitate/matrix strain energy, the precipitate species present and their morphology. The progress in precipitate volume fraction, average radius, particle number and matrix composition can be quantitatively described by performing multicomponent precipitation kinetics calculations in paraequilibrium incorporating: (i) the effects of precipitate/matrix lattice misfit strain and particle aspect ratio, (ii) nucleation at plate boundaries and dislocations and (iii) an appropriate value for the precipitate/matrix interfacial energy, which is the only parameter fitted in the calculation.