Phase Transformation Induced by High Pressure Torsion in the High-Entropy Alloy CrMnFeCoNi.

The forward and reverse phase transformation from face-centered cubic (fcc) to hexagonal close-packed (hcp) in the equiatomic high-entropy alloy (HEA) CrMnFeCoNi has been investigated with diffraction of high-energy synchrotron radiation. The forward transformation has been induced by high pressure torsion at room and liquid nitrogen temperature by applying different hydrostatic pressures and large shear strains. The volume fraction of hcp phase has been determined by Rietveld analysis after pressure release and heating-up to room temperature as a function of hydrostatic pressure. It increases with pressure and decreasing temperature. Depending on temperature, a certain pressure is necessary to induce the phase transformation. In addition, the onset pressure depends on hydrostaticity; it is lowered by shear stresses. The reverse transformation evolves over a long period of time at ambient conditions due to the destabilization of the hcp phase. The effect of the phase transformation on the microstructure and texture development and corresponding microhardness of the HEA at room temperature is demonstrated. The phase transformation leads to an inhomogeneous microstructure, weakening of the shear texture, and a surprising hardness anomaly. Reasons for the hardness anomaly are discussed in detail.

Revealing Grain Boundary Sliding from Textures of a Deformed Nanocrystalline Pd-Au Alloy.

Employing a recent modeling scheme for grain boundary sliding [Zhao et al. Adv. Eng. Mater.2017, doi:10.1002/adem.201700212], crystallographic textures were simulated for nanocrystalline fcc metals deformed in shear compression. It is shown that, as grain boundary sliding increases, the texture strength decreases while the signature of the texture type remains the same. Grain boundary sliding affects the texture components differently with respect to intensity and angular position. A comparison of a simulation and an experiment on a Pd-10 atom % Au alloy with a 15 nm grain size reveals that, at room temperature, the predominant deformation mode is grain boundary sliding contributing to strain by about 60%.

Giant thermal expansion and α-precipitation pathways in Ti-alloys.

Ti-alloys represent the principal structural materials in both aerospace development and metallic biomaterials. Key to optimizing their mechanical and functional behaviour is in-depth know-how of their phases and the complex interplay of diffusive vs. displacive phase transformations to permit the tailoring of intricate microstructures across a wide spectrum of configurations. Here, we report on structural changes and phase transformations of Ti-Nb alloys during heating by in situ synchrotron diffraction. These materials exhibit anisotropic thermal expansion yielding some of the largest linear expansion coefficients (+ 163.9×10-6 to -95.1×10-6 °C-1) ever reported. Moreover, we describe two pathways leading to the precipitation of the α-phase mediated by diffusion-based orthorhombic structures, α″lean and α″iso. Via coupling the lattice parameters to composition both phases evolve into α through rejection of Nb. These findings have the potential to promote new microstructural design approaches for Ti-Nb alloys and ß-stabilized Ti-alloys in general.

Comprehensive Characterization of Extended Defects in Semiconductor Materials by a Scanning Electron Microscope.

Extended defects such as dislocations and grain boundaries have a strong influence on the performance of microelectronic devices and on other applications of semiconductor materials. However, it is still under debate how the defect structure determines the band structure, and therefore, the recombination behavior of electron-hole pairs responsible for the optical and electrical properties of the extended defects. The present paper is a survey of procedures for the spatially resolved investigation of structural and of physical properties of extended defects in semiconductor materials with a scanning electron microscope (SEM). Representative examples are given for crystalline silicon. The luminescence behavior of extended defects can be investigated by cathodoluminescence (CL) measurements. They are particularly valuable because spectrally and spatially resolved information can be obtained simultaneously. For silicon, with an indirect electronic band structure, CL measurements should be carried out at low temperatures down to 5 K due to the low fraction of radiative recombination processes in comparison to non-radiative transitions at room temperature. For the study of the electrical properties of extended defects, the electron beam induced current (EBIC) technique can be applied. The EBIC image reflects the local distribution of defects due to the increased charge-carrier recombination in their vicinity. The procedure for EBIC investigations is described for measurements at room temperature and at low temperatures. Internal strain fields arising from extended defects can be determined quantitatively by cross-correlation electron backscatter diffraction (ccEBSD). This method is challenging because of the necessary preparation of the sample surface and because of the quality of the diffraction patterns which are recorded during the mapping of the sample. The spatial resolution of the three experimental techniques is compared.

Hall-plot of the phase diagram for Ba(Fe1-xCox)2As2.

The Hall effect is a powerful tool for investigating carrier type and density. For single-band materials, the Hall coefficient is traditionally expressed simply by , where e is the charge of the carrier, and n is the concentration. However, it is well known that in the critical region near a quantum phase transition, as it was demonstrated for cuprates and heavy fermions, the Hall coefficient exhibits strong temperature and doping dependencies, which can not be described by such a simple expression, and the interpretation of the Hall coefficient for Fe-based superconductors is also problematic. Here, we investigate thin films of Ba(Fe1-xCox)2As2 with compressive and tensile in-plane strain in a wide range of Co doping. Such in-plane strain changes the band structure of the compounds, resulting in various shifts of the whole phase diagram as a function of Co doping. We show that the resultant phase diagrams for different strain states can be mapped onto a single phase diagram with the Hall number. This universal plot is attributed to the critical fluctuations in multiband systems near the antiferromagnetic transition, which may suggest a direct link between magnetic and superconducting properties in the BaFe2As2 system.

Mechanical properties, structural and texture evolution of biocompatible Ti-45Nb alloy processed by severe plastic deformation.

Biocompatible ß Ti-45Nb (wt%) alloys were subjected to different methods of severe plastic deformation (SPD) in order to increase the mechanical strength without increasing the low Young׳s modulus thus avoiding the stress shielding effect. The mechanical properties, microstructural changes and texture evolution were investigated, by means of tensile, microhardness and nanoindentation tests, as well as TEM and XRD. Significant increases of hardness and ultimate tensile strength up to a factor 1.6 and 2, respectively, could be achieved depending on the SPD method applied (hydrostatic extrusion - HE, high pressure torsion - HPT, and rolling and folding - R&F), while maintaining the considerable ductility. Due to the high content of ß-stabilizing Nb, the initial lattice structure turned out to be stable upon all of the SPD methods applied. This explains why with all SPD methods the apparent Young׳s modulus measured by nanoindentation did not exceed that of the non-processed material. For its variations below that level, they could be quantitatively related to changes in the SPD-induced texture, by means of calculations of the Young׳s modulus on basis of the texture data which were carefully measured for all different SPD techniques and strains. This is especially true for the significant decrease of Young׳s modulus for increasing R&F processing which is thus identified as a texture effect. Considering the mechanical biocompatibility (percentage of hardness over Young׳s modulus), a value of 3-4% is achieved with all the SPD routes applied which recommends them for enhancing ß Ti-alloys for biomedical applications.

Factors influencing the elastic moduli, reversible strains and hysteresis loops in martensitic Ti-Nb alloys.

While the current research focus in the search for biocompatible low-modulus alloys is set on ß-type Ti-based materials, the potential of fully martensitic Ti-based alloys remains largely unexplored. In this work, the influence of composition and pre-straining on the elastic properties of martensitic binary Ti-Nb alloys was studied. Additionally, the phase formation was compared in the as-cast versus the quenched state. The elastic moduli and hardness of the studied martensitic alloys are at a minimum of 16wt.% Nb and peak between 23.5 and 28.5wt.% Nb. The uniaxial deformation behavior of the alloys used is characterized by the absence of distinct yield points. Monotonic and cyclic (hysteretic) loading-unloading experiments were used to study the influence of Nb-content and pre-straining on the elastic moduli. Such experiments were also utilized to assess the recoverable elastic and anelastic deformations as well as hysteretic energy losses. Particular attention has been paid to the separation of non-linear elastic from anelastic strains, which govern the stress and strain limits to which a material can be loaded without deforming it plastically. It is shown that slight pre-straining of martensitic Ti-Nb alloys can lead to considerable reductions in their elastic moduli as well as increases in their total reversible strains.

Thermal stability and phase transformations of martensitic Ti-Nb alloys.

Aiming at understanding the governing microstructural phenomena during heat treatments of Ni-free Ti-based shape memory materials for biomedical applications, a series of Ti-Nb alloys with Nb concentrations up to 29 wt% was produced by cold-crucible casting, followed by homogenization treatment and water quenching. Despite the large amount of literature available concerning the thermal stability and ageing behavior of Ti-Nb alloys, only few studies were performed dealing with the isochronal transformation behavior of initially martensitic Ti-Nb alloys. In this work, the formation of martensites (α' and α″) and their stability under different thermal processing conditions were investigated by a combination of x-ray diffraction, differential scanning calorimetry, dilatometry and electron microscopy. The effect of Nb additions on the structural competition in correlation with stable and metastable phase diagrams was also studied. Alloys with 24 wt% Nb or less undergo a [Formula: see text] transformation sequence on heating from room temperature to 1155 K. In alloys containing >24 wt% Nb α″ martensitically reverts back to ß0, which is highly unstable against chemical demixing by formation of isothermal ωiso. During slow cooling from the single phase ß domain α precipitates and only very limited amounts of α″ martensite form.