Piotr Bobrowski

Computationally-driven engineering of sublattice ordering in a hexagonal AlHfScTiZr high entropy alloy

Multi-principle element alloys have enormous potential, but their exploration suffers from the tremendously large range of configurations. In the last decade such alloys have been designed with a focus on random solid solutions. Here we apply an experimentally verified, combined thermodynamic and first-principles design strategy to reverse the traditional approach and to generate a new type of hcp Al-Hf-Sc-Ti-Zr high entropy alloy with a hitherto unique structure. A phase diagram analysis narrows down the large compositional space to a well-defined set of candidates. First-principles calculations demonstrate the energetic preference of an ordered superstructure over the competing disordered solid solutions. The chief ingredient is the Al concentration, which can be tuned to achieve a D019 ordering on the hexagonal lattice. The computationally designed D019 superstructure is experimentally confirmed by transmission electron microscopy and X-ray studies. Our scheme enables the exploration of a new class of high entropy alloys.

Effect of Hot Rolling and Equal-Channel Angular Pressing on Generation of Globular Microstructure in Semi-Solid Mg-3%Zn Alloy

A combination of hot rolling and equal channel angular pressing (ECAP) was explored to generate globular microstructures in the Mg-3%Zn alloy after re-heating to the semisolid state. It was found that the single-step deformation of as-cast alloy via hot rolling at 350°C with a thickness reduction of 50% refined the alloy microstructure by creating deformation bands of the Mg (α) phase with a size of the order of tenths of micrometers. After re-heating to 630 °C, the microstructure transformed into spheroidal morphologies with an average globule size of 82 μm. An additional deformation of the hot-rolled alloy by the ECAP method at 250 °C further refined the alloy microstructure to sub-micrometer grains of lath and equiaxed shapes. After re-heating of this microstructure to 630 °C the average globule size reached 62 μm, which is roughly 25% smaller than that achieved for the hot-rolled precursor. The role of strain-induced melt activation (SIMA) techniques in generation of globular morphologies in Mg-based alloys after partial re-melting is discussed.

Microstructural Characterization of Yttria-Stabilized Zirconia Sintered at Different Temperatures Using 3D EBSD, 2D EBSD and Stereological Calculations

A set of yttria-stabilized zirconia samples sintered at increasing temperatures was investigated using two-dimensional (2D) and three-dimensional (3D) electron backscatter diffraction (EBSD) techniques to calculate grain size distributions and grain boundary densities. The obtained results were compared to the results of stereological calculations and revealed that mean intercept length, a commonly used stereological parameter, is ca. 20% lower than an average grain diameter derived from 2D and 3D EBSD data. Moreover, the results based on 2D and 3D EBSD analyses were similar to each other in grain boundary density, while the values obtained from the stereological approach were noticeably lower.

Structure of MgLiAl alloys after various routes of severe plastic deformation studied by TEM

Abstract Two MgLiAl alloys consisting either of α (hcp) + β (bcc) phases or only β phase were subjected to twist channel angular pressing TCAP (with helical component) or cyclic compression to a total strain of ∊ = 5 in order to study the effectiveness of various deformation modes on grain refinement. After the first TCAP pass grains of α phase were refined from 30 μm down to about 6 μm and of β phase from initial 200 μm down to 8 μm. MAXStrain cycling led to much finer grains of α and β phases in the range 200 – 300 nm causing higher hardening and indicating higher effectiveness of the process. The Li2MgAl precipitates were refined during severe plastic deformation (SPD) processes and in addition fine particles of hexagonal α phase of size below 100 nm were observed within the β phase showing orientation relationship (0001) α ‖ (011) β. Two-phase material after SPD showed deviation of about 1.5° from the ideal Burgers orientation relationship (0001) α ‖ (011) β.