In situ transmission electron microscopy observations of the monoclinic to tetragonal phase transformation in tetragonal ZrO2

The tetragonal to monoclinic (t → m) transformation and the reverse transformation (m → t) in Y₂O₃ doped ZrO₂ were studied using transmission electron microscopy (TEM). Transformation to the monoclinic phase nucleated at grain boundaries in the tetragonal material and this phase slowly and stably grew. Under a high intensity electron beam the transformation was observed to reverse for certain grains which were only partially transformed and not twinned. Beam heating is suggested as the primary cause for this effect, with compressive stresses from the volume expansion of transformation playing a minor role. After the reverse transformation from m → t ZrO₂ was complete, grains retained no memory of the previous monoclinic nucleation sites for the next t → m cycle.     

Mechanical properties of defective L1_2-Al_3X(X=Sc,Lu) phase:A first-principles study

The mechanical properties of Al₃X(X=Sc,Lu)were studied by density functional theory(DFT).The elastic constants and formation enthalpy indicate that the L1₂-Al₃X(X=Sc and Lu)are mechanically and thermodynamically stable.The bulk moduli and shear moduli show that Al₃Sc has better resistance to volume and shape changes than AI3 Lu.However,the calculated results show that Al₃Lu has better plasticity than Al₃Sc.The properties of structural stability and elastic moduli of the crystal containing four major types of point defects in L1₂-Al₃X(X=Sc and Lu)were calculated.The mechanical properties of point defects show that point defects cause L1₂-Al₃X lattice distortion and change the corresponding elastic constants.Point defects reduce the Young’s,shear and bulk moduli but have little effects on the crystal brittleness and toughness of Al₃Sc and Al₃Lu.Therefore,we have found that Lu addition into aluminum alloys is a very good replacement for expensive Sc addition when the L1₂structures are desired for nucleation or strengthening precipitates in aluminum alloys.     

Thermodynamic consideration of the tetragonal lattice distortion of the L10 ordered phase

In an attempt to understand the thermodynamic consequences of the tetragonal distortion accompanying the L1₀ ordering, we have conducted a theoretical investigation by treating the problem of a rigid lattice free energy model. The problem is treated as an elasticity problem and a generalized formulation is proposed in a form which can be directly utilized to study incoherent two phase equilibria. By use of the Static Concentration Wave (SCW) mean field model as a rigid lattice free energy model, the formulation is applied to the case of our interest. We show that the thermodynamic stability of the L1₀ phase may be significantly influenced by the tetragonal lattice distortion, depending on the magnitude of the associated strain energy relative to the competing positive entropic contribution in the free energy of the stress free L1₀ phase. In association with this, we suggest that the neglect of tetragonal distortion (i.e. the use of a rigid lattice free energy model) could be a source of serious errors particularly for alloys with lower L1₀ ordering transition temperatures (e.g. CuAu and InMg). A prototype phase diagram of the 〈001〉_f.c.c.¹ special point structures, calculated with the tetragonal distortion taken into account, has indeed displayed topological features which are fundamentally different from those of a rigid lattice phase diagram and, furthermore, has reproduced the main topological features of the AuCuAu₃Cu and the InMgIn₃Mg sides of their respective phase diagrams.     

Superelastic behaviour of a fine-grained,yttria-stabilized,tetragonal zirconia polycrystal (Y-TZP)

The microstructure and deformation characteristics of a fine-grained superelastic yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) have been investigated. Both hot indentation and tensile tests were carried out at temperatures between 1273 and 1923K over the strain rate range from 2.7 × 10⁻⁵ to 2 × 10⁻³ s⁻¹. It was found that the material exhibited extensive plasticity at temperatures higher than 1473K; a maximum tensile elongation of over 800% was recorded. Microstructural examination did not indicate the presence of a glassy phase at grain boundaries. Yttrium, however, was found to segregate to the grain boundaries. The microstructure of the Y-TZP was thermally unstable and appreciable grain growth was observed at emperattures higher than 1723 K; the grain growth was enhanced by external stresses, i.e. dynamic grain growth was observed. Grain growth at elevated temperatures resulted in apparent strain rate sensitivity exponents of approximately 0.33 at 1723K. This value decreased with increasing temperature. The grain size-compensated strain rate, however, was found to depend approximately on the square of the flow stress, i.e. to exhibit a true strain sensitivity value of 0.5, which suggests a grain boundary sliding mechanism. Microstructures from samples that were deformed superelastically indicated that grains remained equiaxed; this observation is consistent with a grain boundary sliding mechanism. The activation energy for superplasticity, under the conditions of constant structure, in Y-TZP was calculated to be 720 kJ/mol.     

Cation-pi (Na+-Trp) interactions in the crystal structure of tetragonal lysozyme

Experimental evidence of a cation-pi interaction between a sodium cation (Na+) and the indole ring of residue Trp123 in a structure (2.0 A) of hen egg-white lysozyme is presented. The geometry of the metal ion-pi interaction observed in the protein structure (distance between the aromatic plane and the cation approximately 4 A) is consistent with geometries observed among small molecules crystal structures and quantum chemistry ab initio calculations. The present crystal structure of lysozyme provides unique structural information about the geometry of binding of cations to pi systems in proteins. It shows that the metal ion-pi interaction within proteins is not significantly different from similar bindings found in small molecules and that it can be modeled by theoretical methods.