K.P.D. Lagerlöf

Bio: K.P.D. Lagerlöf is an academic researcher from Case Western Reserve University. The author has contributed to research in topics: Crystal twinning & Dislocation. The author has an hindex of 7, co-authored 8 publications receiving 338 citations.

Plastic Deformation and Solid‐Solution Hardening of Y2O3‐Stabilized ZrO2

Abstract: Fully stabilized cubic ZrO2 single crystals containing various Y2O3 concentrations were deformed at 1400°C in air. The orientation chosen favored (001)(110) slip, which was confirmed by slip-trace analysis and transmission electron microscopy; (111)(110) slip could also be activated. The yield and flow stresses increased with increasing Y2O3 concentration, and stress-strain curves were dominated by a region of zero work hardening from very small strains for all compositions.

On deformation twinning in b.c.c. metals

Abstract: A new model of deformation twinning in bcc metals based on the dissociation of an a2〈111〉 screw dislocations into three co-linear a6〈111〉 partials on three adjacent {112} planes is proposed A short screw segment of this “zonal” dislocation bows out under an applied stress in a fashion similar to a Frank-Read source, where each of the three partials glides on its individuals {112} plane The correlated motion of the three partials on three adjacent {112} planes forms a three layered micro-twin After one Frank-Read zonal dislocation loop forms, the screw segment undergoes cross-slip in a similar fashion as the model proposed by Pirouz for twinning in Si [Scripta metall21, 1463 (1987)] because the formation of a second Frank-Read zonal dislocation loop would produce a stacking sequence of {112} planes associated with a high energy stacking fault However, after cross-slip the screw-segment may again bow out on a new set of parallel {112} planes to form another three layered micro-twin bound by the zonal dislocation loop The repeated cross-slip and bowing out of such zonal dislocation loops results in both lateral growth and thickening of the deformation twin

A zonal dislocation mechanism for rhombohedral twinning in sapphire (α-Al2O3)

Abstract: HRTEM investigations of rhombohedral twins in sapphire (α-Al2O3) indicate that they are “screw twins”; the twin axis η1 is [0 1 11] and involves a 1 6[0 1 11] translation in addition to the 2-fold rotational symmetry. Based on this data, a zonal dislocation mechanism for rhombohedral twinning in sapphire is proposed which predicts the correct shear strain s = 0.202. This mechanism only requires movements of the Al and O ions of less than 50% of the closest AlAl or OO distances in the ideal crystal, although some ions have to move against the shear.

Metastable alumina polymorphs : Crystal structures and transition sequences

Abstract: The available literature on the crystal structure of the metastable alumina polymorphs and their associated transitions is critically reviewed and summarized. All the metastable alumina structures have been identified as ordered or partially ordered cation arrays on the interstitial sites of an approximately close-packed oxygen sublattice (either face-centered cubic or hexagonal close packed). The analysis of the symmetry relations between reported alumina polymorphs having an approximately face-centered cubic packing of the oxygen anions allows for an exact interpretation of all the complex domain structures that have been observed experimentally. Possible mechanisms for the phase transitions between the different alumina polymorphs also are discussed.

Development of Superplastic Structural Ceramics

Abstract: Superplastic structural ceramics (Y-TZP, A1203, Si3N4, and their composites) that can withstand biaxial stretching to large strains have been developed recently. Microstructural design of these ceramics first requires an ultrafine grain size that is stable against coarsening during sintering and deformation. A low sintering temperature is a necessary, but not a sufficient, condition for achieving the required microstructure. In many cases, the selection of an appropriate phase, such as tetragonal phase in zirconia or a phase in silicon nitride, which is resistant to grain growth, is crucial. The use of sintering aids and grain-growth inhibitors, particularly those that segregate to the grain boundaries, can be beneficial. Second-phase particles are especially effective in suppressing static and dynamic grain growth. Another major concern is to maintain an adequate grain-boundary cohesive strength, relative to the flow stress, to mitigate cavitation or grain-boundary cracking during large strain deformation. Existing evidence suggests that a lower grainboundary energy is instrumental in achieving this objective. The selection of an appropriate phase and the tailoring of the grain boundary or liquid-phase composition can sometimes drastically alter the cavitation resistance. Related observations on forming methods, forming characteristics, and sheet formability are also reviewed. The basic deformation characteristics are similar to diffusional creep and are dominated by R. E. Newnham-contributing editor

Growth Twins and Deformation Twins in Metals

Abstract: This article reviews recent basic research on two classes of twins: growth twins and deformation twins. We focus primarily on studies that aim to understand, via experiments, modeling, or both, the causes and effects of twinning at a fundamental level. We anticipate that, by providing a broad perspective on the latest advances in twinning, this review will help set the stage for designing new metallic materials with unprecedented combinations of mechanical and physical properties.

Dynamic transitions from smooth to rough to twinning in dislocation motion

Abstract: The motion of dislocations in response to stress dictates the mechanical behaviour of materials. However, it is not yet possible to directly observe dislocation motion experimentally at the atomic level. Here, we present the first observations of the long-hypothesized kink-pair mechanism in action using atomistic simulations of dislocation motion in iron. In a striking deviation from the classical picture, dislocation motion at high strain rates becomes rough, resulting in spontaneous self-pinning and production of large quantities of debris. Then, at still higher strain rates, the dislocation stops abruptly and emits a twin plate that immediately takes over as the dominant mode of plastic deformation. These observations challenge the applicability of the Peierls threshold concept to the three-dimensional motion of screw dislocations at high strain rates, and suggest a new interpretation of plastic strength and microstructure of shocked metals.

First-principles calculations of intrinsic defects in Al 2 O 3

Abstract: First-principles plane-wave pseudopotential calculations were performed to study electronic structures, structural relaxation, and energetics of intrinsic vacancies and interstitials in ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}.$ In the presence of the intrinsic point defects, extra levels appeared in the band gap. Considering various charge states for the intrinsic point defects, it was found that each point defect is most stable in its fully ionized state. From the formation energies of individual point defects, Schottky, O Frenkel, and Al Frenkel energies were also evaluated and were compared with previous results by experiment and static lattice calculations. Although previous static lattice calculations showed different relative stabilities of Schottky and Frenkel formation, depending on the choice of interatomic potentials, our calculations revealed that the relative values of formation energies are in the order of $\mathrm{Schottky}l\mathrm{Al}$ $\mathrm{Frenkel}l\mathrm{O}$ Frenkel, which is in good agreement with experimental data.