The role of phase interface energy in martensitic transformations: a lattice Monte-Carlo simulation

To study martensitic phase transformation we use a micromechanical model based on statistical mechanics. Employing lattice Monte-Carlo simulations and realistic material properties for shape-memory alloys (SMA), we investigate the combined influence of the external stress, temperature, and interface energy between the austenitic and martensitic phase on the transformation kinetics and the effective material compliance. The one-dimensional model predicts well many features of the martensitic transformation that are observed experimentally. Particularly, we study the influence of the interface energy on the transformation width and the effective compliance. In perspective, the obtained results might be helpful for the design of new SMAs for more sensitive smart structures and more efficient damping systems.Comment: 10 pages, 3 figures, 22 reference

Size Effects in Residual Stress Formation during Quenching of Cylinders Made of Hot-Work Tool Steel

The present work investigates the residual stress formation and the evolution of phase fractions during the quenching process of cylindrical specimens of different sizes. The cylinders are made of hot-work tool steel grade X36CrMoV5-1. A phase transformation kinetic model in combination with a thermomechanical model is used to describe the quenching process. Two phase transformations are considered for developing a modelling scheme: the austenite-to-martensite transformation and the austenite-to-bainite transformation. The focus lies on the complex austenite-to-bainite transformation which can be observed at low cooling rates. For an appropriate description of the phase transformation behaviour nucleation and growth of bainite are taken into account. The thermomechanical model contains thermophysical data and flow curves for each phase. Transformation induced plasticity (TRIP) is modelled by considering phase dependent Greenwood-Johnson parameters for martensite and bainite, respectively. The influence of component size on residual stress formation is investigated by the finite element package Abaqus. Finally, for one cylinder size the simulation results are validated by X-ray stress measurements

Calcium isotope (δ^44/40Ca ) variations of Neogene planktonic foraminifera

Measurements of the calcium isotopic composition (δ44/40Ca) of planktonic foraminifera from the western equatorial Pacific and the Indian sector of the Southern Ocean show variations of about 0.6‰ over the past 24 Myr. The stacked δ44/40Ca record of Globigerinoides trilobus and Globigerina bulloides indicates a minimum in δ44/40Casw (seawater calcium) at 15 to 16 Ma and a subsequent general increase toward the present, interrupted by a second minimum at 3 to 5 Ma. Applying a coupled calcium/carbon cycle model, we find two scenarios that can explain a large portion of the observed δ44/40Casw variations. In both cases, variations in the Ca input flux to the ocean without proportional changes in the carbonate flux are invoked. The first scenario increases the riverine calcium input to the ocean without a proportional increase of the carbonate flux. The second scenario generates an additional calcium flux from the exchange of Ca by Mg during dolomitization. In both cases the calcium flux variations lead to drastic changes in the seawater Ca concentrations on million year timescales. Our δ44/40Casw record therefore indicates that the global calcium cycle may be much more dynamic than previously assumed

A core outcome set for pre‐eclampsia research: an international consensus development study

Objective To develop a core outcome set for pre‐eclampsia. Design Consensus development study. Setting International. Population Two hundred and eight‐one healthcare professionals, 41 researchers and 110 patients, representing 56 countries, participated. Methods Modified Delphi method and Modified Nominal Group Technique. Results A long‐list of 116 potential core outcomes was developed by combining the outcomes reported in 79 pre‐eclampsia trials with those derived from thematic analysis of 30 in‐depth interviews of women with lived experience of pre‐eclampsia. Forty‐seven consensus outcomes were identified from the Delphi process following which 14 maternal and eight offspring core outcomes were agreed at the consensus development meeting. Maternal core outcomes: death, eclampsia, stroke, cortical blindness, retinal detachment, pulmonary oedema, acute kidney injury, liver haematoma or rupture, abruption, postpartum haemorrhage, raised liver enzymes, low platelets, admission to intensive care required, and intubation and ventilation. Offspring core outcomes: stillbirth, gestational age at delivery, birthweight, small‐for‐gestational‐age, neonatal mortality, seizures, admission to neonatal unit required and respiratory support. Conclusions The core outcome set for pre‐eclampsia should underpin future randomised trials and systematic reviews. Such implementation should ensure that future research holds the necessary reach and relevance to inform clinical practice, enhance women's care and improve the outcomes of pregnant women and their babies

A Technology-Enhanced ‘Assessment System for Teaching and Assessment using a Computer Algebra Kernel’ (STACK): The Best Contributions of 2022’s STACK Community Meeting at the Montanuniversitaet Leoben, Austria

After two years of online meetings we had the great pleasure to host the meeting of the STACK Community 2022 at the Montanuniversitaet Leoben in Austria in a hybrid setting. STACK (System for Teaching and Assessment using a Computer algebra Kernel) allows for a wide variety of technology-enhanced teaching, learning and assessment approaches. The accelerated significance of this approach over the last couple of years has been impressively demonstrated at the 2022 community meeting. The best contributions to the meeting have been carefully selected by an international program committee and are collected in this special issue.&nbsp

Special cases of martensite compatibility: A near single-variant habit-plane and the martensite of nanocrystalline NiTi

Lattice parameters measured near the high temperature (~1000°C) bcc α to hcp β transformation in an intermetallic Mo-containing γ-TiAl based alloy indicate a middle valued eigenvalue of the corresponding deformation gradient near 1. Habit-planes calculated under the assumption of a simple slip as lattice invariant shear, agree with experimentally determined orientations of the lens like plates recorded via electron backscattering. By contrast, twinning as invariant lattice shear has been investigated in nanocrystalline NiTi. Here the grain size causes the formation mechanism of the martensite to change from a “herring-bone” morphology faciliting a habit-plane between two twinned laminates and the austenite to a single laminate, which in the nonlinear theory formally cannot form a habit-plane with the austenite. Since this might cause high accommodation strains, the effectiveness of stress accommodation of martensite formed in neighboring grains of a polycrystal is investigated. Subsequent numerical microstructural modeling is outlined. The resulting energetically most favorable transformation sequence yields the transformation kinetics

Special cases of martensite compatibility: A near single-variant habit-plane and the martensite of nanocrystalline NiTi

Lattice parameters measured near the high temperature (~1000°C) bcc α to hcp β transformation in an intermetallic Mo-containing γ-TiAl based alloy indicate a middle valued eigenvalue of the corresponding deformation gradient near 1. Habit-planes calculated under the assumption of a simple slip as lattice invariant shear, agree with experimentally determined orientations of the lens like plates recorded via electron backscattering. By contrast, twinning as invariant lattice shear has been investigated in nanocrystalline NiTi. Here the grain size causes the formation mechanism of the martensite to change from a “herring-bone” morphology faciliting a habit-plane between two twinned laminates and the austenite to a single laminate, which in the nonlinear theory formally cannot form a habit-plane with the austenite. Since this might cause high accommodation strains, the effectiveness of stress accommodation of martensite formed in neighboring grains of a polycrystal is investigated. Subsequent numerical microstructural modeling is outlined. The resulting energetically most favorable transformation sequence yields the transformation kinetics

A mean-field model for transformation induced plasticity including backstress effects for non-proportional loadings

International audienceA viable model for the phenomenon of transformation induced plasticity must be able to predict the strain response to arbitrary, also non-proportional loading paths. The constitutive model presented in this paper is interfacing between the macro- and the mesoscale by stress partitioning in the parent and product phase, using a nonlinear scale transition rule. As one of the key features a mean field tensor representing the orientation strain contribution is introduced taking into account backstress effects. A good agreement of the model results with experiments on a maraging steel is achieved

Transformation hardening and kinetics for stress assisted and temperature driven martensitic transformation in steels

International audienceIn the present study a new phase fraction based transformation hardening and further a stress and temperature dependent transformation kinetics following the postulates in internal variable thermodynamics is presented. The generally formulated kinetics equation is applied to describe the progress of phase transformation in a Cr-Ni-Mo-Al-Ti maraging steel and represents a good tool to predict the continuous change of the slope of isovolume fractions