Theory and Methods for Martensite Design: Session 5
Program Organizers: Greg Olson, Northwestern University; Ricardo Komai, QuesTek Innovations LLC

Thursday 10:20 AM
July 13, 2017
Room: Gold Coast
Location: Hyatt Regency Chicago

Session Chair: Turab Lookman, Los Alamos National Laboratory

10:20 AM  
Applications of Olson-Ghosh Model and Distributed-activation Kinetics Theory in Material Design: Abhinav Saboo1; David Snyder1; Ida Berglund2; 1QuesTek Innovations LLC; 2QuesTek Europe AB and QuesTek Innovations LLC
    The Olson-Ghosh Model and Distributed-activation kinetics theory for heterogeneous Martensite nucleation can be used for prediction of Martensite start temperature, Martensite and retained austenite fractions in steels. These models are being applied to the design of higher performance steels for various applications, as depicted through two case studies. The first study is based on re-design of 17-4PH stainless steel powder for its applications in laser powder bed fusion additive manufacturing to achieve desired performance requirements with minimal processing steps. The second study is based on development of fatigue strength prediction model based on stress-assisted martensitic transformations to be used in design of high performance fatigue resistant steels. Model fidelity and dependencies would be discussed and approach towards model improvement will be suggested.

10:40 AM  
Activation energy of time-dependent martensite formation in steel: Matteo Villa1; Marcel Somers1; 1Technical University of Denmark
    The kinetics of 〖{5 5 7}〗_γ lath martensite formation in (wt-%) 17Cr-7Ni-1Al-0.09C and 15Cr-7Ni-2Mo-1Al-0.08C steels was assessed with magnetometry at sub-zero Celsius temperatures. Samples were cooled to 77 K by immersion in boiling nitrogen to suppress martensite formation. Thereafter, thermally-activated martensite formation was monitored during: (i) isochronal (re)heating at different heating rates; (ii) isothermal holding at temperatures between 120 and 310 K. The activation energy, E_A, of thermally-activated martensite formation was quantified from the results of both isochronal and isothermal tests by applying a Kissinger-like method. In addition, the isothermal data was interpreted applying the approach presented by Borgenstam and Hillert. The results of the independent quantification methods were consistent and indicated an E_A in the range 9-13 kJ mol-1. Thereafter, the two methods were applied to evaluate the data available in the literature. The overall analysis showed that E_A varies in the range 2-27 kJ mol-1 and increases logarithmically with the total fraction of interstitials in the steel.

11:00 AM  
Temporal Correlations and Average Avalanche Shapes from Noise Measurements Obtained during A/M Transformation of Ni2MnGa: László Tóth1; Lajos Daróczi1; Dezső Beke1; 1University of Debrecen
     Simultaneous acoustic and magnetic emission measurements were carried out during thermally induced A/M transformation in Ni2MnGa single crystals in order to investigate temporal correlations between acoustic events caused by structural transformation and between magnetic signals caused by magnetic domain rearrangements. The P(n;τm) distributions of n successive events belonging to the same burst with ti+1-tim waiting times are clearly different from Pind(n;τm) distribution of independent events, demonstrating that correlations exist between acoustic events and between magnetic avalanches. Furthermore, simultaneous measurement of AE and ME enables us to study the correlation between these two signals. Probability distributions of the time delays of consecutive AE and ME signals showed, that there exists a delay time range, where it is more probable that an acoustic event is followed by a magnetic one, than inversely. The scaling behavior of average avalanche shapes for fixed time and for fixed size will also be discussed.

11:20 AM  
{225}γ Habit Planes in Martensitic Steels: From the PTMC to a Continuous Model: Annick Baur1; Cyril Cayron1; Roland Logé1; 1EPFL
    Fine twinned microstructures with {225}γ habit planes are commonly observed in martensitic steels. Based on simple linear algebra concepts, the present study shows that an equibalanced combination of twin-related variants associated to the Kurdjumov-Sachs orientation relationship keeps the {225}γ planes invariant during the FCC-BCC martensitic transformation. The distortion associated to the Kurdjumov-Sachs orientation relationship results from a continuous modeling of the transformation (Cayron, Acta Mater, 69, 189-202, 2015). We also demonstrate that the twin-related variants combination as proposed is equivalent to the Bowles and Mackenzie’s version of the PTMC for the specific {225}γ case. Thus, for the first time, an atomic path can be associated to the PTMC. The present approach allowed the creation of a short animation showing the formation of a thin plate of martensite at an atomic scale.

11:35 AM  
Mathematical Properties of the Microstructures in Materials Satisfying the Cofactor Conditions: Francesco Della Porta1; 1University of Oxford
    In this work, we study from a theoretical point of view the complex microstructures arising in materials satisfying the Cofactor Conditions during thermal cycling. These are particular conditions of geometrical compatibility between phases first introduced by Ball and James (ARMA (1987)). As shown by Song et al. (Nature (2013)), in ZnAuCu, the first material discovered to closely satisfy the Cofactors Conditions, the martensitic microstructures can be very complex and different at every cycle. Starting from simple hypotheses and building on the model by Ball and James (ARMA (1987)), we deduce some invariant mathematical conditions that should be satisfied by the microstructures. These allow us to better understand not only the complex martensitic microstructures but also the curved austenite-martensite interfaces which can be seen for example in ZnAuCu.

11:50 AM  
Crystallography of Martensitic Transformation and Deformation Twinning in Hard-sphere Metals: Cyril Cayron1; 1EPFL IMX LMTM
     The atoms in face-centered cubic, body-centered cubic and hexagonal close-packed structures are geometrically represented with hard-spheres. This hypothesis is used to calculate the continuous atomic paths and lattice distortion from the initial state to the final state [1], the transformations being described with only one unique angular parameter without combining shear matrices. In martensitic iron alloys, specific variant selection rules are sufficient to deduce a) the {225} habit planes in the high carbon steels [2], and b) the {557} habit planes in the low carbon steels [3]. In magnesium, some twinning modes experimentally observed but not predicted by the classical Bevis and Crocker’s theory are calculated; and a solution to the apparent abnormality of the Schmid factor is proposed. [1] C. Cayron, Acta Mater. 111 (2016) 417-441. [2] A. Baur, C. Cayron, R. Logé, under review. [3] C. Cayron, A. Baur, R. Logé, [4] C. Cayron,

12:10 PM Break