Thursday 10:20 AM

July 13, 2017

Room: Gold Coast

Location: Hyatt Regency Chicago

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.

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.

Simultaneous acoustic and magnetic emission measurements were carried out during thermally induced A/M transformation in Ni

Fine twinned microstructures with {225}

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.

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, https://arxiv.org/abs/1606.04257 [4] C. Cayron, https://arxiv.org/abs/1608.07037