Theory and Methods for Martensite Design: Session 1
Program Organizers: Greg Olson, Northwestern University; Ricardo Komai, QuesTek Innovations LLC

Monday 10:40 AM
July 10, 2017
Room: Gold Coast
Location: Hyatt Regency Chicago

Session Chair: Charles Kuehmann, SpaceX/Tesla

10:40 AM  Invited
Design of Heusler-strengthened Shape Memory Alloys for Biomedical Applications: Dana Frankel1; Ida Berglund1; Jason Sebastian1; Gregory Olson2; 1QuesTek Innovations LLC; 2QuesTek Innovations LLC/Northwestern University
    NiTi-based SMAs are used in a wide variety of medical applications, most commonly vascular stents which require excellent fatigue performance to prevent device failure and negative clinical outcomes. The development of novel, low-Ni, precipitation-strengthened SMAs utilizing an Integrated Computational Materials Engineering (ICME) approach, including mechanistic process-structure and structure-property models, allows for the parametric design of a highly biocompatible, radiopaque, and fatigue-resistant SMA. The austenite finish temperature is modeled using a 2-sublattice Redlich-Kister polynomial, while the compositional dependence is captured through a thorough understanding of Heusler phase precipitation kinetics, elemental partitioning, and particle size evolution, informed by advanced characterization techniques such as Atom Probe Tomography (APT). Excellent functional fatigue resistance has been demonstrated in precipitation-strengthened prototypes. Novel processing routes, including rapid solidification techniques, are also investigated in order to mitigate the formation of embrittling phases and refine non-metallic inclusion structures for improved strength and fatigue resistance.

11:20 AM  
Experimental Determination and Modeling of the Msσ Temperature of Austenite in Medium Mn Steels: Fei Huyan1; Jiayi Yan1; Annika Borgenstam1; 1KTH Royal Institute of Technology
    Medium Mn steels show excellent elongation due to the transformation induced plasticity (TRIP) accompanying the martensitic transformation. The highest elongation could be obtained by optimizing the mechanical stability of the austenite quantified using the Msσ temperature. The Msσ temperature is defined as the highest temperature that martensite forms by stress without a yield of the austenite. In the present work, the Msσ temperatures were experimentally determined based on tensile tests at different temperatures for a Fe-0.2C-5Mn (wt %) steel intercritically annealed at 923 K for several different durations. The Msσ temperatures were further modelled based on thermodynamics using the experimental determined C and Mn content especially taking into account the stabilization effect from a small grain size. A good agreement was obtained comparing the model with the experiments.

11:35 AM  
TRIP Titanium Alloy Design: Fan Meng1; Jia-Yi Yan2; Wei Xiong3; Gregory Olson1; 1Northwestern University; 2KTH Royal Institute of Technology; 3University of Pittsburgh
    A novel near-α TRIP titanium alloy Ti-8111Fe for structural applications with both high strength and fracture toughness was computationally designed based on the commercial alloy Ti-5111. Mechanistic models describing the thermodynamics and kinetics of martensitic transformation in titanium alloys were applied to calculate the characteristic Msσ defining the β phase stability at the stress states of uniaxial tension (ut) and crack tip (ct). Desired positive martensitic transformation dilatation was calculated from a room temperature molar volume database developed for β, α’and α” phases. Annealing the alloy at 865oC is predicted to produce an Msσ(ct) of 25oC, about 20% β phase, and a transformation dilatation of 0.27%, compared to -0.55% for Ti-5111. Predicted phase compositions and phase fractions of Ti-8111Fe annealed at 865oC were experimentally validated.

11:50 AM  
The Ms Temperature Calculated with a Thermodynamic based Model Accounting for the Effect of the Prior Austenite Grain Size: Stefan van Bohemen1; Lutz Morsdorf2; 1Tata Steel; 2MPIE
    A thermodynamic based model for predicting the martensite-start temperature (Ms) of steels has been developed that accounts for variations in the austenite grain size (Dγ). This is achieved by introducing two additional energy terms in the physical expression of the critical driving force proposed by Ghosh and Olson. Since grain refinement leads to stronger austenite, a higher driving force and thus a lower Ms is required to initiate the shear transformation. This first mechanism is described by a Hall Petch strengthening term. Secondly, the aspect ratio of martensitic units, i.e. laths, increases when Dγ becomes lower than a critical diameter Dc. This implies more stored energy and therefore a higher driving force is required which also contributes to a decrease of Ms. Model calculations show a good agreement with experimental dependencies between Ms and Dγ.