Phase Transformations and Microstructural Evolution: Ti & Zr, and Steels
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Phase Transformations Committee
Program Organizers: Gregory Thompson, University of Alabama; Rajarshi Banerjee, University of North Texas; Sudarsanam Babu, The University of Tennessee, Knoxville; Deep Choudhuri, University of North Texas; Raju Ramanujan, Nanyang Technological University; Monica Kapoor, National Energy Technology Lab

Tuesday 2:00 PM
February 28, 2017
Room: 16B
Location: San Diego Convention Ctr

Session Chair: Monica Kapoor, National Energy Technology Lab

2:00 PM  Cancelled
Determination of Phase Transformations and Microstructure Evolution of Zr-based Alloys during Thermal Processing: Clinique L. Brundidge1; John Seidensticker1; Tyler Tenkku1; Linda Rishel1; Richard Smith1; 1Bechtel Marine Propulsion Corporation
    Zirconium-based alloys exhibit a transformation between the HCP-α and BCC-β phases in the vicinity of 850oC, depending on exact chemistry and processing conditions. The transformation temperature on cooling from the high temperature phase was observed to vary with cooling rate, affecting the final α-phase microstructure. Understanding these relationships is needed to develop models of microstructural evolution to integrate into fabrication simulation software to aid in the optimization of materials processing. High speed dilatometry was utilized to track the transformation in select Zr-based alloys over a wide range of heating/cooling rates. Time-temperature profiles and dilation behavior at and around the phase transformation were measured and the morphological changes in the transformed products were studied using optical, scanning and transmission electron microscopy. Computational modeling of the basic processes of microstructural evolution has been addressed both at the atomistic and mesoscale in order to link these phenomena to the thermal history.

2:20 PM  
Development of Various Scale Alpha Microstructures in Titanium Alloys: Yufeng Zheng1; Robert Williams1; Rongpei Shi1; Deep Choudhuri2; Talukder Alam2; Rajarshi Banerjee2; Yunzhi Wang1; Hamish Fraser1; 1The Ohio State University; 2University of North Texas
    Beta titanium alloys have attracted considerable attentions due to its refined nature of microstructure manipulated by various thermal/mechanical processes. In our recent study, alpha microstructures of different size scales have been produced in Ti-5Al-5Mo-5V-3Cr (Ti-5553), namely refined, more-refined and super-refined alpha microstructures, by applying various heat treatments, upon the different influences from pre-formed omega phase. The nucleation and growth mechanism of omega phase was firstly studied using high resolution scanning transmission electron microscopy and three-dimension atom probe. Then coupling with phase field modeling, it has been shown that pre-formed omega phase particle can alter the local structure and concentration and therefore provide extra driving force for subsequent alpha precipitation. Specific designed heat treatment can effectively select such extra driving force and therefore produce different fine scale alpha microstructure. Therefore, a thorough understanding of omega phase and its influence on non-transformation pathways controlling alpha microstructures in titanium alloys will be introduced.

2:40 PM  
Hydrostatic Compression Behavior and High-pressure Stabilized b-phase in g-based Titanium Aluminide Intermetallics: Klaus-Dieter Liss1; Xi Li2; Ken-Ichi Funakoshi3; Rian Dippenaar2; Yuji Higo4; Ayumi Shiro5; Mark Reid1; Hiroshi Suzuki6; Takahisa Shobu6; Koichi Akita6; 1Australian Nuclear Science and Technology Organisation; 2University of Wollongong; 3Comprehensive Research Organization for Science and Society (CROSS-Tokai); 4SPring-8, Japan Synchrotron Radiation Research Institute; 5National Institute for Quantum and Radiological Science and Technology; 6Japan Atomic Energy Agency
    Titanium-aluminides find application in modern light-weight, high-temperature aircraft engines, but suffer from poor plasticity during manufacturing and processing. Here we report on an in-situ synchrotron X-ray diffraction study in a large-volume-press of a modern (α2 + γ) material, Ti-45Al-7.5Nb-0.25C, under pressures and temperatures up to 9.6 GPa and 1686 K. At room temperature, the volume response to pressure is accommodated by the transformation γ → α2, expressed by apparently high bulk moduli of both constituent phases. Lattice strain and atomic order are discussed in detail. It is interesting to note that this transformation takes place despite an increase in atomic volume. Upon heating under high pressure, both the eutectoid and γ-solvus transition temperatures are elevated, and a third, cubic β-phase is stabilized above 1350 K. Earlier research has shown that this β-phase is very ductile, essential in near-conventional forging processes. Novel processing routes can be defined from these findings.

3:00 PM  
Kinetics of Low-temperature Spinodal Decomposition in a Fe-Ni-C Martensite: A Discrete Mean-field Model: Philippe Maugis1; Mohamed Gouné2; Frédéric Danoix3; Sophie Cazottes4; Sergiu Curelea4; Myriam Dumont1; 1Aix-Marseille Univ, CNRS, IM2NP; 2CNRS, ICMCB; 3Université de Rouen, CNRS, GPM; 4MATEIS, INSA de Lyon
    The kinetics of low-temperature carbon redistribution in a Fe-Ni-C martensite was investigated by means of hardness measurements, Atom Probe Tomography and Transmission Electron Microscopy. The results suggest that the phase alpha’’-Fe16C2 (with 11.1 at%C) forms from the supersaturated martensite via a spinodal decomposition mechanism, in the time range of a few days at room temperature. To investigate the thermodynamic and kinetic parameters controlling this transformation, a discrete 1D mean-field model has been developed, based on the Cahn-Hilliard approach. The adjustment of the model parameters allowed retrieving the energy of the alpha / alpha’’ interface, as well as the diffusion coefficient of carbon, as function of carbon content, in the supersaturated martensite. At room temperature, the intrinsic diffusion coefficient of carbon is found to be non-monotonous, and negative inside the spinodal region. The effect of the martensite tetragonality is highlighted.

3:20 PM Break

3:40 PM  
Phase-field Simulation of Solidification of High and Medium Manganese Steels: Incorporating the Effects of Convection and of Transformation Strains: Joao Rezende1; Christian Schankies1; Celso Alves1; Dieter Senk1; 1RWTH Aachen
    In the last 15 years the so-called high-manganese steels are increasingly gaining in importance as a material for structural applications. These alloys show at the same time elevated values of tensile strength and of ductility. These properties are achieved due to the specific phase constitution of the materials and the deformation mechanisms associated with those phases. By performing phase-field simulations, we show that, in the case of single phase solidification, the dendrite morphology in the system Fe-Mn-C-Al is strongly influenced by convection in the melt. In addition to this, the strength of this influence depends on the exact chemical composition of the alloy. One important parameter in this respect is the segregation index achieved during solidification in the absence of convection. For the case of two-phase solidification, we take into account the density change of the delta->gamma phase transformation to access the eigenstrain levels which cause problems in continuous casting.

4:00 PM  
Phase Transformation Kinetics of Pressure-vessel Steel Welds Phase Transformation Kinetics of Pressure-vessel Steel Welds : Gideon Obasi1; Dinesh Rathod2; Anastasia Vasileiou2; Ed Pickering2; John Francis2; Mike Smith2; Michael Preuss2; 1 The University of Manchester; 2The University of Manchester
    Microstructural evolution has a pivotal effect on the development of strains during post-weld cooling through a number of physical processes, including volume changes during phase transitions, and transformation-induced plasticity. In this study, we experimentally assess the phase transformation behaviour of SA508 Grade3Class1 steel when subject to typical welding thermal cycles, and compare the results to predictions arising from kinetic models. The transformation products obtained during the thermal cycling comprise retained austenite, ferrite, bainite and martensite. The peak temperature of the thermal cycle, through its effect on austenite grain size, has an influence on the subsequent microstructural evolution during quenching. Specifically, when applying a peak temperature of 900°C, a delay was seen in the formation of bainite due to the prior formation of ferrite. This was not observed when applying a peak temperature of 1200°C due to sluggish ferrite formation, which can be attributed to austenite grain growth at higher temperatures.

4:20 PM  
Phase Transformation, Microstructural Evolution and Property Modification in Rapidly Solidified Grey Cast Iron: Olamilekan Oloyede1; Robert F. Cochrane1; Andrew M. Mullis1; 1University of Leeds
    The phase transformation and microhardness changes of hypoeutectic grey iron subjected to rapid solidification cooled in N2 were studied to showcase the relationship between structure and microhardness of this important engineering material. Droplets samples were prepared, using containerless processing via drop-tube technique. These rapidly cooled samples were collected and sieved into 9 size ranges from ≥ 850 µm to ≤ 53 µm diameters with corresponding estimated cooling rate of ≈ 200 – 23,000 K s –1. The emerged phases and evolved morphologies were characterized using optical and scanning electron microscopy, X-ray diffraction analysis and transmission electron microscopy; while Vickers microhardness tester was used to measure the hardness in the various samples. As the cooling rate increases, the undercooling increased and Martensitic or acicular ferrite structure was observed in the very small droplets which confirms the progressive increase in microhardness of the samples from the as-cast to decreasing droplets sizes.