Phase Transformations and Microstructural Evolution: Ti & Zr
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

Wednesday 2:00 PM
March 1, 2017
Room: 16B
Location: San Diego Convention Ctr

Session Chair: Deep Choudhuri, University of North Texas

2:00 PM  
Investigation of Alpha/Beta Interface Structure in a Titanium Alloy Using Aberration-Corrected Scanning Transmission Electron Microscope: Yufeng Zheng1; Robert Williams1; William Clark1; Hamish Fraser1; 1The Ohio State University
    The structural and chemical nature of alpha/beta interface in titanium alloys plays very important role, because it can influence microstructural evolution by acting as potential nucleation site and providing extra driving force for second phase nucleation and can also affect the deformation mechanism due to its interaction with various defects. In this current work, the crystallography and the structure of the habit plane of refined and coarse alpha precipitates in the beta matrix of Ti-5553 were studied using probe corrected scanning transmission electron microscope. In Z-contrast HAADF-HRSTEM image, for the first time, terrace and disconnection structure on the broad face of both refined and coarse alpha precipitates have been characterized, in which the lengths of terraces (or the spacing of disconnections) and the height of steps at the disconnections can be measured directly. The observations are discussed in terms of an analytical topological model.

2:20 PM  
Influences of Pre-existing Defects on the Morphology and Variant Selection of Precipitates in Alpha/Beta Ti-alloys: Di Qiu1; Rongpei Shi2; Pengyang Zhao2; Weijie Lyu1; Yunzhi Wang2; 1Shanghai Jiao Tong University; 2The Ohio State University
    Pre-existing crystalline defects, such as dislocations, grain boundaries (GBs) and existing precipitates, have been found to have significant influences on the subsequent development of precipitate microstructures during solid state phase transformations. Through phase field simulations incorporating stress fields of various defects as inputs, we show that 1) for 1-dimensional (1-D) single dislocations, edge dislocations interact with alpha nuclei with larger interaction energy than screw dislocations, leading to a greater degree of variant selection (VS); 2) for 2-D low angle GBs modeled by dislocation networks, alpha allotriomorphs and GB side-plates compete with each other during precipitation and the final morphology exhibits a strong correlation with the GB structures and 3) for existing (primary) alpha precipitates, they may induce the nucleation of certain variants of secondary alpha. The simulation results are analyzed in details from the views of energy and crystallography, and compared with experimental observations.

2:40 PM  
Microstructure Evolution and Recrystallization in Linear Friction Welded Titanium Alloys: Riddhiman Bhattacharya1; Thomas Broderick2; John Allison1; 1University of Michigan, Ann Arbor; 2GE Aviation
    Titanium (Ti) alloys are attractive candidates for aerospace structural applications due to their high specific strength and retention of mechanical properties up to temperatures of ~500 degree C. However, their susceptibility to contamination in the molten state necessitates use of solid state processing methods, such as Linear/Translational Friction Welding (LFW/TFW). In this work, micro-mesoscale characterization of similar and dissimilar alpha+beta-Ti welded joints is presented. Evidence of static and dynamic recrystallization has been observed, which are attributed to process variables such as temperature, strain, and strain-rate. These parameters are systematically studied to establish structure-property relationships using Scanning Electron Microscopy (SEM) combined with Electron Back Scattered Diffraction (EBSD) techniques.

3:00 PM  
Primary Alpha Plate Growth in Ti6246: Abigail Ackerman1; David Rugg2; David Dye1; 1Imperial College, London; 2Rolls-Royce plc.
    Ti-6Al-2Sn-4Zr-6Mo has an intricate microstructure resulting from complex thermo-mechanical processing, comprising of large primary and smaller secondary alpha plates surrounded by a beta matrix. The presence of secondary alpha gives significant strength and fatigue benefit, but the details of the processing-microstructure relationships are in practice optimised empirically. Here the (nucleation and) growth of lamellar primary alpha plates during isothermal holding are modelled in terms of the diffusion of the rate controlling species, Mo, away from a growing alpha plate. The velocity-undercooling-phase diagram relationships are determined experimentally using a Gleeble-type electro-thermal-mechanical-tester and compared to the model results.

3:20 PM Break

3:40 PM  
Study on Phase Stability, Correlated Deformation Microstructure and Mechanical Properties in a Metastable β-type Ti-Nb-Zr-Ta-O Alloy: Sumin Shin1; Kenneth Vecchio1; 1University of California, San Diego
    Evolution of microstructure and deformation modes of cold-rolled Ti-23Nb-0.7Zr-0.3O alloy were investigated with different solution treatment. This alloy, referred to as gum metal, has been reported to possess high ductility without work hardening, and low Young’s modulus obtained by cold working metastable β phase. Recent studies suggest that both mechanical twinning and stress-induced phase transformation play an important role in enhancing the work hardening and thus improving the mechanical properties. In this study, we studied the effect of the elemental distribution on the phase stability with the Bo-Md map, and to discuss the deformation modes which possibly enhance the mechanical properties by controlling the deformation microstructure. Analysis of the deformed microstructure using electron backscatter diffraction and X-ray diffraction revealed that deformation bands are composed of {332}<112> twin, {112}<111>twin, and stress-induced martensite (⍺״) phase, and the number of bands decreases with increasing homogeneity of the elemental distribution in the alloy.

4:00 PM  
Phase Formation in Cu - Zn Powder Mixtures Subjected to Ultrasonic Powder Consolidation: Azin Houshmand1; Teiichi Ando1; 1Northeastern University
    Microstructural changes and interdiffusion in Cu – 48 mass% Zn specimens prepared by ultrasonic powder consolidation (UPC) were investigated by optical microscopy, SEM and X-ray diffraction. Fully dense, metallurgically consolidated specimens 4 mm in diameter and about 1 mm in thickness were obtained at nominal consolidation temperatures of 423 - 573 K in just 1 – 4 s. During the few seconds of UPC, significant interdiffusion between Cu and Zn occurred, producing the intermediate gamma brass phase (~ Cu5Zn3). However, beta brass was the intermediate phase to form during UPC when the elemental powder mixture was subjected to ball-milling for a short duration or heat treatment before UPC. The effects of ultrasonic deformation on the phase formation are discussed.

4:20 PM  
The Effect of Aluminum Content on Recrystallization and Grain Growth in Binary Alpha Titanium Alloys: Anna Trump1; John Allison1; 1University of Michigan
    Recrystallization and grain growth are important microstructural phenomena in commercial titanium alloys. In this study, we concentrate on recrystallization and grain growth of the dominant α-phase. The static recrystallization and grain growth behavior of model binary α-Ti alloys were studied by performing room temperature compression tests followed by recrystallization heat treatments. EBSD techniques were used to quantify recrystallization along with texture and grain size evolution. The effects of processing conditions and solute concentration on the static recrystallization kinetics are analyzed and modeled using the JMAK model. Increasing aluminum content was observed to lead to significantly slower static recrystallization and grain growth kinetics. The effects of aluminum content and processing variables on the dynamic recrystallization kinetics and flow stress behavior were also studied using high temperature compression tests. The results from these experimental studies have been used to parameterize a phase-field model of recrystallization and grain growth in single phase titanium.