Phase Transformations and Microstructural Evolution: Steels and Shape Memory, and General
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
Thursday 8:30 AM
March 2, 2017
Location: San Diego Convention Ctr
Session Chair: Monica Kapoor, National Energy Technology Lab
Thermal Stabilization of Bainite: Sk Hasan1; Shiv Singh1; 1IIT Kharagpur
The growth of bainite causes coordinated movement of atoms during the motion of its glissile transformation interface. In case of direct quenching, for a given transformation time, the amount of bainite increases as the transformation temperature decreases within Bs-Ms range of a steel. But in step quenching, after allowing some bainite to be formed at a particular temperature upon up or down quenching to another temperature (both temperatures are within Bs and Ms of the steel), the total amount of bainite decreases than that of directly quenched steel at latter temperature. The reduction of bainite in these cases is attributed to thermal stabilization of bainite. Moreover, it has been found that incomplete transformation to martensite prior to bainite transformation also results thermal stabilization of bainite. Driving force, no. of nucleation sites, transformation strain and variation of austenite yield strength are expected to be the basis of thermal stabilization of bainite.
A Preliminary In-situ TEM Study of Migration Properties of Interfaces between Austenite and Ferrite in a Duplex Stainless Steel: Juan Du1; Frederic Mompiou2; Wen-Zheng Zhang1; 1Tsinghua University; 2CEMES-CNRS and University of Toulouse
The migration properties of interfaces between austenite and ferrite in a duplex stainless steel were investigated by using in-situ transmitted electron microscope at temperature ranging from 725℃ to 900℃. The phase transformation from ferrite to austenite occurs either by the existing interfaces or by nucleation and growth of fresh austenite. Two modes, 1) normal motion and 2) ledge motion (including both lateral motion and normal motion), have been observed during the migration of facets between austenite and ferrite. The moving rate of interfaces along the long axis is much larger than that normal to the long axis. During the migration of interfaces, dislocations emitted from the interface to ferrite matrix were observed frequently, and matrix dislocations moving towards the interfaces may be merged on the interface. However, some matrix dislocations may interact with the tip of austenite lath, which slows down the moving rate.
Characterizing Ni-Ti-Ga Shape Memory Alloys: Oscar Figueroa1; Michele Manuel1; 1University of Florida
Shape memory alloys (SMAs) are class of materials for actuating applications where size is constrained, but high power output is necessary. NiTi alloys are the most commercially viable and are used in a variety of commercial industries. In an effort to expand our understanding of SMAs and their properties, a new system has been designed to characterize the effects of Ga additions to NiTi. While the NiTi phase diagram is well understood, there is little literature on NiTiGa alloys. A series of alloys containing between 1 and 6 at% Ga were characterized to determine phase transformation, microstructural evolution, and mechanical properties of NiTiGa SMAs. Using electron microscopy techniques, spectroscopic chemical analysis, thermal characterization, and mechanical testing, NiTiGa alloys were studied to develop new processing-structure-property relationships. The authors gratefully acknowledge the support of NASA grant NNX12AQ42G.
9:30 AM Cancelled
The Kinetics of Ferromagnetic Tau Phase Formation in Mn-Al Alloys: Ozgun Acar1; Merve Genc1; Ilkay Kalay2; Eren Kalay1; 1METU; 2Cankaya University
The strategical importance of RE elements drives the magnet technologies towards RE-free alloy development. In this context, Mn-Al alloys have recently received much attention due to the availability of forming metastable ferromagnetic- L10 phase. Previous studies showed that this magnetic phase can be formed from high-temperature metastable parent epsilon phase. Several routes were suggested for the formation of tau phase upon solid-state phase transformation however there is no detailed analysis to explore the relevant kinetics and the role of the intermediate epsilon prime phase. In this respect, we investigated the formation of metastable- tau formation in Mn-Al alloys using DSC and in-situ HEXRD at ALBA Synchrotron Source. From the isothermal and non-isothermal approaches the phase transformation hierarchy, nucleation and growth behavior of tau phase from epsilon and epsilon prime were discussed and compared. The role of the intermediate epsilon prime to form the ferromagnetic tau will be discussed in details.
9:50 AM Break
Orientational Dependence of Shock Induced Phase Transition of Single Crystal Copper: Anupam Neogi1; Nilanjan Mitra1; 1IIT Kharagpur
A series of large scale non-equilibrium molecular dynamics (NEMD) simulations of single crystalline copper has been carried out to investigate the shock induced plastic phenomenon, including phase transition, if any. Nucleation of body-centered tetragonal (BCT) phase has been observed to occur in high symmetry crystal orientation, like, <100> at an impact velocity of 1.5 km/s. For the shock wave propagation along the lower symmetry crystal orientation, like, <110> body-centered orthorhombic (BCO) phase has been observed to comprise the deformed microstructure at an impact velocity of 2.0 km/s. In <111> no such solid-to-solid phase transformation has been identified. Adaptive common neighbor analysis (CNA), pair-distribution function, g(r) and also x-ray diffraction pattern analysis techniques has been used as a probing tool to identify those transformed phases with sufficient accuracy. Details analysis and quantification of the nucleated phases will be discussed in this presentation.
The Microstructure Evolution of HAVAR Co-Base Alloy during Cold Rolling: Daniel Moreno1; Shlomo Haroush1; Louisa Meshi2; S Remmenick2; Vladimir Ezersky2; Ido Silverman1; Yaniv Gelbstein2; Roni Shneck2; 1Soreq - nrc; 2Ben-Gurion University
Correlation between mechanical properties and the hardening mechanisms of HAVAR, Co-base alloy was investigated by XRD, SEM, TEM and tensile tests. Annealed alloy dramatically changed due to cold rolled (CR) process inducing very high dislocation density, sub-grains, twins, two coexisting phases, matrix gamma (FCC) phase and epsilon (HCP) phase, stacking faults and cubic M23C6 carbides. The CR induces alternating wide parallel bands of gamma and epsilon phases. The width of these deformation bands is 100-500 nm. The epsilon bands have a uniform internal texture while the gamma bands contain very fine parallel striations of less than 1 nm width. HRTEM revealed intrinsic stacking faults. The strain induced transformation (SIT) forced some grains to create epsilon phase. The variety of planar defects and their high concentration due to cold work is the primary hardening mechanisms which can explain the increased of yield and ultimate tensile stresses by more than four times.