Phase Transformations and Microstructural Evolution: Shape Memory Alloys, and Lightweight Metals Al & Mg
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 8:30 AM
February 28, 2017
Room: 16B
Location: San Diego Convention Ctr

Session Chair: Raju Ramanujan, Nanyang Technological University

8:30 AM  
Phase Field Modeling of Functional Fatigue in Shape Memory Alloys: Yipeng Gao1; Yunzhi Wang1; 1The Ohio State University
    Functional fatigue during thermal and mechanical cycling, which leads to the generation of macroscopic irrecoverable strain and the loss of dimensional stability, is a critical issue that limits the service life of shape memory alloys. By analyzing symmetry breaking during austenite ⇔ martensite transformation cycling, we constructed a phase field model to reveal the physical origin of functional fatigue. The types of defects generated during the transformation cycling are predicted through a combination of crystallography analysis and phase field simulations. They include lattice dislocations and Σ grain boundaries that agree well with experimental observations in typical shape memory alloys. Based on the simulation results, strategies to improve functional fatigue resistance are suggested, and a general design criterion for the balance of functionality and durability of shape memory alloys is proposed.

8:50 AM  
In Situ X-ray Diffraction Investigation of Thermally Induced Martensitic Transformations in High Temperature Shape Memory Alloys: Mohammed Azeem1; Vassili Vorontsov2; Nicholas Jones3; Seema Raghunathan2; David Dye2; 1Manchester University; 2Imperial College London; 3University of Cambridge
    Understanding martensitic transformation pathways and the response of lattice in participating phases en route in a thermally induced setup is essential for optimizing functional stability in shape memory alloys (SMAs). In this investigation we use synchrotron X-ray diffraction to observe the lattice evolution in polycrystalline NiTi, ZrCu and ferromagnetic NiMn based high temperature SMAs. During the course of thermally induced transformation, large strains upto 1.5% were observed in both austenite and martensite. These strains were least in the NiMn based alloys, which also showed a remarkably low thermal hysteresis. The pre-transformation thermal expansion/contraction of martensite lattice along each crystallographic axes was different and a negative coefficient of thermal expansion (CTE) was observed along at least one crystallographic axis in the martensite phase in each system. In case of ferromagenetic NiMn alloys CTE was observed to change near the Curie temperature. Evolution of transformation temperatures and associated hysteresis will be discussed.

9:10 AM  
Martensitic Transformation near Grain Boundaries and PhaseBboundaries in Dual-phase Shape Memory Alloys: Ying Chen1; Rebecca Dar1; 1Rensselaer Polytechnic Institute
    Many polycrystalline Shape Memory Alloys (SMAs) are limited by their inherent intergranular brittleness during martensitic transformation. We focus on a dual-phase SMA in which a ductile non-transforming second phase is precipitated along grain boundaries to plastically accommodate transformation strain in its adjacent volume and alleviate stress concentration. We study strain recovery at austenite/precipitate interfaces as well as at bare grain boundaries using nanoindentation and resolve orientation influence using EBSD mapping. Austenite volume adjacent to its interface with precipitate has the highest strain recovery of all regions tested, while bare grain boundaries exhibit the lowest shape recovery. These preliminary results suggest transformation-interface interactions depend on the interface type, and optimizing austenite/precipitate morphology is promising to enable applications of polycrystalline SMAs with high superelastic recovery and enhanced transformation ductility.

9:30 AM  
Origin of the {332}<113> Twinning System in β Titanium Shape Memory Alloys: Emmanuel Bertrand1; Philippe Castany2; Yang Yang2; Thierry Gloriant2; 1Institut de Matériaux Jean Rouxel (IMN); 2INSA de Rennes
    Superelastic β titanium and α’’ shape memory alloys are subject to numerous deformation mechanisms. The origin of superelastic effect comes from a stress-induced β to α’’ transformation while shape memory effect comes from the selfaccomodating α’’ microstructure which is reoriented when a stress is applied. These alloys are also subject to classical deformation mechanisms such as mechanical twinning and dislocation gliding. {332}<113> twinning system is specific of metastable titanium alloys and is not observed in stable bcc structures. Electron backscattering diffraction, transmission electron microscopy, X-ray diffraction and in situ synchrotron X-ray diffraction characterization are used to discuss the correlation between this specific twinning system and the stress-induced phase transformation. A new twinning system in α’’ microstructure is identified and compared with {332}<113> twinning system.

9:50 AM  
Modeling the Superelastic Behavior in Small-scale ThCr2Si2-type Crystals: Ian Bakst1; John Sypek2; Hang Yu3; Paul Canfield4; Seok-Woo Lee2; Christopher Weinberger1; 1Colorado State University; 2University of Connecticut; 3Drexel University ; 4Iowa State University
    Crystals of the ThCr2Si2-type structures have long been investigated for superconductivity. Recently nano-indentation experiments have shown that small-scale crystals of CaFe2As2 can exhibit superelastic behavior with recoverable strains of over 10%. In this talk, we demonstrate that this behavior can be attributed to the phase transition inherent to these materials. Using density functional theory in conjunction with analytical models, we are then able to demonstrate that the superelastic behavior depends on the type of loading and that is confined to uniaxial loading. The behavior of CaFe2As2 is then compared to LaRu2P2 which highlighting differences in their superelastic responses. These responses are compared to recent experimental results.

10:10 AM Break

10:30 AM  
Strengthening Model to Optimize Coarsening Resistant Q and θ’-Phase Precipitates in Al-Si-Mg-Cu Cast Alloys: Andrew Bobel1; Mike Walker2; Greg Olson1; 1Northwestern University; 2General Motors
    The age hardening behavior of heat-treatable Al-Si-Mg-Cu cast alloys is complicated by the various strengthening precipitate morphologies that exist, while optimization of these aluminum alloys is convoluted by the competition between Q-phase rods and θ’-phase plates. Analysis of the effect of particle morphology on strength enhancement has been undertaken by modification of the Orowan-Ashby equation for <100> oriented finite rods in addition to an appropriate version for similarly oriented plates. A coarsening model was derived using machine learning software based on 3-D atom-probe tomography (3DAPT) and TEM measurements of Q-phase rods in both wrought and cast aluminum alloys. Strengthening model predictions of tensile yield strength are in excellent agreement with experimental observations over a wide range of aluminum alloy systems. The developed models allow predicting the required particle morphology and volume fraction necessary to achieve target property goals in the design of future aluminum alloys.

10:50 AM  
Structure, Mechanical Properties and Corrosion Behavior in a Powder-processed Icosahedral-phase-strengthened Aluminum Matrix Nano-composite: Mark Aindow1; Benjamin Bedard1; Iuliana Cernatescu2; Alexis Ernst1; Mauricio Gordillo3; Aaron Nardi4; Thomas Watson2; 1University of Connecticut; 2Pratt and Whitney; 3FEI Corporation; 4United Technologies Research Center
    Nanocomposite powder particles of aluminum with dispersed icosahedral quasicrystals were produced by gas atomization from an Al-Cr-Mn-Co-Zr alloy. Bulk dispersion-strengthened material was obtained from the powder by blind-die compaction and forging. The material exhibited an attractive combination of room temperature mechanical properties with a dynamic elastic modulus of 90.5 GPa, a tensile yield strength of 690 MPa with 6% elongation to failure, and a high cycle fatigue life of 109 cycles at an alternating stress of 207 MPa. The material also exhibited significant potential for elevated temperature applications with a modulus of 75 GPa and yield strength of 400 MPa at 300˚C. The material also exhibits a resistance to pitting corrosion in saline environments that is superior to that of other high-strength Al alloys. Preliminary data obtained from cold spray trials using the nanocomposite powder have shown that these mechanical and chemical properties can also be exploited in coatings.

11:10 AM  
The Effect of Aluminum Content on Recrystallization and Grain-growth of Magnesium Alloys: Aeriel Murphy1; John Allison1; 1University of Michigan
    Grain size has a significant influence on mechanical properties of magnesium alloys and thus recrystallization and grain growth are important microstructural evolution processes in these materials. Static recrystallization, grain-size, and texture evolution have been investigated using Electron Back-Scatter Diffraction (EBSD) in pure Mg, Mg-4Al and Mg-9Al. Static recrystallization, grain growth and texture evolution have been characterized on uniaxially compressed cylinders after annealing at temperatures ranging from 150 to 400°C for 1 to 240 minutes. The level of dynamic recrystallization present in the as-received extruded condition was also characterized in these alloys. The influence of composition, twinning, and grain orientation on static recrystallization and grain growth kinetics have been quantified and will be discussed in this presentation.

11:30 AM  
Mg-Sc-based Alloy and Its Functionality: Daisuke Ando1; Yukiko Ogawa; Yuta Takeuchi1; Yuji Sutou; Junichi Koike1; 1Tohoku University
    Mg alloys have been expected as a next generation structural material for decade years. However, because of low formability, low corrosion resistance and high cost, Mg alloys have not been used widely yet. Therefore, in order to break the wall, our group have attempted to add some functionality, such as high strength, super-elasticity and shape memory effect into Mg alloys using metastable BCC phase in Mg-Sc alloys. This alloy shows ultra-high strength after aging due to fine HCP precipitation from BCC matrix. Furthermore, the alloys show super-elasticity of 4.4% at -150°C and shape recovery upon heating. The shape memory properties are caused by reversible martensitic transformation. Its density is around 2 g/cm3, which is one-third less than that of practical TiNi Shape memory alloy. The study shows a possibility to use metastable BCC phase for novel microstructural control and adding functionality into Mg alloys.