Mechanical Response of Materials Investigated through Novel In-situ Experiments and Modeling: Session II
Sponsored by: TMS Structural Materials Division, TMS: Thin Films and Interfaces Committee, TMS: Advanced Characterization, Testing, and Simulation Committee
Program Organizers: Saurabh Puri, VulcanForms Inc; Amit Pandey, Lockheed Martin Space; Dhriti Bhattacharyya, Australian Nuclear Science and Technology Organization; Dongchan Jang, Korea Advanced Institute of Science and Technology; Jagannathan Rajagopalan, Arizona State University; Josh Kacher, Georgia Institute of Technology; Minh-Son Pham, Imperial College London; Robert Wheeler, Microtesting Solutions LLC; Shailendra Joshi, University of Houston
Monday 2:00 PM
March 15, 2021
Room: RM 17
Location: TMS2021 Virtual
Session Chair: Amit Pandey, Lockheed Martin Space
2:00 PM
Low Temperature Failure Mechanism of [001] Niobium Micropillars under Uniaxial Tension: Seok-Woo Lee1; Gyuho Song1; Nicole Aragon2; Ill Ryu2; 1University Of Connecticut; 2University of Texas at Dallas
Micromechanical studies have shown that the plasticity of metals at the micrometer scale is size-dependent and is controlled by the intermittent operation of dislocation sources, which leads to new phenomena such as size-affected strength and jerky plastic flow. However, the effects of low temperature on source-controlled plasticity and its related failure mechanism have not been clearly understood, yet. In this work, our in-situ cryogenic micro-tensile tests, transmission electron microscopy, and dislocation dynamic simulations on bcc niobium micropillars reveal that the suppression of cross-slip at low temperature prevents dislocation multiplication, leading to dislocation starvation. Then, dislocation nucleation occurs until stress concentration at a slip step nucleates a crack, the propagation of which leads to catastrophic brittle failure. Thus, the competition between dislocation nucleation and crack nucleation controls a failure process. Our results provide an insight in understanding of a failure process of metal micropillars at low temperature under uniaxial tension.
2:20 PM
Quantifying Electron Beam Effects during In-situ TEM Nanomechanical Tensile Testing on Aluminum Thin Films: Sandra Stangebye1; Olivier Pierron1; Joshua Kacher1; 1Georgia Institute of Technology
Transmission electron microscopy (TEM) imaging relies on high energy electrons for atomic scale resolution, however, the electrons themselves interact with and may alter the material being imaged. Using an in situ TEM MEMS-based nanomechanical tensile testing technique, the effect of the electron beam is investigated while deforming nanocrystalline aluminum thin films. Samples tested with repeated e-beam exposure (30 seconds exposed and 30 seconds no exposure) resulted in beam-induced stress relaxation and a three-time increase in plastic strain rate when the specimen is exposed to the e-beam. The observed effect was seen at 80 and 300 kV TEM accelerating voltages indicating that the effect is not due to knock on damage. True activation volume measurements increase on average from 9b3 with the beam on to 13b3 with the beam off. The differences in activation volume provides evidence that the e-beam may be changing the active deformation mechanism.
2:40 PM
Deformation Tests of Al Thin Films Using In-situ TEM and Molecular Dynamics Simulations: Lucia Bajtosova1; Rostislav Králik1; Barbora Krivská1; Jozef Veselý1; Jan Fikar2; Miroslav Cieslar1; 1Charles University; 2Ústav Fyziky Materiálů AV ČR, v.v.i.
Thin metallic films are common components used in fabrication of micromechanical systems with expanding use in various areas of applications including consumer electronics, automotive, medical or communication applications. The materials used in these components are frequently subjected to a range of operating velocities, frequencies and accordingly high strain rates during vibrations. Therefore it is necessary that the mechanical properties of these materials meet strict requirements and they need to be thoroughly tested. Small scale localized material properties can be revealed by in-situ deformation testing in transmission electron microscopes using a holder with a fully integrated picoindenter. This method allows direct observation of nano-scaled samples during mechanical measurements and simultaneous acquisition of load-displacement curves. Molecular dynamics simulations were used to predict the behavior of aluminum-based thin films during deformation experiments and results were compared with corresponding experimental results.
3:00 PM
In-situ TEM Investigation of the Electroplasticity Phenomenon in Ti-6Al: Xiaoqing Li1; Shiteng Zhao2; John Turner2; Karen Bustillo2; Rohan Dhall2; Andrew Minor1; 1University of California, Berkeley; 2Lawrence Berkeley National Laboratory
Electroplasticity is a phenomenon in which applied pulsed electric fields during deformation result in reduced flow stress and increased formability in metals. In this work, in situ TEM electromechanical tests of Ti-6Al tensile samples on electrical push-to-pull devices were performed in order to correlate direct observations of nanostructure change with both mechanical data and applied electrical. By analyzing the frame-by-frame videos with the recorded mechanical and electrical data, we can compare the deformation mechanisms and dislocation behaviors during the pulsing period. We found, upon applying electrical current, many slip directions in the HCP system are activated and dislocation nucleation is less concentrated spatially, which indicating the effect of electrical current can make the plastic deformation on the metal piece more uniform. More theoretical analysis about the short-range order rearrangement by applied electrical current and local lattice expansion measurement will be discussed to evaluate Joule heating effect from electron wind effect.
3:20 PM
Giant Superelasticity in SrNi2P2 Micropillars via Lattice Collapse and Expansion: Shuyang Xiao1; Vladislav Borisov2; Guilherme Gorgen-Lesseux3; Gyuho Song1; Roser Valentí2; Paul Canfield3; Seok-Woo Lee1; 1University Of Connecticut; 2Goethe University; 3Iowa State University
An elastic strain limit of most crystalline solids is less than one percent because the permanent shape change usually occurs at a very small strain. In order to obtain a large elastic strain limit, crystalline solids need to undergo a reversible structural transition. In this work, we show that a SrNi2P2 single crystal micropillar exhibits the ultrahigh compressive elastic strain limit over 17% via double lattice collapse and expansion. High-resolution transmission electron microscopy revealed the co-existence of two different crystal structures, and density functional theory shows that each structure is collapsed at a different stress state. This superelastic deformation process is repeatable over 104 cycles. In comparison to other superelastic crystalline solids, the elastic strain limit of SrNi2P2 is nearly the largest ever, and this result suggests that a new group of superelastic materials could be potentially discovered in ThCr2Si2-structured intermetallic compounds that can exhibit the similar structural transition.
3:40 PM
Ripplocations: A Novel Deformation Mechanism in Layered Crystalline Solids: Hussein Badr1; 1Drexel University
When basal planes of layered crystalline solids, LCS - such as graphite, mica, phyllites or the MAX phases, among others - are loaded edge-on in compression, they typically fail by kink bands formation. The latter were long assumed to be caused by basal dislocations. Recently we made the case that bulk ripplocations are the operative deformation mechanism in most LCSs. In other words, we have been making the case that atomic layers, like other layered systems such as playing cards, laminated composites, etc.- deform by constrained buckling. Using HRTEM, we and others, presented direct evidence for bulk ripplocations in Ti3SiC2, phyllosilicates and graphite. We also showed that ripplocations are the key mechanism for kinking non-linear elasticity and for c-axis strain exhibited by LCSs. We further applied a folding mechanics model that appears to be valid over ten orders of magnitude in scale.