Zhongyuan and Shuyang attended the MRS Fall 2022!
Multi-Stage Superelasticity in SrNi2P2 Intermetallic Compound via Lattice Collapse and Expansion and the Influence of Cryogenic Temperature
Shuyang Xiao, Vladislav Borisov, Adrian Valadani, Guilherme Gorgen-Lesseux, Roser Valenti, Paul Canfield, Seok-Woo Lee
Abstract: Superelastic crystalline solids are able to recover their original shape even after the large amount of deformation is applied. The shape recovery of most superelastic crystalline solids usually occurs through a reversible phase transition between martensitic and austenitic phase. Although this shear process provides up to ~10% of elastic strain, there are several issues that limit their elastic performance. Usually, this shear process requires high activation temperature to reverse the phase transition. So, it is difficult to exhibit superelasticity at cryogenic temperature. Also, the shear process often drives dislocations to move and causes large accumulation of dislocations, which eventually prevent the reversal of phase transition. Therefore, it is desirable to seek out a new class of superelastic material that can overcome these issues through a completely different superelasticity mechanism. In this presentation, we report our first discovery of the giant superlelasticity in a novel intermetallic compound SrNi2P2 via a unique phase transition process, lattice collapse-expansion and the influence of cryogenic temperature.
SrNi2P2 single crystals were grown using solution-growth technique, and in-situ uniaxial compression and tension tests on [001]-oriented SrNi2P2 micropillars, which were fabricated using focused-ion-beam milling, were conducted at room temperature, 200K, and 100K. Room temperature mechanical testing revealed the giant superelasiticity with a remarkably huge elastic strain limit near 20% if both compression and tension are considered. Up to our knowledge, this giant elastic strain corresponds to one of the highest elastic strain limits ever reported for crystalline solids. This excellent superelastic performance is achieved through multiple-stage lattice collapse and expansion under compression and tension by forming and breaking P-P bonds in two co-existing different crystal structures in SrNi2P2. Based on Density Function Theory (DFT) calculation and High-Resolution Transmission Electron Microscope (HRTEM), P-P bonds are formed or broken partially depending on the stress state, leading to the multi-stage lattice collapse and expansion. Fraction of P-P bonds seems to be controlled primarily by entropy under a given stress, so the total number of P-P bonds is determined by thermodynamics under a give stress and temperature. Unlike shear process of martensite-austenite transition, no dislocation is involved in the superelasticity process of SrNi2P2. Simple making and breaking bond process seems to guarantee ultrahigh fatigue life with presumably nearly infinite number of cycles. Our state-of-the-art in-situ cryogenic nanomechanical testing revealed that forming and breaking P-P bonds is strongly sensitive to cryogenic temperature. At lower temperature, phase transition becomes easier under compression but more difficult under tension. This difference can be explained by thermal contraction. Due to the thermal contraction at lower temperature, which reduces P-P distance, making P-P bonds becomes easier, but breaking P-P bonds becomes more difficult. One noticeable result is that even under cryogenic environment (~100K), SrNi2P2 still exhibit superelasticity, implying that SrNi2P2 is capable of shape recovery at very low temperature unlike conventional superelastic crystalline solids such as Ni-Ti intermetallic compounds. Our experimental and computational results provide a fundamental insight into understanding the superelasticity mechanisms of SrNi2P2. Furthermore, our work suggests its strong potential as a cryogenic superelastic material that could be useful to develop an impact resistance material for aerospace engineering under cryogenic environment.
Micro-Mechanical Characterization on Amorphous Carbon and Its Nanoporous Structures
Zhongyuan Li, Ayush Bhardwaj, James Watkins, Seok-Woo Lee
Abstract: Carbon-related nanostructured materials have a strong potential as a mechanical reinforcement material due to their strong ionic/covalent C-C bond, which restricts the motion of dislocation, the carrier of plasticity. However, their brittleness and relatively high density limits their structural application. In this study, therefore, we have developed amorphous carbon with almost no short-range ordering and its nanoporous structures, where the ligand thickness is only around 10nm, by rapid thermal annealing of Brush block copolymer and Phenol formaldehyde. To evaluate the mechanical response, we perform the nanoindentation tests and in-situ compression tests on a micropillar, which was fabricated by utilizing focused-ion-beam milling. Both fully dense and nanoporous structures exhibit a large compressive fracture strain up to ~40% with a significantly high work hardening rate. Raman spectroscopy before and after compression test revealed that this unusual plasticity results from dynamic change in distribution of sp2 and sp3 bonds. During plastic deformation, the number of sp2 bonds decreases but the number of sp3 bonds increases. Carbon atoms, which have sp2 bonds, seem to be reconfigured to form sp3 bond, which is thermodynamically preferable under compression. In addition, we found that both compressive fracture strength and compressive maximum strength are nearly independent of porosity unlike the hardness that scales with the porosity. We think that the porosity-independent strength could be related to the size-affected strength of nanoscale ligand. Larger pores reduce the density, but the size reduction of ligand enhances its strength. These two contributions compensate each other, leading to the negligible change in strength of the entire nanoporous structure. Interestingly, all structures, both fully dense and nanoporous structures, in our study demonstrate the modulus of resilience much higher than most engineering materials due to their low Young’s modulus, implying that amorphous carbon materials have an exceptional capability to absorb and release the mechanical energy per a given volume. However, yield strength of our fully dense amorphous carbon was found to be slightly lower than that of amorphous carbon published in other works. Complete diffuse diffraction pattern indicates that our amorphous carbon possesses the almost complete random arrangement of carbon. This liquid-like structure could be beneficial to lower mass density but does not seem to be desirable to obtain high strength. It would be necessary to control the distribution of a short-range ordering to obtain the improved yield strength without sacrificing the mass density and the ductility much by optimizaing the synthesis condition or performing the post thermal-mechanical treatment. All these efforts will pave a new pathway to create light, strong, and tough carbon nanostructures for their future structural applications.
