Shock Compression of Body-Centered Cubic Metals from the Atomistic to Continuum Scale: Iron and Molybdenum

Citation

Gandhi, Vatsa Bhupeshkumar (2023) Shock Compression of Body-Centered Cubic Metals from the Atomistic to Continuum Scale: Iron and Molybdenum. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/kwf1-7y79. https://resolver.caltech.edu/CaltechTHESIS:05052023-185856720

Abstract

Fundamental understanding of material behavior under extreme conditions is crucial for designing high strength, light weight, and high temperature resistance materials, and for modeling planetary physics problems such as behavior of the core and impact phenomena. Under extreme conditions, materials not only exhibit a different mechanical, thermal, and failure response but can also undergo structural changes, such as phase transformations, which significantly alters their material properties. This motivates studying their dynamic response and developing constitutive models for applications such as hypersonics, high speed manufacturing, impact and blast of structures, aircraft and spacecraft shielding, meteorite impact, and collision of planets. Despite the importance, experimental investigations of shock induced phase transitions, inelastic material behavior, and elastic-plastic anisotropy under multi-axial stress states and at microscopic length scales of metals still remains largely unexplored. Thus, the focus of this thesis is on the shock compression behavior of body-centered cubic (BCC) metals, specifically iron and molybdenum, under compression-shear loading and at the atomistic-continuum spatial scales. In particular, the role of solid-solid phase transformation of body-centered cubic (BCC) iron on material strength and the orientation dependence of single crystal molybdenum on its elastic-plastic transition is investigated.

Iron in its high pressure hexagonal close-packed (HCP) ϵ-phase is critical in geological and planetary applications such as inner cores of rocky planets and hypervelocity impacts of asteroids, and meteorites. Thus, understanding plasticity behavior of iron under these condensed matter states is important to develop more accurate models for such applications and to understand deformation mechanisms of inner planetary cores. Because the ϵ-phase is unstable, iron reverts to its ambient α-phase (BCC) upon release making it difficult to probe the strength behavior using conventional methods. Additionally, solid-solid phase transformations provide a unique opportunity to study material strength as they are crucial for expanding the design space for various load-bearing applications. In the first part of the thesis, the pressure dependent dynamic strength behavior of both the ambient BCC α-phase and high-pressure HCP ϵ-phase of iron at strain rates on the order of 1 X 10⁵ s⁻¹ and pressures up to 42 GPa is investigated. Pressure shear plate impact experiments are conducted using a sandwich configuration to decouple the effect of pressure and shear thereby allowing to probe shear strength once the sample reaches an equilibrated state of pressure but prior to release. The strength of the ϵ-phase is observed to be more than double the strength of α-phase possibly due to microstructural evolution during phase transformation. Additionally, the evolution of yield properties with pressure, temperature, and strain is presented for the first time, enabling more accurate modeling of extreme deformation phenomena associated with iron-rich celestial bodies such as planetary collisions.

Molybdenum, its alloys, and other body-centered cubic (BCC) refractory metals are critical in geological and planetary applications such as structural properties of terrestrial planetary composition, formation of the earth-moon system, and hypervelocity impacts of rocky planets. Additionally, the high temperature specific strength, creep resistance, and ductility of BCC refractory metals make them ideal for aerospace and armor/anti-armor applications. Under high strain-rate inelastic loadings, the macroscopic response of these metals is often influenced by the atomistic mechanisms including dislocation motion and deformation twinning. Current material models rely on investigations that involve continuum measurements followed by postmortem microstructural analysis of recovered samples. However, these may not reflect the material behavior during the passage of the shock wave and, thus, requires real-time in-situ atomistic characterization to link the microstructure to macroscopic response. In the second part of the thesis, plate impact experiments coupled with both laser interferometry continuum measurements and in-situ dynamic Laue x-ray diffraction (XRD), at the Advanced Photon Source (APS), are conducted on single crystal molybdenum. Here, the role of crystal orientation, either [100] or [111], on deformation mechanisms during the elastic-plastic transition and the steady state response is explored at pressures ranging from 9-19 GPa. Complementary simulation methodology is developed to analyze the evolution of the Laue diffraction spots captured during impact. By extracting the lattice strain and stresses from XRD images, dislocation slip along [110]〈111〉 and [112]〈111〉 is found to be the probable deformation mechanism during compression with negligible anisotropy observed at the Hugoniot state. For the first time, real-time evidence of molybdenum undergoing deformation twinning along [112̅]〈111〉 during shock release beyond a critical pressure of 16 GPa irrespective of the loading orientation is presented.

Thesis Files