Phase transformation-related phenomena under compression and shear of ceramics
Abstract
One of the ArmyÕs main research directions in strong ceramics is to achieve unprecedented energy absorption and stress relaxation under intense dynamic loadings. The goal of this project is to search for, study, and explore new phase transformation related material properties and phenomena under high pressure and shear of ceramics that lead to enhanced energy absorption and stress relaxation. In particular, to advance multiple experimental, theoretical, and numerical approaches and study of three recently revealed but unexplored phenomena: Transformation-induced plasticity (TRIP) phenomenon revealed for phase transformation (PT) from hexagonal hBN to superhard wurtzitic wBN. A cascading mechanism of structural changes revealed for PT hBNÀwBN. PT induced by rotational plastic instability revealed for PT from rhombohedral rBN to superhard cubic cBN. Due to large transformation strain and high pressure, the energy absorption for these PT is estimated to be more than two orders of magnitude larger than for PT in shape memory alloys. If small hBN particles are embedded in strong ceramics, the PT in them will absorb the estimated energy serving as a mechanism of inelastic deformation, thus, increasing toughness (transformation toughening) and relaxing stresses. In-situ synthesis of superhard BN during the PT drastically increases the strength of composite ceramics. To achieve our goal, the following objectives will be completed: 1. Perform an experimental study of plastic flow and PTs with emphasis on the above phenomena in hBN and rBN with different initial degree of two-dimensional disordering under compression and shear in rotational diamond anvil cell (RDAC) under various pressure-shear loading paths. Perform similar exploratory studies for PT from hexagonal and rhombohedral graphite to hexagonal and cubic diamond and for BN and graphite within SiC matrix. In-situ xray diffraction (XRD) with synchrotron radiation will be used to study kinetics of disordering and PTs to superhard BN and carbon phases. 2. Develop an advanced nanoscale phase field approach (PFA) for interaction between dislocation plasticity and stress-induced PT in BN and graphite, and apply this PFA to a finite element method (FEM) simulation of nucleation and evolution of superhard nanostructure. 3. Develop a microscale approach to coupled strain-induced PTs and disordering based on micromechanics treatment and nano to micro transition. 4. Develop a macroscale model for coupled plastic flow, PTs, and disordering, as well as FEM algorithms and subroutines for implementation of this model. 5. Calibrate, verify, and iteratively improve the nano, micro, and macroscale models based on RDAC experiments and known atomistic and experimental data. Perform a FEM study of coupled stress-strain fields, strain-induced PTs, and disordering at the macroscale in a sample under various compression and shear loadings in RDAC. A comparison with RDAC experiments and iterative model improvements will allow us to determine PT criteria, kinetics of PTs and disordering, and pressure and disordering-dependent yield strength. Calibrated models will be used for planning experiments, modeling TRIP, cascading structural changes, and PT induced by rotational plastic instabilities, and evaluate energy absorption and stress relaxation. We will also seek for and study new strain-induced phases, phenomena, and unprecedented material properties.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Oct 16, 2018
- Source ID
- W911NF1710225
Entities
People
- Valery I. Levitas
Organizations
- Army Contracting Command
- Iowa State University
- United States Army