Date: Friday, May 14th
Presenter: Dr. Marcus Young,
Synchrotron X-ray diffraction sources offer the unique ability to directly measure lattice parameters and, therefore, internal elastic strains of all phases within the bulk (rather than near surface) of metal alloys. In this presentation, two different metal alloy systems (ultrahigh-carbon steels and pseudoelastic NiTi shape memory alloys) examined by synchrotron X-ray diffraction will be discussed.
The first of these alloys, ultrahigh-carbon steel (UHCS), can be classified as a metal matrix composite (MMC) consisting of spherical Fe3C particles in a Fe matrix. Metal matrix composites (MMCs) are of technological importance for a variety of applications. One important aspect of MMCs is their unique mechanical behavior, which is controlled by the load transfer occurring between matrix and reinforcement. Load transfer is affected by the mismatch in stiffness between matrix and reinforcement, by plastic deformation of the metallic matrix and by damage of the ceramic reinforcement or its interface with the matrix. In this presentation, the micromechanics of load transfer in this MMC using high-energy synchrotron x-rays in conjunction with in-situ tensile loading to failure will be discussed. Predictions from analytical models (based on rule-of-mixture) and numerical models (based on the finite-element method) will be compared with experimental strain measurements.
The second alloy is a pseudoelastic NiTi shape memory alloy (SMA), which has the unique ability to fully recover from deformation up to about 8% strain due to a phase transformation from austenite to martensite. Because of their good mechanical and functional properties, NiTi SMAs are currently used in a wide range of medical and technological applications. Here, an ultrafine-grained pseudoelastic NiTi SMA wire examined using synchrotron X-ray diffraction during in-situ uniaxial tensile loading and unloading will be presented. The macroscopic stress-strain curve will be discussed in light of phase volume fractions and lattice strains in various crystallographic directions in the austenitic B2, martensitic R, and martensitic B19? phases and it will be shown that the phase transformation occurs in a localized manner.