Bartlomiej Winiarski

Laser shock peening on Zr-based bulk metallic glass and its effect on plasticity: Experiment and modeling

The Zr-based bulk metallic glasses (BMGs) are a new family of attractive materials with good glass-forming ability and excellent mechanical properties, such as high strength and good wear resistance, which make them candidates for structural and biomedical materials. Although the mechanical behavior of BMGs has been widely investigated, their deformation mechanisms are still poorly understood. In particular, their poor ductility significantly impedes their industrial application. In the present work, we show that the ductility of Zr-based BMGs with nearly zero plasticity is improved by a laser shock peening technique. Moreover, we map the distribution of laser-induced residual stresses via the micro-slot cutting method, and then predict them using a three-dimensional finite-element method coupled with a confined plasma model. Reasonable agreement is achieved between the experimental and modeling results. The analyses of serrated flows reveal plentiful and useful information of the underlying deformation process. Our work provides an easy and effective way to extend the ductility of intrinsically-brittle BMGs, opening up wider applications of these materials.

Microyielding of Core-Shell Crystal Dendrites in a Bulk-metallic-glass Matrix Composite

In-situ synchrotron x-ray experiments have been used to follow the evolution of the diffraction peaks for crystalline dendrites embedded in a bulk metallic glass matrix subjected to a compressive loading-unloading cycle. We observe irreversible diffraction-peak splitting even though the load does not go beyond half of the bulk yield strength. The chemical analysis coupled with the transmission electron microscopy mapping suggests that the observed peak splitting originates from the chemical heterogeneity between the core (major peak) and the stiffer shell (minor peak) of the dendrites. A molecular dynamics model has been developed to compare the hkl-dependent microyielding of the bulk metallic-glass matrix composite. The complementary diffraction measurements and the simulation results suggest that the interface, as Maxwell damper, between the amorphous matrix and the (211) crystalline planes relax under prolonged load that causes a delay in the reload curve which ultimately catches up with the original path.

Novel implementations of relaxation methods for measuring residual stresses at the micron scale

The advent of dual-beam focused ion beam - field emission gun scanning electron microscope (FIB-SEM) imaging and milling systems in combination with digital image correlation (DIC) analysis has made it possible to make very fine excisions and to record the resulting displacements with nanometre precision. This has led to novel implementations of macroscale residual stress measurement methods to the microscale, for example, micro-slotting, micro-hole drilling and so on. This review article outlines the different methods for residual stress mapping at the micron scale highlighting their relative merits and limitations with the aim of guiding the user towards the most appropriate techniques for a given application. These open new possibilities for residual stress measurement not just at the continuum scale but even for mapping stresses within grains. In addition, recent advances in milling throughput promised by new dual-beam plasma focused ion beam systems promise to help bridge the gap between the conventional macroscale methods and their micron-scale counterparts.

Developments in Large Volume 3D Analysis via P-FIB: EBSD & EDS

The integration of electron backscattered diffraction (EBSD) and energy dispersive spectroscopy (EDS) on scanning electron microscopes (SEM) is an increasingly common method of characterising materials. EDS offers chemical quantification and spatial distribution of the elements whilst EBSD enables microstructural characterisation. The integration of these two techniques with simultaneous acquisition, as in the AZtec platform, enables full material characterisation and data correlation within a single user interface.

On the Three-Dimensional Microstructure of Martensite in Carbon Steels

The mechanical properties of high-performance steels are often reliant on the hard martensitic structure. It can either be the sole constituent e.g. in tool steels, or it can be part of a multi-phase structure as e.g. in dual-phase steels. It is well-known that the morphology of martensite changes from lath to plate martensite with increasing carbon content. The transition from lath to plate is however less known and in particular the three-dimensional (3D) aspects in the mixed lath and plate region require more work. Here the current view of the 3D microstructure of martensite in carbon steels is briefly reviewed and complemented by serial sectioning experiments using a focused ion beam scanning electron microscope (FIB-SEM). The large martensite units in the Fe-1.2 mass% C steel investigated here are found to have one dominant growth direction, less transverse growth and very limited thickening. There is also evident transformation twinning parallel to the transverse direction. It is concluded that more 3D analysis is required to understand the 3D microstructure of martensite in the mixed lath and plate region and to verify the recently proposed 3D phase field models of martensite in steels.