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Metal additive manufacturing (AM) techniques are powder-based, layer by layer methods which can directly build 3D structures onto substrates with complex geometries. They offer a unique ability to dynamically mix materials during the deposition process and produce functionally graded structures, new composite microstructures and perhaps even new material classes. Some of the challenging issues related to the energy beam based process are the very high heating and cooling rates, leading to non-equilibrium microstructures, which are usually harder, less ductile, and often exhibit high residual stresses; the strongly textured, anisotropic microstructures inherited from the solidification conditions; or the pronounced residual stresses resulting from the large thermal gradients in the AM fabricated parts. However, the very rapid consolidation of the material in a small material volume and the achieved high solidification rates allow for the manufacture of components containing meta-stable materials. In this talk some relevant results of the AM related research at Empa will be presented. The first part of the presentation deals with the development and characterization of a novel oxide dispersion strengthened (ODS) titanium aluminide alloy (Ti-45Al-3Nb ODS) for beam-based AM processes. The alloy design and selection process was supported by computational thermodynamics based on the CALPHAD approach, taking into account requirements for processing as well as long term alloy behavior under service conditions. Besides, an in situ method to study the behavior of alloys during rapid heating and cooling combining laser heating with synchrotron micro X-ray diffraction (microXRD) and high-speed imaging was developed. In the second part, the feasibility of producing metal-diamond composites by SLM was studied. A Cu-Sn-Ti alloy powder was mixed with 10-20 vol.% artificial, mono-crystalline diamonds. The influence of the processing parameters on the density and microstructure of the composites as well as on the stability of the diamonds was studied. It was shown that stable specimens containing intact diamonds could be produced. [more]

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Developing materials for optical and electrical applications often requires an understanding of the relationships between processing and point defects. Atomic scale relationships such as these are frequently elusive to understand due to a lack of characterization techniques. In this work, I will show examples of nanoscale and atomic scale characterization of oxide and semiconductor point defects determined through atom probe tomography (APT). Specifically, oxygen stoichiometries in oxygen and proton conducting oxides can be directly related to the electrical conductivities where grain boundaries dominate transport. Laser assisted APT also allows for unique opportunities for measuring atomic diffusion where thermal transport can assist transformations from metastable states. Using a “Dynamic” atom probe, atomic scale diffusion can be quantified at the atomic scale in 3-dimensions using a combination of APT and in-situ electron diffraction with a temporal resolution of better than 1 ns. An in-situ STEM / APT instrument engineered and constructed at CSM will be detailed as well as opportunities for using such data for improved APT data reconstruction. [more]

Thermodynamically consistent phase field approach (PFA) for multivariant martensitic phase transformations (PTs) and twinning for large strains is developed [1,2]. A thermodynamic potential is introduced, which allowed us to describe each martensite-martensite (i.e., twin) interface with a single order parameter [3]. These theories are utilized for finite element simulation of various important problems [1-4]. PFA to dislocation evolution was developed during the last decade and it is widely used for the simulation of plasticity at the nanoscale. Despite significant success, there are still a number of points for essential improvement. In our work [5,6], a new PFA to dislocation evolution is developed. It leads to a well-posed formulation and mesh-independent solutions and is based on fully large-strain formulation. Our local potential is designed to eliminate stress-dependence of the Burgers vector and to reproduce desired local stress-strain curve, as well as to obtain the mesh-independent dislocation height H for any dislocation orientation. The gradient energy contains an additional term, which excludes localization of dislocation within height smaller than H but disappears at the boundary of dislocation and the rest of the crystal; thus, it does not produce interface energy and does not lead to a dislocation widening. Problems for nucleation and evolution of multiple dislocations along the multiple slip systems are studied. The interaction between PT and dislocations is the most basic problem in the study of martensite nucleation and growth. Here, a PFA is developed to a coupled evolution of martensitic PTs and dislocations [7,8], including inheritance of dislocation during direct and reverse PTs. It is applied to studying the hysteretic behavior and propagation of an austenite-martensite interface with incoherency dislocations, the growth and arrest of martensitic plate for temperature-induced PTs, the evolution of phase and dislocation structures for stress-induced PTs, and the evolution of dislocations and high pressure phase in a nanograined material under pressure and shear [7-9]. In particular, possibility to reduce PT pressure by an order of magnitude, obtained in our experiments on BN, was confirmed in simulations. Short review of PFAs to other structural changes will be made, including melting of nanoparticles, superheating with ps and fs lasers, interface stresses and nonequilibrium energy, and PT between two solids via intermediate melt within solid-solid interface. 1. V. I. Levitas, V. A. Levin, K. M. Zingerman, & E. Freiman, Phys. Rev. Lett. 103, 025702 (2009). 2. V. I. Levitas, Int. J. Plasticity 49, 85-118 (2013). 3. V. I. Levitas and A. M. Roy, Phys. Rev. B 91, 174109 (2015). 4. V. A. Levin, V. I. Levitas, K. Zingerman & E. Freiman, Int. J. Solids & Struct. 50, 2914-28 (2013). 5. V. I. Levitas and M. Javanbakht, Phys. Rev. B., Rapid Commun. 86, 140101 (2012). 6. V. I. Levitas and M. Javanbakht, J. Mech. Phys. Solids, DOI: 10.1016/j.jmps.2015.05.009 (2015). 7. V. I. Levitas and M. Javanbakht, J. Mech. Phys. Solids, Parts 1 and 2, DOI: DOI:10.1016/j.jmps.2015.05.005 and DOI:10.1016/j.jmps.2015.05.006 (2015). 8. V. I. Levitas and M. Javanbakht, Appl. Phys. Lett. 102, 251904 (2013). 9. V. I. Levitas and M. Javanbakht, Nanoscale 6, 162 - 166 (2014). [more]

The austenite-ferrite transformations are a key metallurgical tool to tailor properties of advanced low-carbon steels. Even though significant progress has been made to develop knowledge-based process models for the steel industry it remains critical to improve the predictive capabilities of these models by developing next generation modelling approaches with a minimum of empirical parameters. Computational materials science now offers tremendous opportunities to formulate microstructure evolution models containing fundamental information on the underlying atomistic mechanisms that can be implemented across different length and time scales. The phase transformation kinetics depends critically on interface migration rates which are significantly affected by the presence of alloying elements, e.g. Mn, Mo and Nb in steels. Here, an approach is illustrated that links atomistic scale models for the solute-interface interaction with phase field modelling and conventional diffusion models. The overall status of this multi-scale phase transformation model approach will be analyzed for intercritical annealing of dual-phase steels and the rapid heat treatment cycles in the heat affected zone of linepipe steels. [more]

The processing regime relevant to superplasticity in the Ti-6Al-4V alloy is identifed. The effect is found to be potent in the range 850 to 900 deg ?C at strain rates between 0.001/s and 0.0001/s. Within this regime, mechanical behaviour is characterised by steady-state grain size and negligible cavity formation; electron backscatter diffraction studies confirm a random texture, leaving grain boundary sliding as the overarching deformation mechanism. Outside of the superplastic regime, grain size refinement involving recrystallisation and the formation of voids and cavities cause macroscopic softening; low ductility results. Stress hardening is correlated to grain growth and accumulation of dislocations. The findings are used to construct a processing map, on which the dominant deformation mechanisms are identified. Physically-based constitutive equations are presented which are faithful to the observed deformation mechanisms. Internal state variables are used to represent the evolution of grain size, dislocation density and void fraction. Material constants are determined using genetic-algorithm optimisation techniques. Finally, the deformation behaviour of this material in an industrially relevant problem is simulated: the deformation of diffusion-bonded material for the manufacture of hollow, lightweight structures, e.g. those used for fan blades of aeroengines. [more]