Boundary engineering of bulk nano-crystalline/nano-twin materials by high strain rate dynamic plastic deformation (BENTMAT)

The strength of polycrystalline materials increases with decreasing grain size, following in general the well-known Hall-Petch relation (inverse relationship). Therefore, to refine the grains in materials, in particular to get nano crystalline (NC) materials (average grain size < 100 nm), is an important strengthening strategy. However, strength and ductility (materials ability to strain before fracture) are usually opposing properties, namely, the higher the strength, the lower the ductility and vice versa. One efficient approach to increase the ductility of ultrafine-grained materials is supposed to be the generation of a large quantity of nano-sized mechanical twins. In this project, in addition to more conventional severe plastic deformation (SPD) approaches (e.g. equal channel angular pressing (ECAP), an innovative metal processing approach, high strain rate dynamic plastic deformation (DPD), is explored, to produce bulk nano-crystalline/nano-twin materials with an unprecedented combination of strength and ductility, where DPD is realized in a Droptower impact system at SimLab, NTNU/SINTEF. In particular, the twinning behaviour of a commercial purity Ti subjected to room temperature DPD has been studied. Close investigations (by SEM-EBSD) on the crystallographic nature of the twin boundaries have interestingly revealed a new mechanism to form twin boundaries, different from the conventional twinning in metals, which always includes a fast nucleation and growth process (close to the speed of sound). In Al-alloys deformation twinning is rarely observed, due to the generally high stacking fault energy. Nevertheless, experiments have demonstrated that a notable amount of deformation twins actually can be found in coarse-grained Al-7Mg alloy deformed by (DPD). The mechanisms by which these twins are formed, have been investigated by transmission and scanning electron microscopy (TEM/SEM), and the role of Mg alloying further investigated by first-principles calculations. The tendency for mechanical twinning strongly depends on characteristics of Generalized Planar Fault Energy (GPFE) curve (unstable stacking fault energy vs unstable twinning energy). First-principles calculations show that both Mg and vacancies (introduced e.g. by large deformations) segregate to stacking faults (SF) in Al, and interestingly there is a synergy between the two. However, although the intrinsic SF energy decreases along with an increased twinning propensity, it can be concluded that deformation twinning in general is extremely difficult for Al-Mg alloys. Impurity segregations can also strongly influence the properties of grain boundaries (GBs) in alloys. To clarify the effect of Mg and Cu doping on the strength of Al GBs, systematic first-principles calculations have been performed to investigate the segregation behavior at special symmetrical tilt grain boundaries (STGB). It is found that both Mg and Cu tend to segregate to Al GBs, and decrease the GB energy. However, while ab-initio tensile testing shows that Mg solutes lead to embrittlement of the STGB, Cu segregations have a strengthening effect on specific Al GBs. Impurity elements also play a crucial role in the electrical properties of multi-crystalline(mc) -Si based instruments. Most impurities, including transition metals and light elements (e.g. C, O, N, etc.), have detrimental effect on the mc-Si based solar cell efficiency. In contrast, P and As are important n-type dopants, which are usually ion implanted in Si wafers to enhance overall conductivity for nano-scale semiconductor applications. First-principles calculations of the segregation behavior of C, P and As along a set of special (CSL) GBs in mc-Si, show that the segregation tendency depends strongly on the structural order of the boundaries, where less order favor more segregation. This is important information as basis for optimized grain boundary engineering of these materials. SPD experiments by ECAP have been carried out on different alloys including Al-5Cu alloy, Al-8Zn alloy and Al-6Bi-8Zn alloy. It is found that an Al-5Cu alloy processed by ECAP may provide a beneficial combination of strength and ductility, and specific processing conditions (#ECAP passes and aging heat treatment), to achieve this have been established. For Al-6Bi-8Zn alloy (potential bearing alloy), it is revealed that soft Bi particles have a strong influence on enhancing grain refinement during ECAP. I addition to the knowledge and understanding gained with respect to the specific methods and alloys investigated in this project, important competence and skills of generic character have been obtained, both with respect to various SPD methods, specimen preparation and advanced nano-/microstructure characterization of various metals and alloys. Not at least import experience and skills with respect to first principles calculations and their use in structural materials design and engineering have been established.

In this project, an innovative metal processing approach, high strain rate (102~104 s-1) dynamic plastic deformation (DPD), will be explored, to produce bulk nano-crystalline/nano-twin materials with an unprecedented combination of strength and ductility. DPD is going to be realized in Droptower impact system (impact velocity 0.8-24m/s), Pneumatic accelerator (impact velocity~25m/s) and Pendulum accelerator (kicking machine) apparatus at SIMLab, NTNU. A big advantage of DPD over other severe plastic defo rmation (SPD) techniques is that, a much lower accumulated deformation strain (~3.0) is needed to generate nano structured metal materials. In the present project DPD will be applied to process face centered cubic (FCC) materials like Cu, Al and hexagon al close packed (HCP) Ti, Mg and their alloys. The main objective is to obtain a fundamental understanding of the deformation behavior and the formation mechanism of nano-sized grains and micro-scale and nano-scale twins in different materials subjected t o high strain rate DPD. The influence of strain, strain rate, deformation temperature, grain size, crystal structure and stacking fault energy (SFE) of different materials on the formation of nano twins will be studied by using advanced nano/microstructur e characterization and X-ray diffraction. First principles atomistic simulations on different types of deformation-twin boundaries will be carried out and the interfacial energy will be calculated. Processing-structure-property relationships of different nano-twinned (NT) and nano-crystalline (NC) materials prepared by the DPD method will be established by tensile testing and nano indentation. Split Hopkinson Pressure Bar (SHPB) and Split Hopkinson tension Bar (SHTB) testing will also be used to study the high strain rate deformation behaviors of the NC and NT materials. The role of nano-twin boundaries on the deformation behavior and mechanical properties of the materials will be addres