Investigation of the electroplastic effect using nanoindentation

A promising approach to deform metallic-intermetallic composite materials is the application of electric current pulses during the deformation process to achieve a lower yield strength and enhanced elongation to fracture. This is known as the electroplastic effect. In this work, a novel setup to study the electroplastic effect during nanoindentation on individual phases and well-defined interfaces was developed. Using a eutectic Al-Al2Cu alloy as a model material, electroplastic nanoindentation results were directly compared with macroscopic electroplastic compression tests. The results of the micro- and macroscopic investigations reveal current induced displacement shifts and stress drops, respectively, with the first displacement shift/stress drop being higher than the subsequent ones. A higher current intensity, higher loading rate and larger pulsing interval all cause increased displacement shifts. This observation, in conjunction with the fact that the first displacement shift is highest, strongly indicates that de-pinning of dislocations from obstacles dominates the mechanical response, rather than solely thermal effects

Dislocation-mediated plasticity in the Al2_{2}Cu {\theta}-phase

The deformation behaviour of the intermetallic Al$_{2}$Cu-phase was investigated using atomistic simulations and micropillar compression, where slip on the unexpected {211} and {022} slip planes was revealed. Additionally, all possible slip systems for the intermetallic phases were further evaluated and a preference for the activation of slip systems based on their effective interplanar distances as well as the effective Burgers vector is proposed. The effective interplanar distance corresponds to the manually determined interplanar distance, whereas the effective Burgers vector takes a potential dislocation dissociation into account. This new order is: {211}1/2, {022}1/2 and {022}, {110}, {310}, {022}, {110}1/2, {112} and {112}1/2 from high to low ratio of deff/beff. Also, data on the critical resolved shear stresses of several of these slip systems were measured.Comment: 27 pages, 17 figure

Plasticity of the C15-CaAl2 Laves phase at room temperature

The room temperature plasticity of the cubic C15 CaAl2 Laves phase was investigated using nanomechanical testing and electron microscopy. The correlation between slip traces in the vicinity of nanoindents and crystallographic orientation data allowed us to gain statistical data on the activated slip and crack planes for 10 different crystallographic orientations. Slip on {111} and {112} planes was found to be most favourable for all orientations, whereas cracks predominantly occurred on {112} planes. A constant hardness of 4.9 ± 0.7 GPa and an indentation modulus of 85.5 ± 4.0 GPa for all investigated orientations for a constant strain rate and a strain rate sensitivity of 0.028 ± 0.019 were measured. Micropillar compression tests and transmission electron microscopy confirmed slip on {111} and {112} planes with a Burgers vector of  type. This allowed to determine the critical resolved shear stresses as 0.99 ± 0.03 GPa for {111} and 0.97 ± 0.07 GPa for {112} slip

Mechanical properties of cold sintered ZnO investigated by nanoindentation and micro-pillar testing

Characteristic densification in cold sintered microstructures could also have a strong influence in defining their mechanical response. For the first time, nanoindentation and micro-pillar testing is used to study these details. Based on our recent work, we selected cold sintered (250 °C, ∼ 99 % dense) and conventionally sintered (900 °C, ∼ 98 % dense) ZnO samples. Hardness, elastic modulus and compressive stress of cold sintered samples were measured to be 5.5 GPa ± 0.5, 100 GPa ± 5 and ∼ 1.2 GPa, respectively. Same values for conventionally sintered ZnO were 4.8 GPa ± 0.6, 109 GPa ± 6 and 0.8 GPa, respectively. The distinctive nature of grain boundary regions in cold sintered samples were found to influence the deformation behavior of these samples, as confirmed by TEM investigations. Our study reveals the potential of cold sintering and use of selected testing techniques as suitable choice

Co-deformation between the metallic matrix and intermetallic phases in a creep-resistant Mg-3.68Al-3.8Ca alloy

The microstructure of Mg-Al-Ca alloys consists of a hard intra- and intergranular eutectic Laves phase network embedded in a soft α-Mg matrix. For such heterogeneous microstructures, the mechanical response and co-deformation of both phases under external load are not yet fully understood. We therefore used nano- and microindentation in combination with electron microscopy to study the deformation behaviour of an Mg-3.68Al-3.8Ca alloy.We found that the hardness of the Mg2Ca phase was significantly larger than the α-Mg phase and stays constant within the measured temperature range. The strain rate sensitivity of the softer α-Mg phase and of the interfaces increased while activation volume decreased with temperature. The creep deformation of the Mg2Ca Laves phase was significantly lower than the α-Mg phase at 170 °C. Moreover, the deformation zone around and below microindents was dependant on the matrix orientation and was influenced by the presence of Laves phases. Most importantly, slip transfer from the α-Mg phase to the (Mg,Al)2Ca Laves phase occurred, carried by the basal planes. Based on the observed orientation relationship and active slip systems, a slip transfer mechanism from the soft α-Mg phase to the hard Laves phase is proposed. Further, we present implications for future alloy design strategies

Sustainable Materials Science and Green Metallurgy (Sustainable Materials and Metallurgical Science & Engineering)

Sustainable Materials and Metallurgical Science & Engineerin