Ambient-temperature time-dependent deformation of cast and additive manufactured Al-Cu-Mg-Ag-TiB2 (A205).

A dual-stage indentation test at ambient temperature including a constant indentation load rate followed by a constant indentation load-hold segment was employed to assess the time-dependent plastic deformation of cast and additive manufactured Al-Cu-Mg-Ag-TiB2 alloys in as-fabricated and T7 conditions at room temperature. Optical microscopy, scanning electron microscopy, electron backscattered diffraction, and transmission electron microscopy techniques were used to study the microstructure of the samples and to correlate the microstructure with the creep properties. That is, the indentation load/displacement/time data from depth-sensing indentation creep were combined with the advanced microstructural assessments to analyze the controlling mechanisms of creep in as-cast, as-built, and T7 samples. Expectedly, the microstructure of samples manufactured by different methods was substantially different in terms of the grain size and the distribution of TiB2 particles. The θ'', θ' and Ω phase were formed in all heat-treated samples; however, the density of Ω phase was higher in the cast-T7 samples. Distinct microstructure and precipitation density resulted in different indentation-derived properties, both cast and AM samples at T7 condition showed enhanced creep resistance compared to their as-manufactured counterparts. The main controlling mechanism of creep deformation was found to be dislocation creep based on the indentation-derived creep stress exponent values.

Indentation-derived creep response of cast and laser powder bed fused AlSi10Mg alloy: Air temperature.

Compared with time-consuming conventional uniaxial tensile/compressive creep experiments, depth-sensing indentation testing is considered a reliable, and convenient testing technique to assess the time-dependent plastic deformation of materials in a reasonable time scale. In the present study, we report the ambient (room) temperature indentation creep properties of additively manufactured (i.e., laser-powder bed fused) and cast AlSi10Mg alloy at as-fabricated and different post-fabrication heat treatment states. The indentation creep testing parameters (i.e., dwell time, peak indentation load, and indenter shape) were optimized to adequately represent the creep response (time-displacement) variations for different material conditions. To this end, dual-stage, constant loading rate followed by constant-load holding, pyramidal indentation experiments were performed at a loading rate of 10 mN/s, a peak load of 200 mN, and a dwell time of 400 s. Besides, electron backscattered diffraction was performed to evaluate the manufacturing process (selective laser melted versus cast)/ post-fabrication heat treatment/ texture/ creep properties relationships for the studied AlSi10Mg alloy. Also, the indentation hardness, indentation strain rate sensitivity, indentation creep exponent, and activation volume were analyzed to study and confirm the mechanism of indentation creep. The calculated high values of creep stress exponents (i.e., >10) are attributed to dislocation-reinforcing particle interaction as the controlling mechanism of the creep which agrees with this assumption that AlSi10Mg is indeed an in-situ metal matrix composite with eutectic silicon as the reinforcing particles.

An overview of microscale indentation fatigue: Composites, thin films, coatings, and ceramics.

There are many applications from computer hardware and sensors to thin films and coatings in which parts are fabricated in small sizes and low thicknesses. Most of these applications could undergo cyclic loading and unloading conditions during their operation. Therefore, cyclic and fatigue evaluations of these components are an essential topic and should be fully understood. In these cases, due to the dimensional limitations, conventional methods of the fatigue experiments encounter challenges and mostly are not accurate or applicable. Nano- and micro-indentation fatigue tests are considered non- or semi-destructive experiments that have opened a new approach to study the cyclic response of these small-sized specimens and thin films. The objective of the present review paper is to evaluate a convenient, reliable, and non-destructive testing approach in the assessment of fatigue (cyclic) response of materials on a small scale. Along with conventional bulk scale fatigue testing methods (i.e. reverse bending, pull-push, multi-axial bending), the depth-sensing indentation testing technique can be employed to study the cyclic behavior of metallic and non-metallic materials especially when a limited volume of the material is available. In this paper, we tried to cover most of the previous studies performed on indentation fatigue of composites, thin films, coatings, and ceramics along with associated discussions and main findings. We covered the physics behind the indentation and the difference between the indentation and conventional fatigue analyses. Followed by that, microstructural evaluations of some of the studies are provided to give readers more insights into this approach. In most applications, the indentation fatigue technique could be a reliable solution due to its accuracy, simplicity, and nondestructive approach in finding out the fatigue and cyclic behavior of materials having a small size or volume. It is worth noting that the loading mode in the indentation fatigue is completely different than the traditional (bulk-scale) fatigue as the tensile segment of the load cycle is not produced in the indentation fatigue (it is a compression-compression loading cycle). Therefore, the controlling mechanisms of failure between small-scale fatigue and bulk-scale fatigue may not be the same.