ABSTRACT
This additive manufacturing benchmarking challenge asked the modelling community to predict the stress-strain behavior and fracture location and pathway of an individual meso-scale (gauge dimensions of approximately 200 µm thickness, 200 µm width, 1mm length) tension specimen that was excised from a wafer of nickel allow IN625 manufactured by laser powder bed fusion (L-PBF). The data used for the challenge questions and answers are provided in a public dataset (https://data.nist.gov/od/id/mds2-2587). Testing models against the data is still possible, although a good-faith blinded prediction should be attempted before reading this article, as the results are contained herein. The uniaxial tension test was pin loaded, conducted at quasi-static strain rates under displacement control, and strain was measured via non-contact methods (digital image correlation). The predictions are challenging since the number of grains contained in the thickness of the specimen are sub-continuum. In addition, pores can be heterogeneously distributed by the L-PBF process, as opposed to intentionally seeded defects. The challenge provided information on chemical composition, grain and subgrain structure (surface-based measurements via electron backscatter diffraction and scanning electron microscopy) and pore structure (volume-based measurements via X-Ray computed tomography) along the entire gauge length for the tension specimen. During the challenge, prediction responses were collected from six different groups. Prediction accuracy compared to the measurements varied, with elastic modulus and strain at ultimate tensile strength consistently over-predicted, while most other values were a mix of over- and under-predicted. Overall, no one model performed best at all predictions. Failure-related properties proved quite challenging to predict, likely in part due to the data provided as well as the inherent difficulty in predicting fracture. Future directions and areas of improvement are discussed in the context of improving model maturity and measurement uncertainty.
ABSTRACT
The elastic-plastic properties of mesoscale electrodeposited LIGA Ni alloy specimens are investigated as a function of specimen size, strain rate, and material composition. Two material compositions are studied: a high-strength fine-grained Ni-Fe alloy and a high-ductility coarse-grained Ni-Co alloy. The specimens have thicknesses of approximately 200 µm and gauge widths ranging from 75 µm to 700 µm. Tensile tests are conducted at strain rates of 0.001/s and 1/s using tabletop loading apparatuses and digital image correlation (DIC) for strain measurement. For each test condition, the apparent Young's modulus, yield strength, ultimate tensile strength, and strain hardening exponent and strength coefficient are extracted from the tensile tests. The true strains to failure are also assessed from fractography. Size, rate, and composition effects are discussed. For most properties, the statistical scatter represented by the standard deviation exceeds the measurement uncertainty; the notable exceptions to these observations are the apparent Young's modulus and yield strength, where large measurement uncertainties are ascribed to common experimental factors and material microplasticity.
ABSTRACT
Photolithographically defined thin film Au dots were used as micro fiducial markers for digital image correlation (DIC), to enable two-dimensional strain measurement of 200 µm-thick LIGA (Lithographie, Galvanformung, Abformung) nickel alloys. Due to the sensitivity of electrodeposited films' microstructure and properties on the processing conditions, characterization of LIGA mechanical properties continues to be necessary for microsystems commercialization. DIC offers advantages over laser-based strain measurement techniques but creating suitable speckle patterns on specimens with dimensions under a millimeter is challenging. The material surface roughness itself is often used as the speckle pattern, or micro- or nanoparticles are applied to the surface. But for materials with highly polished surfaces, such as commercial LIGA alloys, the surface roughness is not always suitable, while application of particles still poses technical challenges in uniformity and reproducibility. We fabricated freestanding tensile specimens, with gauge sections 700 µm wide × 3 mm long × 200 µm thick, from electrodeposited Ni-10% Co using a commercial LIGA process, and conducted microtensile tests at strain rate 0.001 s-1. Designing and fabricating arrays of randomly oriented 1.5 µm-thick Au dots on the specimens provided a suitable way to obtain full-field surface strains over the entire gauge lengths and was reproducible from one specimen to another. Microfabricated fiducial markers therefore can be a useful surface-preparation approach for investigating micromechanical behavior, particularly plasticity and fracture, of LIGA films using DIC.
ABSTRACT
Two different LIGA electrodeposited nickel alloys displayed distinct fracture modes after meso-scale tensile testing. The Ni-Co alloy failed in a ductile manner, while the Ni-Fe alloy failed in a more brittle-appearing manner. Various factors affecting the fracture are discussed; it was determined that the fracture mode did not depend upon the strain rate but did depend upon the sample geometry. The difference in the microstructure is likely the cause of the difference in fracture mode, as the Ni-Co alloy is fine-grained, while the Ni-Fe alloy is nano-grained and likely failed by a creep-like mechanism.
ABSTRACT
Two-phase liquid-cooling technologies incorporating micro/nanostructured copper or silicon surfaces have been established as a promising thermal management solution to keep up with the increasing power demands of high power electronics. However, the reliability of nanometer-scale features of copper and silicon in these devices has not been well investigated. In this work, accelerated corrosion testing reveals that copper nanowires are not immune to corrosion in deaerated pure hot water. To solve this problem, we investigate atomic layer deposition (ALD) TiO2 coatings grown at 150 and 175 °C. We measured no difference in coating thickness for a duration of 12 days. Using a core/shell approach, we grow ALD TiO2/Al2O3 protective coatings on copper nanowires and demonstrate a preservation of nanoengineered copper features. These studies have identified a critical reliability problem of nanoscale copper and silicon surfaces in deaerated, pure, hot water and have successfully demonstrated a reliable solution using ALD TiO2/Al2O3 protective coatings.
