Small-scale fracture behavior of four model alloy systems were investigated in the order of increasing microstructural complexity, namely: (i) a Ni-based Bulk Metallic Glass (Ni-BMG) with an isotropic amorphous microstructure; (ii) a single-phase high entropy alloy, HfTaTiVZr, with body centered cubic (BCC) microstructure; (iii) a dual-phase high entropy alloy, AlCoCrFeNi2.1, with eutectic FCC (L12) -BCC (B2) microstructure; and (iv) a Medium-Mn steel with hierarchical microstructure. The micro-mechanical response of these model alloys was investigated using nano-indentation, micro-pillar compression, and micro-cantilever bending. The relaxed Ni-BMG showed 6% higher hardness, 22% higher yield strength, and 26% higher bending strength compared to its as-cast counterpart. Both the as-cast and corresponding relaxed BMGs showed stable notch opening and blunting during micro-cantilever bending tests rather than unstable crack propagation. However, pronounced notch weakening was observed for both the structural states, with the bending strength lower by ~ 25% for the notched samples compared to the un-notched samples. Deformation behavior of HfTaTiVZr was evaluated by micropillar compression and micro-cantilever bending as a function of two different grain orientations, namely [101] and [111]. The [111] oriented micropillars demonstrated higher strength and strain hardening rate compared to [101] oriented micropillars. The [111] oriented micropillars showed transformation induced plasticity (TRIP) in contrast to dislocation-based planar-slip for the [101] oriented micropillars, explaining the difference in strain hardenability for the two orientations. These differences in deformation behavior for the two orientations were explained using Schmid factor calculations, transmission electron microscopy, and in-situ deformation videos. For the dual-phase AlCoCrFeNi2.1 high entropy alloy, the L12 phase exhibited superior bending strength, strain hardening, and plastic deformation, while the B2 phase showed limited damage tolerance during bending. The microstructure and deformation mechanisms were characterized for a few different medium-Mn steels with varying carbon (0.05-0.15 at%) and manganese (5-10 at%) content. The alloy with 10 at% Mn and 0.15 at% C (1015 alloy) showed hierarchical microstructure of retained austenite and ferrite with lamellae 200 nm to 300 nm wide. Micro-pillar compression at different strain levels for this alloy revealed that deformation in austenite is primarily accommodated through transformation to martensite, thereby increasing the strain hardening rate.
| Identifer | oai:union.ndltd.org:unt.edu/info:ark/67531/metadc2179335 |
| Date | 07 1900 |
| Creators | Jha, Shristy |
| Contributors | Mukherjee, Sundeep, Rout, Bibhudutta, Du, Jincheng, Mishra, Rajiv, Srivilliputhur, Srinivasan |
| Publisher | University of North Texas |
| Source Sets | University of North Texas |
| Language | English |
| Detected Language | English |
| Type | Thesis or Dissertation |
| Format | Text |
| Rights | Public, Jha, Shristy, Copyright, Copyright is held by the author, unless otherwise noted. All rights Reserved. |
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