Abstract

BCC metals are commonly used throughout the world and understanding their deformation behavior, especially at the sub-grain level, is essential for their continued use in technological advancements. Correctly and confidently characterizing the active slip systems in BCC materials has been a difficult task throughout past research. The research described in this thesis utilizes high resolution digital image correlation (HRDIC) and relative displacement ratio (RDR) analysis to accurately characterize active slip systems in large grained BCC tantalum and provides new insights into dislocation nucleation sites, relative CRSS values for {110} and {112} slip systems, the correlation between GB transmission factors and strain gradients, the relative length of NBGZs, and slip transmission. A 99.99% pure tantalum oligo sample was sputtered with gold and remodeled to provide high resolution data points to be used in HRDIC. The high resolution of the gold remodeled samples combined with a RDR analysis made it possible to confidently identify active slip systems during tensile deformation at room temperature. One of the observations from this analysis was the discrepancy between the observed active slip systems and those predicted from a simple single-CRSS Schmid's Law. By considering the active systems observed in grains with a range of orientation, it was concluded that the {112} slip systems have a higher CRSS than the {110} by 6.7%. Independent CPFE simulations and experiments on single crystal samples of the same material, agreed with our findings establishing a range of increased CRSS for {112} of 3.9%-7.1%. These conclusions are compared with the small number of available estimates of the CRSS ratio, and lie in between the value of equal CRSS used by most modelers, and experimental estimates of 15-25% higher for {112}. The identified active slip systems were also used in the Luster and Morris equation to calculate each GBs transmissivity factor - an estimate of strain incompatibility between neighboring grains. Results indicate that there is an inverse correlation between GB transmissivity and strain gradient slope, as well as a positive correlation between GB transmissivity and slip trace reorientation for some GBs. Only one instance of slip transmission was observed from the 24 GBs analyzed, suggesting it is an uncommon occurrence in BCC tantalum. An analysis of the length of the NBGZ in relation to slip and strain gradients was compared to previous studies and suggests the relative and absolute length of the NBGZ changes with grain size, at least for large length scales. Strain gradients for each side of the GB were measured and results indicated steep negative strain gradient slopes that suggest dislocation nucleation in the GBs and propagation towards the interior of the grain. When compared against the transmissivity factor, an inverse relationship was found to exist between strain gradients and high transmissivity factors.

Degree

MS

College and Department

Mechanical Engineering

Rights

https://lib.byu.edu/about/copyright/

Date Submitted

2022-04-07

Document Type

Thesis

Handle

http://hdl.lib.byu.edu/1877/etd12053

Keywords

BCC, slip system, dislocation theory, RDR, digital image correlation, critical resolved shear stress, grain boundary transmissivity

Language

english

Included in

Engineering Commons

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