Doctoral Dissertations

Date of Award

12-2008

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Peter K. Liaw

Committee Members

Yanfei Gao, Hahn Choo, John D. Landes, Hampparsum Bozdogan

Abstract

The objectives of the thesis research are 1) to study the strain rate effect on the deformation behavior of bulk-metallic glasses (BMGs), 2) to investigate the loading mode effect on the fatigue behavior of BMGs.

The main results are obtained as follows. At room temperature, during the inhomogeneous deformation, BMGs show the strain softening caused by the increase of the free volume in the shear bands. However, BMGs have a high-energy state and trend to crystallize, especially in the shear bands with the maximum shear stress and the increasing temperature. The instability of BMGs makes it possible for strain hardening. Thus, The deformation behavior of BMGs is affected by the stucture stability. The BMG with a relatively worse thermal stability, the low crystallization activation energy and high Avrami exponent would lead to the strain hardening phenomenon at the high strain rate.

Loading modes affect the fatigue life, fatigue-endurance limits, and fracture mechanisms of BMGs. Under four-point-bend fatigue, the fracture is an open-mode fracture (Mode I). However, under compression-compression fatigue, the fracture is a shear-mode fracture (Mode II). The fatigue life for the mode-I fracture is shorter than that for the mode-II fracture. The tension stress can accelerate the fatigue failure, and the compression stress can slow down the fatigue failure. The fatigue deformation and failure mechanisms under the different stress states will be discussed. According to the statistics analysis of the literature results, the fatigue failure probability of BMGs at a specific stress ratio could be predicted using the Logistic Regression.

Under static loading, shear bands generally form when the stress is higher than the yield stress. Shear bands could also be observed when the maximum fatigue stress is much lower than the yield stress, which could be caused by the fatigue softening. In order to prove the fatigue softening, four-point-bend fatigue is investigated. Many shear bands are observed on the tension and compression surfaces after fatigue test. However, the distribution of the shear bands is different for the various sections of the four-pointbend- fatigue sample. The shear-band spacing is relatively small on the tension surface within the inner pins. The shear-banding spacing is relatively large on the tension surface between inner and outer pins. No shear bands are observed outside the outer pins. The compression tests are investigated for the different sections after the four-point-bend fatigue. It is shown that the strength is the lowest for the inner pins part, the highest for the outside the outer pins part, and the medium for the part between inner and outer pins, which gives the direct proof for the fatigue softening.

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