Doctoral Dissertations

Date of Award

8-2005

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Raymond A. Buchanan

Committee Members

Peter K. Liaw, Charlie R. Brooks, Charles Feigerle

Abstract

Bulk metallic glasses (BMGs) represent an emerging class of materials with an amorphous structure and a unique combination of properties. Some of these outstanding properties include exceptionally high strength, large elastic deformation, near-net-shape formability, and superplasticity. However, these materials are not commonly used in structural applications because of a lack of plasticity and a lack of clarity in terms of deformation and failure mechanisms. Furthermore, the electrochemical behavior of these materials with and without loading is not well defined. Thus, the objectives of this study were to define and model the electrochemical and mechanical behaviors of BMGs, in addition to the interactions between these.

The electrochemical behaviors of Zr-, Ti-, and Ca-based BMGs have been studied in various environments. Moreover, the electrochemical behaviors of several common, crystalline materials have also been characterized in the same environments to facilitate comparisons. In general, the Zr- and Ti-based BMG alloys demonstrated relatively good general corrosion resistance in all of the environments. Mean corrosion penetration rates (CPRs) were found to be less than 30 μm/year for these alloys. On the other hand, the Ca-based BMG alloys were found to be highly active with CPRs ranging from 300 – 5700 μm/year in a non-aggressive 0.05 M Na2SO4 electrolyte. Furthermore, most of these alloys were found to be susceptible to localized corrosion in these environments. However, the Zr- and Ti-based BMG alloys exhibited relatively high, positive values for both pitting overpotentials (ηpit) and protection overpotentials (ηpp). The Zr-based BMG commonly known as Vitreloy 105 (Vit 105) was selected for further studies. This material was fabricated at the Oak Ridge National Laboratory by arc-melting and drop-casting into a water-cooled, copper mold. Mechanical characterization of this alloy was conducted through four-point bend fatigue testing, as well as tensile testing with in situ thermography. Fatigue testing in air revealed that both the fatigue lives at various stresses and the fatigue endurance limit are similar to those reported for this material in uniaxial fatigue.

This result alone demonstrates that the great differences in fatigue results reported in the literature are not due to differences in testing geometry. In fact, the larger scatter observed in four-point bend fatigue at a given stress range was found to be due to variations in material quality. Thus, material quality is believed to be the primary reason for the great differences in fatigue behavior of various BMG alloys that have been reported in the literature since 1995. After the electrochemical and mechanical behaviors of the Vit 105 BMG alloy were defined separately, the corrosion-fatigue behavior of this alloy was studied. Corrosion-fatigue tests were conducted under identical conditions as those utilized during fatigue testing in air. However, in this case, the environment was a 0.6 M NaCl electrolyte, identical to one of the environments in which the electrochemical behavior was previously defined. The environmental effect was found to be significant at most stress levels, with decreasing effects at higher stress levels due to decreasing time in the detrimental environment. Furthermore, the corrosion-fatigue endurance limit was found to be severely depressed to a stress range of less than 400 MPa. Again, the variation in the corrosion-fatigue data at a given stress range was found to be primarily dependent upon material quality. In addition, the crack-initiation locations were observed to shift from the inner span, in air, to the outer loading pins in the 0.6 M NaCl electrolyte. This shift in initiation locations was due to wear at the outer pins that removed the passive layer, which promoted pitting and crack initiation.

Cyclic-anodic-polarization tests were conducted during cyclic loading to elucidate the effect of cyclic stresses on the electrochemical behavior. It was found that a stress range of 900 MPa resulted in active pitting at the open-circuit potentials. Thus, ηpit had shifted from high, positive values in the static condition to low, negative values under cyclic loading. Next, the degradation mechanism was examined by anodic and cathodic polarization. While cathodic polarization extended the fatigue life, anodic polarization severely degraded the fatigue life. Based upon these dramatic shifts in the fatigue lives at 900 MPa, it was concluded that the degradation mechanism is stress-assisted dissolution, not hydrogen embrittlement.

Finally, tensile tests were conducted with the Vit 105 BMG alloy with in situ infrared (IR) thermography to observe the evolution of shear bands during deformation. More importantly, the length, location, sequence, temperature evolution, and velocity of individual shear bands have been quantified through the use of IR thermography. This study revealed that multiple shear bands can initiate, propagate, and arrest within the sample during a single tensile test, contrary to popular belief. After arrest, many shear bands were reactivated at a later time and higher stress and propagated before arresting again. The velocity of shear band propagation was estimated to be a minimum of 1 m/s.

The temperature profiles along the axis of shear band propagation were found to continually decrease from the point of initiation to the point of arrest. This gradual decrease in the temperature as the shear band propagates suggests that arrest occurs because the driving mechanism slowly decreases until it is exhausted. A maximum temperature increase of approximately 2.6°C was observed in association with the propagation of shear bands. However, this temperature change is likely an underestimate of the actual increase in temperature generated by the shear band due to the limited temporal and spatial resolution of the IR camera and rapid heat conduction in the sample.

Finally, the maximum temperature of a shear band has been shown to be the best predictor of the shear band length out of all of the parameters examined in this study. Based upon this correlation, it can be concluded that the final failure must have occurred when a critical shear-band temperature was attained in one or more of the shear bands, preventing the arrest of the shear band before it attained a critical length.

Based upon all of these studies on a variety of BMG alloy systems, it is obvious that these materials are extremely sensitive to both material quality and surface defects. Therefore, future research on the improvement of BMG alloys should be focused on these areas. However, these materials possess a unique collection of desirable properties despite these drawbacks. Thus, it is possible that the shortcomings of this novel class of materials can be remedied through further study and understanding.

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