Aydiner, Cahit Can (2004) Investigation of thermal tempering in bulk metallic glasses. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-04192004-120604
Bulk metallic glasses are recent advanced materials which generate residual stresses due to rapid cooling from their surfaces during processing. These stresses arise from the thermal gradients that form within the sample at and above the glass transition region. A typical processing of BMGs involves feeding the alloy melt into a mold followed by severe quenching. The formation and nature of these stresses are analogous to the residual stresses due to the thermal tempering of silicate glasses. This analytical-experimental study investigates the thermal tempering phenomenon in BMGs for the first time.
One of the best glass forming metallic alloys, Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 (Vitreloy 1TM), is employed in this study. First, the best technique for the high-resolution measurement of residual stresses in BMGs is determined to be the crack compliance method. Second, the formation of the stresses is modeled with three different levels of viscoelastic phenomenology, namely, an instant freezing model, a viscoelastic model and a structural model. The first is a simplistic analytical model to estimate residual stresses whereas the structural model accounts for the temperature history dependence of the glassy structure. The constitutive laws for the viscoelastic and structural models are incorporated into the finite element method (ABAQUSTM software package) allowing the application of these models to complex geometries. To increase the accuracy of the analysis, the 'correct' temperature evolution in the sample during processing has to be input to these 'mechanical' models. Therefore, the heat transfer problem during the casting process of the BMG is analyzed in detail. Accuracy also requires a detailed knowledge of the thermal parameters of the material as a function of temperature; thus, some attention is also devoted to their measurement. At the end, calculated and measured stresses are compared and good agreement is achieved. BMGs are demonstrated to be capable of generating very high (around 400 MPa) compression on their surfaces. The study also yielded valuable physical insight into the thermal tempering process itself. It is seen that this process exhibits significant discrepancies in BMGs compared to its analogy in silicate glasses. For instance, the transient tensile stresses that develop in the latter are shown to be lacking in the BMGs. Another discrepancy between the two materials is that the density of BMGs is uniform across the sample cross section in contrast to that found in silicate glasses. Overall, this investigation developed sufficient understanding of the thermal tempering phenomenon in BMGs to establish it as a viable process to manipulate properties.
|Item Type:||Thesis (Dissertation (Ph.D.))|
|Subject Keywords:||bulk metallic glass; crack compliance method; residual stress; structural relaxation; thermal tempering; viscoelasticity|
|Degree Grantor:||California Institute of Technology|
|Division:||Engineering and Applied Science|
|Major Option:||Applied Mechanics|
|Thesis Availability:||Restricted to Caltech community only|
|Defense Date:||11 December 2003|
|Author Email:||can (AT) caltech.edu|
|Default Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Imported from ETD-db|
|Deposited On:||22 Apr 2004|
|Last Modified:||26 Dec 2012 02:37|
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