Thermal Tempering of Bulk Metallic Glasses
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THERMAL TEMPERING OF BULK METALLIC GLASSES Cahit C. Aydiner, Ersan Üstündag Department of Materials Science, California Institute of Technology, Pasadena, CA 91125 USA ABSTRACT The recent development of multi-component alloys with exceptional glass forming ability has allowed the processing of large amorphous metal samples. The possibility of formation of thermal tempering stresses during the processing of these bulk metallic glass (BMG) specimens was investigated using the (1) instant freezing, and (2) viscoelastic models. Both models yielded similar results although from vastly different approaches. It was shown that fast convective cooling of Zr41.2Ti13.8Cu12.5Ni10Be22.5 plates could generate significant compressive stresses on the surfaces balanced with mid-plane tension. The crack compliance method was then employed to measure the stress profiles in a BMG plate that was cast in a copper mold. These profiles were roughly parabolic suggesting that thermal tempering was indeed the dominant residual stress generation mechanism. However, the magnitude of the measured stresses (with peak values of only about 1.5% of the yield strength) was significantly lower than the modeling predictions. Possible reasons for this discrepancy are described in relation to the actual casting process and material properties. INTRODUCTION Multi-component metallic alloys with superb glass formation ability have recently been developed allowing, for the first time, the processing of large specimens with amorphous structure. An important question that arises with bulk production is the nature and magnitude of processing-induced residual stresses. The BMG σX (Z) processing typically involves casting an alloy into a mold followed by severe quenching. This procedure σm can lead to large thermal gradients due to the low thermal conductivity of BMG. In addition, during glass transition the alloy exhibits large changes in its 0 Z viscosity within a small temperature range. All of these can lead to "thermal tempering" which generates compressive surface residual stresses balanced with mid-plane tension. σ S
Figure 1. A typical residual stress profile across the thickness of a large plate due to thermal tempering: surface compression (σs) is balanced with mid-plane tension (σm). The inplane stresses are equibiaxial and function of the thickness coordinate (Z) only.
A similar phenomenon was observed previously in silicate glasses . Thermal tempering in these glasses was mostly studied for an infinite plate geometry that is cooled from both sides. Figure 1 demonstrates the typical temper stress profile resulting from this process. In-plane coordinates are X and Y while Z is the out-ofplane coordinate. Thermal-tempering-induced residual stresses are equibiaxial in the X-Y plane for this infinite plate problem. The material functions needed for advanced modeling of thermal tempering are not yet
available for BMGs. For this reason, the simple instant freezing model  was employed first in this study for preliminary ca