Modeling and measurement of residual stresses in a bulk metallic glass plate

The recent advent of multi-component alloys with exceptional glass forming ability has allowed the processing of large metallic specimens with amorphous structure. The possibility of formation of thermal tempering stresses during the processing of these bulk metallic glass (BMG) specimens was investigated using two models: (i) instant freezing model, and (ii) viscoelastic model. The first one assumed a sudden transition between liquid and elastic solid at the glass transition temperature. The second model considered the equilibrium viscosity of BMG. Both models yielded similar results although from vastly different approaches. It was shown that convective cooling of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 plates with high heat transfer coefficients could potentially generate significant compressive stresses on the surfaces balanced with mid-plane tension. The crack compliance (slitting) 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. The extremely low residual stresses measured in these BMG specimens, combined with their high strength and toughness, serve to further increase the advantages of BMGs over their crystalline metal counterparts.