Use of elastic waves to study relaxation in metallic glasses

Abstract

Metallic glasses may be formed in a large number of metal alloys by quenching the appropriate liquid at rates of the order of 10⁶ K s⁻¹. This is usually achieved by melt spinning which produces thin ribbons or tapes of the material. In their as-quenched state, metallic glasses are highly metastable with respect to not only the crystalline state, but also a denser glassy structure of lower energy. Consequently, on subsequent annealing these glasses undergo a number of subtle structural changes usually described as ‘structural relaxations’. We have used a sensitive pulse-echo technique to monitor the changes in Young's modulus during these relaxations. The total change in modulus is high (∼ 10%) and the change during isothermal annealing can be fitted to an expression of the type <disp-formula> <mml:math> <mml:mrow> <mml:mi>∆</mml:mi> <mml:mi>E</mml:mi> <mml:mo>=</mml:mo> <mml:mfrac> <mml:mrow> <mml:mi>k</mml:mi> <mml:mi>T</mml:mi> </mml:mrow> <mml:mi>α</mml:mi> </mml:mfrac> <mml:mo>(</mml:mo> <mml:mtext>ln</mml:mtext> <mml:mfrac> <mml:mi>t</mml:mi> <mml:mrow> <mml:msub> <mml:mi>t</mml:mi> <mml:mn>0</mml:mn> </mml:msub> </mml:mrow> </mml:mfrac> <mml:mo>+</mml:mo> <mml:mn>1</mml:mn> <mml:mo>)</mml:mo> </mml:mrow> </mml:math> </disp-formula> The value of E at full relaxation depends on the annealing temperature during relaxation. By subsequent temperature cycling it is possible to alter the values of E reversibly between those characteristic of different annealing temperatures. As a consequence it is possible to perform a crossover experiment commonly seen for the refractive index of oxide glasses. The results are discussed in terms of the topological defects and chemical short-range ordering present in metallic glasses.