Dependence of Modulus and Hardness on the Annealing Conditions of Pt 57.5 Cu 14.7 Ni 5.3 P 22.5 Bulk Metallic Glass
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MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2019.30
Dependence of Modulus and Hardness on the Annealing Conditions of Pt57.5Cu14.7Ni5.3P22.5 Bulk Metallic Glass Zheng Chen, Amit Datye, P. Aidan Brooks, Madison Sprole, Jittisa Ketkaew, Sungwoo Sohn, Jan Schroers, and Udo D. Schwarz Department of Mechanical Engineering and Materials Science, Yale University, New Haven, CT 06520, U.S.A.
ABSTRACT
The mechanical properties of metallic glasses are often tuned by annealing, which influences these properties by adjusting the relaxation and/or crystallization status of the glasses. Here, we studied the hardness and modulus of Pt57.5Cu14.7Ni5.3P22.5 bulk metallic glass annealed at different temperatures by nanoindentation, where the annealing gives the material different fictive temperatures and fractions of crystallization. It is found that both reducing the fictive temperature of a fully amorphous sample and increasing the degree of crystallization in a partially crystallized sample increase hardness and modulus. Combining the two approaches, elevated hardness and modulus values are found for composite materials containing both crystalline and amorphous phases when they are compared to chemically identical alloys featuring similar percentages of crystalline and amorphous phases that have been prepared by annealing at higher temperatures. Our findings indicate that the mechanical properties of the platinum-based alloys can be customized by processing them with targeted heat treatments.
INTRODUCTION Isothermal annealing of bulk metallic glasses (BMGs) can structurally relax them into a metastable quasi-equilibrium liquid state characterized by the fact that initial comparatively fast re-ordering processes fade out and structural changes subsequently occur on much longer time scales. Since the material’s overall structural state reached at this transitional point depends for a given alloy composition solely on the chosen annealing temperature, it is fully characterized by specifying its value, which is referred to as the material’s ‘fictive temperature’ Tf. Continued annealing first further changes the
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amorphous structure by enabling the material to reach states of lower total energy before partially and eventually fully crystallizing the alloy. The related structural changes are expected to affect mechanical properties such as modulus and hardness, but it is presently not clear how much the two properties will change through this process. To shed light on the effect of structural states on mechanical properties, we used nanoindentation to measure the modulus and hardness changes of Pt 57.5Cu14.7Ni5.3P22.5 1, a platinum-based BMG alloy system, after exposing samples to different thermal treatments. Relaxation and crystallization effects are separated by isothermally
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