The Effect of Composition on the Thermodynamics, Structure, Mechanical Properties and Atomic Motion of (Pd-Pt)_42.5Cu₂₇Ni_9.5P₂₁ Alloys

The Pd_42.5Cu₂₇Ni_9.5P₂₁ alloy is one of the best metallic glass formers that has been discovered so far, reaching critical casting thicknesses of up to 80 mm. According to basic hard sphere models Pt should be able to replace Pd in this alloy. In fact, Pt_42.5Cu₂₇Ni_9.5P₂₁ shows significant structural differences compared to the Pd based counterpart<b>¹</b> with a reduced critical thickness of only 20 mm, despite both alloys behave very similarly if it comes to the temperature dependence of the overall kinetics (fragility), which is expressed by the viscosity or a-relaxation time of the supercooled liquid.<br/>To study the differences, a series of (Pd-Pt)_42.5Cu₂₇Ni_9.5P₂₁ alloys is prepared, in which Pd is gradually replaced by Pt<b>²</b>. For these alloys the thermodynamic functions are assessed namely specific heat capacity, enthalpy, entropy and Gibbs free energy revealing that the driving force for crystallization increases with the increase of the Pt content, which is in line with the decreasing critical casting thickness when the amount of Pt increases and the amount of Pd decreases in the alloy. In addition, the Pt-richer alloys are thermodynamically more fragile than the Pd-rich alloys, which is revealed by a larger specific heat capacity and a faster drop of the configurational entropy in the Pt-richer alloys. This together with structural investigations using high energy x-ray diffraction (HEXRD) leads to the conclusion that the structure of the Pt rich alloys is dominated by its change in medium range order whereas the Pd-rich alloy is dominated and constricted by its extraordinary short range order that is already pronounced at high temperatures in the equilibrium liquid<b>²</b>. The mechanical properties change drastically from a ductile behavior on the Pt-rich side to an embrittlement with increasing Pd content and decreasing Pt concentration. Nanoindentation investigations together with the HEXRD studies reveal that the embrittlement with increasing Pd-content can be connected to the structural changes. The increase in brittleness is quantitatively related to an increase of rigid 3-atom connections of adjacent icosahedral clusters³.<br/>Since the two alloys show similar kinetic fragilities but different thermodynamic and structural fragilities we extensively used XPCS to study the atomic dynamics of the two extreme cases of the series, namely the alloy Pd_42.5Cu₂₇Ni_9.5P₂₁ and the alloy Pt_42.5Cu₂₇Ni_9.5P₂₁ not only as a function of temperature but also as a function of the wave vector. The structural fragilities correlate with the temperature dependence of the stretching parameter describing the decay of the density fluctuations. This implies that the evolution of dynamical heterogeneities in supercooled alloys is determined by the rigidity of the melt structure. We find also that the atomic motion not only reflects the topological order but also the chemical short-range order, which can lead to a surprising slowdown of the α-process at the mesoscopic length scale. These results will contribute to the understanding of the glass transition, which is still missing<b>⁴</b>.<br/><br/>References:<br/>1. Oliver Gross, Nico Neuber, Alexander Kuball, Benedikt Bochtler, Simon Hechler, Maximilian Frey, Ralf Busch, Communications Physics 2, 83 (2019).<br/>2. N. Neuber, O Gross, M Frey, B Bochtler, A Kuball, S Hechler, I Gallino, Ralf Busch,<br/>Acta Materialia 220, 117300 (2021).<br/>3. Nico Neuber, Maryam Sadeghilaridjani, Nandita Ghodki, Oliver Gross, Bastian Adam, Lucas Ruschel, Maximilian Frey, Saideep Muskeri, Malte Blankenburg, Isabella Gallino, Ralf Busch, Sundeep Mukherjee, Scientific Reports 12, 17133 (2022).<br/>4. Nico Neuber, Oliver Gross, Maximilian Frey, Benedikt Bochtler, Alexander Kuball, Simon Hechler, Fan Yang, Eloi Pineda, Fabian Westermeier, Michael Sprung, Florian Schäfer, Isabella Gallino, Ralf Busch, Beatrice Ruta, Communications Physics 5, 316 (2022).