Changes In Chemical Short-range Order Research Articles

Anomalous structure-property relationships in metallic glasses through pressure-mediated glass formation

Metallic glasses are commonly found to favor denser packing structures and icosahedral order in experiments, simulations, and theoretical models. Here we present a molecular dynamics simulation study of Cu-Zr metallic glasses, prepared through a pressure-mediated pathway. The resulting glasses exhibit anomalous structure-property relationships; these glasses are less energetically stable, concomitant with a denser atomic packing and a significant increase in icosahedral short-range order. The enhanced icosahedral order is shown to be accompanied by a pressure-mediated change in chemical short-range order. The results demonstrate that in amorphous alloys (nonmonatomic), theoretical frameworks of the two-order-parameter model must be generalized to account for chemical degrees of freedom.

CEMS studies of structural modifications of metallic glasses by ion bombardment

Fe76Mo8Cu1B15 and Fe74Nb3Cu1Si16B6 amorphous metallic alloys were exposed to ion bombardment with nitrogen ions and protons to ensure different degree of radiation damage. The radiation damage profiles were calculated in the “full cascade” mode. Conversion electron Mossbauer spectrometry was employed to scan structural modifications in the surface regions of the irradiated alloys. In Fe76Mo8Cu1B15, the irradiation with 130 keV N+ has caused a significant increase of the hyperfine magnetic fields and isomer shift due to changes in topological and chemical short-range order (SRO), respectively. No appreciable effects were revealed after bombardment with 80 keV H+ ions. Fe74Nb3Cu1Si16B6 amorphous metallic alloy was irradiated by 110 keV N+ and 37 keV H+ and only changes in chemical SRO were revealed after bombardment with nitrogen ions. The observed alternations of the structure depend primarily on the total number of displacements of the resonant atoms which are closely related to the fluence as well as type and energy of the incident ions.

Ion bombardment of Fe-based amorphous metallic alloys

Fe74Cu1Nb3Si16B6 amorphous metallic alloy is investigated after ion irradiation by 110 keV N + and 593 MeV Au ions. The depth-profiles of the radiation damage were calculated by the SRIM2008 code. Applicability of transmission and conversion electron Mossbauer effect measurements to distinguish between the bulk and surface radiation damage is demonstrated by using different irradiation conditions. The investigated alloy is characterized by ferromagnetic interactions. The implantation does not depict appreciable changes of the samples’ surfaces. Changes in chemical short-range order (SRO) are revealed in N + irradiated alloys. Heavy Au ions caused pronounced effects in the position of the net magnetization though no impact on SRO was observed. After annealing, structural relaxation and annealing-out of the irradiation-induced stresses caused the rotation of the net magnetization back to its original position.

Influence of chemical short-range order on atomic diffusion in Al–Ni melts

We use inelastic neutron scattering and molecular dynamics simulation to investigate the chemical short-range order (CSRO), visible through prepeaks in the structure factors, and its relation to self-diffusion in Al–Ni melts. As a function of composition at 1795K, Ni self-diffusion coefficients from experiment and simulation exhibit a nonlinear dependence with a pronounced increase on the Al-rich side. This comes along with a change in CSRO with increasing Al content that is related to a more dense packing of the atoms in Ni-rich Al–Ni systems.

Reversible structural relaxation in FeBSi amorphous alloys

Reversible structural relaxation in two FeBSi amorphous alloys, Fe 78B 7Si 15 and Fe 75B 10Si 15, has been studied through measurements of the electrical resistance and relaxation enthalpy using a differential scanning calorimeter. Both alloys exhibited reversible changes of these physical properties due to the reversible relaxation. Larger reversible changes were observed in Fe 78B 7Si 15 than in Fe 75B 10Si 15. This result can be well explained by the idea that the reversible relaxation is caused by changes in chemical short-range order among iron and silicon atoms according to Dubois and Le Caer's structural model of amorphous alloys. The activation energy spectrum characterizing the reversible relaxation process was obtained from reversible enthalpy changes during isothermal annealing at 500 K. It results in an asymmetric gaugassian-like distribution with a peak maximum at 1.7 eV.

Structural relaxation in PdGe metallic glass

We have measured small reversible changes of electrical resistance which occur following a small temperature change applied to an amorphous Pd 78Ge 22 alloy. Reversibility of this relaxation was demonstrated by the resistance returning (nearly) to the original value after repeated temperature cycling and equilibration. The reversible characteristic of the relaxation suggests that the mechanism is the change in chemical short-range order (CSRO) with temperature. Thus the measurements provide information on atomic mobility in this metallic glass. The relaxation kinetics indicate a spectrum of relaxation times which is best described as log-normal but skewed towards longer times. Parameters describing the breadth and shift in this spectrum with temperature are described. We also observe an irreversible resistance change superimposed on the reversible changes. The kinetics of the irreversible change are found to obey the equation for free volume relaxation proposed by van den Beukel. However, this model fails to describe the slowing of the CSRO kinetics that occurs relatively early in the annealing process.