JiaHao Li

Different behaviours of amorphization induced by ion mixing

Ion mixing of multiple metal layers was conducted at 77 K in five selected binary metal systems, i.e. the Ag-Cr, Co-Cu, Cu-Fe, Fe-Mo and Fe-Nd systems, which all have a nearly zero or positive heat of formation (Δ H For ). TEM examination showed a common microscopic feature of localized amorphization after high dose ion irradiation, i.e. the amorphous phase was frequently interspersed in a crystalline matrix. EDS analysis was therefore carried out to determine the actual compositions of the phases formed. It revealed that all local amorphous regions resided within rather narrow composition ranges. The narrow range found here for amorphization is probably the reason that amorphous alloy formation was difficult in these systems. After comparing these observations with those reported earlier for the some 30 systems so far studied, it is found that the behaviour of ion mixing induced amorphization, and hence the binary metal systems, can be classified into two categories. The first category contains the readily amorphous phase forming systems and usually features very limited terminal solid solubilities along with large negative heat of formation. Typical examples are the Au-Ti, Ni-Mo, Ni-Nb and Ru-Zr systems. In these systems amorphization can be achieved over a wide composition range once a minimum dose is reached. For systems in the second category, amorphization is only allowed in a rather narrow composition range, as in the systems studied here. This is also the case when a system has a large solid solubility, even though its Δ H For is rather negative, e.g. the Mo-Ru and Au-V systems. In such situations, the results of phase formation may be sensitive to the composition as well as to the ion irradiation dose. The possible mechanism responsible for the different behaviors of phase formation by ion mixing are also discussed.

Spark plasma sintering and thermal conductivity of carbon nanotube bulk materials

Carbon nanotube (CNT) bulk samples were fabricated by spark plasma sintering (SPS), which, as a rapid consolidation technique, preserved the phase structure and diameter of cylindrical tubules of the CNTs even at high temperatures of up to 2000°C. The thermal conductivity of the resultant bulk samples was measured by the conventional laser-flash method, and the corresponding thermal conductivity was found to be as low as 4.2W∕m∕K at room temperature. This low thermal conductivity of the CNT bulk materials was explained on the basis of multiple physical elements including intensive tube–tube interactions. CNT bulk materials may find potential applications as thermoelectric materials that require low thermal conductivity, but high electrical conductivity.

Two-step crystal growth mechanism during crystallization of an undercooled Ni50Al50 alloy.

Crystallization processes are always accompanied by the emergence of multiple intermediate states, of which the structures and transition dynamics are far from clarity, since it is difficult to experimentally observe the microscopic pathway. To insight the structural evolution and the crystallization dynamics, we perform large-scale molecular dynamics simulations to investigate the time-dependent crystallization behavior of the NiAl intermetallic upon rapid solidification. The simulation results reveal that the crystallization process occurs via a two-step growth mechanism, involving the formation of initial non-equilibrium long range order (NLRO) regions and of the subsequent equilibrium long range order (ELRO) regions. The formation of the NLRO regions makes the grains rather inhomogeneous, while the rearrangement of the NLRO regions into the ELRO regions makes the grains more ordered and compact. This two-step growth mechanism is actually controlled by the evolution of the coordination polyhedra, which are characterized predominantly by the transformation from five-fold symmetry to four-fold and six-fold symmetry. From liquids to NLRO and further to ELRO, the five-fold symmetry of these polyhedra gradually fades, and finally vanishes when B2 structure is distributed throughout the grain bulk. The energy decrease along the pathway further implies the reliability of the proposed crystallization processes.

Formation of the Ni–Zr–Al Ternary Metallic Glasses Investigated by Interatomic Potential through Molecular Dynamic Simulation

Under the framework of second moment approximation of the tight binding theory, a realistic interatomic potential is first developed for the Ni–Zr–Al ternary metal system and then applied to predict the glass-forming ability of the system through molecular dynamics simulation. It is found that when the composition falls into the hexagonal region defined by six vertexes of Ni 20 Zr 80 Al 0 , Ni 0 Zr 65 Al 35 , Ni 0 Zr 25 Al 75 , Ni 20 Zr 0 Al 80 , Ni 40 Zr 0 Al 60 , and Ni 77 Zr 23 Al 0 , the super-saturated solid solution becomes unstable and spontaneously turns into the disorder state, i.e., the metallic glass state. The defined composition region could be considered as a quantitative glass-forming ability, within which the Ni–Zr–Al ternary metallic glass is predicted to be energetically favored to form. Interestingly, the prediction based on the interatomic potential matches well with experimental observations.

Proposed truncated Cu–Hf tight-binding potential to study the crystal-to-amorphous phase transition

Proposed truncated Cu–Hf tight-binding potential was constructed by fitting the physical properties of Cu, Hf, and their stable compounds, i.e., Cu5Hf, Cu8Hf3, Cu10Hf7, and CuHf2. Based on the constructed potentials, molecular dynamics simulations were carried out to compare the relative stability of the crystalline solid solution and the disordered state. Simulation results not only reveal that the physical origin of crystal-to-amorphous transition is the crystalline lattice collapsing when the solute atoms exceeding the critical concentration, but also predict that the glass forming range (GFR) of the Cu–Hf system is 21–77 at. % Cu, which covers the GFRs determined by various metallic glass-producing techniques. Ion beam mixing experiments of the Cu–Hf system were conducted using 200 keV xenon ions and the results show that a uniform amorphous phase can be obtained in the Cu23Hf77 sample, matching well with the GFR determined by the interatomic potential, which, in turn, provides additional evidence to ...

An icosahedral phase formed during annealing of ion-irradiated Fe-Mo amorphous films

An icosahedral phase has been observed in the Fe-Mo system for the first time. The new icosahedral phase was formed during thermal annealing of ion-irradiated Fe-Mo amorphous films, which were prepared by sputtering from an Fe50Mo50 alloy ingot. The atomic concentration of this phase was determined by in situ energy-dispersive spectra (EDS) to be around Fe40Mo60 which has been proved to be a favourable composition for ion-induced amorphisation in this system.

CEMS study of irradiation effects on the chemical bonding of SnO2

Abstract For a comparative study of ion irradiation effects on the chemical bonding of metal oxides, SnO2 thin films were irradiated by 180 keV120Sn ions and 80 keV N+ ions, and analyzed by conversion electron Mossbauer spectroscopy (CEMS) and ESCA techniques. The results show that only 120Sn ion irradiation causes appreciable rupture of the SnO bonds. This phenomenon is interpreted by a proposed thermal spike model. Theoretical estimates are consistent with the experimental results.

Simplified thermodynamic method to predict the glass formation of the ternary transition metal systems

By considering the different effects of enthalpies on the glass formation of ternary transition metal systems, a thermodynamic method is proposed to predict the glass-forming regions as well as the glass-forming abilities. Cu–Zr–Ti and ten more ternary systems, as well as the corresponding binary subsystems, were studied, and the predictions agree well with the experimental observations.

Oscillating behavior of high-pressure stability observed in the immiscible Co–Cu system by first-principles calculation

Based on the density functional perturbation theory, the phonon spectra of the immiscible Co–Cu compound under different pressures are calculated. It is found that the CoCu3 compound can be stabilized under certain high pressure, further confirming the recently proposed concept of high-pressure alloying between immiscible elements [Dubrovinskaia et al., Phys. Rev. Lett. 95, 245502 (2005)]. Interestingly, alternately appearing imaginary phonons are observed under different pressures, suggesting that there exists an oscillation behavior of the stability of the CoCu3 compound under high pressure, which deepens the understanding of the concept of high-pressure alloying between immiscible elements.

Atomic approach to the optimized compositions of Ni–Nb–Ti glassy alloys with large glass-forming ability

In the present study, we first constructed a long-range empirical potential for a ternary Ni–Nb–Ti system consisting of fcc, bcc and hcp metals, and then employed the verified potential in atomic simulations to study the formation of Ni–Nb–Ti glassy alloys. Atomic simulations derived a pentagon composition region of the Ni–Nb–Ti system, within which glassy alloys are inclined to form. Furthermore, the driving force for the transformation from a crystalline solid solution to a glassy alloy could be correlated to the glass-forming ability (GFA) of a particular alloy. The GFA of ternary Ni–Nb–Ti alloys with various compositions were assessed on the basis of atomic simulation results, obtaining an optimized composition region, within which the alloys possess larger GFA, as well as an optimum composition with the largest GFA. The optimum alloy is expected to be the most stable and the size of the obtainable glass may be the largest in the ternary Ni–Nb–Ti system. The optimized composition region and optimum composition could provide guidance to design the compositions of Ni–Nb–Ti metallic glasses.