Shijun Zhao

Defect energetics of concentrated solid-solution alloys from ab initio calculations: Ni0.5Co0.5, Ni0.5Fe0.5, Ni0.8Fe0.2 and Ni0.8Cr0.2

Using ab initio calculations and special quasirandom structures, we have characterized the distribution of defect formation energy and migration barrier in Ni-based solid-solution alloys: Ni_{0.5}Co_{0.5}, Ni_{0.5}Fe_{0.5}, Ni_{0.8}Fe_{0.2} and Ni_{0.8}Cr_{0.2}. As defect formation energies depend sensitively on elemental chemical potential, we have developed a computationally efficient method for determining it which takes into account the global composition and local short-range order. We find that Fe has the biggest alloy effects for Ni among these four elements. Our results show that the distribution of migration energies for vacancies and interstitial have a region of overlap, which will facilitate the recombination between them.

Local Structure and Short-Range Order in a NiCoCr Solid Solution Alloy

Multielement solid solution alloys are intrinsically disordered on the atomic scale, and many of their advanced properties originate from the local structural characteristics. The local structure of a NiCoCr solid solution alloy is measured with x-ray or neutron total scattering and extended x-ray absorption fine structure (EXAFS) techniques. The atomic pair distribution function analysis does not exhibit an observable structural distortion. However, an EXAFS analysis suggests that the Cr atoms are favorably bonded with Ni and Co in the solid solution alloys. This short-range order (SRO) may make an important contribution to the low values of the electrical and thermal conductivities of the Cr-alloyed solid solutions. In addition, an EXAFS analysis of Ni ion irradiated samples reveals that the degree of SRO in NiCoCr alloys is enhanced after irradiation.

Pressure-induced fcc to hcp phase transition in Ni-based high entropy solid solution alloys

A pressure-induced phase transition from the fcc to a hexagonal close-packed (hcp) structure was found in NiCoCrFe solid solution alloy starting at 13.5 GPa. The phase transition is very sluggish and the transition did not complete at ∼40 GPa. The hcp structure is quenchable to ambient pressure. Only a very small amount (<5%) of hcp phase was found in the isostructural NiCoCr ternary alloy up to the pressure of 45 GPa and no obvious hcp phase was found in NiCoCrFePd system till to 74 GPa. Ab initio Gibbs free energy calculations indicated the energy differences between the fcc and the hcp phases for the three alloys are very small, but they are sensitive to temperature. The critical transition pressure in NiCoCrFe varies from ∼1 GPa at room temperature to ∼6 GPa at 500 K.

Delayed damage accumulation by athermal suppression of defect production in concentrated solid solution alloys

ABSTRACT A combined experimental and computational evaluation of damage accumulation in ion-irradiated Ni, NiFe, and NiFeCoCr is presented. A suppressed damage accumulation, at early stages (low-fluence irradiations), is revealed in NiFeCoCr, with a linear dependence as a function of ion fluence, in sharp contrast to Ni and NiFe. This effect, observed at 16 K, is attributed to the complex energy landscape in these alloys that limits defect mobility and therefore enhances defect interaction and recombination. These results, together with previous room-temperature and high-temperature investigations, suggest ‘self-healing’ as an intrinsic property of complex alloys that is not a thermally activated process. IMPACT STATEMENT A combined experimental and computational evaluation reveals a remarkable delayed damage accumulation due to significant athermal suppression of defect production in ion-irradiated concentrated solid solution alloys at 16 K. GRAPHICAL ABSTRACT

X-ray absorption investigation of local structural disorder in Ni1-xFex (x=0.10, 0.20, 0.35, and 0.50) alloys

Defect energetics in structural materials has long been recognized to be affected by specific alloy compositions. Local structural distortion greatly affects the physical properties and performance of alloys. To reveal the atomic-level lattice distortion, the local structures of Ni and Fe in Ni1-xFex (x = 0.10, 0.20, 0.35 and 0.50) solid solution alloys were measured with extended X-ray absorption fine structure (EXAFS) technique. The EXAFS measurements have revealed that the bond length of Fe with surrounding atoms is 0.01–0.02 A larger than that of Ni with its neighbors in the alloys. Both the lattice constant and the interatomic distance of the nearest neighbors increase with the addition of Fe content in the solid solutions. The local bonding environments in Ni1-xFex alloys were also calculated from ab initio and compared with the experimental results.