Structural relaxation and rejuvenation in a metallic glass induced by shot-peening

The metallic glass Pd₄₀Cu₃₀Ni₁₀P₂₀ in as-cast or pre-annealed states was shot-peened successively at room temperature or at 77 K. The structural state of the glass was characterized by the relaxation spectrum measured in a differential scanning calorimeter. Mechanically induced relaxation of the as-cast glass and mechanically induced rejuvenation of pre-annealed samples are both more evident at 77 K than at 298 K, enabling deductions about the underlying mechanisms. The relaxation spectrum of the glass as a function of temperature displays two broad maxima, which occurring at the higher temperature is attributed to the part of the free-volume distribution associated with flow defects. In samples shot-peened at 77 K, the stored energy after deformation can be as high as 20% of the cold work. Shot-peening simultaneously generates flow defects within shear bands and destroys them in the matrix between bands: whose effect dominates is principally dependent on the initial state of relaxation of the glass. Shot-peening of partially crystallized samples appears capable of breaking up and dispersing crystallites without inducing any further significant crystallization.     

A thermodynamic model for nanocrystallization in bulk metallic glasses

The structural complexity of glass-forming alloys, which generally contain more than three components, can lead by partial crystallization during annealing to a dispersion of nanocrystals in an amorphous matrix, giving the material a very high mechanical strength. In the present study, the evolution of the driving force for crystallization is expressed as a function of the composition and the chemical potentials of the components. Application to Zr₆₀Al₁₀Cu₃₀ and Zr₆₀Al₁₀Cu₂₀Pd₁₀ bulk metallic glasses shows that the first crystallization step leads to a metastable equilibrium between nanocrystals of an intermetallic and a percolating amorphous phase. The effects of the number of components and of chemical bonding on the fraction crystallized is analysed and discussed.     

Self-assembly-assisted growth of two-dimensional Penrose tiling

An algorithm for the growth of two-dimensional Penrose tiling, based on symmetry operations on a seed rhombus, is discussed and demonstrated. Independent of empirical matching rules, as suggested by Penrose [Bull. Inst. Math. Appl. 10 266 (1974)], and also overcoming the scaling-down operation, as proposed by Ramachandrarao et al. (Acta Crystallogr. A 47 210 (1991)], the present algorithm follows a mechanism akin to self-assembly and can be continued ad infinitum.     

Deformation twinning and plastic recovery in Cu/Ag nanolayers under uniaxial tensile straining

Molecular dynamics simulations of nanoscale multilayered Cu/Ag metals have been conducted under uniaxial tensile straining to elucidate the out-of-plane deformation behaviour relevant to tensile spallation experiments. In nanolayers with pristine architectures, plastic dissipation prior to crack nucleation is suppressed owing to the high hydrostatic tensile stress state. The presence of sources for micro-twinning partials, such as micro-cracks or columnar grain boundaries, result in abundant deformation twin lamellae across multiple Cu/Ag interlayers. Plastic recovery is observed during unloading of the nanolayers.     

Formation of FeNi with L10-ordered structure using high-pressure torsion

FeNi with the L1₀-ordered structure is formed over an astronomical timescale in meteorites. In this study, severe plastic deformation using high-pressure torsion (HPT) is employed for the production of the L1₀ structure in the laboratory. Its formation is confirmed by X-ray diffraction analysis and transmission electron microscopy. Processing of elemental nanopowders by HPT is an effective method for the formation of the L1₀ phase, which is enhanced by the addition of Co to FeNi or annealing after HPT. The formation of the phase must be associated with enhanced diffusion through HPT.     

Loss of coherency of the alpha/beta interface boundary in titanium alloys during deformation

The loss of coherency of interphase boundaries in two-phase titanium alloys during deformation was analyzed. The energy of the undeformed interphase boundary was first determined by means of the van der Merwe model for stepped interfaces. The subsequent loss of coherency was ascribed to the increase of interphase energy due to absorption of lattice dislocations and was quantified by a relation similar to the Read–Shockley equation for low-angle boundaries in single-phase alloys. It was found that interphase boundaries lose their coherency by a strain of approximately 0.5 at T = 800°C.     

The kinetics of infralow-frequency viscoelastic internal friction induced by irreversible structural relaxation of a metallic glass

This letter presents the results of internal-friction measurements of Co 70 Fe 5 Si 15 B10 metallic glass at temperatures 400K h T h 700K and frequencies 0.01 Hz h f h 0.05Hz. It is shown that Maxwellian viscoelastic damping is predominant in this case. The nature of this damping is determined by irreversible structural relaxation. The kinetic relaxation law is derived.     

A phase-field model for deformation twinning

We propose a phase-field model for modeling microstructure evolution during deformation twinning. The order parameters are proportional to the shear strains defined in terms of twin plane orientations and twinning directions. Using a face-centered cubic Al as an example, the deformation energy as a function of shear strain is obtained using first-principle calculations. The gradient energy coefficients are fitted to the twin boundary energies along the twinning planes and to the dislocation core energies along the directions that are perpendicular to the twinning planes. The elastic strain energy of a twinned structure is included using the Khachaturyan's elastic theory. We simulated the twinning process and microstructure evolution under a number of fixed deformations and predicted the twinning plane orientations and microstructures.     

General analytical solution for stress intensity factor of a hypocycloid hole with many cusps in an infinite plate

In this article, the problem for the determination of the displacement functions and the stress intensity factors (SIFs) around a boundary of a hypocycloid hole with cusps in an infinite elastic plate subject to normal and shear stresses are presented. A hole with cusps (hypocycloid) is mapped onto a unit circle and the modified complex potential is used for solving the relevant boundary value problems. An analytical solution for the SIF of a hypocycloid hole is obtained. For a special case, our results agree with others.