Plastic deformations in crystal, polycrystal, and glass in binary mixtures under shear: Collective yielding

Using molecular dynamics simulation, we examine the dynamics of crystal, polycrystal, and glass in a Lennard-Jones binary mixture composed of small and large particles in two dimensions. The crossovers occur among these states as the composition c is varied at fixed size ratio. Shear is applied to a system of 9000 particles in contact with moving boundary layers composed of 1800 particles. The particle configurations are visualized with a sixfold orientation angle alpha_j(t) and a disorder variable D_j(t) defined for particle j, where the latter represents the deviation from hexagonal order. Fundamental plastic elements are classified into dislocation gliding and grain boundary sliding. At any c, large-scale yielding events occur on the acoustic time scale. Moreover, they multiply occur in narrow fragile areas, forming shear bands. The dynamics of plastic flow is highly hierarchical with a wide range of time scales for slow shearing. We also clarify the relationship between the shear stress averaged in the bulk region and the wall stress applied at the boundaries.Comment: 17 pages, 15 figures, to appear in Physical Review

Effect of composition on the mechanical properties of newly developed Ti<SUB>2</SUB>AlNb-based titanium aluminide

Among titanium aluminides Ti2AlNb-based alloys have received considerable attention in recent years, as potential materials for high-temperature applications in jet engines. Several compositions based on Ti-22Al-25Nb with molybdenum, silicon, zirconium additions were developed. The investigated microstructures consist of bi-lamellar O-Ti2AlNb structure with α2-Ti3Al nodules, surrounded by a B2 matrix. Effects of Aluminium and Niobium contents are, especially, examined on tensile properties, on creep strength and on oxidation resistance. It is shown that a high Al/Nb ratio is required for a good oxidation resistance, but is associated with a loss of ductility. Reducing Nb results in a decrease of creep strength that can be compensated by zirconium and/or molybdenum additions which improve the creep resistance significantly without impairing ductility or yield stress

Microstructure and mec-hanical properties of orthorhombic alloys in the Ti-Al-Nb system

The current understanding of the metallurgy of the orthorhombic alloys in the Ti-Al-Nb system is reviewed with emphasis on tensile and creep properties of ternary alloys. It is shown that increasing the Nb content of alloys from 15 to 27 at% at a constant Al level significantly increases both the tensile and creep properties of equiaxed as well as lath structures, while small changes in Al content have a large effect on creep. For a given alloy composition, the amount of B2(β) phase and its distribution and the scale of O laths influences tensile properties, while creep properties depend on the volume fraction of equiaxed α2/O phase present in the structure as well as the size of O laths

Monotonic and low cycle fatigue behavior of an O+B2 alloy at high temperatures

Low cycle fatigue behavior of an O+B2 alloy was evaluated at 650 degrees C in ambient atmosphere under fully reversed total axial strain controlled mode. Three different microstructures, namely equiaxed O plus aged B2 (fine O plates in B2 matrix), lenticular O laths plus aged B2 and a pancake composite microstructure comprising equiaxed alpha 2, lenticular O and aged B2, were selected to study the effect of microstructure on low cycle fatigue behavior in this class of alloys. Distinct well-defined trends were observed in the cyclic stress-strain response curves depending on the microstructure. The cyclic stress response was examined in terms of softening or hardening and correlated with microstructural features and dislocation behavior. Fatigue life was analyzed in terms of standard Coffin-Manson and Basquin plots and for all microstructures a prevailing elastic strain regime was identified, with a single slope for microstructures equiaxed and composite and a double slope for lenticular O laths