High repetition rate femtosecond laser heat accumulation and ablation thresholds in cobalt-binder and binderless tungsten carbides

Femtosecond (fs) laser ablation has been studied for the potential of fast, high precision machining of difficult-to-machine materials like binderless tungsten carbide. Obstacles that have limited its efficiency include melting from heat accumulation (HA), particle shielding, and plasma shielding. To address HA without shielding effects, high repetition rate (57.4 MHz), ultra-low fluence fs laser irradiation is performed to study the incubation effect and subsequent HA-ablation threshold of fine-grained tungsten carbides. Exposure times on the order of 100 ms were conducted in air with fluences (1.82 to 9.09 mJ/cm2) two orders of magnitude below the single fs pulse ablation thresholds reported in literature (0.4 J/cm2). Heat accumulation at high repetition rate explains the ultra-low fluence melt threshold behavior resulting in melt crowns around ablated holes and grooves. The results of this study aid in predicting heat buildup in high repetition rate laser irradiation for applications that wish to achieve high ablation rates of difficult-to-machine, ultrahard materials and help enable shaping of binderless tungsten carbide for use in applications too extreme for bindered tungsten carbide

Computational modeling of effects of alloying elements on elastic coefficients

Models for composition and temperature dependencies of single-crystal elastic stiffness coefficients are developed and applied to the Al12Mg17 and hexagonal closed-packed solution phases in the Mg–Al system based on data from first-principles calculations. In combination with models for multi-phases, the bulk, shear, and Young’s moduli of Mg–Al alloys are predicted and compared with available experimental data in the literature. It is noted that both phase transition and grain boundary sliding may play important roles in the elastic coefficients as a function of temperature

Dislocation–twin interactions in nanocrystalline fcc metals

Dislocation interaction with and accumulation at twin boundaries have been reported to significantly improve the strength and ductility of nanostructured face-centered cubic (fcc) metals and alloys. Here we systematically describe plausible dislocation interactions at twin boundaries. Depending on the characteristics of the dislocations and the driving stress, possible dislocation reactions at twin boundaries include cross-slip into the twinning plane to cause twin growth or de-twinning, formation of a sessile stair-rod dislocation at the twin boundary, and transmission across the twin boundary. The energy barriers for these dislocation reactions are described and compared

Grain refinement vs. crystallographic texture: Mechanical anisotropy in a magnesium alloy

A magnesium alloy was subjected to severe plastic deformation via an unconventional equal channel angular extrusion route at decreasing temperatures. This method facilitates incremental grain refinement and enhances formability by activating dynamic recrystallization in the initial steps and suppressing deformation twinning. Compression experiments in three orthogonal directions demonstrated high strength levels in the processed sample, up to 350 MPa in yield and 500 MPa in ultimate strengths. Notable flow stress anisotropy is correlated with the processing texture and microstructure

First-principles calculations of twin-boundary and stacking-fault energies in magnesium

The interfacial energies of twin boundaries and stacking faults in metal magnesium have been calculated using first-principles supercell approach. Four types of twin boundaries and two types of stacking faults are investigated, namely, those due to the (1 0 1 1) mirror reflection, the (1 0 1 1) mirror glide, the (1 0 1 2) mirror reflection, the (1 0 1 2) mirror glide, the I1 stacking fault and the I2 stacking fault. The effects of supercell size on the calculated interfacial energies are examined. Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Magnesium; Interfaces; Twinning; First-principles calculation Magnesium alloys are increasingly being used in a wide range of applications due to their light weight and high strength. One of the current research frontiers on Mg alloys is to understand, estimate and improve their low plastic formability to operate under increasingly demanding conditions. At the atomic scale, the plastic formability is closely related to the ease of the formation of planar defects along the close-packed planes, namely, twin and stacking faults Since we employ a first-principles approach with periodic boundary conditions, the interfaces due to twinning and stacking faults are modeled using a supercell. In fact, the crystallographic theory of twinning [2] is rather complicated for a hexagonal close-packed (hcp) metal. For the special cases of (1 0 1 1) and (1 0 1 2) twins, following Morris et al

Tantalum sheet with improved copper-tantalum co-deformability

Poor co-deformation behavior of commercial polycrystalline tantalum (Ta) sheet used for tin diffusion barriers in Nb3Sn composite superconductors leads to the use of more Ta than is necessary in these conductors, and to premature Ta layer fracture during wire drawing. These problems arise because the Ta layer deforms nonuniformly in the composite conductor as it is reduced in thickness. The origin of the problem resides in the microstructure of the Ta and the co-deformation mechanics of relatively strong BCC Ta with surrounding weaker and more ductile FCC Cu. In an attempt to remedy this problem, 25mm square bars of Ta were processed by multi-axis severe plastic deformation (SPD) via equal channel angular extrusion (ECAE), then rolled to sheet and annealed. The SPD processing refined the microstructure and reduced nonuniformities in grain size and texture. Measurements of grain size, microstructural uniformity, Ta layer thickness, and Cu-Ta interface roughness are reported. The Ta sheet incorporating an SPD processing step during its manufacture co-deforms more uniformly with pure Cu than does commercially available Ta sheet