China Science

Micron-sized aluminum pillars, fabricated by focused-ion beam milling, were subjected to compression in a nanoindenter using a flat-ended tip to examine their deformation behavior. The deformation was jerky and the statistical distributions of the sizes of the bursts, their occurrence frequency, as well as the stresses at which they occurred were analyzed. The burst size was found to increase with stress in an approximately exponential manner. Post-mortem transmission electron microscopy investigation of the dislocation structures revealed that the dislocation density of the micro-pillars did not grow significantly after severe deformation. Based on the experimental observations, a Monte Carlo model was developed to describe the stochastic nature of deformation of these micro-pillars.

K.S. Nga?A.H.W. Ngan aEmail:hwngan@hku.hk
[a]Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, China

A considerable amount of B2 phase with a cellular morphology is retained in a 4Zr–4Nb-containing TiAl-based alloy. Heterogeneous precipitation of ordered ? from B2 is found to occur readily after HIPping: B2 ? ? with the B82-structure in cell regions and B2 ? ? with the D88-structure in cell-wall regions. Congregated D88-? domains and particles form as a network surrounding the well-developed B82-? cells. The heterogeneous formation of different ? variants is caused by a heterogeneous distribution of Zr + Nb elements across B2, which plays an important role in stabilizing vacancies and promotes the formation of D88-?. Fine D88-? particles are also observed to precipitate from the B82-? cell matrix after ageing at 700 °C for 1000 h, showing a transformation path of ? ? B2 ? B82-? ? D88-? for the aged cells. The heterogeneous formation of a D88-? network and B82-? cells is found to be detrimental to ductility and fatigue strength. A very brittle fine-grained TiAl alloy is produced as a result.

Z.W. Huang aEmail:z.w.huang@bham.ac.uk
[a]School of Materials Science; Engineering, Southwest Jiaotong University, Chengdu, Sichuan 610031, PR China;[b]Department of Metallurgy; Materials, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK

The anisotropy of fracture toughness in AA2139 (Al–Cu–Mg) alloy sheet has been investigated via synchrotron radiation computed tomography of arrested cracks in Kahn tear test pieces for different loading cases. The three-dimensional distribution and morphology of pores and defects in the as-received state are seen to be anisotropic, with chains of voids and void elongation in the L (longitudinal) direction. For toughness testing in L–T orientation (T is long transverse), voids ahead of the crack grow and link in the L direction. In T–L tests, voids ahead of the crack tip also grow in the loading direction, although a high degree of alignment is retained in the L direction. The present work provides quantitative microstructural data that can be used as input for and validation of recent idealized model formulations, and it is shown that the measured void dimensions and evolution are consistent with measured toughness anisotropy.

T.F. MorgeneyeraEmail:tm504@soton.ac.uk?M.J. Starinka?I. Sinclaira
[a]Materials Research Group, School of Engineering Sciences, University of Southampton, Southampton SO17 1BJ, UK;[b]Alcan Centre de Recherches de Voreppe, BP 27, 38341 Voreppe Cedex, France

Constitutive equations are constructed for single-crystal nickel-based superalloys. Account is taken of dislocation glide in the channels of the matrix phase (referred to as ?) of the face-centred cubic (fcc) type, dislocation climb at the interfaces with the reinforcing L12 precipitates (referred to as ??) and the processes leading to cutting of the interfaces by dislocation ribbons via stacking fault shear of the type. A treatment of ribbons produced by the combination of channel dislocations by an appropriate set of dislocation reactions is developed. The model allows the following features of superalloy creep to be recovered: dependence upon microstructure and its scale, effect of lattice misfit, internal stress relaxation, incubation phenomena, the interrelationship of tertiary and primary creep, and vacancy condensation leading to damage accumulation. Using the model, the creep deformation behaviour of the single-crystal superalloy CMSX-4 is studied, with emphasis on the interrelationship between primary and tertiary creep. It is shown that the values for the various parameters used in the modelling are physically reasonable and are related to the microstructure and its evolution during creep. The creep anisotropy prevalent in these materials due to primary creep is recovered correctly.

A. MaaEmail:a.ma@imperial.ac.uk?D. Dyea?R.C. Reedb Email:r.reed@birmingham.ac.uk
[a]Department of Materials, Imperial College London, Prince Consort Road, London SW7 2BP, UK;[b]Department of Metallurgy; Materials, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK

Analytical transmission electron microscopy was used to characterize precipitation processes in a model Al–4.0Cu–0.3Mg (wt.%) alloy aged at 200 °C. The evolution of microstructure was more complex than previously reported and involved processes common both to the binary Al–Cu and ternary Al–Cu–Mg systems. We report precipitation of a novel orientation of the common intermediate phase ??, which we have designated . Isomorphous with the well-known body-centred tetragonal Al2Cu phase that occurs in Al–Cu-based alloys, we observed the following novel orientation: , . A trace of Mg was associated with the orientation and we suggest that this is a consequence of the role that Mg atom clusters play during nucleation. For the first time, we also report the occurrence of the ? phase (Al5Cu6Mg2) in a new orientation analogous to that of , designated as and . These results are discussed in terms of microstructural design for age hardening in Al–Cu–Mg base alloys, much of which involves the ?? and ? precipitate phases.

During fusion welding, the presence of sulfur in steel often affects heat and fluid flow in the weld pool and its geometry. While the role of sulfur during welding of stainless steel plates with the same sulfur content is well understood, welding of stainless steel plates containing different concentrations of sulfur has not yet received proper attention. Here we report an experimental and modeling investigation of gas tungsten arc butt welding of stainless steel plates containing different sulfur concentrations. The main variables studied were sulfur concentrations in the two plates, welding current and welding speed. The results show significant shift of the fusion zone toward the low sulfur steel. The asymmetric fusion zone profile with respect to the original joint interface could be quantitatively explained through numerical modeling of heat transfer and fluid flow considering a bead shift observed experimentally.

S. Mishraa?T.J. Lienertb?M.Q. Johnsonb?T. DebRoyc Email:debroy@psu.edu
[a]Department of Metallurgical Engineering; Materials Science, Indian Institute of Technology Bombay, Mumbai, India;[b]Materials Science; Technology: Metallurgy Group, Los Alamos National Laboratory, Los Alamos, NM, USA;[c]Department of Materials Science; Engineering, The Pennsylvania State University, University Park, PA, USA

The objective of the study is to understand the nanoscale near-surface deformation response of two polymer nanocomposite systems with significant differences in ductility during nanoscratching with a Berkovich indenter using a load of 1 mN and a scratch velocity of 1 ?m s?1. An accompanying objective is to investigate the commonality in surface deformation behavior between nano- and microscale deformation to reinforce the underlying fundamental principles governing surface deformation. An understanding of surface deformation response is accomplished through determination of physical and mechanical properties, structural characterization and electron microscopy analysis of surface deformation tracks and residual plastically deformed structures. The study suggests for the first time that the understanding derived from microscale surface deformation studies can be extended to nanoscale surface deformation. The microscale response in a polypropylene-based system is characterized by periodic multiple ripple-type deformation tracks that form via a mechanism identical to the periodic single-ripple-type tracks during nanoscale deformation. Similarly, in a less ductile polyethylene-based system, the periodic parabolic tracks and ironing mode of deformation during microscale deformation tend to be significantly reduced in intensity, with ironing being the primary deformation mechanism at the nanoscale. The surface deformation topography suggests that both micro- and nanoscale response is material specific. Additionally, the study suggests that reinforcement of polymers with nanoclay is a viable route to significantly decrease the susceptibility of polymeric materials to micro- and nanoscale deformation and can be discussed in terms of physical and mechanical properties of materials notably percentage crystallinity and elastic recovery.

Q. Yuana?N. Ramisettia?R.D.K. MisraaEmail:dmisra@louisiana.edu
[a]Center for Structural; Functional Materials, University of Louisiana at Lafayette, P.O. Box 44130, Lafayette, LA 70504-4130, USA;[b]Department of Chemical Engineering, University of Louisiana at Lafayette, P.O. Box 44130, Lafayette, LA 70504-4130, USA

The nucleation (to a limited extent), growth and coarsening behavior of Cu-rich precipitates in a concentrated multicomponent Fe–Cu-based steel aged at 500 °C from 0.25 to 1024 h is investigated. The temporal evolution of the precipitates, heterophase interfaces, matrix compositions and precipitate morphologies are presented. With increasing time, Cu partitions to the precipitates, Ni, Al and Mn partition to the interfacial region, whereas Fe and Si partition to the matrix. Coarsening time exponents are determined for the mean radius, R(t), number density, NV(t), and supersaturations, which are compared to the Lifshitz–Slyzov–Wagner (LSW) model for coarsening, modified for concentrated multicomponent alloys by Umantsev and Olson (UO). The experimental results indicate that the alloy does not strictly follow UO model behavior. Additionally, we delineate the formation of a Ni–Al–Mn shell with a stoichiometric ratio of 0.51:0.41:0.08 at 1024 h, which reduces the interfacial free energy between the precipitates and the matrix.

R. Prakash Kolli aEmail:rpkolli@nalco.com?David N. SeidmanaEmail:d-seidman@northwestern.edu
[a]Department of Materials Science; Engineering, Northwestern University, Northwestern University Center for Atom-Probe Tomography [NUCAPT], Evanston, IL 60208, USA