Microstructural evolution, twinning, nanoporous superalloys and deformation mechanisms

February 20, 2011

[1] In situ evaluation of the microstructure evolution during rapid hardening of an Al–2.5Cu–1.5Mg (wt.%) alloy

A Deschamps et al

The kinetics of the microstructural evolution during ageing at 200 °C of an Al–2.5%Cu–1.5%Mg (wt.%) alloy is evaluated using nuclear magnetic resonance and in situ small-angle X-ray scattering. This alloy is known to exhibit “rapid hardening” upon artificial ageing, where 50–70% of the total age hardening increment is reached within minutes at elevated temperatures. It is shown that formation of small solute-rich entities which correspond to Cu–Mg clusters or GPB zones occurs within seconds at 200 °C. These entities have radius not, vert, similar0.5 nm, volume fraction not, vert, similar2% and Cu concentration not, vert, similar30 at.%. Within the stage of constant yield strength (rapid hardening plateau) at this temperature, they are shown to be the dominant constituent of the precipitate microstructure and to be extremely stable in terms of size and volume fraction. The S phase is observed to nucleate within the hardening plateau, but remains a minor constituent until the end of this stage. Based on the quantitative measurement of their size and volume fraction, the obstacle strength of these solute-rich clusters is calculated to be about one-tenth of the Orowan strength. The energy required to shear these clusters (not, vert, similar0.5 eV) is shown to be compatible with existing atomistic calculations of similar objects.

[2]Effects of surface damage on twinning stress and the stability of twin microstructures of magnetic shape memory alloys

Markus Chmielus et al

Twinning is the primary deformation mechanism in magnetic shape memory alloys (MSMAs). Obstacles such as inclusions, precipitates and defects hinder or even prevent twin boundary motion in the bulk of Ni–Mn–Ga MSMA single crystals. Here, we study the effect of surface damage on the mechanical properties and twin structure of Ni–Mn–Ga single crystals. Any methods that produce defects may be considered for modifying the near-surface microstructure. In this study deformations were produced by grinding and mechanical polishing using abrasive particles. The amount of damage was characterized with X-ray diffraction: damage causes peak broadening. Deformation and damage localized near the surface increases the twinning stress. Surface damage stabilizes a densely twinned microstructure. The twins are thin but extend over the entire sample and allow a large strain to be accommodated at moderate stress. This effect is critical for preventing damage accumulation in high-cycle magnetomechanical actuation and for achieving high dynamic performance.

[3] Production of nanoporous superalloy membranes by load-free coarsening of γ′-precipitates

B Hinze et al

Nickel-based superalloys are predominantly used as structural materials in high-temperature applications due to their exceptional high-temperature strength based on precipitation hardening by the coherent γ′-phase. In modern superalloys, the γ′-phase fraction can amount to 75%, at which γ′-precipitates align as cubes parallel to the left angle bracket0 0 1right-pointing angle bracket directions of the crystal lattice. At high temperatures and under a mechanical load, e.g. during service in gas turbines, the γ′-cubes coalesce to γ′-rafts, generating an interpenetrating microstructure of γ and γ′. By extracting one phase of this interpenetrated network, nanoporous superalloy membranes containing channel-like interconnected pores are produced. So far, this can only be achieved by applying simultaneously thermal and mechanical loads during tensile creep deformation. Here, a new production process is presented. Due to internal stresses, load-free aging of single-crystalline superalloys, e.g. CMSX-4, also generates an interpenetrating microstructure of γ and γ′. This can be utilized to manufacture nanoporous superalloy membranes in the absence of an external mechanical load. The advantage of this process is its simplicity and the potential to fabricate larger membranes than possible by the costly tensile creep deformation process currently used.

[4] Deformation mechanisms of nanograined metallic polycrystals

Saada and Kruml

This paper is an attempt to discuss the relevance of the physical concepts used to describe the plastic flow behaviour of a wide class of nanograined metallic polycrystals, by critically analyzing recent experimental observations on nanograined Ni and Cu. The paper focuses on the description of the elastic–plastic transition, and of the strain rate sensitivity. Using the generally accepted assumption that plastic flow results from dislocation nucleation at grain boundaries, it is shown that two deformation regimes must be distinguished: a nucleation-controlled mechanism at low strain rates, and a combined nucleation and propagation mechanism at high strain rates. At low deformation rates, the average nucleation rate is determined either from knowledge of the stress–strain curve, or from analysis of creep or relaxation kinetics. The strain rate sensitivity is shown to be related to the effect of stress on the nucleation rate.

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