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RISS 인기검색어
- 검색키워드
- 저자 : Lavenstein, Steven (검색결과 2 건)
- 무료
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High frequency <i>in situ</i> fatigue response of Ni-base superalloy René-N5 microcrystals
Lavenstein, Steven,Crawford, Bryan,Sim, Gi-Dong,Shade, Paul A.,Woodward, Christopher,El-Awady, Jaafar A. Elsevier 2018 Acta materialia Vol.144 No.-
<P><B>Abstract</B></P> <P>A novel <I>in situ</I> scanning electron microscope (SEM), high frequency fatigue testing methodology is developed using a combination of laser milling, focused ion beam fabrication and nanoindentation. This methodology is used to investigate crack initiation, propagation, fracture, fatigue life, and the mechanical response of microcantilever samples of a Ni-based superalloy (René-N5) under different cyclic strain amplitudes. The crack initiation and propagation in the microcantilever is monitored by observing changes in the beam's dynamic stiffness and continuous SEM imaging. The dynamic stiffness response of the micro-beams exhibits a transition from softening to hardening at a critical strain amplitude of 7 × <SUP> 10 − 3 </SUP> . Theoretical analysis indicates that this transition corresponds to the stress required to shear <SUP> γ ′ </SUP> precipitates. SEM imaging reveals the evolution of significant extrusions, intrusions, and slip traces during cyclic loading above this critical strain amplitude. Below this strain amplitude, very little surface roughening is observed. In addition, the measured dynamic stiffness is observed to exhibit two regimes of decrease after crack initiation. These two regimes correspond to short and large crack propagation. Finally, an overall increase in fatigue life is observed when comparing to bulk scale experiments on nickel-base superalloys. It is proposed that this is an inherent size effect in the small-volume, single crystal specimens tested.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
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Anomalous hardening in magnesium driven by a size-dependent transition in deformation modes
Sim, Gi-Dong,Kim, Gyuseok,Lavenstein, Steven,Hamza, Mohamed H.,Fan, Haidong,El-Awady, Jaafar A. Elsevier 2018 Acta materialia Vol.144 No.-
<P><B>Abstract</B></P> <P>Here, we report a comprehensive study that combines <I>in situ</I> scanning electron microscopy experiments and atomistic simulations to quantify the effect of crystal size on the transformation in deformation modes in a-axis oriented Mg single crystals at room temperature. The experimental results indicate that the deformation is dominated by the nucleation and propagation of tensile twins. The stress required for twin propagation was found to increase with decreasing sample size, showing a typical “smaller is stronger” behavior. Furthermore, an anomalous increase in strain hardening is first reported for microcrystals having diameters larger than ∼18 μm, which is induced by twin-twin and dislocation-twin interactions. The hardening rate gradually decreases toward the bulk response as the microcrystal size increases. Below 18 μm, deformation is dominated by the nucleation and propagation of a single tensile twin followed by basal slip activity in the twinned crystal, leading to no apparent hardening. In addition, molecular dynamics simulations indicate a transition from twinning mediated plasticity to dislocation mediated plasticity for crystal sizes below a few hundred nanometers in size. A deformation mechanism map for twin oriented Mg single crystals, ranging from the nano-scale to bulk scale is proposed based on the current simulations and experiments. The current predicted size-affected deformation mechanism of twin oriented Mg single crystals can lead to better understanding of the competition between dislocations plasticity and twinning plasticity.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
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