1,720,988 research outputs found
Pseudoelasticity in Fe3Ga with boron-A combined atomistic-micromechanical treatment
Full text linkAbstract The Fe3Ga base alloy is known to exhibit pseudoelasticity and the solute hardened Fe3GaB holds considerable promise as well. The present work aims at developing a theoretical model to establish the critical twinning stress in Fe3Ga with varying boron concentration. The theoretical model is based on the atomistic-micromechanical approach where we utilize density functional theory and Peierls Nabarro formalism at an atomic scale and the Eshelbian anisotropic elasticity at microscale. Using local strain measurements at the grain scale, we also show the experimental evidence of effect of boron in elevating the twinning stress in Fe3Ga. The work calculates the interaction energies associated with the presence of boron in octahedral, tetrahedral and xenohedral sites in the D03 lattice and the transition from octahedral to xenohedral sites upon twinning. The model distinguishes the elevation of twinning stress depending on the interstitial site and the transition between the sites
Shape memory functionality under multi-cycles in NiTiHf
No full textThe functionality of NiTiHf (25%Hf) shape memory alloy (SMA) was studied under isobaric temperature cycling experiments. Both the local and overall strains, determined with digital image correlation (DIC), displayed excellent stability and no measurable unrecovered elongation. The external stress levels were higher than 250 MPa and transformation temperature was 420 °C which exceeds other NiTi based alloys, demonstrating the outstanding potential of the NiTiHf alloys
High temperature shape memory behavior of Ni50.3Ti25Hf24.7 single crystals
No full textIn this work we present the high temperature functional behavior of the new Ni50.3Ti25Hf24.7 shape memory alloy (SMA). Very high transformation strains were measured during isobaric experiments at temperatures up to Af = 420 °C. For the [111]B2 orientation in tension, we measured with digital image correlation (DIC) averaged transformation strain of 5.15%, while in small domains 7.74%. For the [011]B2 orientation in compression, we measured averaged transformation strain of 4.6%, locally 5.3%. The remarkable results in terms of actuation strains at T > 400 °C define the Ni50.3Ti25Hf24.7 alloy as one of the most promising shape SMA for high temperature applications
Fatigue crack propagation in haynes 230: A comparison between single and polycrystal crack closure levels
No full textAn experimental campaign was developed to evaluate fatigue crack growth in Haynes 230. The effects of plasticity induced crack closure were investigated with Digital Image Correlation. In particular, crack opening levels were measured with the digital extensometer technique, which allowed the evaluation of crack flanks relative displacements. Experimental results were compared with a reference da/dn-ΔKeff curve and with the data of a previous study, which analyzed single crystal propagation. It was found that the adoption of crack closure local measurements provided an accurate estimation of crack propagation driving forces, since all the experimental points from single crystals and polycrystals collapse onto the da/dn-ΔKeff curve
Shape memory strains and temperatures in the extreme
No full textIt is well known that the achievement of high transformation strains in shape memory alloys (SMAs) has been curtailed by plastic deformation mediated via dislocation slip. In particular, the utilization of SMAs at high temperatures is also hindered by plastic slip. In this paper, an overview of the most important SMAs is provided by constructing transformation strain, transformation temperature, and slip resistance plots to put existing works in perspective. To this plots, we added results on NiTiHf alloys which impart both high temperature capability and high slip resistance at unprecedented levels. The remarkable finding is that NiTiHf alloys can undergo transformation strains near 20% and transformation temperatures exceeding 400 °C
Slip nucleation in single crystal FeNiCoCrMn high entropy alloy
No full textWe present an experimental and theoretical study of slip nucleation in high entropy FeNiCoCrMn alloy which possesses superb mechanical properties. Uniaxial compression experiments were conducted on the 59 ̄1 oriented single crystals. The single crystals permit evaluation of critical resolved shear stress (CRSS), and using Digital Image Correlation strain measurements it is possible to track the slip nucleation precisely. The experimental value of 175 MPa for CRSS is in excellent agreement with 178 MPa obtained utilizing an advanced atomistic-modified Peierls-Nabarro modeling formalism. This close agreement demonstrates the efficacy of our methodology and has implications in design of new high entropy alloys
Ultrahigh tensile transformation strains in new Ni50.5Ti36.2Hf13.3 shape memory alloy
No full textWe report on unprecedented transformation strains exceeding 20% in tension for Ni50.5Ti36.2Hf13.3 shape memory alloy (SMA). The strain measurements were made at multiscales utilizing advanced digital image correlation. The display of excellent strain reversibility in shape memory (isothermal deformation between Mf and Af), isobaric thermal cycling (between Mf and Af), and superelasticity experiments (deformation above Af) confirms a wide range of functionality. The ultrahigh strains in [111] orientation exceed the lattice deformation theory predictions possibly pointing to contributions from mechanical twinning effects. The high strength levels and large strains result in very high work outputs compared to other SMAs
Fatigue crack growth in Haynes 230 single crystals: An analysis with digital image correlation
Full text linkFatigue crack growth was investigated in Haynes 230, a nickel-based superalloy. Aniso- tropic stress intensity factors were calculated with a least squares algorithm using the dis- placements obtained from digital image correlation. Crack opening/sliding levels were measured by analysing the relative displacement of crack flanks. Reversed crack tip plastic zones were calculated adopting an anisotropic yield criterion. The strains measured in the reversed plastic zone by digital image correlation showed a dependence on crystallo- graphic orientation. Finally, a finite element model was adopted to examine plasticity around the crack tip. Results were compared with the experimentally observed strains
Shape memory behavior in Fe3Al-modeling and experiments
Full text linkThe Fe3Al alloy with D03 structure exhibits large recoverable strains due to reversible slips. Tension and compression experiments were conducted on single crystals of Fe3Al, and the onset of slip in forward and reverse directions were obtained utilizing high-resolution digital image correlation technique. The back stress provides the driving force for reversal of deformation upon unloading, resulting in a superelastic phenomenon as in shape memory alloys. Using density functional theory simulations, we obtain the energy barriers (GSFE - generalized stacking fault energy) for 1 1 0〈1 1 1〉 and 1 1 2〈1 1 1〉 slips in D03 Fe3Al and the elastic moduli tensor, and undertake anisotropic continuum calculations to obtain the back stress and the frictional stress responsible for reversible slip. We compare the theoretically obtained slip stress magnitudes (friction and back stress) with the experimental measurements disclosing excellent agreement
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