1,721,016 research outputs found
High-temperature superelasticity of Ni50.6Ti24.4Hf25.0 shape memory alloy
Full text linkChanges in transformation temperatures and high-temperature mechanical behavior of the new Ni50.6Ti34.4Hf25.0 alloy were determined after selected aging treatments. Differential scanning calorimetry was used to compare the homogenized, solution-treated and aged specimens for a wide combination of aging temperatures and times. Isothermal deformation experiments were conducted measuring reversible transformation strains with digital image correlation. Extraordinary superelasticity was found at test temperatures up to 300 °C for the specimen aged at 500 °C for 4 h
Esperienze nell’applicazione di tecniche “compression pre-cracking” per prove di propagazione frattura
No full textConsiderando tecniche di progettazione moderne quali la “Damage Tolerance”, la caratterizzazione delle curve di propagazione è uno degli elementi fondamentali, per la valutazione e la progettazione dell’integrità strutturale dei componenti in servizio, insieme alla performance dei controlli non distruttivi e alla determinazione dei carichi di esercizio. La zona della soglia di propagazione risulta poi particolarmente critica in quanto è lecito pensare che la maggior parte della vita di un componente avvenga nel suo ambito.
Recentemente, è stato però dimostrato che le metodologie tradizionali per la determinazione della soglia di propagazione, quali il “K-decreasing” e il “Kmax=const.”, influenzano il risultato in maniera non conservativa, ovvero possono produrre soglie di propagazione più alte di quelle naturali del materiale. E’ quindi stata proposta una metodologia innovativa basata sulla pre-criccatura in compressione. Tale tecnica, data la sua recente introduzione, è stata finora applicata a pochi casi applicativi. I materiali considerati ad oggi sono le leghe di alluminio, le leghe di titanio e gli acciai ad alta resistenza sollecitati mediante geometrie M(T) e C(T) e a rapporti di ciclo positivi.
La presente memoria si è quindi incentrata nell’applicazione della metodologia innovativa all’acciaio dolce strutturale A1N (tipicamente utilizzato per produrre assili ferroviari) in forma di provini SE(B) sollecitati da un ampio raggio di rapporti di ciclo tra R=-2 e R=0.85
Digital image correlation-based analysis of strain accumulation on a duplex g-TiAl
No full textThis paper discusses the experimental results obtained for a duplex g-TiAl alloy deformed at room temperature. High resolution digital image correlation was used to measure local strain fields. The characterization of the microstructure was performed using optical microscopy and a precise correlation between the strain fields and the microstructure was found. High local strains were measured along well-oriented lamellar grains, in particular for the g-phase. Furthermore, the analysis of the two- dimensional strain tensor shows that the largest strain component is the shear strain in the plane of the lamellar grains. This study aims to provide further understanding of the deformation behavior of g- TiAl alloy
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
Molecular dynamics modeling of NiTi superelasticity in presence of nanoprecipitates
No full textThe presence of nano-sized coherent precipitates is well known to have crucial impact on the mechanical behaviors of a broad class of superelastic alloys. As a representative material, the pseudoelasticity of austenitic NiTi alloy in presence of a lenticular coherent Ni4Ti3 precipitate is investigated using atomic scale simulations. We predict the local stress gradient at the matrix-precipitate interface induced by inter-lattice atomic disregistry. The calculated stress distribution conforms to the latest high resolution electron microscopy measurements in the literature. Due to the presence of the local disturbance fields, the preference for activating different martensitic variants, given the uni-directionality thereof, is influenced substantially. The resultant constitutive attributes are thus observed to undergo adjustments in terms of reduced transformation stress, strain and hysteresis in general agreement with experimental literature
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