The present work emphasises the potentialities of dislocation dynamics simulations for performing analyses of crystal plasticity and obtaining information that cannot be reached by experiment. As an example, a study is presented of parameters that are critical for modeling strain hardening in face-centred cubic crystals, the mesoscopic coefficients of interaction between non-coplanar slip systems.
A 12-T magnetic field was introduced during the austenitic decomposition of a high carbon steel at various cooling rates. It was found that the magnetic field applied can reduce the amount of proeutectoid cementite, increase the lamellar spacing of pearlite and decrease the frequencies of low angle misorientations in ferrite. The experimental results of this work prove the shift of the eutectoid point to high carbon side and high temperature side by the application of a magnetic field.
La permeabilite et la diffusivite de l'hydrogene dans l'acier inoxydable 304 augmente comme le degre de sensibilisation croit. Ceci est du a la depletion en chrome de la region des joints de grains. Confirmation par un modele de l'effet de la sensibilisation. Determination a partir de ce modele de la diffusivite dans la zone des joints de grains. La solubilite apparente augmente avec le temps de sensibilisation
The establishment of structure-performance relationship of catalysts during a full life cycle is highly desirable for catalyst design and understanding of nature of active sites. As a case study, lower olefins synthesis directly from CO2 hydrogenation (CTO) was performed over iron catalyst and the catalyst structure evolution during its life cycle has been comprehensively studied. Particularly, operando techniques allow direct observation of the continuous phase transition during activation (Fe2O3 → Fe5C2), reaction and deactivation (Fe5C2 → Fe3O4), and regeneration (Fe3O4 → Fe5C2) process, respectively. After a combined CO2-CO regeneration, 91.8% of the initial activity and 97.7% of the initial C2–C4 olefins selectivity were recovered. Our results indicate that the Fe5C2 phase is the active phase for C2–C4 olefins production. The iron carbide phases are irreversibly oxidized to Fe3O4 under CTO conditions, which is the major factor responsible for the catalyst deactivation. This study not only reveals the dynamic structure evolution of iron catalyst during the full life cycle in CTO, but also endows deep insight into the underlying deactivation mechanism.
Il existe quatre types de precipites dans l'acier ferritique avant recuit. Des variations microstructurales importantes apparaissent lors d'un recuit a 600 C ou a des temperatures superieures. Mise en evidence lors d'un recuit a 700 C, d'une reaustenisation et d'une transformation martensitique. Un recuit a 800C conduit a la precipitation de carbures
Determination par mesures de la conductivite electrique et par frottement interne des temperatures de transformation martensitique et premartensitique en fonction du temps de vieillissement (1 a 100 heures) dans l'alliage vieilli a 400 C. Comparaison entre les deux methodes de mesure
Etablissement d'un modele physique pour decrire le processus de clivage dans un acier bas carbone permettant une meilleure comprehension du parametre qui dicte le comportement de rupture par clivage.
Observation par microscopie optique, microscopie electronique en transmission et spectrometrie dispersive d'un eutectique quasi-binaire dans le systeme ternaire Al-Ni-Re. L'eutectique consiste en une faible fraction volumique de fibres α-Re dans une matrice β-NiAl. Determination de la composition eutectique. Determination de la tenacite a la rupture a temperature ambiante de β-NiAl
Determination pour les materiaux polycristallins de symetrie hexagonale ou rhomboedrique des orientations des macles les plus probables du point de vue du modele de coincidence. Aucune restriction n'est imposee pour les parametres cristallins (c/a), mais seules les valeurs les plus basses de la multiplicite Σ sont considerees. Presentation de 3 exemples pour la symetrie hexagonale dans WC, Ti et Zr
Using forward recoil energy spectroscopy (FRES), it was found that evaporated films of a variety of metals can absorb on the order of 1 at.% deuterium (D). This is several orders of magnitude greater than would be expected based on lattice solubility, showing that these films contain a large concentration of defects which trap D. The nature of this trapping was studied using iron as a model system. The defects which trapped D annealed out at ∼500 K and were present in higher concentrations in polycrystalline films than in single crystalline films. Trap concentration decreased with increasing substrate temperature during deposition. These results indicate that the traps may be associated with either voids or grain boundaries.
Hardened surface layers such as those produced by ion implantation can inhibit the emergence of persistent slip bands (PSBs) at the free surface during low cycle fatigue, thereby extending crack initiation life. They have little effect, however, on bulk PSB formation. PSBs nucleated in the interior eventually impinge on the underside of the surface layer, producing localized cyclic shear strains in the film which lead to film rupture and rapid crack initiation. As a result, increases in fatigue life are modest. The present finite element model analysis shows that PSBs carrying plastic strains characteristic of fatigued f.c.c. metals produce stresses in an elastic surface layer which may exceed 1% of the surface film shear modulus. The detailed results are consistent with phenomenological data on surface damage accumulation during low cycle fatigue of ion-beam-modified nickel. Implications for the design of surface microalloys for inhibition of fatigue crack initiation are discussed.
Etude experimentale par une technique isopiestique. L'activite du manganese montre une deviation positive par rapport a la loi de Raoult dans les alliages riches en chrome, et varie progressivement vers une deviation negative dans les alliages riches en cobalt
This study examined stressful life events and other predictors associated with remission from DSM-IV drug dependence involving cannabis, cocaine, hallucinogens, heroin, inhalants, non-heroin opioids, sedatives, stimulants, tranquilizers, or other drugs. Waves 1 and 2 of the National Epidemiologic Survey on Alcohol and Related Conditions were used to examine the prevalence and predictors of past-year remission status. Among U.S. adults with previous (i.e., prior-to-past-year) drug dependence (n = 921) at baseline (wave 1), the prevalence of past-year remission status at wave 1 was: abstinence (60.5%), asymptomatic drug use (18.8%), partial remission (7.1%), and still drug dependent (13.5%). Similarly, the prevalence of past-year remission status three years after baseline at wave 2 was: abstinence (69.1%), asymptomatic drug use (15.5%), partial remission (8.4%), and still drug dependent (7.0%). Remission three years after baseline at wave 2 was much more likely among formerly drug dependent U.S. adults who abstained from drug use at baseline (wave 1) relative to those who reported asymptomatic drug use, partial remission, or remained drug dependent. Design-based weighted multinomial logistic regression analysis showed that relative to abstinence, past-year stressful events at baseline (wave 1) predicted higher odds of partial remission and drug dependence at both waves 1 and 2. This is the first national study to examine the potential role of stressful life events associated with remission from drug dependence. Although the majority of those who reported previous drug dependence transitioned to full remission, a sizeable percentage were either still drug dependent or in partial remission. Higher levels of stressful life events appear to create barriers to remission and should remain a focus for relapse prevention programs.
Les resultats de l'etude experimentale realisee sur l'aluminium renforce par des dispersions d'Al 2 O 3 indiquent que celui-ci peut etre recristallise en une structure a gros grains grâce a un procede en deux etapes. Cette structure fournit une resistance plus faible en compression qu'une structure a grains fins
Les macles produites lors de la compression sont stables durant la deformation par traction.Lors de cette traction,la deformation plastique apparait soit par glissement de dislocations,soit par creation de nouveaux macles mecaniques.Les dislocations et les macles peuvent facilement traverser les joints des sousgrains.
This chapter reviews the historical evolution of phase transformations understanding as it was developed in steels and other ferrous alloys. The focus is on the discoveries, dead ends, confusions, controversies, and achievements in the 1880–1925 period when the age-old ‘hardening problem’ in steel was pursued using metallography, thermal analysis, and the Gibbs phase rule. The shift in paradigm regarding metal structure and phase transformations was completed in the following period, 1925–1970, when breakthroughs afforded by X-ray diffraction and other techniques shed new light on all transformations. The evolving interactions of physical metallurgy with chemistry, physics, and other fields will be highlighted.
Phase transformations of β-Ti to α-Ti was studied in Ti-43at%Al using X-ray diffraction, optical microscopy, scanning electron microscopy, electron microprobe and transmission electron microscopy. Two types of non-equilibrium transformation mechanisms were observed to occur, Widmanstatten plate precipitation and massive transformation. Rapid quench from 1400°C can lead to the precipitation of Widmanstatten plates of α-Ti with the appearance of parallel plates at grain boundaries and basketweave plate colonies within grains. In addition, massive transformation was found to occur at prior β-Ti grain boundaries, resulting in the featureless microstructure and irregular grain boundaries. In all cases, the high temperature α-Ti phase was ordered into α2 having antiphase boundaries present within massive grains or Widmanstatten plates.
The fatigue tests in air show that persistent slip bands (PSB's) and cracks nucleate very early at special grain boundaries. At stress amplitudes for which no persistent slip band nucleation was observed in single crystals and which were oriented for single slip, cracks still nucleate at grain boundaries. The endurance limit for special bicrystals lay 50% below the endurance limit of polycrystals. In air, the cracks nucleate at the boundary but propagate within the PSB. From the stress amplitudes at which PSB's nucleate in single crystals and in bicrystals, which have one grain with the same orientation as the single crystals, the additional shear stress due to elastic anisotropy was measured to be 55 MPa. These tests yield an understanding of the behavior of short cracks, which nucleate at special boundaries but cannot propagate further, if they hit an unfavorably oriented boundary for which higher local stresses for propagation were needed.
Instead of the quasi-chemical approach with nearest neighbor interactions VAA, VAB and VBB, only one Morse potential function has been considered. In our model, the disordered alloys were assumed to be the assembly of one type of atoms named here pseudo-atoms, having average properties, which are allowed to interact with each other through Morse potential functions. Using these potentials, pressure-volume relations were calculated for α-phase CuAl alloys.
