MARTENSITIC TRANSFORMATION INDUCED BY TENSILE STRESS PULSES

La cinétique de la transformation martensitique induite par un pulse de tension tractive, produit par la reflection d'une onde de choc compressive sur une surface libre, a été etudiée dans un interval de temps d'à peu près une microseconde. Deux alliages ont étés employés : Fe-32% Ni - 0,035%C et Fe-22,5% Ni - 4% Mn. Le component hydrostatique de tension interagit avec la dilatation (0,04) de la martensite et augmente la temperature de transformation spontanée . Les essais d'impact ont étés conduits en variant la temperature ou bien la durée du pulse à une pression constante. Il a été demontré que la transformation martensitique, normalement considérée athermique dans l'alliage Fe-Ni-C, montre une nature isothermique dans le regime de microsecondes. La fraction transformée augmente avec la diminuition de température et avec l'augmentation de la durée de pulse, à une pression constante. A partir des résultats obtenus, les paramètres cinetiques et energetiques de la transformation martensitique dans les deux alliages ont étés obtenus et comparés aux théories en existance.The kinetics of martensitic transformation induced by a tensile stress pulse (produced by reflection of compressive shock waves at free surfaces) in time durations in the microsecond range were studied in Fe-32wt%Ni- 0.035wt%C and Fe-22.5wt%Ni-4wt%Mn alloys. The tensile hydrostatic component of stress interacts with the dilatational strain (0.04) of the martensitic transformation and increases the Ms temperature. Shock impact experiments were conducted by varying either the temperature or pulse duration at a constant pressure. The martensitic transformation, normally considered to be athermal (exhibiting burst characteristics) in the Fe-Ni-C alloys, exhibits an isothermal nature in the microsecond regime in both alloys. The fraction transformed at a constant impact pressure increases with decrease in temperature at a constant pulse duration, and increase in pulse duration at a constant temperature. From the results obtained, the isothermal kinetic parameters and the energetics of the martensitic transformation were obtained and interpreted in terms of the existing theories

SYNTHESIS OF METASTABLE PHASES BY SHOCK-IMPACT GENERATED TENSILE STRESS PULSES

La technique de pulse tractive a été employée pour étudier les transformations de fase stable-metastable dans le système equiatomique Niti. Des tensions hydrostatiques tractives de 1,7 et 2,85 GPa et durée de 0,5 µs ont induit la transformation martensitique vers la structure monoclinique (avec forme aciculaire) ; cette structure a la forme de "tweed" après transformation thermique. A 5,1 GPa, l'alliage souffre l'écaillage. Les produits de transformation ont étés analisés par rayons X, calorimétrie differentielle de balayage et microscopie electronique par transmission.A tensile pulse technique is used for the study of stable-to-metastable phase transformations in an equiatomic NiTi alloy. For tensile hydrostatic stresses of 1.7 and 2.85 GPa, and 0.5 µs pulse duration at room temperature, the NiTi alloy transforms martensitically to a monoclinic structure with "acicular" morphology, unlike the "tweed" morphology of thermal martensite. At 5.1 GPa, the alloy undergoes spalling. The transformation products are analyzed by x-ray diffraction, differential scanning calorimetry, and transmission electron microscopy

Static and dynamic mechanical properties of epoxy-based multi-constituent particulate composites

Multi-phase particulate composites consist of individual particles of more than one material dispersed throughout and held together by a polymer binder. The mechanical and physical properties of the composite depend on the properties of the individual components; their loading density; the shape and size of the particles; the interfacial adhesion; residual stresses; and matrix porosity. These multi-phase particulate composites systems, particularly those with high fill densities, have not typically been studied to determine the effects of microstructural features on properties. In this paper, we present our investigation of the influence of particle size and dispersion on the static and dynamic mechanical response of these multi-phase (n > 2) polymer-metal composites. The low and high strain rate compressive strengths are determined using an MTS load frame and a split Hopkinson pressure bar, respectively, and the elastic properties were studied using dynamic mechanical analysis. The results are analyzed using a factorial design of experiments to determine the effect of aluminum and nickel volume percent and aluminum particle size on the compressive strength as a function of strain rate