Showing papers by "Marc A. Meyers published in 1990"

Observation of an adiabatic shear band in AISI 4340 steel by high-voltage transmission electron microscopy

Abstract: Adiabatic shear bands, formed in a hollow AISI 4340 steel cylinder subjected to dynamic expansion by means of an explosive charge placed in its longitudinal axis, were characterized. The adiabatic shear bands formed in this quenched and tempered steel were of the classical “transformed” type. Scanning electron microscopy (SEM) of etched surfaces revealed that alignment of the lamellae along the direction of shear seems to be the event that precedes shear localization. The transmission electron microscopy of a “white”-etching shear band having undergone a shear strain of approximately 4 revealed that it containedX (Fe5C2) carbides in a martensitic structure. These carbides were observed to form on (112) internal microtwins. Grains could not be resolved inside of the shear band, but they could be observed in the surrounding matrix material. A traverse of the shear band was made, and there existed no definite boundary between the matrix and the shear band. No evidence of a transformation to austenite was observed. Heat transfer calculations were conducted to help explain the features observed inside of the shear band. It is concluded that the “white”-etching bands, commonly referred to in the literature as “transformed” bands, do not exhibit a transformation at values of shear strain of up to 4. The enhanced reflectivity is an etching artifact and is possibly due to microstructural changes, a very small grain size, and carbide redissolution in the bands.

Effect of metallurgical parameters on shear band formation in low-carbon (∼0.20 Wt Pct) steels

Abstract: With the objective of establishing the effects of the metallurgical condition on the propensity to form adiabatic shear bands, low-carbon steels (AISI 1018 and 8620) having widely different temperability responses were subjected to impact by cylindrical projectiles in the velocity range of 450 to 1050 m/s. These steels received a variety of mechanical and thermal treatments that provided a wide range of microstructures and mechanical responses. The propensity for shear band formation was strongly dependent on the mechanical response. It was measured by counting the length of shear bands per cross section. Microstructural characterization of the bands revealed that white-etching bands were only observed in the quenched and quenched-and-tempered conditions.

Formation of Controlled Adiabatic Shear Bands in AISI 4340 High Strength Steel

Abstract: : Adiabatic shear banding is one of the predominant failure modes of ultrahigh strength (UHS) steels under high rates of deformation. Though these bands have been previously studied, the specific factors governing when, how, or if a certain material fails by shear band localization are still relatively unknown. This article examines these questions in terms of the microstructure using a controlled shear strain. Four microstructures of equal hardness (Rc52) with different carbide distributions were produced in a VAR 4340 steel by varying the normalizing temperature (one hour at 845 C, 925 C, 1010 C, or 1090 C). A short duration reaustenitizing treatment was subsequently used to produce similar prior austenite grain sizes; this was followed by a 200 C temper. The controlled shear strain was introduced by the use of a hat-shaped specimen loaded in a split Hopkinson bar rig, producing a shear zone at local strain rates up to 5 x 0.00005. The use of mechanical stops to arrest the deformation process allowed the development of the shear bands to be studied under controlled stress conditions. Transmission electron microscopy (TEM) studies revealed a microcrystalline structure within the shear band with a crystalline size of 8nm to 20nm. A gradual change from the microcrystalline structure within the band to lath martensite was observed. Tensile unloading cracks were observed in the sheared regions. A normalizing temperature of 925 C produced the microstructure with the greatest resistance to unstable shearing.

Reaction-assisted shock consolidation of RSR Ti–Al alloys

Abstract: A new method for the shock consolidation of hard metallic powders has been successfully tested. This method extends the process developed by Sawaoka and Akashi for the processing of ceramics (U.S. Patent 4,655,830) to metallic powders. Shock-activated reactions between elemental mixtures of niobium and aluminum powders were used to chemically induce bonding between difficult-to-consolidate intermetallic TiAl compound powder particles. The highly exothermic reactions activated by the passage of shock waves form an intermetallic binder phase which assists in the consolidation of the very hard TiAl alloy powders. Shock impact experiments were carried out utilizing a twelve-capsule shock recovery system in which a plane wave generating lens is used for accelerating a flyer plate to velocities of 1.7 and 2.3 km/s. With these impact velocities, sufficient shock pressures are generated in the powders, contained in capsules, to result in shock-induced reactions between the elemental powders of the mix. Fully dense compacts were successfully recovered and were subsequently characterized by optical, transmission, and scanning electron microscopy, x-ray diffraction, and microhardness testing. Transmission electron microscopy revealed both microcrystalline and amorphous regions in the reaction zone. In one instance, the amorphous material crystallized under the heating effect of the electron beam. Shock induced reaction between elemental powders and with the TiAl powders, producing ternary compounds, was also observed.