Recovery and recrystallization in Cold-Rolled Al-SiCw composites

Recrystallization behavior has been studied in 50, 70, and 90 pct cold-rolled silicon carbide whisker-reinforced aluminum composites containing fine aluminum-oxide particles. The distribution of aluminum-oxide particles in the composites was not uniform. Macrohardness measurement, transmission electron microscopy (TEM), and light microscopy were used in the investigation. It was found that in regions with low aluminum oxide content, the introduction of SiC whiskers resulted in recovery reactions during and after cold rolling and in an increase in the growth rate of subgrains during annealing. These recovery reactions were enhanced by an increase in the degree of deformation. The number density of nuclei varied between areas with different contents of aluminum-oxide particles. The impingement of nuclei and recovery reactions limited the growth of nuclei, resulting in a process where recrystallization (growth of nuclei by high-angle grain boundary migration) and extended recovery (growth of subgrains) took place simultaneously. The relative amounts of recrystallization and extended recovery that occur simultaneously affect the recrystallization kinetics as well as the grain size distribution and texture after recrystallization.     

Flow localization and shear band formation in a precipitation strengthened austenitic stainless steel

Flow localization and shear band formation in a γ′ strengthened austenitic stainless steel, JBK-75, was investigated by means of compression of reduced-gage-section, cylindrical specimens. The specimens were deformed in a strain rate range of 10³ to l0³s^-1 and from 650 °C to 1100 °C. The alloy exhibited an extreme susceptibility for macroscopic localized flow when the microstructure contained fine γ′ precipitates on the order of 10 nm. At high strain rates localized flow in the precipitate containing structures took the form of macroscopic, transgranular shear bands. At low strain rates the localized flow occurred mainly in precipitate-free zones along high angle grain boundaries. Localized flow was always associated with the occurrence of dynamic recrystallization. Other mechanisms for flow localization, including the effects of adiabatic heating on γ′ precipitate dissolution, are discussed.     

A structural study of the influence of pressure on shear band formation

Experiments have been performed to show the influence of applied pressure on the formation of localized shear bands in a naturally aged Al-Zn-Mg alloy. The results indicate that the onset of localization is insensitive to applied pressure but that the development of the bands is pressure sensitive indicating that dilational damage occurs during the growth of bands. Detailed electron microscopy studies indicates that although the overall macroscopic orientation of the bands may be nonrational they consist of sequential regions of localized shear on crystallographic planes and narrow regions of multi-slip.     

Micro and macroscopic aspects of shear band formation in internally nitrided single crystals of Fe-Ti-Mn alloys

The formation of localized shear bands in single crystals of internally nitrided alloys of Fe-Ti-Mn was studied experimentally and theoretically. The experimental studies included observation of the dislocation substructures that are formed at and near shear band/matrix interfaces along with documentation of the crystallography of the localized shearing process. Computational studies of tensile deformation of the crystals using the finite element method, that incorporated a large strain, strain rate dependent constitutive theory for crystalline slip, were carried out and the results were compared to the experimental observations. Mechanical tests, and in situ metallographic observations of shear band formation, showed that localized shearing occurs while the materials are continuously hardening and before “damage”, through microfracture, begins. Electron diffraction and imaging of the deformation substructures near shear band interfaces showed that an important part of the localization mechanism is nonuniform lattice reorientations that cause a “geometrical softening” of the lattice. The numerical studies are in good agreement with the experiments and demonstrate the role of material strain hardening and strain rate sensitivity.     

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

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.     

Microstructural study of adiabatic shear band

This article presents a study of the microstructural development of the adiabatic shear band in an HY-100 steel. The steel was deformed at a high strain rate by ballistic impact, and subsequent metallographic observations along with electron microscopy were performed. A number of white- etched shear bands were found near the perforated region, and three typical microstructural features of the adiabatic shear band were observed: elongated grain structure at the boundary between the shear band and matrix, fine equiaxed grain structure with high dislocation densities in the middle of the shear band, and relatively coarse-grained structure located between the above two structures. These microstructures might be formed in an extremely short time by the combined effects of the large temperature rise and the highly localized deformation. Since very complex phenomena might occur within the shear band, possible mechanisms, such as dynamic recovery and strain-induced dynamic phase transformation, are suggested to explain the micro- structural development of the adiabatic shear band.     

Shear band formation in plane strain compression

Shear band formation during plane strain compression of single crystals and polycrystals of an Al-3 wt% Cu alloy was studied. X-ray and electron diffraction, optical microscopy, and electron (TEM/STEM) microscopy were used to document the structure and micromechanisms of the localization process. Complementary finite element studies were performed using the measured single crystals' single slip system strain hardening data. The experimental observations and the computed deformation response are in very close agreement, and indicate that localization, through macroscopic shear band formation, occurs in continuously strain hardening, damage-free material. Shear band formation was preceeded by the development of very coarse slip which, like the shear bands, propagated across entire grains and, in single crystals, across the entire crystal. The computations and experiments showed that geometrical softening, caused by nonuniform lattice reorientation, is an important micromechanical influence on the localization process in both single crystals and polycrystals. Also, the propagation of shear bands across grain boundaries in polycrystals was looked at experimentally and computationally. Particular attention was paid to the crystallography of shear band transmission through grain boundaries.     

Segregation band formation in Al-Si die castings

Banded defects are often found in high-pressure die castings. These bands can contain segregation, porosity, and/or tears, and changing casting conditions and alloy are known to change the position and make-up of the bands. Due to the complex, dynamic nature of the high-pressure die-casting (HPDC) process, it is very difficult to study the effect of individual parameters on band formation. In the work presented here, bands of segregation similar to those found in cold-chamber HPDC aluminum alloys were found in laboratory gravity die castings. Samples were cast with a range of fraction solids from 0 to 0.3 and the effect of die temperature and external solid fraction on segregation bands was investigated. The results are considered with reference to the rheological properties of the filling semisolid metal and a formation mechanism for bands is proposed by considering flow past a solidifying immobile wall layer.     

Adiabatic shear localization in titanium and Ti-6 pct Al-4 pct V alloy

Ballistic impact experiments were conducted on 12.5 mm thick commercial purity titanium and Ti-6 pct Al-4 pct V alloy plates using steel “stepped” projectiles with 10.5 mm diameter. The impact velocities varied between 578 m per second and 846 m per second, and a flash X-ray technique was used to determine projectile velocity and to assure the normality of impact. The microstructural damage mechanisms associated with impact (shear band formation, shock wave propagation, and dynamic fracture) were analyzed by optical, and scanning and transmission electron microscopy. Elliptical and spherical cavities were observed along the bands. Microindentation hardness differences between the bands and adjacent regions were slight for the targets; for the projectiles, the hardness in the band was significantly lower than that of surrounding regions. Observation of the fractured regions along the bands showed unique features indicating possible melting. Transmission-electron microscopy of a shear band in titanium revealed microcrystalline features (∼0.3 μm diameter) with poorly defined grain boundaries. Formerly Graduate Student, Department of Metallurgical and Materials Engineering, New Mexico Institute of Mining and Technology, Socorro, NM 87801     

The effect of textures and grain shape on the shear band formation in rolled F.C.C. metals

In rolled f.c.c. metals, the commonly observed textures are {110}〈001〉, {110}〈112〉, and {112}〈111〉. A theoretical Taylor factor calculation was made to give the influence of these three textures on plastic deformation. The combined effect of textures and grain shape on the shear band formation and fracture in rolled f.c.c. metals were discussed and compared with experimental results. It is concluded that among the three textures studied, the Goss {110}〈001〉 textured grains in rolled f.c.c. metals are easier deformed than the others, which gives an explanation of the form of shear bands usually observed in rolled f.c.c. metals.