Subject index (NOTE: 294/1–2 and 294/3)

In this study, the effect of aluminum (Al) addition to a manganese (Mn) steel Fe-12Mn-0.5C in regard to the change in stacking fault energy (SFE) and the consequent evolution of deformation microstructure and texture were investigated during cold rolling. An analysis of the texture and microstructure was performed to understand the deformation micro-mechanisms. Deformation micro-mechanisms were substantiated by the estimation of dislocation density and the arrangement of dislocations in the deformed microstructure by X-ray line profile analysis, which revealed significant changes in the dislocation structure with the addition of Al. Three stages of deformation mechanism were observed in all Al-added compositions. In the early stages of deformation, slip as well as twinning prevailed. In the intermediate stage, twinning took over completely and at large strains, macroscopic shear bands became the dominant deformation mode. An increase in the propensity of nanometer-sized deformation twins was observed with rolling strain. However, the addition of Al decreased the overall twin fraction in the deformed microstructure. The theoretical twinning stress was calculated to explain the crucial role of SFE on the occurrence of deformation twins in these steels. The deformation texture was predominantly of the brass type for all the Al-added compositions; however, appreciable differences were seen with Al content. The 〈111〉//ND γ-fiber, which develops in Al-free Fe-12Mn-0.5C, completely disappeared in 3 wt pct Al-containing material.

Microstructure and mechanical properties of Mg-Gd-Y-Zn-Mn alloy sheets processed by large-strain high-efficiency rolling

The effect of strain rate on mechanical properties and microstructure of a metastable FeMnCoCr high entropy alloy

Deformation mechanism at large strains in a high-Nb-containing TiAL at room temperature

Effects of nanocrystalline and ultrafine grain sizes on constitutive behavior and shear bands in iron

A Massively Parallel Spatially Resolved Stochastic Cluster Dynamics Method for Simulations of Irradiated Materials

Three-dimensional finite transient deformations of polycarbonate (PC) panels impacted at low velocity by a hemispherical-nosed rigid cylinder have been studied by using the commercial finite element software ls-dyna with a thermo–elasto–viscoplastic material model for the PC incorporated in it as a user defined subroutine. The implementation of the subroutine has been verified by comparing analytical and numerical solutions of simple initial-boundary-value problems. The mathematical model of the low velocity impact problem has been validated by comparing the computed and the experimental results for the maximum deflection and time histories of the centroidal deflection. It is found that the initial slope of the reaction force between the impactor and the panel versus the indentation for a curved panel can be nearly 20 times that for the flat panel of the same thickness as the curved panel. For the impact velocities considered, it is found that the maximum effective plastic strain in the PC shell near the center of impact and the dominant deformation mode there strongly depend on the panel curvature, the panel thickness, and the impact speed. Effects of the panel curvature, the panel thickness, and the impact speed on stresses and strains developed in a panel are delineated. This information should help designers of impact resistant transparent panels such as an airplane canopy, automobile windshield, and goggles. However, damage initiation and propagation, and the final indentation induced in the clamped panels have not been computed.

In this study, a preliminary investigation about the grain size effect in machining of polycrystalline copper structures at atomistic scale is carried out using molecular dynamics simulation. Four copper structures with different grain sizes are chosen for simulation. The four structures consist of 16, 64, 128, and 256 grains, and the corresponding equivalent grain sizes are 13.6, 8.6, 6.8, and 5.4 nm, respectively. The results show that significant smaller forces are required to machine the copper workpiece in both the tangential and thrust directions as the grain size decreases. The magnitude of equivalent stress distribution also becomes smaller with the decrease of grain size. It disagrees with the commonly accepted strengthening effect (i.e., the Hall-Petch relation) for polycrystalline materials as a result of grain size reduction. This phenomenon can be explained by the inverse Hall-Petch relation proposed in literature in recent years. According to the new relation, the polycrystalline material strength decreases as the grain size decreases within a threshold value. This can be further attributed to the fact that the dominant deformation mode is changed from dislocation movement to other mechanisms such as grain boundary sliding with very fine nano-structured polycrystalline.

The Kirchhoff elastic theory of thin filaments with spontaneous curvature is employed in the understanding of the onset of the kink transitions observed in short DNA rings. Dynamical analysis shows that when its actual curvature is less than some threshold value determined by the spontaneous curvature, a circular DNA will begin to buckle to other shapes. The observable and the dominant deformation modes are also determined by dynamical instability analysis, and the different effects of Zn2+ and Mg2+ ions on DNA configurational properties are qualitatively discussed.

Uniaxial tension and compression tests were conducted on as-grown [0001] Mg single crystals and on [0001] single crystals containing an alien distribution of basal dislocations to elucidate their effect on the deformation mechanisms and strain hardening. During tensile deformation, the basal dislocations induce elasto-plastic transition and micro-yielding that precede the gross deformation by twinning. They decrease the stress for twin nucleation, the tensile strength, and the failure strain compared to the virgin single crystals. The weakening of the microstructure and early failure are attributed to the higher density of micro cracks formed in the crystals with alien basal dislocations. The extraneous basal dislocations alter entirely the deformation mechanisms and strain hardening characteristic of [0001] single crystals under compression. They suppress the pyramidal slip as a dominant deformation mode and promote the gross deformation by cooperative basal slip and twinning during extended deformation plateau. The deformation twinning is of anomalous type, nucleating and propagating with the assistance of extraneous basal dislocations at the negative Schmid Factor and against the imposed compressive strain. τtw≈2.1±0.2MPa is found as the basal slip-assisted twinning stress in Mg.

In-situ neutron diffraction studies on high-temperature deformation behavior in a CoCrFeMnNi high entropy alloy

For thermoforming of continuous fabric reinforced thermoplastics, fabric shearing is the dominant deformation mode. The maximum shearing angle depends on the textile architecture and it is expressed as a locking angle, i. e., the smallest possible angle between warp and weft yarns. Wrinkling and buckling occur when the locking angle is exceeded or when too high compression stresses are introduced in the fabric during forming. To avoid these undesired effects membrane stresses must be introduced into the fabric, e.g., by using blankholders. In the first part of this paper a new pneumatic clamping device will demonstrate the advantages of easy handling and flexible force variation. The second part deals with a technology to form and join continuous fiber reinforced fabrics in one step: Tailored Blank Technology. This method reduces the manufacturing steps and offers the advantage to produce parts with different thicknesses or to add profiles and inserts to the main sheet. First results about the temperature windows will be discussed for glass fiber reinforced polypropylene.

Roll forming is a continuous process in which a flat strip is shaped to the desired profile by sequential bending in a series of roll stands. Because of the large variety of applications of roll forming in the industry, Finite Element Analysis (FEA) is increasingly utilized for roll forming process design. Bending is the dominant deformation mode in roll forming. Sheet materials used in this process are generally temper rolled, roller- or tension- leveled. These processes introduce residual stresses into the material, and recent studies have shown that those affect the material behavior in bending. In this study a numerical model of the temper rolling (skin passing) process was used to determine a residual stress distribution in a dual phase, DP780, steel strip. A 5-stand roll forming process for the forming of a V-section was modeled, and the effect of various thickness reduction levels in the temper rolling process on the final shape defects was analyzed. The results show that a small thickness reduction in the temper rolling process decreases the maximum bow height but the final springback angle increases. It is also shown that reasonable model accuracy can be achieved by including the residual stress information due to temper rolling as initial condition in the numerical modeling of a roll forming process.

Mechanical anisotropy and deformation mechanisms in an AlCuZr superplastic alloy

Strain softening, grain boundary migration and dynamic recrystallization of nickel during high temperature low cycle fatigue: Chen, S. and Gottstein, G.Acta Metall. Dec. 1988, 36, (12), 3093–3101