Materials science and engineering

During FY-96, work within the Materials Science and Engineering Thrust Area was focused on material modeling. Our motivation for this work is to develop the capability to study the structural response of materials as well as material processing. These capabilities have been applied to a broad range of problems, in support of many programs at Lawrence Livermore National Laboratory. These studies are described in (1) Strength and Fracture Toughness of Material Interfaces; (2) Damage Evolution in Fiber Composite Materials; (3) Flashlamp Envelope Optical Properties and Failure Analysis; (4) Synthesis and Processing of Nanocrystalline Hydroxyapatite; and (5) Room Temperature Creep Compliance of Bulk Kel-E

Severe plastic deformation through adiabatic shear banding in Fe-C steels

Severe plastic deformation is observed within adiabatic shear bands in iron-carbon steels. These shear bands form under high strain rate conditions, in excess of 1000 s{sup -1}, and strains in the order 5 or greater are commonly observed. Studies on shear band formation in a ultrahigh carbon steel (1.3%C) are described in the pearlitic condition. A hardness of 11.5 GPa (4600 MPa) is obtained within the band. A mechanism is described to explain the high strength based on phase transformation to austenite from adiabatic heating resulting from severe deformation. Rapid re-transformation leads to an ultra-fine ferrite grain size containing carbon principally in the form of nanosize carbides. It is proposed that the same mechanism explains the ultrahigh strength of iron-carbon steels observed in ball-milling, ball drop tests and in severely deformed wires

Laminated metal composite formed from low flow stress layers and high flow stress layers using flow constraining elements and method of making same

This invention relates to a laminated metal composite, comprising alternating layers of low flow stress material and high flow stress material, and formed using flow constraining elements around each low flow stress layer; and a method of making same. A composite is a combination of at least two chemically distinct materials with a distinct interface separating the two materials. A metal matrix composite (MMC) is a composite material composed of a metal and a nonmetallic reinforcing agent such as silicon carbide (SiC) or graphite in continuous or discontinuous fiber, whisker, or discrete particulate form. A laminate is a material composed of several bonded layers. It is possible to have a laminate composed of multi-layers of a single type of material bonded to each other. However, such a laminate would not be considered to be a composite. The term {open_quotes}laminated metal composite{close_quotes} (LMC), as used herein, is intended to include a structural material composed of: (1) layers of metal or metal alloys interleaved with (2) a different metal, a metal alloy, or a metal matrix composite (MMC) containing strengthening agents

Superplasticity in laminated metal composites

Several studies have shown the possibility of achieving superplastic behavior in laminated metal composites consisting of alternating layers of superplastic and non-superplastic materials. Achieving high rate sensitivity in such a laminate requires the appropriate choice of component materials and component volume fraction as well as deformation under appropriate conditions of strain rate and temperature. The first investigators to study this behavior were Snyder et al. [1], who demonstrated that a non-superplastic material (interstitial free iron) could be made superplastic by lamination with a superplastic material (fine-grained ultrahigh carbon steel (UHCS)). Other laminates in which superplasticity has been observed in a non-superplastic material include UHCS/stainless steel and UHCS/aluminum bronze. In these studies, tensile tests were conducted with the tensile axis parallel to the layers. High strain rate sensitivities were observed and are associated with high tensile ductilities. However, as observed by Tsai et al. [2], obtaining high strain rate sensitivity is a necessary but not sufficient condition for high elongations. Tsai et al. studied the UHCS/brass laminate and found that, despite a strain rate sensitivity exponent of 0.5, only about 60% elongation was obtained. The low tensile ductility resulted from brittle, intergranular fracture of the brass. Once cracking started in the brass, cracks penetrated into the UHCS and premature failure resulted. Thus high elongations requires achieving high strain rate sensitivity as well as avoiding brittle fracture in the less ductile layer. In addition to tension, other deformation modes, including compression [3] and co-extrusion [4], have been studied for deformation response under conditions of high strain rate

Nano-subgrain Strengthening in Ball-milled Iron

The strength and deformation behavior of ball-milled, iron-base materials containing nano-scale subgrains have been evaluated. As reported by several authors, nanosubgrains form during the early stages of ball milling as a result of severe plastic deformation inherent in the ball milling process. The strength for these nano-scale subgrains are compared with the strength of larger-scale subgrains in iron and iron-base alloys produced by traditional mechanical working. The data covers over 2 orders of magnitude in subgrain size (from 30 nm to 6 {micro}m) and shows a continuous pattern of behavior. For all materials studied, the strength varied as {lambda}{sup -1}, where {lambda} is the subgrain size. Strengthening from subgrains was found to breakdown at a much smaller subgrain size than strengthening from grains. In addition, the ball-milled materials showed significant strengthening contributions from nano-scale oxide particles. Shear bands are developed during testing of ball-milled materials containing ultra-fine subgrains. A model for shear band development in nano-scale subgrains during deformation has also been developed. The model predicts a strain state of uniaxial compression in the shear band with a strain of -1.24. Subgrains are shown to offer the opportunity for high strength and good work hardening with the absence of yield point behavior

Influence of Iron Oxide Particles on the Strength of Ball-Milled Iron

Detailed microstructural and mechanical property studies of ball-milled iron, in the powder and consolidated states, are reviewed and assessed. The analyses cover three and one-half orders of magnitude of grain size (from 6 nm to 20 mm) and focus on the influence of oxide particles on the strength. The study includes the early work of Koch and Yang, Kimura and Takaki and continues with the more recent work of Umemoto et al and Belyakov, Sakai et al. It is shown that the major contributors to strength are the nanooxide particles. These particles are created by adiabatic shear banding during ball-milling leading to a bimodal distribution of particles. The predicted strength from particles, {sigma}{sub p}, is given by {sigma}{sub p} = B {center_dot} (D*{sub S}){sup -1/2} where D*{sub S} is the surface-to-surface interparticle spacing, and B = 395 MPa {center_dot} {micro}m{sup -1/2}. A model is proposed that accounts for the influence of the bimodal particle size distribution on strength

Microstructural investigation on an Al 6061 T6 alloy subjected to ballistic impact C

Ballistic impact generates significant modifications in the microstructural patterns. High strain rate and local high temperature conditions work together in opposite way: the first causes strain hardening, while the second factor produces softening. Moreover, after the impact, the cooling process is responsible of other local modifications on the arrangement of dislocations and precipitates. Therefore an experimental analysis on Al 6061 T6 cut from the edge of a component subjected to ballistic impact has been carried on in order to investigate on the microstructural modifications. Considerations about the influence on the mechanical behavior and on the fracture propagation are reported. The crystallographic textures and the misorientation featuring the grains play in fact a significant role in the fracture mechanism. The comparison between the texture situation before and after the impact can allow to evaluate the localized straining of the material and to point out its dissipation efficiency as a function of the distance from the damaged surfaces © 2011 Published by Elsevier Ltd. Selection and peer-review under responsibility of ICM1

The c/a Ratio in Quenched Fe-C and Fe-N steels - a Heuristic Story

The body-centered tetragonal (BCT) structure in quenched Fe-C steels is usually illustrated to show a linear change in the c and a axes with an increase in carbon content from 0 to 1.4%C. The work of Campbell and Fink, however, shows that this continuous linear relationship is not correct. Rather, it was shown that the body-centered-cubic (BCC) structure is the stable structure from 0 to 0.6 wt%C with the c/a ratio equal to unity. An abrupt change in the c/a ratio to 1.02 occurs at 0.6 wt%C. The BCT structure forms, and the c/a ratio increases with further increase in carbon content. An identical observation is noted in quenched Fe-N steels. This discontinuity is explained by a change in the transformation process. It is proposed that a two-step transformation process occurs in the low carbon region, with the FCC first transforming to HCP and then from HCP to BCC. In the high carbon region, the FCC structure transforms to the BCT structure. The results are explained with the Engel-Brewer theory of valence and crystal structure of the elements. An understanding of the strength of quenched iron-carbon steels plays a key role in the proposed explanation of the c/a anomaly based on interstitial solutes and precipitates