J. Wadsworth

Plasticity and structural instability in a bulk metallic glass deformed in the supercooled liquid region

Abstract The deformation behavior of a bulk amorphous Zr–10Al–5Ti–17.9Cu–14.6Ni alloy was characterized in the supercooled liquid region. The alloy was observed to exhibit Newtonian behavior at low strain rates but to become non-Newtonian at high strain rates. Structures of the amorphous material, both before and after deformation, were examined using X-ray diffraction and high-resolution electron microscopy. Experimental results showed the presence of nanocrystallites in the deformed samples, suggesting that the non-Newtonian behavior was associated with the concurrent crystallization of the amorphous structure during deformation; that is, a mixed crystalline-plus-amorphous structure was being tested. A mechanistic model based upon structural evolution has been developed to interpret the observed non-Newtonian behavior.

Effect of cell morphology on the compressive properties of open-cell aluminum foams

The mechanical properties of open-cell 6101 aluminum foams with different densities (∼5–10%) and morphologies (4–16 cells cm−1) were characterized in compression. It was found that density is the primary variable controlling the modulus and yield strength of foams. The effects of other variables such as cell size and shape were also studied. Whereas the cell size appears to have a negligible effect on the strength of foams, at a fixed density, the cell shape was shown to effect the strength of foams. In the present paper, theoretical models are offered to explain the differences in modulus and strength caused by the differences in cell shape and size.

Superplastic properties of a fine-grained yttria-stabilized tetragonal polycrystal of zirconia

Summary Superplasticity in fine-grained Y-TZP, as first reported by Wakai et al. has been demonstrated to be reproducible. In addition, in the present study, a new, high elongation value of 350% has been obtained at 1550°C. The microstructure of the Y-TZP appears to be stable at 1450°C, but grain growth takes place in specimens tested at 1550°C.

Strain rate-dependent deformation in bulk metallic glasses

Abstract Metallic glasses have metastable structures. As a result, their plastic deformation is dependent upon structural dynamics. In the present paper, we present data obtained from Zr-base and La-base metallic glasses and discuss the kinetic aspects of plastic deformation, including both homogeneous and heterogeneous deformation. In the case of homogeneous deformation (typically occurring in the supercooled liquid region), Newtonian behavior is not universally observed and usually takes place only at low strain rates. At high strain rates, non-Newtonian behavior is usually observed. It is demonstrated that this non-Newtonian behavior is associated with in situ crystallization of the amorphous structure. In the case of heterogeneous deformation (occurring at room temperature), deformation is controlled by localized shear banding. The plastic deformation of a La-base metallic glass is also investigated using instrumented nanoindentation experiments over a broad range of indentation strain rates. At low rates, the load-displacement curves during indentation exhibit numerous serrations or pop-ins, but these serrations become less prominent as the indentation rate is increased. Using the tip velocity during pop-in as a gauge of serration activity, we find that serrated flow is only significant at indentation strain rates below a certain critical value.

Nanoimprint of gratings on a bulk metallic glass

The authors demonstrate that optical gratings with 600 and 1500nm periods on a Pd40Ni40P20 bulk metallic glass (BMG) can be faithfully imprinted in air from Si dies. Results of scanning electron microscopy, atomic force microscopy, and optical diffraction analysis show the fine line feature of ∼150nm. The gratings have smooth and uniform surface profiles with comparable optical properties as the original Si dies. The BMG gratings can be further used to imprint the second-generation replicas on polymethylmethacrylate. Thereby, BMG is a suitable material not only for imprinting nanostructured parts such as gratings, but also as a good die material for nanoimprints.

Superplastic behavior of a Zr-10Al-5Ti-17.9Cu-14.6Ni metallic glass in the supercooled liquid region

The mechanical behavior of metallic glasses is characterized by either inhomogeneous or homogeneous deformation. Inhomogeneous deformation usually occurs when a metallic glass is deformed at room temperature (i.e., low temperature) and is characterized by the formation of localized shear bands, followed by the rapid propagation of these bands, and sudden fracture. Consequently, when a metallic glass is deformed under tension it exhibits only limited macroscopic plasticity ( 0.70T{sub g}), and the material can often exhibit significant plasticity, especially in the supercooled liquid (SL) region. In summary, large tensile ductility can generally be obtained in the supercooled liquid region from a metallic glass with a large {Delta}T. The exact deformation mechanism, however, and in particular whether an alloy deforms by Newtonian viscous flow remains a controversial issue. The purpose of this paper is to report an example of non-Newtonian behavior in a cast amorphous Zr-10Al-5Ti-17.9Cu-14.6Ni alloy in the supercooled liquid region.

Harper-dorn creep in pure metals

Abstract Evidence is presented that Harper-Dora (H-D) viscous creep is a diffusion-controlled, dislocation creep process. The factors influencing H-D creep are the same as those influencing power-law creep, namely dislocation density, subgrain or grain size, stacking fault energy, and elastic modulus. H-D creep has been observed, and is analyzed, for seven different metals: Al, Pb, Sn, α-Zr, α-Ti, α-Fe, and β-Co. In addition, seven metals (α-Fe, Ni, Ag, β-Co, Cu, Mo, and Cr) which were originally believed to be controlled by diffusional (Nabarro-Herring) creep at low stresses, are shown to be controlled by Harper-Dorn creep. An internal stress assisted creep model is presented which correlates data in both the power-law and H-D creep regimes. The model predicts, quantitatively, the conditions under which H-D creep will dominate plastic flow.

Ancient and modern laminated composites — from the Great Pyramid of Gizeh to Y2K ☆

Abstract Laminated metal composites (LMCs) have been cited in antiquity; for example, an iron laminate that may date as far back as 2750 BC was found in the Great Pyramid in Gizeh in 1837. A laminated shield containing bronze, tin, and gold layers is described in detail by Homer. Well-known examples of steel laminates, such as an Adze blade, dating to 400 BC can be found in the literature. The Japanese sword is a laminated composite at several different levels and Merovingian blades were composed of laminated steels. Other examples are also available, including composites from China, Thailand, Indonesia, Germany, Britain, Belgium, France, and Persia. The concept of lamination to provide improved properties has also found expression in modern materials. Of particular interest is the development of laminates including high-carbon and low-carbon layers. These materials have unusual properties that are of engineering interest; they are similar to ancient welded Damascus steels. The manufacture of collectable knives, labeled “welded Damascus,” has also been a focus of contemporary knife makers. Additionally, in the former Soviet Union, laminated composite designs have been used in engineering applications. Each of the above areas will be briefly reviewed, and some of the metallurgical principles will be described that underlie improvement in properties by lamination. Where appropriate, links are made between these property improvements and those that may have been present in ancient artifacts.

Application of the divorced eutectoid transformation to the development of fine-grained, spheroidized structures in ultrahigh carbon steels

Abstract This paper describes a two-step thermomechanical processing method for obtaining fully spheroidized structure in UHC steels. The first step is a hot a warm working (HWW) procedure to obtain spheroidized and fine proeutectoid carbides. The second step incorporates a D ivorced E utectoid T ransformation ( DET ). This treatment is called DET and includes a well-defined soaking time and soaking temperature fro austenitizing followed by air cooling. When deformation is imparted to the UHC steel, after austenitizing and during air cooling, it is called DETWAD ( D ivorced E utectoid T ransformation W ith A ssociated D eforamation).

Enhanced ductility in coarse-grained Al-Mg alloys

Enhanced ductilities,i.e., values of tensile ductility exceeding those normally expected in metallic alloys, have been observed at warm temperatures in coarse-grained Al-Mg alloys which exhibit viscous-glide controlled creep. Numerous tests have been conducted in order to quantify this phe-nomenon over wide ranges of temperature and magnesium concentration. The contributions of strain-rate sensitivity and strain hardening have been analyzed in relation to the observed tensile ductilities. It is shown that an analysis based only on flow instability in tension cannot be used to predict failure in a unique manner.