D.R. Lesuer

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.

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.

Influence of microstructure on tensile properties of spheroidized ultrahigh-carbon (1.8 Pct C steel

Ultrahigh-carbon steel (UHCS) containing 1.8 pct carbon was processed to create microstructures consisting of fine-spheroidized carbide particles (0.2- to 1.5-μm size range) within a fine-grained ferrite matrix (0.3- to 5-μm range) through a variety of thermomechanical processing and heat-treatment combinations. Tensile ductility, yield, and fracture strengths, and strain-hardening behavior were evaluated at room temperature. Yield strengths ranged from 640 to 1450 MPa, and uniform tensile elongation ranged from 3 to 23 pct. Quantitative analyses revealed that a Hall-Petch type relationship exists between the yield strength and the ferrite grain size and carbide particle size within grain interiors. The fracture strength, on the other hand, was found to be uniquely dependent on the coarse carbide particle size typically found at grain boundaries. Data from other investigators on spheroidized carbon steels were shown to correlate well with the data for the UHCS (1.8 pct C) material. It was shown that the tensile ductility will increase when the difference between the fracture strength and the yield strength is increased and when the strain-hardening rate is decreased. The basis for the trends observed is that the tensile ductility is limited by the fracture process that appears to be dictated by the nucleation of cracks at large carbide particles. The results obtained indicate that UHCSs have significant potential for sheet applications where high strength and good ductility are primary requirements.

Pearlite in ultrahigh carbon steels: Heat treatments and mechanical properties

Two ultrahigh carbon steel (UHCS) alloys containing 1.5 and 1.8 wt pct carbon, respectively, were studied. These materials were processed into fully spheroidized microstructures and were then given heat treatments to form pearlite. The mechanical properties of the heat-treated materials were evaluated by tension tests at room temperature. Use of the hypereutectoid austenite-cementite to pearlite transformation enabled achievement of pearlitic microstructures with various interlamellar spacings. The yield strengths of the pearlitic steels are found to correlate with a predictive relation based on interlamellar spacing and pearlite colony size. Decreasing the pearlite interlamellar spacing increases the yield strength and the ultimate strength and decreases the tensile ductility. It is shown that solid solution alloying strongly influences the strength of pearlitic steels.

α Grain size and β volume fraction aspects of the superplasticity of Ti-6Al-4V

Abstract The elevated temperature deformation behaviour of Ti-6Al-4V has been investigated in the temperature range between 775 and 925 °C and for true strain rates 2 × 10−5 to 0.005 s−1. Region III was characterized by m = 0.13 and Q = 602 kJ mol−1. Region II is subdivided into a low temperature region IIa and a high temperature region IIb. Region IIa is characterized by m = 0.77, Q = 216 kJ mol−1 and relatively low ductility while region IIb is characterized by n = 1.3, and identical value of Q and high ductility. Low and high β-phase proportions are responsible for regions IIa and IIb respectively. An apparent region I was accounted for by concurrent grain growth utilizing a grain size sensitivity parameter which is dependent on the β-phase volume fraction. The strain rate-temperature regimes of regions IIa, IIb and III are illustrated in a “region” map.

Dynamic compaction of aluminum nanocrystals

Abstract We investigated shock-compaction behavior of nanocrystalline aluminum ( n -Al) powder with an average particle size of about 50–70 nm. The starting powders were analyzed using various analytical tools, including transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The initial Al particles were found to be coated naturally by a uniform, thin (∼ 5 nm) near-stoichiometric, amorphous oxide layer. Shock compaction was carried out at 2–3 GPa (20–30 kbars) to obtain high-density disks. The microstructure of samples prior to and after consolidation were both examined. Despite substantial deformation during dynamic compaction, the surface oxide remained intact at the pressures employed and prevented a metallurgical bond between the nano-sized particles. Theoretical simulations of the dynamic compaction of n -Al were also carried out using an Eulerian hydrocode. The results were in good agreement with experimental observations.

The case for ultrahigh-carbon steels as structural materials

Ultrahigh-carbon steels (UHCSs) are low-alloyed plain carbon steels containing 1–2.1% carbon. These steels have remarkable structural properties when processed to achieve fine ferrite grains with fine spheroidized carbides. They can be made superplastic at intermediate temperatures. Further, they can be made hard with compression toughness and strong with good tensile ductility at ambient temperatures. Contrary to conventional wisdom, UHCSs are ideal replacements for currently used high-carbon (0.5–1 % carbon) steels because they have comparable ductility but higher strength and hardness. In this article, examples of structural components formed from fine-grained spheroidized UHCSs are illustrated, and other potential structural applications are reviewed. These steels can be laminated with other metal-based materials to achieve superplasticity, high impact resistance, exceptionally high tensile ductility, and improved fatigue behavior.

Layer thickness effect on ductile tensile fracture

Laminated metal composites containing equal volume percentage of ultrahigh carbon steel (UHCS) and brass were prepared in three different layer thicknesses (750, 200, and 50 μm) by press- bonding and rolling at elevated temperature and were tensile tested at ambient temperature. A dramatic increase in tensile ductility (from 13 to 21 to 60 pct) and a decrease in delamination tendency at the UHCS-brass interfaces were observed as the layer thickness was decreased. The layer thickness effect on ductility is attributed to residual stress whose influence on delamination is decreased as the layer thickness is decreased. Suppression of delamination inhibits neck for- mation in the UHCS layers, allowing for extended uniform plasticity. For a given layer thick- ness, the tensile ductility decreases as the ratio of hardness of component layers is increased.

The effects of the α/β phase proportion on the superplasticity of Ti-6Al-4V and iron-modified Ti-6Al-4V

Abstract The effect of the α/β phase ratio on the superplasticity of iron-modified Ti6Al4V has been studied using elevated temperature stepped-strain rate tests and stepped-temperature tensile tests. The behavior of these alloys was found to be consistent with the models proposed by Suery and Baudelet (Res. Mechanica, 2 (1981) 163–173) and Chen (B. Baudelet and M. Suery (eds.), International Conference on Superplasticity, Editions du Centre de la Recherche Scientifique, Paris, 1985, pp. 5.1–5.20) with regard to the rheological hardening of the softer β phase due to the presence of the harder α phase. The effect of alloy composition on the deformation resistance of the β phase could be accounted for by considering alloy element partitioning when estimating the diffusivity. The effect of phase hardening on the strain rate was found to be a function of fβq where fβ is the proportion of the β phase and q is a constant equal to 1.3 for fβ above a critical value (fcr) and to −1.6 below fcr. cr is the point at which a reversal in the role of the phases occurs and was found to be moderately sensitive to the concentration of iron.

Processing, structure, and properties of a rolled, ultrahigh-carbon steel plate exhibiting a damask pattern

Abstract A plate of ultrahigh-carbon steel (UHCS) was processed by hot and warm rolling, according to the Wadsworth–Sherby mechanism, to produce damask surface markings. The surface markings produced by this industrial processing method are similar to those of historical Damascus steels, which are also of hypereutectoid composition. The microstructure of the UHCS with damask contains fine, spheroidized carbides and a discontinuous network of proeutectoid carbides along former-austenite grain boundaries, which give rise to a surface pattern visible with the unaided eye. Tensile tests at room temperature measured tensile strengths and ductilities, which depend on sample orientation relative to the rolling direction of the plate. Hot and warm rolling causes a directional microstructure, giving rise to both an elongated, directional damask pattern and a directional dependence for strength and ductility. A maximum tensile ductility of 10.2% was measured at 45° relative to the rolling direction. The plate material was subjected to heat treatments creating pearlitic and martensitic microstructures, which retain visible damask patterns.