Al2O3/Y-TZP particulate composites with compositions of between 20 and 80 vol.% Y-TZP were produced by tapecasting, lamination, and sintering. The processing methods employed resulted in fine grain sizes with only small variations between the composites produced. The resulting particulate composites were tested in compression at a temperature of 1350 °C over strain rates from 10−5 to 3.16×10−4 s−1. Microstructural changes during testing were minor. Stress exponents were measured to the range from approximately two to three, which are consistent with published data on similar materials from tensile experiments. Models of composite creep behavior are compared to the experimental data over the full range of compositions studied. A constrained isostrain model is found to provide better predictive capabilities than either an unconstrained model, an isostress model, or a rheological model. Furthermore, the constrained isostrain model provides the most reasonable predictions for creep rates of 100% Al2O3 and 100% Y-TZP materials.
An ultrahigh-carbon steel containing 1.3 wt.% C (UHCS-1.3C) was processed to obtain spheroidized and pearlitic microstructures. Spheroidized microstructures exhibit carbide particle size and ferrite grain size distributions which are invariable with austenitizing temperature below 870 °C. Pearlitic microstructures exhibit prior austenite grain sizes and pearlite colony sizes which increase and interlamellar spacings which decrease with increasing austenitizing temperatures above 870 °C. Plane-strain fracture toughness, KIv, was measured at room temperature for all heat-treated materials. In the case of spheroidized UHCS-1.3C, fracture toughness does not change significantly with austenitizing temperature. The fracture toughness of UHCS-1.3C processed for pearlitic microstructures decreases with increasing austenitizing temperature. Austenite grain size, pearlite colony size, and interlamellar spacing are evaluated for influence on the fracture toughnesses associated with pearlitic microstructures. It is found that the primary microstructural parameter controlling fracture toughness is the cleavage facet size, which typically spans several pearlite colonies. The size of cleavage facet size is primarily controlled by the austenite grain size. The influences of pearlite colony size and interlamellar spacing are mino
Diffusion-controlled-creep processes are used to assess the creep behavior of dispersion and solute hardened materials at coarse and fine grain sizes. It is shown that the creep behavior of a dispersion strengthened (DS) Al-Mg alloy is similar to the creep behavior of pure Al-Mg alloys. Both materials show dislocation climb and dislocation solute-drag contributions to creep. It is shown that the threshold stress for creep for these materials is a function of the mobile dislocation density, of the dislocation velocity and of the concentration of solute atoms in the dislocation core. It is, therefore, appropriate to describe the threshold stress as the threshold strain rate. It is shown that the same value of the diffusion-compensated strain rate for the threshold stress is obtained for slip in DS Al-Mg material as in a fine-grained Al-Mg alloy, where grain-boundary sliding is the principal deformation process. This is evidence that grain boundary sliding is accommodated by dislocation creep.
Keywords: Creep; Grain-size effects; Solute effects; Threshold stress
Fully pearlitic steels are of great importance in a number of extremely demanding structural applications, in large part because of their combination of strength and toughness. Strength and toughness are controlled by the microstructures developed in pearlitic steels, especially interlamellar spacing, pearlite colony size, and prior austenite grain size. This article reviews the effects of these microstructural features on the yield strength and toughness of fully pearlitic steels, the importance of hypereutectoid alloy compositions for increasing the strength of fully pearlitic steels.
One experimental and five commercial aluminum alloys were tested in tension at elevated temperatures (225 °C to 500 °C) over a range of strain rates (2×10−5 to 10−1 s−1). The experimental alloy contained 5 wt pct Zn with a balance of Al. The commercial alloys included AA 5182, 5754, 7150, 6111, and 6022. Two 5182 materials were examined, one produced by standard ingot-processing methods and the other by continuous casting. The 5754 and 5182 alloys exhibited a deformation regime consistent with solute-drag creep for values of diffusivity-compensated strain rate less than 1013 m−2. Within this regime, the 5754 and ingot-metallurgy 5182 materials exhibited tensile ductilities up to 140 pct. The continuously cast 5182 material exhibited lower ductility in this regime than the 5754 and ingot-metallurgy 5182 materials, despite similar stress exponents. Ductility was reduced in the continuously cast 5182 because of significant dynamic grain growth and cavitation. The 7150, Al-5Zn, 6111, and 6022 materials exhibited significantly higher stress exponents and lower tensile ductilities thanthe 5000-series materials.
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.
Ultrahigh-carbon steel; Damask; Pearlite; Mechanical properties; Properties
Cavitation was examined in an Al–Mg solid-solution alloy deformed in tension at 400 °C under conditions providing solute-drag creep, which can produce tensile ductilities from 100% to over 300%. Two nondestructive evaluation techniques were employed to measure the extent of cavitation: ultra-high-resolution x-ray computed
tomography and pulse-echo ultrasonic evaluation. Subsequent to nondestructive evaluation, the sample was sectioned for examination by standard metallographic
techniques. Metallographic examination confirmed that both nondestructive techniques accurately indicated the extent of cavitation. Ultrasonic testing provided a practical
means of distinguishing material with cavities from that without cavities. Ultra-high-resolution x-ray computed tomography provided an accurate three-dimensional
image of internal cavitation.
Tensile ductilities consistently in excess of 100 percent are produced at elevated temperatures in aluminum solid-solution alloys containing magnesium. The alloys that produce such enhanced ductility include commercially available 5XXX-series alloys and course-to fine-grained Al-Mg alloys.
Several binary and ternary Al alloys containing from 2.8 to 5.5 wt pct Mg were tested in tension at elevated temperatures (200 °C to 500 °C) over a range of strain rates (10−4 to 2.0 s−1). Tensile ductilies of up to 325 pct were obtained in binary Al-Mg alloys with coarse grains deformed in the solute-drag creep regime. Under test conditions in which solute-drag creep controls deformation, Mg in concentrations from 2.8 to 5.5 wt pct neither affects tensile ductility nor influences strain-rate sensitivity or flow stress significantly. Strength is shown to increase with increasing Mg concentration, however, in the power-law-breakdown regime. The solute-drag creep process, which leads to superplastic-like elongations, is shown to have no observable grain-size dependence in a binary Al-Mg material. Ternary alloying additions of Mn and Zr are shown to decrease the strain-rate sensitivity during solute-drag creep, negatively influencing ductility. An important cause of reduced ductility in the ternary alloys during creep deformation is found to be a transition from necking-controlled failure in the binary alloys to cavitation-controlled failure in the ternary alloys investigated. An increase in ternary element concentration, which can increase the relative volume percentage of proeutectic products, increases cavitation.