S.R. Nutt

Al-Mg alloy engineered with bimodal grain size for high strength and increased ductility

Al–7.5Mg powders were cryomilled, then consolidated and extruded to produce bulk nanostructured material. The extrusions had a tensile yield strength of 641 MPa and an ultimate strength of 847 MPa. Additional samples were prepared by combining cryomilled powder unmilled Al–7.5Mg, resulting in extrusions with high strength and increased ductility.

Interfacial properties of polymer composites measured by push-out and fragmentation tests

Abstract The interfacial properties for E-glass/epoxy composites were measured using push-out tests and single fiber fragmentation tests. Theoretical models for both stress-based and energy-based criteria were used to interpret the experimental results. Fibers treated with γ-aminopropyl-triethoxysilane (γ-APS) showed higher bond strength (∼1.7 times higher) and interfacial toughness (∼1.9 times higher) than those of unsized E-glass based composites. However, the average interfacial toughness obtained from fragmentation tests was about six times higher than that obtained from push-out tests. Considering the analytical frameworks employed to interpret the values measured in the present work, the fragmentation test is a more appropriate method to obtain interfacial energy for polymeric composites, but both methods are appropriate for relative measurements of interface strength.

PM synthesis and properties of steel foams

Abstract Steel foam steel was synthesized by a powder metallurgical route, resulting in densities less than half that of steel. Process parameters for foam synthesis were investigated, and two standard powder formulations were selected consisting of Fe–2.5%C and 0.2 wt.% foaming agent (either MgCO3 or SrCO3). Using the PM approach, foams with relative density ranging between 0.38 and 0.64 were obtained. Compression tests were performed on annealed and pre-annealed foam samples of different density to determine mechanical response and energy absorption behavior. The stress–strain response was strongly affected by annealing, which reduced the carbon content and converted much of the pearlitic structure to ferrite. These microstructural changes led to a more ductile response during compressive loading, in which a long stress plateau typically occurred after initial yielding. In fact, annealed steel foams behaved much like aluminum foams under compressive loading, despite pore structures that were considerably more coarse than those reported for aluminum foams.

Anisotropy and strain localization in steel foam

Abstract Steel foam fabricated by a powder metallurgical process was tested in uniaxial compression. The closed-cell foam samples exhibited anisotropy in compression, a phenomenon that was caused primarily by the ellipsoidal cell shapes within the foam. Yield strengths were 3× higher in the transverse direction than in the longitudinal direction. Yield strength also showed a power-law dependence on relative density ( n ≅1.8). Compressive strain was highly localized and occurred in discrete bands that extended transverse to the loading direction. The deformation bands were comprised of collapsed cells, and deformation occurred in a sequential manner with repeating cycles of yield, collapse and densification of cells. Foam densification commenced when all of the foam was converted into deformation bands.

Microstructural investigation on B4C/Al-7093 composite

Abstract The microstructures of B4C/Al-7093 composites synthesized using the Boralyn technique were investigated. The present results show that the B4C particles were distributed uniformly in the aluminum matrix. SEM fractographic investigation revealed that particle cracking was a dominant damage mechanism, indicating a strong B4C/Al interfacial bond which promoted high mechanical properties in the composite. TEM and EDS analyses showed MgO particles at and near the B4C/Al interfaces. The amount of MgO significantly influenced the microstructure of the matrix. The formation of MgO in the composite resulted in the depletion of the Mg atoms in the matrix and furthermore, the suppression of precipitation during the aging process.

Strain rate sensitivity and defects in steel foam

Abstract Steel foams fabricated by a powder metallurgical process were subjected to uniaxial compression tests to explore the dependence on strain rate. The yield strength of foam samples showed stronger strain rate dependence at higher strain rates, while the energy absorption increased linearly with strain rate. Measurements of cell wall curvature showed that an increased mean curvature correlated with a reduction in yield strength, and foam strengths generally fell below predictions of the Gibson–Ashby theory. Morphological defects (corrugations of cell walls) also reduced yield strength and altered the dependence on density.

A Comparison of the Corrosion Behavior of Nanocrystalline and Conventional Al 5083 Samples

Abstract Pitting scan (PS) and electrochemical impedance spectroscopy (EIS) techniques were used to evaluate the corrosion behavior of nanocrystalline (NC) samples and conventional Al 5083 (UNS A95083) in three solutions with different chloride concentrations. The PS obtained in 0.5 N sodium chloride (NaCl) and in 0.5 M sodium sulfate (Na2SO4) + 0.1 N NaCl indicated that pitting should occur in these solutions. These results were confirmed by EIS data that were recorded for up to 25 days. For exposure to 0.5 M Na2SO4 + 0.05 N NaCl the pitting (Epit) and protection (Eprot) potentials determined after exposure for 2 h suggested that pitting would not occur in this solution. However, the initial impedance spectra were indicative of pitting. Tests carried out after exposure for 2 days and 3 days showed passive behavior, but starting with exposure for 5 days, pitting was again indicated for all three samples. PS performed at the end of the EIS tests suggested that this result was due to the increase in corrosi...

Effects of process parameters on steel foam synthesis

The effects of simple process parameters used for making steel foams by powder metallurgical (P/M) synthesis were investigated. Different powder blending methods and melting times were employed and the effects on the geometric structure of steel foam were examined. Powder blending was critical to achieving a homogeneous distribution of foaming agent because of the density disparity between steel and foaming agent powders. Dispersion of the foaming agent affected the pore size distribution of the expanded foams. Foam expansion was accomplished by melting powder compacts, and the time above the melt temperature affected the pore structures within the foams. With increasing melt time, pores coalesced, leading to the eventual collapse of the foam. Inserting interlayer membranes in the powder compacts inhibited coalescence of pores and produced foams with more uniform cell size and distribution.

Thermal stability in nanostructured Al-5083/SiCp composites fabricated by cryomilling

Abstract Nanostructured Al-5083/SiCp composites, which consist of an ultrafine grained Al-5083 matrix reinforced with nanosized SiC particles, were fabricated via a cryomilling plus consolidation process. The thermal stability of these composites was studied by investigating the effects of annealing temperature on tensile properties and Vickers hardness of the composites and grain growth in the Al-5083 matrixes, with the annealing temperature ranging up to 873 K. The experimental results indicate that, after annealing at temperatures up to 773 K (0·90 T m, where T m is 862 K, the melting onset temperature of the Al-5083 matrixes), the strength of the composites did not decrease significantly, while the grain size of the Al-5083 matrixes did not show a noticeable increase. The grains in some areas of the Al-5083 matrixes did not grow even after annealing at 873 K (1·01 T m). The high thermal stability is attributed to the dispersion of native oxide particles observed at the lamellar interfaces created by cryomilling.

Influence of hot isostatic pressing on microstructure and mechanical behaviour of nanostructured Al alloy

Abstract Aluminium alloy AA 5083 [Al–4·4Mg–0·7Mn–0·15Cr (wt-%)], powder was ball milled in liquid nitrogen via the cryomilling method to obtain a nanocrystalline (NC) structure. Samples of the powder were hot vacuum degassed to remove interstitial contaminants, then consolidated by hot isostatic pressing (HIPing) at six temperatures (from 0·46Tm to 0·89Tm), before being high strain rate forged (HSRF) to produce plate material. The microstructure was analysed at the different processing stages. The compressive properties of the as HIPed material, plus tensile properties of the final product were studied. Despite grain growth during HIPing, an ultrafine grain (UFG) structure was retained in the consolidated material, which consequently had increased strength over conventionally processed AA 5083. As the HIP temperature was increased, the density increased. Strength changes were minimal in compression and tension with varying HIP temperature, once near full density was attained at 275°C (∼0·64TM). Yield strength data indicate negligible variation in the grain size of the materials.