Siddhartha Roy

Mechanical properties of cellular solids produced from hollow stainless steel spheres

Mechanical properties of cellular hollow sphere structures are studied in this work. The material was fabricated by coating the metallic powder slurry on expanded polystyrol (EPS) spheres, drying, forming under compression, debinding, and final sintering of the spheres to each other. Longitudinal elastic wave velocities were measured using ultrasound phase spectroscopy while compression tests were carried out up to a homologous temperature of 0.6. Dependence of the relative Young’s modulus on the relative density is similar to conventional open-cell foams. Compression stress–strain plots show the three stages of elastic deformation, plateau, and densification. With increasing temperature the overall level of the compressive stress–strain plots shifts to lower stresses. The hollow sphere solids show slightly better high temperature strength in comparison to the base metal. However, due to the considerable scatter in the experimental data points, this improvement seems to be insignificant. Structural observations on samples deformed to within the plateau region clearly show the heterogeneous progress of deformation.

RESIDUAL STRESSES IN NOVEL METAL/CERAMIC COMPOSITES EXHIBITING A LAMELLAR MICROSTRUCTURE

The aim of this study is to analyze the mechanics of a new class of metal/ceramic composite on a mesoscopic length scale. These composites are produced by melt infiltration of porous ceramic preforms produced by freeze-casting and subsequent sintering. This production route has a high application potential since it offers a cost-effective way to obtain composites with ceramic content in the range 30-70 vol%. The as-produced material exhibits a hierarchical domain structure with each domain composed of alternating layers of metallic and ceramic lamellae. Residual stresses present in all phases of the composite produced by infiltrating alumina preforms with a eutectic aluminium-silicon alloy have been measured. Integral as well as spatially resolved measurements were carried out on single domain samples at the high-energy, energy-dispersive diffraction (EDDI) beamline at the synchrotron radiation source BESSY (Berlin, Germany). Results show that strongly fluctuating residual stresses are introduced by the production process, which can be rationalized taking into account the thermal expansion mismatch of alloy and preform.

Effect of Phase architecture on mechanical properties of interpenetrating metal/ceramic composites

The aim of this article is to compare the influence of the phase architecture on the mechanical properties of two interpenetrating MMCs with same metallic and ceramic phases and similar phase contents. One of the composites was fabricated by infiltrating freeze-cast alumina preforms, while the other composite was fabricated by infiltrating open porous alumina foam. Tests were carried out to determine the three longitudinal elastic constants, elastic-plastic flow behavior under compression and mechanism of internal load transfer under compression. Results show that phase morphology has a significant influence on the composite mechanical properties. Highest stiffness and compressive strengths are observed along the freezing direction in the freeze-cast MMC and this result from the significantly higher fraction of load carried by the alumina phase in this MMC type. Foam based MMC shows more isotropic behavior and its properties lie between the longitudinal and transverse properties for freeze-cast MMC.

Load Partitioning Study in a 3D Interpenetrating AlSi12/Al2O3 Metal/Ceramic Composite

Internal load transfer in an interpenetrating metal/ceramic composite has been studied in this work using energy dispersive synchrotron X-ray diffraction. One of the samples was loaded in tension and the other one in compression. In each case, the sample was first loaded into the elastic-plastic regime, unloaded to zero stress, and reloaded beyond the prior maximum stress. Results show that at stress amounts greater than 100 MPa aluminum deforms plastically and the load is transferred to alumina and silicon. Unloading and reloading typically show reverse plastic deformation, Bauschinger effect and strain hardening in aluminum.

Mechanical properties of innovative metal/ceramic composites based on freeze-cast ceramic preforms

This article provides an overview of recent research carried out about the mechanical properties of an innovative metal/ceramic composite fabricated by squeeze-casting liquid Al-12Si in freeze-cast alumina preforms. The composite consists of domains made of alternating metallic and ceramic lamellae. Elastic and elastic-plastic deformation behavior and mechanism of internal load transfer under external compressive stresses in individual domains with different orientations were studied. Results show that the stiffness is highest along the freezing direction and lowest along the direction perpendicular to it. When compressed along directions parallel to freezing direction, the composite is strong and brittle. When compressed along other directions, the behavior is controlled by the soft metallic alloy. Studies of internal load transfer under external compressive stress along different directions show that the mechanism of internal load transfer differs significantly when loading direction changes from that along the freezing direction to the direction perpendicular to it.