S. Sangal

Synthesis of Aluminium–Cementite Metal Matrix Composite by Mechanical Alloying

Cementite powder was prepared from elemental iron and graphite powder by mechanical alloying (MA) in a specially built dual-drive planetary mill. The phase evolution, particle-size distribution, and morphology of particles were studied during 40 hours grinding period. X-ray diffraction (XRD) shows formation of cementite and other iron carbides along with elemental iron after milling, whereas after annealing only cementite is present. Initially particle size increases with milling due to ductility of iron powder and then reduces with further milling. Al-cementite composite was synthesized by mixing cementite with Al powders, and then by hot pressing or cold compaction and sintering. XRD analysis of Al–Fe3C composite shows Fe3C, FeAl, Al, and other iron carbides along with Al4C3 after sintering. Scanning electron microscope (SEM) micrograph of hot-pressed samples shows excellent compatibilility between Al matrix and cementite particles.

A Process of Notch Wavy Rolling for Strengthening Metal Sheets

Wavy roll design was employed for strengthening 1 mm thin austenitic stainless steel coil sheet by cold rolling without further reduction in thickness. This steel possesses high corrosion resistance and high ductility. Initially, the sheets were rolled into sine wave shape (wave amplitude <2 mm) and then flattened using conventional cold rolling mill. Such a process cycle was repeated for four times successfully and the mechanical properties were measured after each cycle. The yield strength increased from 255 to 931 MPa with corresponding decrease in elongation from 45% to only 17% after the fourth cycle of severe cold working. Tensile strength and hardness values increased from 753 MPa and 185 HV to 973 MPa and 371 HV, respectively. The micro-to-nano-scale resolution structures, obtained by optical and atomic force microscope (AFM), were used to explain the variation in properties during this manufacturing process and to propose schematically the deformation mechanism.

Strengthening of a thin austenitic stainless steel coil by cold wavy rolling with no magnetic and dimensional changes

A comparison of the effects of wavy rolling and cold rolling on microstructure variation, phase evolution, tensile and magnetic properties of a thin coil of Fe-18.47Cr-8.10Ni-0.94Mn austenitic stainless steel was made at room temperature. Wavy rolling led to strengthening with no change in magnetic property and thickness, unlike the conventional cold rolling that changed all these properties by deformation induced martensitic transformation, in addition to substructure evolution. The yield strength of 413 MPa and magnetic saturation 3.7 emu/g under mill-annealed condition increased, respectively, to 1208 MPa and 11.8 emu/g, upon four cycles of wavy rolling. While the maximum yield strength of 1790 MPa could be achieved by combining this stage of four cycles of wavy rolling with subsequent 50% conventional cold rolling, the magnetic saturation increased to 73.3 emu/g by deformation induced martensitic transformation caused by the latter.

On the fracture of small samples under higher order strain gradient plasticity

In this work we perform Finite Element simulations within the framework of large deformation elasto-viscoplasticity on a material that is sensitive to the gradients of plastic strain and incorporates a single intrinsic length scale parameter. Both small scale yielding simulations and those on a finite sized sample show that large stress enhancements can occur at the tip of a notch due to gradient effects. The amount of plastic strain and opening stress that can be expected at the notch tip depends on an interplay between the notch radius, specimen dimensions and boundary conditions. It is shown that cleavage can be the favored criterion for failure in even a ductile material when the notch radius is small compared to the intrinsic length scale. Moreover, for large intrinsic length scales, failure may not always initiate at a notch but may be triggered away from it due to the presence of a boundary impermeable to dislocations.