K. Mondal

Large apparent compressive strain of metallic glasses

Very high plastic strains exceeding 20% in uniaxial compression tests, which show an inflection in the stress–strain curves of Zr-based bulk metallic glasses, are shown to be a spurious effect due to mechanical interlocking of cracks with shear bands. This type of stress–strain behaviour is misinterpreted as an actual deformation in the literature. The effect of mechanical interlocking is explained by fractographic analysis.

Gibb’s free energy for the crystallization of glass forming liquids

This letter gives an approximation for the Gibb’s free energy change for the crystallization of undercooled liquid (ΔG). The present ΔG expression is arrived at by the modification of existing models. Calculations based on the present expression have shown that the ΔG values obtained for seven glass forming alloys (Au81.4Si18.6, La50Al30Ni20, Mg65Cu25Y10, Mg50Ni30La20, Pd40Ni40P20, Zr65Cu17.5Ni10Al7.5, and Zr41.2Ti13.8Cu12.5Ni10Be22.5) compare well with the experimental ones in all the cases with a maximum deviation of 6% up to the glass transition temperature (Tg). The present expression is able to calculate the ΔG values accurately in a variety of glass forming alloys ranging from binary to quinary systems with a wide range of reduced glass transition temperature (Trg=Tg/Tm) of 0.45–0.75.

Relation Between Open Circuit Potential and Polarization Resistance with Rust and Corrosion Monitoring of Mild Steel

The present work discusses continuous corrosion assessment from a unique correlation of open circuit potential (OCP) and linear polarization resistance with rust formation on mild steel after prolong exposure in 3.5% NaCl salt fog environment. The OCP measurement and linear polarization tests were carried out of the rusted samples only without the removal of rust. It also discusses the strong influence of the composition, fraction, and morphology of the rust layers with OCP and linear polarization resistance. The rust characterization was done after the measurement of OCP and linear polarization resistance of the rusted steel samples. Therefore, monitoring of both the OCP and linear polarization resistance of the rusted mild steels coupled with rust characterization could be used for easy and dynamic assessment of the nature of corrosion.

Influence of Subsurface Structure on the Linear Reciprocating Sliding Wear Behavior of Steels with Different Microstructures

The present work investigates the influence of subsurface microstructure on the linear reciprocating sliding wear behavior of a number of steels with ferrite-pearlitic, pearlitic, bainitic, and martensitic microstructures under dry unlubricated condition. The change in the underlying microstructure with depth from worn-out surface of steel sample intimately relates to the associated hardness variation and wear volume. The present paper is not about comparison of wear resistance of steels with different structures; rather it is on mutual influence of wear and substructure for individual microstructure. Inherent toughness of the matrix and ability of microstructural components to get deformed under the reciprocating action of the ball decide the wear resistance of the steels. Bainite has good amount of stability to plastic deformation. Ferrite shows severe banding due to wear action. Work hardening renders pearlite to be wear resistant. Temperature rise and associated tempering of martensite are observed during wear.

Porous copper template from partially spark plasma-sintered Cu-Zn aggregate via dezincification

Present work deals with the preparation of spark plasma-sintered Cu-Zn aggregate (5, 10 and 20 wt% Zn) with interfacial bonding only starting from elemental powders of Cu and Zn (99.9% purity) and subsequently making of porous template of Cu by dezincification. Sintering is done so as to achieve only interfacial bonding with the aim to maintain maximum potential difference between the Cu and Zn particles during dezincification process in various solutions, viz. 1 N HCl and 3.5 wt% NaCl solutions. X-ray diffraction, optical microscopy and SEM-EDS are carried out to examine microstructural evolution and subsequent changes in hardness with sintering temperatures and different Zn percentages. Dezincification and pore formation are conducted on sintered 0.5 mm thick 12 mm diameter disc samples. The size, distribution and nature of pores in porous templates of Cu are then investigated using optical microscopy and SEM-EDS analysis.

Relook on fitting of viscosity with undercooling of glassy liquids

The present approach is on the modification of viscosity fitting of undercooled liquid as a function of undercooling. The method consists of finding analytical solution of three arbitrary constants of the Vogel–Fulcher–Tamman (VFT) equation by choosing three viscosity data at three critical temperatures for an undercooled liquid. Three critical temperatures are liquidus temperature (Tl), crystallization onset temperature (Tx) and glass transition temperature (Tg). The experimental viscosity data at or very near to these three critical temperatures (depending on the availability in literature) have been utilized to achieve the analytical solution. The analytical solution of VFT equation is further examined by selecting the experimental data points away from the critical temperatures in order to check their (Tl,Tx and Tg) significance towards the solution. Total absolute error (TAE) and total squared error (TSE) values obtained from the present method with respect to the experimental viscosity data in the temperature range between Tl and Tg are very much comparable and in most of the cases lower than that of existing ‘best-fit’ cited in the literature for a number of glassy alloys. Moreover, this method interestingly enables us to find the fragility parameters for a number of glassy alloys and convincingly explain their true glass forming abilities (GFA).

On the Possibility of Occurrence of Anisotropy in Processing of Cu-CNT Composites by Powder Metallurgical Techniques

Copper - multiwall carbon nanotubes (MWCNT) composite was processed by powder metallurgical processing technique. Pure copper powder and MWCNT were mechanically alloyed by high energy milling to produce Cu-MWCNT composite powder. The composite powder was subsequently consolidated by vacuum hot pressing. Characterization studies were conducted along axial (hot pressing direction) and radial (transverse) directions. Microstructural observations of the processed composite revealed random distribution of MWCNT in axial direction and aligned distribution in radial direction. The structure property correlation was established and it revealed certain degree of anisotropy in mechanical and electrical properties of the composite.

Nanoporous Ag template from partially sintered Ag-Zn compact by dezincification

A novel approach is followed to successfully fabricate nanoporous thin Ag template using partial sintering of elemental Ag and Zn (both have 99.9% purity) and subsequent dezincification. The starting materials for dezincification are partially sintered Ag-Zn aggregates (2.5, 5 and 10 wt% Zn). Partial sintering is done in order to achieve only interfacial bonding with the aim to maintain maximum potential difference between Ag and Zn particles during dezincification process in 1 N HCl and 3.5 wt% NaCl solutions. Two different dissolution methods, namely, simple immersion for 45 days and electrochemical way (holding the sample at critical potential), are employed. Electrochemical polarization tests are carried out to determine the critical potential for subsequent chrono-amperometry. X-ray diffraction, optical microscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy are carried out to examine microstructural evolution, size, distribution and nature of pores in sintered aggregate as well as in template.

Electrochemical passivation behaviour of nanocrystalline Fe80Si20 coating in borate buffer solution

Passivation behaviour of nanocrystalline coating (Fe80Si20) obtained by in situ mechanical alloying route is studied and compared with that of the commercial pure iron and cast Fe80Si20 in sodium borate buffer solution at two different pH values (7·7 and 8·4). The coating reveals single passivation at a pH of 7·7 and double stage passivity at a pH of 8·4. The first passive layer is due to the dissolution mechanism and second passivity is related to stable passivation. The cast sample shows single stage passivity in the solution of pH 8·4. The difference in the passivation behaviour for the cast alloy (Fe80Si20) and the coating is related to the presence of highly iron-enriched localized regions, formed during the processing stage of coating.

Wear behaviour of bainitic rail and wheel steels

The paper presents a short review on the past research and recent trends in the development of rail and wheel steels using bainitic morphology from the angle of mechanical and wear performances. The enhancement in mechanical properties as well wear resistance of the bainitic rail and wheel steels is very promising. In addition, the mechanical behaviour of a bainitic rail steel and a wheel steel along with their linearly reciprocating dry sliding wear characteristics in comparison to the existing pearlitic rail and ferritic–pearlitic wheel steels used in Indian Railways is discussed. The steels were subjected to austempering heat treatment at different temperatures and durations in the bainitic region as determined from the TTT diagrams of the respective steels. Bainitic steels show better mechanical properties and wear resistance due to the typical morphology of bainitic microstructural.