P.C. Chakraborti

Analysis of deformation induced martensitic transformation in stainless steels

Abstract Many studies monitoring the formation of martensite during the tensile deformation of austenite report data which are, in principle, affected by both the applied stress and the resulting plastic strain. It is not clear in these circumstances whether the transformation is stress induced (i.e. the stress provides a mechanical driving force) or whether the generation of defects during deformation helps nucleate martensite in a scenario better described as strain induced transformation. The authors demonstrate in the present work that a large amount of published data relating the fraction of martensite to plastic strain can in fact be described in terms of the pure thermodynamic effect of applied stress.

Connection between deformation-induced dislocation substructures and martensite formation in stainless steel

We report a quantitative connection between the strain amplitude dependencies of the organisation of dislocation cell substructures with the formation of deformation-induced martensite during cyclic plasticity of austenitic stainless steel at ambient temperature.

Preparation of alumina–silica–nickel nanocomposite by in situ reduction through sol–gel route

Abstract Ceramic based composites with dispersion of nano sized metal/metal carbide particles have generated wide technological interest for their improved mechanical properties — hardness, fracture strength as well as fracture toughness, superior electrical properties and magnetic properties. In the present investigation alumina–silica gels have been prepared along with nickel chloride and dextrose distributed in the nanometric pores of the gel. The gels are prepared with different molar proportions of alumina and silica containing 5 wt% of nickel chloride and 50 wt% excess dextrose. During heat treatment at a temperature of 9008C for half an hour in nitrogen atmosphere, nickel chloride is reduced to metallic nickel by in situ generated hydrogen in the silica–alumina matrix. X-ray analyses indicate that no nickel chloride reduction is possible upto 50 mol% silica in alumina–silica matrix. Beyond this range, higher the silica content, higher is the reduction of nickel chloride. The presence of metallic nickel has been substantiated further by SAD analysis. Particle size analysis based on X-ray diffraction as well as transmission electron micrograph shows the presence of nickel particles of size ,20 nm distributed in the alumina–silica nanocomposite.

Effect of grain size on austenite stability and room temperature low cycle fatigue behaviour of solution annealed AISI 316LN austenitic stainless steel

Abstract The present paper investigates completely reversed room temperature low cycle fatigue (LCF) behaviour of solution annealed austenitic stainless steel AISI 316L with two different grain sizes of 90 and 139 μm developed by solution annealing treatment at 1050 and 1150°C respectively and at six strain amplitudes ranging between ± 0·375 and ± 1·00%. Complete cyclic hardening has been observed for both the grain sizes. While fine grained steel shows an improvement in cyclic life compared with that of coarse grained steel for strain amplitudes ± 0·375 and ± 0·50%, and perfectly follows the Coffin–Manson (C–M) behaviour within the experimental domain, higher cyclic life with bilinear C–M behaviour is observed in the case of coarse grained steel at ± 0·625% strain amplitude and above. Optical microscopy of fatigue fracture surfaces reveals the formation of martensite on cyclic straining predominantly at higher strain amplitudes.

Effect of phase separation on the fracture toughness of SiO2-B2O3-Na2O glass

Fracture toughness of glass is usually poor, due to the absence of grain boundaries and discontinuities. The compositions of the glass studied are in the phase separated region of SiO2-B2O3-Na2O system. The interface between the glass in glass separation enhances the fracture toughness. The increase in the connectivity of phase separated regions causes increase of fracture toughness from 0.98 through 1.43 to 1. 54 MPam1/2.

Extraction of Region Contour in SEM Fractographs

In the domain of material science, quantitative fractography is an analytical tool to study the characteristics of a fracture surface. The inception of Scanning Electron Microscope (SEM) has motivated the researchers toward the quantitative analysis of such surface. Due to fracture, new surfaces are evolved and voids are also formed. Extraction of such regions (surface/void) from SEM fractographs is of immense importance as it enables the subsequent characterization of the surfaces and the study of void distribution. To carry out the analysis, image processing tools are being applied by the researchers mostly on a case to case basis. Thus, well founded image processing technique to cater the specific need is still lacking. In this work, we have proposed a scheme to determine the closed contour of the regions denoting the surface or void. The proposed methodology relies on the systematic combination of basic techniques of image processing to accomplish the task in an automated manner.

Asymmetric Cyclic Behaviour of Fine- and Coarse-Grained Commercially Pure Copper and Aluminium

In the present study, uniaxial ratcheting behaviour of annealed copper and aluminium with two different grain sizes has been investigated. Engineering stress-control tests have been performed at various combinations of stress amplitudes and mean stresses. To account for grain size variation, stress amplitudes were selected keeping the constant ratio of equivalent stress amplitude and tensile strength. It is found that ratcheting life of fine- and coarse-grained copper and aluminium is inversely related to both stress amplitude and mean stress. However, the effect of stress amplitude on ratcheting life is more as compared to mean stress for both the FCC metals. It is also find out that the ratcheting life of fine-grained FCC metals is more compared to coarse-grained FCC metals. The ratcheting strain rate follows three stages irrespective of stress combination and grain size of both FCC metals. It is observed that for both the FCC metals, the average steady state ratcheting strain rate follows perfect power relationship with cycles to failure.

Ductile Tearing Resistance Indexing of Automotive Grade DP 590 Steel Sheets: EWF Testing Using DENT Specimens

The essential work of fracture (EWF) method has been explored for indexing the ductile tearing resistance of DP 590 automotive grade dual-phase steel sheet both in longitudinal (L-T) and transverse (T-L) orientations. The simplest possible test and analysis procedures have been adopted. The EWF method is found to be eminently suitable for routine quality control and product development purposes for such materials. Areas for further research for improving the experimental strategy are highlighted. For the investigated steel sheet, the estimated tearing resistance is found to be distinctly higher for the L-T orientation compared to the T-L orientation; the reason thereof merits further investigation.

An Automated Approach for Volume Fraction Measurement of Titanium Alloy using Digital Image Processing

Physical properties like strength, toughness, ductility, hardness, corrosion resistance of a metal or alloy is strongly affected by the micro-structure of the material. The micro-structures evolve out of the thermo-mechanical process applied on the material and phases present undergoes transformation. Proper identification of phases and their quantitative measurement play an important role in the field of material characterization. Most of the commercial softwares available for this purpose relies on the intensity to segment the phases and analyzes the same. But it may fail to discriminate the phases if transformation results into the phases with overlapped intensity. In this work, we have experimented with heat treated Titanium alloy that consists of α/β phases. The transformation results into isolated island like α phase and lamellar α/β phase. There exists an overlap in intensity. It is also important to study the distribution of α and β in this lamellar matrix. In this work, along with intensity based thresholding, geometrical property is applied to segment the phases and carry out the analysis. Experiment is carried out with multiple field of views of Titanium micro-structure and obtained satisfactory output.

Digital image correlation for grain scale strain measurement in interstitial free high strength steel

Deformation measurement of materials and structures subjected to various loading conditions is an important task of experimental solid mechanics. Apart from the widely used point wise strain gauge technique, various full field non-contact optical methods are used for this purpose. In this work, an automated scheme to measure the grain level deformation in tensile deformed interstitial free high strength steel has been introduced. The method is based on digital image correlation technique. The proposed scheme utilised high resolution scanning electron images of the specimen surface that are sequentially captured during tensile loading. It is found that grain to grain there is large variation in deformation response at a given load. The present work also reveals point to point variation of strain within a grain interior.