Department of Design and Automation

Vellore Institute of Technology (VIT) is a private university located in&nbsp;Tamil Nadu, India. Founded in 1984, as Vellore Engineering College, the institution offers 20 undergraduate, 34 postgraduate, four integrated and four research programs. It has campuses in Vellore, Amravati, Bhopal and Chennai.

VIT is one of the top ranked private universities in India according to NIRF, THE and QS Rankings.&nbsp;Govt. of India has recognized&nbsp;VIT, Vellore as an&nbsp;Institution of Eminence. This has allowed VIT to take independent quality initiatives and move up in world ranking.

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VIT University

Centre for Innovative Manufacturing Research

World University root node

Vellore Campus

Physics of Metals and Metallography

A Comparative Study on Misorientations to Determine the Extent of Recrystallization in Pure ETP Copper

Experimental fracture mechanics at the microscale became an indispensable tool for understanding and developing advanced material systems. In case of linear elastic fracture mechanics, stringent requirements are typically only warranted for very brittle materials. The material properties of semi-brittle materials might be accessible by elasto-plastic fracture mechanics. However, challenges exist in determining the crack length, in producing geometry and notch geometry, in defining of the initiation toughness and in extracting the size independent crack resistance curves. In this study, we assess current approaches of measuring the fracture toughness of semi-brittle materials by elasto-plastic fracture mechanics. We investigate the notch geometry (through thickness notch and bridge notch), the notch depth and the method of determining in situ the crack length for ultrafine grained tungsten. Further challenges due to the overlap of sample size and crack process zone are identified. Finally, we propose a workflow for analyzing the elasto-plastic fracture toughness of material systems at the microscale. © 2020 The Authors

Fulltext

Materials and Design

Experimental conditions affecting the measured fracture toughness at the microscale: Notch geometry and crack extension measurement

Microscale mechanical behaviour of single crystalline Barium Titanate (BaTiO3), a ferroelectric ceramic was studied by uniaxial in situ micropillar compression and nanoindentation. It was observed that pillars below 1 µm diameter reached the theoretical strength of BaTiO3 whereas larger pillars yielded at lower stress values with multiple stress drops confirming slip activity. A size scaling exponent of 0.96 ± 0.09 was estimated for BaTiO3 which is close to one of the soft fcc metals. The material's strength, hardness and deformation behaviour did not show any dependence on the character of the ferroelectric domain, within our error bars. © 2020

Scripta Materialia

Effect of size and domain orientation on strength of Barium Titanate

The fracture behavior of a white etching layer formed on the rail surface in pearlitic steels during the rail-wheel contact is investigated using indentation-based microcantilever fracture tests. The sample thickness is in the order of 5 μm. The local fracture toughness of the white etching layer, its neighboring brown etching layer, martensite and pearlite with similar chemical composition are determined and compared to ferritic steels. All samples show stable crack growth accompanied by significant plasticity at the crack tip. The toughnesses scale inversely with the microhardness. The white etching layer exhibits a toughness of 16.0 ± 1.2 MPa m1/2 which is in the same range as the fully martensitic steel. It is shown that the local fracture toughness can be roughly estimated based on the Vickers hardness of the white etching layer. Also, an estimation of a critical defect size in white etching layers which considerably furthers the understanding of crack initiation is made in this study. Furthermore, various criteria for analyzing the elasto plastic fracture toughness are compared. © 2019 The Authors

Micro fracture investigations of white etching layers

Failure in engineering materials like steels is strongly affected by in-service deleterious alterations in their microstructure. White Etching Layers (WELs) are an example of such in-service alterations in the pearlitic microstructure at the rail surface. Cracks initiate in the rails due to delamination and fracture of these layers and propagate into the base material posing severe safety concerns. In this study, we investigate the microscale fracture behaviour of these WELs. We use in situ elastic-plastic fracture mechanics using J-integral to quantify the fracture toughness. Although usually assumed brittle, the fracture toughness of 21–25 MPa√m reveals a semi-brittle nature of WELs. Based on a comparison of the fracture toughness and critical defect size of WELs with the undeformed pearlitic steels, WELs are detrimental for rails. In the micro fracture tests, WELs show crack tip blunting, branching, and significant plasticity during crack growth due to their complex microstructure. The fracture behaviour of the WELs is governed by their microstructural constituents such as phases (martensite/austenite), grain size, dislocation density and carbon segregation to dislocations and grain boundaries. We observed dislocation annihilation in some martensitic grains in the WELs which also contributes to their fracture behaviour. Additionally, the strain-induced transformation from austenite to martensite affects the crack growth and fracture. © 2019 Acta Materialia Inc.

Acta Materialia

In situ study on fracture behaviour of white etching layers formed on rails

Machining of aerospace and biomedical grade titanium alloys has always been a challenge because of their low conductivity and elastic modulus. Different machining methods and parameters have been adopted for high precision machining of titanium alloys. Machining of titanium alloys can be improved by microstructure optimization. The present study focuses on the effect of microstructure on machinability of Ti6Al4V alloys at different cutting speeds. Samples were subjected to different annealing conditions resulting in different grain sizes and local micro-strains (misorientation). Cutting forces were significantly reduced after annealing; consequently, sub-surface residual stresses were reduced. Deformation twinning was also observed on samples annealed at a higher temperature due to larger grain size. Initial strain free grains and deformation twinning during machining reduces the cutting force at higher cutting speed. © 2017, The Korean Society of Mechanical Engineers and Springer-Verlag Berlin Heidelberg.

Journal of Mechanical Science and Technology

Effect of microstructure and cutting speed on machining behavior of Ti6Al4V alloy

Titanium alloys are most extensively used in biomedical application, which need high degree of dimensional accuracy and good machinability to reduce manufacturing cost. Both of these necessities can be addressed by surface and sub-surface modification without altering the mechanical properties in core of the material. The laser treatment is one of best tool to obtain controlled surface modification. In present paper the laser treatment directionality effect on subsurface modification is studied and correlated with machinability of Ti6Al4V. The reduction in cutting force variation is observed for unidirectional laser treated sample, which contribute to enhance the machinability of the alloy. © 2016 The Authors

Procedia Manufacturing

Influence of Laser Heat Treatment on Machinability of Ti6Al4V Alloy

Asymmetry in hexagonal crystal structure makes the occurrence of slip strongly dependent on the texture of sample. In titanium, which has a c/a ratio less than ideal, slip occurs preferentially on prismatic slip system. However other slip systems may get activated depending on the resolved shear stresses. In this paper we present results from plane strain compression experiments where the same area of the sample was imaged before and after deformation to document changes in microstructure. We then compare these results with a simple calculation of plastic strain based on activation of various slip systems depending on their respective critical resolved shear stresses. We show that incorporation of a strain rate dependent hardening parameter provides a reasonable match with the experimentally observed deformation behaviour of various grain orientations. © Published under licence by IOP Publishing Ltd.

IOP Conference Series: Materials Science and Engineering

Loading and texture bias on the competitive slip activity for basal and prismatic slip systems in HCP alloys

The effect of orientation and interaction with neighboring grains in polycrystalline Ti6Al4V deformation are quantified using interrupted deformation coupled with microstructure analysis. It is shown that softer grains accommodate more strain compared to hard basal grains, which can be explained in terms of their resolved shear stress and critical resolved shear stress. Grains with similar orientations follow different orientation trajectories during deformation depending on their misorientation with respect to neighboring grains. The influence of near neighbor interaction is quantified using a new parameter, average nearest neighbor misorientation (NNM). The NNM accurately predicts similarities and differences in the evolution of grain orientations during deformation. © 2015 Elsevier B.V.

Materials Science and Engineering A

Quantitative analysis of orientation and near neighbor interaction effects during deformation of polycrystalline Ti6Al4V

An understanding of the plastic deformation behavior of Ti6Al4V (Ti64) is of great interest because it is used in aerospace applications due to its high specific strength. In addition, Ti alloys have limited slip systems due to hexagonal crystal structure; hence twinning plays an important role in plastic deformation. The present work focuses upon the grain size effect on plastic deformation behaviour of Ti64. Various microstructures with different grain size were developed via annealing of Ti64 alloy in α-β phase regime (825°C and 850°C) for 4 hours followed by air cooling. The deformation behavior of these samples was investigated at various deformation temperature and strain rate conditions. Detailed microstructure studies showed that (i) smaller grains undergoes twinning only at low temperature and high strain rate, (ii) large grain samples undergo twinning at all temperatures & strain rates, though the extent of twinning varied. © (2015) Trans Tech Publications, Switzerland.