Effect of Process Variables on the Microstructural Features for As-Cast Magnesium Alloys

The solidification pattern for a material is strongly dependent on the process parameters, such as cooling rate, thermal gradient, solidification time, mold shape and dimensions, mold filling rate, superheat temperature etc. Thus, the obtained microstructural features have a direct functional relation with these initial variables. In this study, two commercial Mg alloys (AM60B and AZ91D) were studied under different solidification conditions to characterize the influence of cooling rate, thermal gradient, growth velocity, Niyama criterion, solidification time and mold dimensions on microstructural features such as secondary and tertiary dendrite arm spacing, grain size, porosity, pore shape and size, local morphological and phase variations. Porosity, grain size and dendrite arm spacing were measured and correlated with the process variables recording during the casting process. It was determined that the process of mold filling and solidification are simultaneous in nature and they significantly affect the development of microstructure and its dependency on the process parameters. This significantly affects the obtained porosity values and their variation along the casting. These results clearly indicate that rate of filling, nature of flow o f liquid and shape o f the mold greatly affect the solidification process and thereby the microstructure. Any predictive solidification model, treating these factors singularly wouldn’t be appropriate to pre-determine the microstructure and properties of the casted component

Strengthening mechanisms in high entropy alloys:Fundamental issues

High entropy alloys (HEAs), offering a multi-dimensional compositional space, provide almost limitless design opportunities surpassing the frontiers of structural materials development. However, an in-depth appraisal of the fundamental materials physics behind strengthening in HEAs is essential in order to leverage them to achieve greater flexibility in application oriented materials design. This viewpoint paper concentrates on issues regarding inherent compositional fluctuations in HEAs and corresponding impact on strengthening is highlighted. In particular, metal physics based design criteria in multi-phase HEAs are discussed and comparisons between multi-phase and single-phase HEAs are drawn.</p

High Entropy Alloys:Ready to Set Sail?

Over the past decade, high entropy alloys (HEAs) have transcended the frontiers of material development in terms of their unprecedented structural and functional properties compared to their counterpart conventional alloys. The possibility to explore a vast compositional space further renders this area of research extremely promising in the near future for discovering society-changing materials. The introduction of HEAs has also brought forth a paradigm shift in the existing knowledge about material design and development. It is in this regard that a fundamental understanding of the metal physics of these alloys is critical in propelling mechanism-based HEA design. The current paper highlights some of the critical viewpoints that need greater attention in the future with respect to designing mechanically and functionally advanced materials. In particular, the interplay of large compositional gradients and defect topologies in these alloys and their corresponding impact on overall mechanical response are highlighted. From the point of view of functional response, such chemistry vis-à-vis topology correlations are extended to novel class of nano-porous HEAs that beat thermal coarsening effects despite a high surface to volume ratio owing to retarded diffusion kinetics. Recommendations on material design with regards to their potential use in diverse applications such as energy storage, actuators, and as piezoelectrics are additionally considered.ISSN:2075-470

Twinning induced spatial stress gradients:Local versus global stress states in hexagonal close-packed materials

Length scale dependent microstructural heterogeneities serve as effective pathways in engineering materials for providing simultaneous strength-ductility enhancement. In this regard, hexagonal close-packed (hcp) materials that exhibit a combination of slip and multiple twinning modes potentially act as ideal candidates that generate heterogeneous microstructures. However, such an inhomogeneous distribution of crystallographic defects also results in build-up of spatially heterogeneous local stress gradients that can be distinct from globally applied stress state. In particular, stress fields arising at the vicinity of deformation twins and due to their interaction with grain interfaces often act as precursors to damage nucleation in most hcp metals and alloys. Hence, assessment of such local stresses and their overall impact on plasticity becomes necessary in order to understand the relationship between twinning and fracture in hcp materials. The current work utilizes commercially pure titanium (cp-Ti) as a model material to investigate the impact of twinning induced stress gradients on the local mechanical response. By means of correlative multiscale structural characterization and local stress gradient measurements, we establish a definitive relationship between applied stress vis-à-vis local stress on the local plasticity behavior ahead of a {112¯2} compression twin-grain boundary intersection in cp-Ti. Additionally, the role of twin interfacial structure for tension and compression twinning modes are experimentally determined and their corresponding impact on the local stress fields and associated twin migration mechanisms is assessed.</p

Secure Cloud Storage Scheme Based On Hybrid Cryptosystem

This paper presents a secure cloud storage scheme based on hybrid cryptosystem, which consists of Elliptic Curve Cryptography (ECC), Advanced Encryption Standard (AES), and one-way hash function. Here, the data owner exports large volume of encrypted data to a cloud storage provider. The exported encrypted data is over-encrypted by the cloud storage provider, and the data is sent to the requesting user. An existing hybrid cryptosystem based dynamic key management scheme with hierarchical access control has been incorporated in our scheme. The key management scheme groups users in various security classes, and helps to derive efficiently, as well as directly the secret keys of the lower order security classes. The incorporated key management scheme in our proposed scheme incurs low computational, communication, and storage overheads for key generation, and derivation purposes. The security analysis, and the simulation results run on the AVISPA tool (formal security verification tool) show that the proposed scheme is protected from the adversaries. This scheme is useful in `owner-write-users-read\u27 application areas, and the end users may use resource-constrained wireless mobile devices securely in this proposed scheme

BCC-FCC interfacial effects on plasticity and strengthening mechanisms in high entropy alloys

Al0.7CoCrFeNi high entropy alloy (HEA) with a microstructure comprising strain free face-centered cubic (FCC) grains and strongly deformed sub-structured body centered cubic (BCC) grains was subjected to correlative nanoindentation testing, orientation imaging microscopy and local residual stress analysis. Depending on the geometry of BCC-FCC interface, certain boundaries indicated appearance of additional yield excursions apart from the typically observed elastic to plastic displacement burst. The role of interfacial strengthening mechanisms is quantified for small scale deformation across BCC-FCC interphase boundaries. An overall interfacial strengthening of the order of 4GPa was estimated for BCC-FCC interfaces in HEAs. The influence of image forces due to the presence of a BCC-FCC interface is quantified and correlated to the observed local stress and hardness gradients in both the BCC and FCC grains

Measurement of spatial stress gradients near grain boundaries

A correlative method based on electron back scattered diffraction and focused ion-beam-digital image correlation slit milling technique was used to quantitatively determine spatially resolved stress profiles in the vicinity of grain boundaries in pure titanium. Measured local stress gradients were in good agreement with local average misorientation and experimentally calculated geometrically necessary dislocation densities. Stress profiles within few hundred to thousand nanometers near the grain boundary display a local minimum, followed by a typical Hall-Petch type variation of "one over square root of distance". The observed trends allude to local stress relaxation mechanisms active near grain boundaries. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Experimental determination and theoretical analysis of local residual stress at grain scale

Grain/phase boundaries contribute significantly to build up of residual stresses, owing to varied plastic/thermal response of different grain orientations or phases during thermomechanical treatment. Hence, accurate quantification of such local scale stress gradients in commercial components is important in understanding their mechanical performance. The current work introduces a correlative method utilizing Electron Back Scattered Diffraction and Focused Ion Beam-Digital Image slit milling methodology to accurately determine spatially resolved stress profiles in the vicinity of grain boundaries using commercially pure titanium as a model material. Measured local stress gradients were in good agreement with local misorientation values. The role of dislocation-grain boundary interactions on buildup of local stress gradients is elucidated. Stress profiles near grain boundaries initially display non Hall-Petch characteristics, followed by a typical Hall-Petch type variation of “one over square root of distance”. The observed trends allude to local stress relaxation mechanisms very close to the grain boundaries. The findings indicate that grain scale stress gradients can be significant in terms of playing a crucial role in macroscopic fatigue behavior