Ultrafine-grained microstructure of a QE22 alloy prepared by Friction Stir processing (FSP) is isochronally annealed to study the thermal stability and grain growth kinetics. The FSPed microstructure of QE22 alloy is thermally stable under ultrafine-grained regime up to 300 °C and the activation energy required for grain growth is found to be exceptionally high as compared to conventional ultrafine-grained magnesium alloys. The high thermal stability and activation energy of the FSPed QE22 alloy is due to Zener pinning effect from thermally stable eutectic Mg12Nd and fine precipitates Mg12Nd2Ag and solute drag effect from segregation of Neodymium (Nd) solute atoms at grain boundaries. © 2016 Elsevier B.V.

Sushanta Kumar Panigrahi

Department of Mechanical Engineering

Md Faseeulla Khan

Activation energy

Chemical activation

Drag

Friction

Friction stir welding

Grain boundaries

Growth kinetics

Magnesium

Magnesium alloys

microstructure

Rare earths

Silver

Silver alloys

stability

Thermodynamic stability

tribology

Friction stir processing

MAGNESIUM RARE-EARTH ALLOYS

SOLUTE DRAG EFFECTS

ULTRA FINE GRAINED MICROSTRUCTURE

ZENER-PINNING

Grain growth

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

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IIT Madras

Materials Science and Engineering A

Magnesium matrix in-situ composites with hybrid TiC+TiB2 reinforcement are potential materials for automobile and aerospace applications. It is essential to establish the processing map of such composites as these materials may undergo several thermo-mechanical processes while manufacturing engineering components. In the current work, AZ91 Magnesium matrix TiC+TiB2 reinforces hybrid in-situ along with its base counterpart were developed and subjected to solutionization followed by peak aging treatment. The safe processing zone for both base and in-situ composite at all heat treatment conditions have been established through processing map based on dynamic materials model (DMM) by conducting hot compression tests at various temperatures (250 °C – 450 °C) and strain rates (0.001 s−1 – 10 s−1). Two regions: stable and instable were identified from processing map for all the material conditions. Dynamic recrystallization is the main dominant mechanism in stable region, whereas instable region is characterized by twin and intergranular cracks. To understand the mechanism of hot deformation behavior, activation energy is calculated based on constitutive model for all material conditions. The developed in-situ composite in peak aged condition is found to possess higher activation energy (∼230 kJ/mol) as compared to base alloy (∼126 kJ/mol). A correlation between constitutive model and processing map have been established with an emphasis on their microstructures. © 2018 Elsevier B.V.

Journal of Alloys and Compounds

Deformation behavior and processing map development of AZ91 Mg alloy with and without addition of hybrid in-situ TiC+TiB2 reinforcement

The bulk ultrafine grained (UFG) AA6063/4 wt.%SiC composite sheets with varying reinforcement size of SiC (12 μm (coarse), 1 μm (fine) and 45 nm (nano)) have been developed with a novel hybrid process of stir casting and cryo-severe plastic deformation process (cryorolling). The influence of SiC particle size during development of UFG composites and its effect on microstructural modification, strengthening mechanism and failure mechanism was studied in detail. The resultant effect of cryorolling and size of SiC particles has resulted in significant microstructural refinement (less than 350 nm in all UFG composites) and accumulation of high dislocation density; which resulted in improvement in tensile strength as 88%, 108% and 141% in UFG composites reinforced with coarse, fine and nano particles respectively as compared to the unreinforced base alloy. A good agreement in theoretical and experiment yield strength of UFG coarse and fine composites is observed with a variation in UFG nano composite. © 2018 Elsevier B.V.

Development and strengthening mechanisms of bulk ultrafine grained AA6063/SiC composite sheets with varying reinforcement size ranging from nano to micro domain

The application of AZ91 magnesium alloy is limited because of dendritic β-Mg17Al12 phase which degrades mechanical properties and causes high tension to compression yield asymmetry (R). To overcome this, a severe plastic deformation (SPD) based hybrid process has been implemented in this study, to develop in-situ AZ91 + TiC-TiB2 composite. This results in redistribution of β-Mg17Al12 phase on the grain boundaries along with notable grain refinement. The combined effect of in-situ reinforcement and grain refinement due to SPD process resulted in simultaneous enhancement of strength and ductility. Further, intense grain refinement and presence of TiC-TiB2 reinforcement in the grain boundary region is found to increase the stress concentration along the grain boundary which hinders twin nucleation and significantly reduces the R value from 1.42 (as-cast condition) to 1.04 (SPDed in-situ composite). The underlying mechanism of significant property enhancement in the developed material has been correlated with the tension and compression tests and microstructures. © 2018 Elsevier B.V.

Microstructural modification and its effect on strengthening mechanism and yield asymmetry of in-situ TiC-TiB2/ AZ91 magnesium matrix composite

In the present work, influence of in-situ TiC-TiB2 reinforcement on age hardening behavior and mechanical properties of TiC-TiB2/AZ91 composite was investigated. The base and in-situ composite materials were solution treated at 400 °C for 24 h and then aged at different temperatures up to 100 h. The kinetics of age hardening results revealed that the addition of in-situ TiC-TiB2 reinforcement increase the age hardening kinetics of the composite as compared to AZ91 magnesium alloy. The improvement in age hardening kinetics is attributed to the presence of high dislocation density resulted from the mismatch of coefficients of thermal expansion between in-situ TiC-TiB2 reinforcement and magnesium matrix. The precipitation behavior of Mg17Al12 phase in both AZ91 alloy and TiC-TiB2/AZ91 composites were established. After age hardening, base and in-situ composite materials show significant improvement in ultimate tensile strength by 47.72% and 66.49% respectively as compared with the base monolithic alloy. The enhancement of mechanical properties in the base and in-situ composites is explained in detail by analyzing their fractographs and microstructures. © 2018

Materials Characterization

Effect of in-situ (TiC-TiB2) reinforcement on aging and mechanical behavior of AZ91 magnesium matrix composite

Cast magnesium-metal matrix composites are widely used in automotive and aerospace industries due to high strength-to-weight ratio and good damping properties. In the present work, a novel hybrid method has been adopted to fabricate TiC-TiB2 reinforced magnesium matrix composites. The reinforcement is formed in-situ from elemental Ti and B4C powders and molten Mg-Al-Zn alloy without any addition of a third phase metal powder such as aluminum. Results show that the distribution of TiC and TiB2 reinforcing phases in the magnesium matrix is more uniform when the composite is fabricated at 900 °C for 2 h. The base and composite materials were subjected to homogenization treatment which resulted in dissolution of β-Mg17Al12 phase into α-Mg matrix and enhances the strength and ductility by 22% and 50% in base and 17% and 50% in composite respectively. The enhancement of mechanical properties in the homogenized in-situ composites is explained in detail by analyzing the fractographs and microstructures of the material. © 2016 Elsevier Ltd

Materials and Design

Synthesis, characterization and mechanical properties of in-situ (TiC-TiB2) reinforced magnesium matrix composite

The microstructure, mechanical properties and fracture behavior of an as-received QE22 alloy have been investigated under different thermal conditions, including solution treated (ST), under aged (UA), peak aged (PA) and over aged (OA) conditions. A significant increase in hardness of 27%, yield strength of 60% and ultimate tensile strength of 19% was observed in peak aged sample as compared to solution treated sample. The improvements of mechanical strength properties are mainly associated with the metastable λ and β' precipitates. Grain growth was not observed in the ST samples after subjecting to UA and PA treatments due to the presence of eutectic Mg12Nd particles along the grain boundaries. In over aged sample, significant grain growth occurred because of dissolution of eutectic phase particles. Different natures of crack initiation and propagation were observed under different thermal conditions during tensile testing at room temperature. The mode of failure of solution treated sample is transgranular, cleavage and twin boundary fractures. A mixed mode of transgranular, intergranular, cleavage and twin boundary failure is observed in both peak aged and over aged samples. © 2015.

Fulltext

Journal of Magnesium and Alloys

Age hardening, fracture behavior and mechanical properties of QE22 Mg alloy

Strain-induced abnormal grain growth was observed along the gage length during high-temperature uniaxial tensile testing of rolled Mg-Al-Zn (AZ31) sheet. Effective strain and strain rates in biaxial forming of AZ31 sheets also affected the nature of grain growth in the formed sheet. For the uniaxial testing done at 400 C and a strain rate of 10-1 s-1, abnormal grain growth was prevalent in the gage sections that experienced true strain values between 0.2 and 1.0. Biaxial forming of AZ31 at 5 × 10 -2 s-1 and 400 C also exhibited abnormal grain growth at the cross sections which experienced a true strain of 1.7. Uniaxially tested sample at 400 C and a strain rate of 10-3 s-1, however, showed no abnormal grain growth in the gage sections which experienced true local strain values ranging from 1.0 to 2.3. The normalized flow stress versus temperature and grain size compensated strain rate plot showed that the deformation kinetics of the current AZ31 alloy was similar to that reported in the literature for AZ31 alloys. Orientation image microscopy (OIM) was used to study the texture evolution, grain size, and grain boundary misorientation during uniaxial and biaxial forming. Influence of deformation parameters, namely strain rate, strain, and temperature on grain growth and refinement were discussed with the help of OIM results. © 2013 Springer Science+Business Media New York.

Journal of Materials Science

Influence of strain and strain rate on microstructural evolution during superplasticity of Mg-Al-Zn sheet

Metallurgical and Materials Transactions A

Grain Boundary Segregation of Rare-Earth Elements in Magnesium Alloys

Scripta Materialia

Role of RE in the deformation and recrystallization of Mg alloy and a new alloy design concept for Mg–RE alloys

Achieving excellent thermal stability and very high activation energy in an ultrafine-grained magnesium silver rare earth alloy prepared by friction stir processing

Journal	Data powered by TypesetMaterials Science and Engineering A
Publisher	Data powered by TypesetElsevier Ltd
ISSN	09215093
Open Access	No