B.S. Murty

High-Entropy Alloys: Basic Concepts

High-entropy alloys (HEAs) have emerged as an important class of materials in the last one decade with immense potential both at the level of fundamental understanding and also in terms of applications. This chapter deals with the basic concepts that underpin this new class of alloys. It starts with the definition of HEAs and how they differ from other multicomponent alloys such as Ni-base superalloys. Then it gives a classification of alloys in terms of the number of components. A notation that is commonly used to represent composition in these alloys, particularly the nonequiatomic alloys with alloying additions, is brought out. Finally, four core effects of HEAs are elucidated, namely, high-entropy, severe lattice distortion, sluggish diffusion, and cocktail effects.

Aluminum-Based Cast In Situ Composites: A Review

In situ composites are a class of composite materials in which the reinforcement is formed within the matrix by reaction during the processing. In situ method of composite synthesis has been widely followed by researchers because of several advantages over conventional stir casting such as fine particle size, clean interface, and good wettability of the reinforcement with the matrix and homogeneous distribution of the reinforcement compared to other processes. Besides this, in situ processing of composites by casting route is also economical and amenable for large scale production as compared to other methods such as powder metallurgy and spray forming. Commonly used reinforcements for Al and its alloys which can be produced in situ are Al2O3, AlN, TiB2, TiC, ZrB2, and Mg2Si. The aim of this paper is to review the current research and development in aluminum-based in situ composites by casting route.

Effect of grain size on dielectric and ferroelectric properties of nanostructured Ba 0.8 Sr 0.2 TiO 3 ceramics

Barium strontium titanate (Ba0.8Sr0.2TiO3, BST) nanocrystalline ceramics have been synthesized by high energy ball milling. As the sintering temperature increases from 1200 °C to 1350 °C, the average grain size of BST ceramics increases from 86 nm to 123 nm. The X-ray diffraction (XRD) studies show that these ceramics are tetragonal. The phase and grain size of the sintered pellets have been estimated from the XRD patterns, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images. The effect of grain size on dielectric and ferroelectric properties is studied. The dielectric and piezoelectric parameters are greatly improved at room temperature with increase in grain size. The Curie transition temperature is found to shift slightly towards higher temperatures as the grain increases from 86 nm to 123 nm. The coercive field decreases and the remnant polarization and spontaneous polarization increase as the grain size of BST nano ceramics increases. These ceramics are promising materials for tunable capacitor device applications.

SYNTHESIS OF LEAD FREE SODIUM BISMUTH TITANATE (NBT) CERAMIC BY CONVENTIONAL AND MICROWAVE SINTERING METHODS

Dielectric studies were carried out on a lead free Sodium Bismuth Titanate, NBT, (Na0.5Bi0.5TiO3) composition. The material was synthesized by conventional ceramic method (CS) and microwave sintering (MS). The presence of single phase has been confirmed by X-ray diffraction and scanning electron microscopy of NBT ceramic. The later technique (MS) resulted in material with high density, dielectric properties and improved microstructure. The transition temperature was observed slightly higher for microwave sintered (MS) material. Longitudinal modulus measurements are very sensitive property to identify the phase transitions in ceramics. Longitudinal modulus (L) measurements were also employed on these samples in the frequency 136 kHz and can be studied in the wide temperature range 30°C to 400°C. The elastic behavior (L) showed a break at two temperatures (~200°C and 350°C) in both the conventional and microwave sintered ceramics. In NBT ceramics, permittivity anomalies are connected to modulus anomalies. The results are correlated with the dielectric measurements. This behavior explained in the light of structural phase transitions in the ferroelectric ceramics.

Phase Formation in Equiatomic High Entropy Alloys: CALPHAD Approach and Experimental Studies

CALPHAD approach has been used to predict the stable phases, their relative amounts and compositions in multicomponent equiatomic high entropy alloys. The results show a good match between the predictions and experimental results on the phase formation for two equiatomic high entropy alloys (CrCoCuNi and CrCuMnNi alloys) prepared by mechanical alloying, considering the kinetic constraints of the non-equilibrium processing route.

Fabrication and Response of Al70Y16Ni10Co4 Glass Reinforced Metal Matrix Composites

Al70Y16Ni10Co4 glass was developed through powder metallurgy route. Al-based glass-reinforced metal matrix composites were produced by hot pressing and simultaneous extrusion of these powders.The consolidation parameters were selected from systematic calorimetric studies. The response of these composites is superior mechanical properties, and these properties increases with glassy reinforcement. Remarkable improvement in the wear properties were observed when subjected to abrasion test. The abrasive wear rate has been decreased from 2.46 × 10−4 m3/m for commercial Al-Mg-Si (6061) extruded alloy to 0.373 × 10−4 m3/m for 50 vol% glass reinforced 6061 composite. Similar improvements were observed with glass-reinforced aluminum matrix composites. Plowing as well as pullout of particles during abrasion tests corresponds to the wear of these composites.

Applications of Nanomaterials

Historically, there are several recorded instances of technologies that have revolutionized human civilization. From the invention of automobile wheels to the printing press, technological revolutions have resulted in remarkable improvement in the quality of life and have eventually led to societal transformations. With nanotechnology promising to impact almost every sector (Fig. 4.1), it is popularly believed that this could be the next revolution.

Interplay Between Residual Stresses, Microstructure, Process Variables and Engine Block Casting Integrity

The replacement of nodular cast iron with 319 type aluminum (Al) alloys in gasoline engine blocks is an example of the shift towards the use of lighter alloys in the automotive industry. However, excessive residual stress along the cylinder bore may lead to bore distortion, significantly reducing engine operating efficiency. In the current study, microstructure, mechanical properties and residual stress were characterized along the cylinder bridge of engine blocks following thermal sand reclamation (TSR), T7 heat treatment, and service testing of the casting. Neutron diffraction was effectively used to quantify the residual stress along both the Al cylinder bridge and the adjacent gray cast iron cylinder liners in the hoop, radial, and axial orientations with respect to the cylinder axis. The results suggest that an increase in cooling rate along the cylinder caused a significant refinement in microstructure at the bottom of the cylinder. In turn, this suggested an increase in alloy strength at the bottom of the cylinder relative to the top. This increased strength at the bottom of the cylinder likely reduced the susceptibility of the cylinder to rapid relief of residual stress at elevated temperature. In contrast, the coarse microstructure at the top of the cylinder likely triggered stress relief at an elevated temperature.

Production, Kinetic Study and Properties of Fe-Based Glass and Its Composites

Fe-based glassy powders with four different compositions of varying copper content were produced by mechanical alloying of elemental powder mixtures. The thermal stability as well as the crystallization kinetics was investigated using differential scanning calorimetry in both isochronal and isothermal modes. The isochronal and isothermal activation energies have a similar value (∼560–570 kJ/mol). In addition, the Johnson–Mehl–Avrami (JMA) analysis shows that the transformation is diffusion controlled three-dimensional process, and the crystallization proceeds with increasing nucleation rate. Metal matrix composites were synthesized through powder metallurgy methods by uniaxial hot pressing and subsequent extrusion of commercially pure Fe powders, blended with Fe-glass reinforcement. The resultant properties of the composites strongly depend on the composition of the glassy reinforcement.

Ferromagnetic-Dielectric Ni 0.5 Zn 0.5 Fe 1.9 O 4−δ /PbZr 0.52 Ti 0.48 O 3 Particulate Composites: Electric, Magnetic, Mechanical, and Electromagnetic Properties

Novel ferromagnetic-dielectric particulate composites of Ni0.5Zn0.5Fe1.95O4−δ (NZF) and PbZr0.52Ti0.48O3 (PZT) were prepared by conventional ceramic method. The presence of two phases in composites was confirmed by XRD technique. The variations of dielectric constant (𝜀) with frequency in the range of 100 kHz–1 MHz at room temperature and also with temperature at three different frequencies (50 kHz, 100 kHz, and 500 kHz) were studied. Detailed studies on the dielectric properties were done confirming that the magnetoelectric interaction between the constituent phases may result in various anomalies in the dielectric behaviour of the composites. It is proposed that interfaces play an important role in the dielectric properties, causing space charge effects and Maxwell-Wagner relaxation, particularly at low frequencies and high temperatures. The piezoelectric d33 constant was studied at room temperature, and the d33 constant value decreased with ferrite content. Magnetic properties like B-H loops traces were studied to understand the saturation magnetic (Ms) and magnetic moment (𝜇𝐵) of the present particulate composites. The magnetoelectric (ME) output was measured by varying dc bias magnetic field. A large ME output signal of 2780 mV/cm Oe was observed in the composite having 50% ferrite. The temperature variation of longitudinal modulus (L) and internal friction (Q−1) of these particulate composites at 104 kHz was studied in the temperature range 30°C–420°C by the composite oscillator technique. Longitudinal modulus showed a sharp minimum, and internal friction exhibits a sharp peak at ferroelectric-paraelectric phase transition. These ferroelectric-dielectric particulate composites were prepared with a view to using them as ME sensors and transducers.