Mageshwari Komarasamy

Effect of Microstructure on the Deformation Mechanism of Friction Stir-Processed Al0.1CoCrFeNi High Entropy Alloy

Grain refinement from several millimeters in as-received (AR) condition to the range of 0.35–15 μm was achieved by friction stir processing (FSP). Due to the sluggish nature of atomic diffusion in high entropy alloys (HEAs), the FSP region exhibited an immense variation in microstructure which was directly attributed to the accumulated plastic strain during FSP. In accordance with the Hall–Petch relationship, yield strength (YS) has increased by a factor of four after grain refinement while maintaining large uniform elongation (UE). The Kocks–Mecking plot indicated different deformation mechanisms operative in both FSP and AR conditions.

Lattice strain framework for plastic deformation in complex concentrated alloys including high entropy alloys

Abstract Alloys involving multiple solutes where the concentrations are such that it becomes difficult to identify a “solvent”, such as the so called “high entropy alloys”, have the potential for interesting combinations of properties. A key question relates to the fundamental mechanisms of plastic deformation in these alloys. A simple lattice strain framework is proposed for complex concentrated alloys to address the energetics and kinetics of dislocations and twins. It is argued that the lattice strain in highly concentrated alloys raises the base energy of the crystal and thereby reduces the additional energy required to nucleate dislocations and twins. However, the kinetics of dislocation motion are dampened by the lattice strain and local energy variations. This is reflected in lower values of activation volume. This framework can be used to design non-equiatomic high entropy alloy matrixes that enhance the properties achieved thus far.

Thermo-mechanical response of single-phase face-centered-cubic AlxCoCrFeNi high-entropy alloy microcrystals

ABSTRACT The response of [100]-oriented single-crystal face-centered-cubic Al0.1CoCrFeNi and Al0.3CoCrFeNi high-entropy alloy (HEA) microcrystals tested from 293 to 573 K by means of in situ micro-compression is reported. At all temperatures, plasticity is governed by dislocation slip with significant strain hardening and intermittent strain bursts observed. A model, which is in good agreement with experimental measurements, is also developed to predict the effects of Al concentration, temperature, and crystal size on the strength of HEAs. The model interestingly predicts a softening response with an increase in Al concentration when the crystal size is ≤0.1 µm. Finally, this model can guide the development of advanced HEAs for small-scale applications. GRAPHICAL ABSTRACT IMPACT STATEMENT In situ scanning electron microscopy (SEM) experiments are performed to quantify the thermo-mechanical response of AlxCoCrFeNi microcrystals. A physics-based model is also proposed to predict the strength of high-entropy alloys as a function of crystal size, temperature, and Al concentration.

Chapter 8 – Physical Metallurgy-Based Guidelines for Obtaining High Joint Efficiency

This chapter provides guidelines to obtain high joint efficiency friction stir welds in 7XXX Al alloys. The guidelines are based on physical metallurgy as well as FSW process conditions.

Chapter 4 – Temperature Distribution

This chapter provides an analysis of the temperature distribution in various weld zones. Results from both the experimental observations and numerical simulations are presented. The peak temperature at nugget and HAZ is tabulated at the end of the chapter. Location-specific peak temperature is critical for postweld heat treatment response and overall mechanical performance of the weld.

Chapter 6 – Mechanical Properties

The thermal cycle during the welding determines the microstructural evolution which in turn controls the mechanical property variation across the weld zones. This chapter provides a summary of the mechanical properties of the friction stir welds, such as hardness evolution, tensile and fatigue properties. Effects of FSW process parameters, thermal boundary conditions during FSW, and post-weld heat treatment on the mechanical properties are discussed in detail. A comprehensive list of joint efficiency values is provided at the end of the chapter.

Chapter 5 – Microstructural Evolution

Microstructural evolution links to the final properties because of the thermal cycle. In this chapter, both the grain size and precipitate evolution in various weld zones are presented. A summary of these microstructural changes are tabulated at the end of the chapter to capture the key features. Precipitate strengthening dominates over grain boundary strengthening in 7XXX Al alloys. Therefore, capturing the details of precipitate evolution is critical for connecting microstructure to mechanical properties.

Chapter 9 – Summary and Future Outlook

Joining of the high-strength aluminum alloys via solid-state friction stir welding (FSW) has been exceedingly successful. An overall summary of the book and probable future directions are discussed briefly in this chapter.

Chapter 7 – Corrosion

Apart from the mechanical properties of the friction stir welding (FSW) joints, corrosion is a critical property as the structural integrity of the components deteriorates with time as the environmental damage initiates and propagates. A brief summary of the corrosion behavior of the various weld zones is presented in this chapter.

Friction Stir Welding—Overview

An overview of the friction stir welding process is presented in this chapter. This chapter introduces various zones in the FSW weld, common terminologies, along with a brief discussion of the material flow during FSW and possible defect formation in these welds.