Design of ternary high-entropy aluminum alloys (HEAls)

https://doi.org/10.1016/j.jallcom.2021.161836Get rights and content

Highlights

  • A novel category of Al alloys coined High-Entropy Aluminum Alloys (HEAls) was introduced.

  • The idea of introducing high entropy concept in conventional Al alloys was demonstrated.

  • Al–4Zn–4Mg and Al–4.5Zn–4.5Mg (mol.%) alloys as HEAls were designed and studied.

  • Both alloys show high stiffness and high strength after solution treatment and have excellent impact-resistance after artificial aging.

Abstract

Two novel aluminum alloys were designed and studied by incorporating concepts of high entropy and conventional aluminum alloys. The alloys: Al–4Zn–4Mg and Al–4.5Zn–4.5Mg (mol.%) have a single-phase FCC matrix with a maximized concentration of the alloying elements. CALculation of PHAse Diagrams (CALPHAD) approach was applied to determine the compositions and the design of heat treatment schedules. Both alloys show high stiffness and high strength after solution treatment and have excellent impact-resistance after artificial aging. The phase stabilities and strengthening mechanisms were investigated using Optical Microscopy (OM), Scanning Transmission Electron Microscopy (STEM), and X-Ray Diffraction (XRD). The results show that the GP zones and T-phase nanoparticles are mainly responsible for the high stiffness-high strength and impact resistance properties.

Introduction

Aluminum alloys are widely used in aerospace, automotive, and manufacturing industries. Aluminum alloys are characterized by low density, high strength, good electrical and thermal conductivity, and good mechanical properties that can be obtained through various heat treatment schedules [1], [2], [3]. The 2XXX series of aluminum alloys, which mainly contain copper as the principal alloying element, possess a good combination of high strength and toughness. They are utilized in aircraft manufacturing by cladding with the 6XXX series (mainly alloyed with magnesium and silicon) to obtain good corrosion resistance. The 5XXX series aluminum alloys, in which magnesium is the primary alloying agent, show superb corrosion resistance properties that have made them a great choice for construction and marine applications [1]. By the addition of copper, chromium, manganese, and other elements into the Al–Zn–Mg system, known as the 7XXX series, the strength can significantly be increased [4], [5]. Though much work and improvements have been made over the years, we find that with light-weighting of our infrastructure (in order to reduce our carbon footprint), the need for property improvements is pushing the boundaries of existing alloys; thus, new pathways are needed.

High-Entropy Alloys (HEAs) were developed fifteen years ago, which is composed of different elements with various ratios [6], [7]. The configurational entropy of mixing in HEAs is high so that a single-phase solid solution can be stabilized. Moreover, HEAs have shown mechanical properties that exceed those in conventional alloys [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. In this work, we have applied the concept of high entropy for strengthening Al alloys. Therefore, a new category of aluminum alloys was designed that we termed High-Entropy Aluminum Alloys (HEAls). Several alloys were designed based on this concept using Integrated Computational Materials Engineering (ICME); the Al–Zn–Mg system showing much promise. The CALculation of PHAse Diagrams (CALPHAD) approach was applied to design the alloys as well as their respective heat treatment schedules. It is worth mentioning that the CALPHAD approach relies on the thermodynamic databases to calculate phase stabilities by modeling the Gibbs free energy of each individual phase in a system. Therefore, this approach can be applied as a powerful tool to predict the properties of a material at a determined condition [21], [22], [23], [24].

Section snippets

High-entropy aluminum alloys (HEAls)

Yeh et al. [25] first proposed a strict definition for HEAs as solid solutions containing at least five equiatomic principal elements. Later, this definition was broadened by having a concentration of each element between 5 and 35 at% [25]. Different studies showed that a random solid solution could be achieved even if the alloying elements are not in an equiatomic ratio [26], [27], [28]. Non-equiatomic HEAs have been validated by He et al. [29] in the Co–Cr–Fe–Ni system. Subsequently, HEAs

ICME approach to design HEAls with FCC matrix

The ICME approach is a powerful method to accelerate materials design, in which the computational modeling and predictions can guide experiments to reduce trial and error attempts [38], [39], [40]. In this work, the CALPHAD approach was applied to find the proper elements, the concentration of each, phase stabilities, and heat treatment schedules based on the “HEAls with FCC matrix” design strategy. Thermo-Calc® [41], [42], along with the TCAL6 database, were utilized as a CALPHAD tool. The

Experimental procedures

The charge was prepared utilizing 99.9% purity aluminum, Al–50Mg master alloy, and 99.99% purity zinc and melted in an induction furnace. The melt was degassed with nitrogen for 20 min and then cast at 760 °C into a 400 °C-preheated permanent steel mold to manufacture standard 2-inch gauge length tensile bars. An electric furnace was utilized for solution treatment and aging of the samples, which were quenched in cold water. Tensile tests were conducted using an Instron 5500 with a strain rate

Phase stability simulations

Phase diagrams of Al–Mg and Al–Zn systems, calculated and plotted using Thermo-Calc®, are shown in Fig. 1. As can be seen, Zn and Mg show a high solubility level in Al, with Zn having a large FCC solid solution range. Fig. 2 shows the isothermal phase diagrams of Al–Zn–Mg ternary system at four different temperatures: 250, 350, 450, and 550 °C. At the Al-rich corner, the FCC solid solution region expands with increasing temperature until the liquid phase starts to interfere. The Al–Zn–Mg system

Conclusions

The concept of high entropy was successfully combined with the conventional aluminum alloys to demonstrate designing of a novel category of aluminum alloys named HEAls. Applying the CALPHAD approach, the composition of the candidate HEAls, along with proper heat treatment schedules, were determined. Two HEAls: Al–4Zn–4Mg and Al–4.5Zn–4.5Mg (mol.%) exhibit high stiffness-high strength properties subsequent to solution treatment. The two alloys will attain impact-resistance as a result of

CRediT authorship contribution statement

Mohammad Asadikiya: Writing – original draft, preparation, Writing – review & editing, Project Administration, Experiments, Investigation, Visualization, Formal analysis, Validation. Yifan Zhang: Visualization, Investigation. Libo Wang: Experiments. Diran Apelian: Writing – review & editing. Yu Zhong: ws.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

The authors thank the consortium members of the Advanced Casting Research Center (ACRC) for their financial support, encouragement, and guidance, and specifically, the chair of the focus group of this project, Dr. Lin Zhang of FCA. We also thank the support we received from IMRI at UCI for the use of STEM facilities, specifically, Prof. Xiaoqing Pan.

Data Availability Statement

All the data required to reproduce this work is provided in the manuscript.

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