Medium Entropy Alloys and Methods of Preparing the Same

This application claims priority to and the benefit of Korean Patent Application No. 10-2024-0046146 filed with the Korean Intellectual Property Office on Apr. 4, 2024, the entire contents of which are incorporated herein by reference.

The present invention relates to a medium entropy alloy and a manufacturing method thereof, and more specifically, to a medium entropy alloy and a manufacturing method thereof capable of improving the tensile characteristics of an alloy by refining the multiphase of the medium entropy alloy.

A typical metal alloy is composed of a major element and a small amount of alloying elements. As the alloying elements are added, the possibility of forming an intermetallic compound increase. The intermetallic compound can weaken the mechanical properties, such as causing brittleness of the material.

High-entropy alloys (HEAs) and medium-entropy alloys (MEAs) are multi-element alloys that have a plurality of elements as their main elements, unlike conventional alloys that are composed of a main element and other elements. Since high-entropy alloys and medium-entropy alloys have a plurality of elements as their main elements, they can dramatically increase the number of alloys that can be designed compared to existing alloys, and since they contain equivalent plurality of alloy elements, they have high solute efficiency. In addition, the high-entropy alloys and medium-entropy alloys also exhibit excellent mechanical properties utilizing multiple and synergistic strengthening mechanisms including deformation twinning-induced plasticity (TWIP), transformation-induced plasticity (TRIP), grain boundary strengthening, dislocation density strengthening, and precipitation hardening.

Conventional alloys are divided into high-entropy alloys, medium-entropy alloys (MEAs), and low-entropy alloys (LEAs) according to the compositional entropy (ΔSconf) of the alloy system obtained by the following relationship equation 1. If [Configurational Entropy≥1.5 R], it is classified as a high-entropy alloy, if [1.5 >Configurational Entropy≥1.0 R], it is classified as a medium-entropy alloy, and if [1.0 R>Configurational Entropy], it is classified as a low-entropy alloy.

(R: gas constant, Xi: mole fraction of i element, n: number of constituent elements)

In the case of conventional high-entropy alloys and medium-entropy alloys, expensive constituent elements such as Co, Cr, Fe, Mn, and Ni-based FCC high-entropy alloys and W, Nb, Mo, Ta, and V-based BCC high-entropy alloys are widely used, so their price competitiveness is low, and it has been difficult to commercially use them because heavy elements are mainly used.

However, recently, interest in lightweight structural materials has increased in various fields such as the aerospace industry, mobility industry, and wearable device industry, and research on medium entropy alloys that add lightweight alloy elements or serve as the basis for major elements is attracting attention. Al and Ti are widely used as lightweight structural materials for medium entropy alloys. In the case of Ti, it has excellent strength-to-weight ratio and is widely used in aerospace and other demanding industries, but it has a problem in that its thermal conductivity is lower than that of metals such as aluminum because its thermal conductivity is relatively low at 21.9 W/(m·K).

In contrast, Al has an excellent strength-to-weight ratio, is highly price competitive, and has a significantly lower density than Ti, so it is attracting attention in industries such as aerospace and vehicle design where a 1 g weight reduction can result in significant energy savings.

However, in the case of Al, when other alloy elements are added, it easily forms an intermetallic compound, and the intermetallic compound is brittle and can cause sudden fracture during a tensile test. Therefore, research and development on an Al-based medium entropy alloy with sufficient tensile characteristics is necessary.

The technical object that the present invention seeks to solve is to provide a medium entropy alloy of lightweight structural material capable of improving tensile characteristics.

Another technical object that the present invention seeks to solve is to provide a medium entropy alloy manufacturing method having the aforementioned advantages.

An embodiment is a medium entropy alloy including an Al-rich FCC phase, a Zn-rich HCP phase, and an intermetallic compound, wherein the Al-rich FCC phase, the Zn-rich HCP phase, and the intermetallic compound include Al, Zn, and Cu, and the intermetallic compound satisfies the following Equation 1.

(In Equation 1, A represents the atom % of Al in the intermetallic compound, B represents the atom % of Cu in the intermetallic compound, and C represents the atom % of Zn in the intermetallic compound.)

The medium entropy alloy can satisfy the following Equation 2.

(In Equation 2, the first peak is the peak intensity when 2θ is 36±5°, the fourth peak is the peak intensity when 2θ is 65±5°, and the fifth peak is the peak intensity when 2θ is 78+5°, in the X-ray diffraction analysis (XRD) spectrum.)

With atom % as reference, the ratio of Zn to Al can be 0.8 to 1.2.

With atom % as reference, the ratio of Cu to Al can be 0.2 to 1.0.

The medium entropy alloy may contain Al: 33.3 to 45.0%, Zn: 33.3 to 45.0%, Cu: 10 to 33.3% and other impurities in atom %.

The medium entropy alloy may contain a triple phase, and the triple phase may include a first phase having Al as a primary component; a second phase having Zn as a primary component; and a third phase with an Al-based intermetallic compound and a Cu-rich.

The first phase may include an Al-rich FCC phase.

The second phase may contain a Zn-rich HCP phase.

The composition of the first phase can include Al: 45 to 60%, Zn: 35 to 50%, Cu: 5 to 10% and impurity of the balance in atom %, with the entire mole number of the first phase as a reference.

The composition of the second phase can include Al: 5 to 20%, Zn: 65 to 80%, Cu: 10 to 25% and impurity of the balance in atom %, with the entire mole number of the second phase as a reference.

The composition of the third phase can include Al: 40 to 55%, Zn: 5 to 20%, Cu: 35 to 50%, and inevitable impurities, with the entire mole number of the third phase as a reference.

The Al based intermetallic compound may include AlCu and Al2Cu.

The yield strength of the medium entropy alloy at room temperature 298K can be 350 to 380 MPa.

The tensile strength of the medium entropy alloy at room temperature 298K can be 405 to 470 MPa.

The elongation of the medium entropy alloy at room temperature 298K can be 1.2 to 5.0%.

The Al and Zn can satisfy the following Equation 3.

(Here, [X] means atom % of X)

Another embodiment of the present invention may include the steps of: manufacturing an ingot by casting a mixed powder including Al, Zn, and Cu; quenching the ingot; subjecting the quenched ingot to severe plastic deformation; and performing a post heat treatment for 2 to 50 minutes.

The severe plastic deformation process may include a High-Pressure Torsion (HPT) process or an Equal Channel Angular Pressing (ECAP) process.

The load applied during the High-Pressure Torsion (HPT) process can be 38 to 50 tons; and the overall rotation speed can be 5 to 40 times.

The heat treatment temperature of the post heat treatment step can be 250 to 350° C.

A medium entropy alloy according to an embodiment can form a triple phase and have a solid solution effect. In addition, the room temperature mechanical characteristics can be improved due to the phase interface strengthening effect.

A medium entropy alloy according to another embodiment of the present invention has finer and more evenly distributed intermetallic compounds than the ingot through severe plastic deformation and a post heat treatment process. Additionally, mechanical characteristics can be improved by bypassing crack propagation and obtaining excellent ductility.

The terms such as first, second, and third are used to describe various portions, components, regions, layers, and/or sections, but various parts, components, regions, layers, and/or sections are not limited to these terms. These terms are only used to distinguish one part, component, region, layer, or section from another part, component, region, layer, or section. Accordingly, a first part, component, region, layer, or section described below may be referred to as a second part, component, region, layer, or section without departing from the scope of the present invention.

Terminologies as used herein are to mention only a specific exemplary embodiment, and are not to limit the present invention. Singular forms used herein include plural forms as long as phrases do not clearly indicate an opposite meaning. The term “including/comprising/containing” as used herein concretely indicates specific characteristics, regions, integer numbers, steps, operations, elements, and/or components, and is not to exclude presence or addition of other specific characteristics, regions, integer numbers, steps, operations, elements, and/or components.

When any portion is referred to as being “above” or “on” another portion, any portion may be directly above or on another portion or be above or on another portion with the other portion interposed therebetween. In contrast, when any portion is referred to as being “directly on” another portion, the other portion is not interposed between any portion and another portion.

Unless defined otherwise, all terms including technical terms and scientific terms as used herein have the same meaning as the meaning generally understood by a person of an ordinary skill in the art to which the present invention pertains. Terms defined in a generally used dictionary are additionally interpreted as having the meaning matched to the related art document and the currently disclosed contents and are not interpreted as ideal or formal meaning unless defined.

Also, unless otherwise stated, % means wt %, and 1 ppm is 0.0001 wt %.

In this specification, the term “combination thereof(s)” described in a Markush format expression means one or more mixtures or combinations selected from the group consisting of components described in the Markush format expression, and means including one or more selected from the group consisting of the components.

Below, an embodiment is described in detail so that a person of ordinary skill in the technical field to which the present invention belongs can easily carry out the present invention. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention.

A medium entropy alloy according to an embodiment is a medium entropy alloy including an Al-rich FCC phase, a Zn-rich HCP phase, and an intermetallic compound, wherein the Al-rich FCC phase, the Zn-rich HCP phase, and the intermetallic compound include Al, Zn, and Cu, and the intermetallic compound satisfies the following Equation 1.