A Brief History of Alloys and the Birth of High-Entropy Alloys
Alloys have helped and defined the march of civilization for over five millennia. The progress from native metals and native alloys to the accidental discovery of arsenical bronzes is a remarkable story. Numerous combinations of alloying elements were tried usually based on one principal element. Higher amounts of alloying were also used in alloys such as high-tin bronzes and ultrahigh carbon steels. From such concentrated binary alloys to multicomponent alloys marked a major advance resulting in alloy steels and superalloys in the last century. Nevertheless there was one element in major proportion in all these alloys. A revolutionary step in alloying occurred just before the beginning of the third millennium CE when Jien-Wei Yeh and Brian Cantor independently thought of multicomponent equiatomic or near-equiatomic alloys. Surprisingly, many of these alloys turned out to be solid solutions similar to the bronzes of the third millennium BCE. They have breathed new life into materials world promising an extraordinarily rich family of alloys.
Keywords
Native alloys; binary alloys; multicomponent alloys; high-entropy alloys
1.1 Introduction
Alloying is the greatest gift of metallurgy to humankind. The English language insists on unalloyed pleasures, thereby implying that the sensation of pleasure must be pure and not admixed with other emotions. Exactly the opposite rules in metallurgy, where pure metals have few uses but lot more upon alloying. The power of this idea of alloying is not confined to metals. The same principle of alloying applies in polymers and ceramics. It can be carried further by mixing two classes of materials to create a variety of composites.
The civilizational journey of humankind began with the discovery of native metals such as gold and copper as pure metals. Nowadays we have access to an incredible number and variety of materials. Ashby map (Ashby, 2011) shown in Figure 1.1 gives a panoramic view of the development in the use of materials over 10 millennia. A graphic depiction of the different classes of materials from ceramics to metals, polymers, and more recently to composites is vividly displayed. The passage from discovery through development to design of materials can be noted. Ashby’s (2011) map in term of strength versus density shown in Figure 1.2 demonstrates the filling of material–property space in a vivid fashion from 50,000 BCE up to the present scenario. In time scale, the largest filling has occurred in the past 50 years during which envelopes of metals, ceramics, and composites had a large expansion, and new envelopes of synthetic polymers and foam materials take a significant space. But, the filled area also seems to approach some fundamental limits beyond which it is difficult to go further (Ashby, 2011).
In many ways, the history of alloying is the history of metallurgy and materials science. Books and treatises have been written. An elegant and brief history is by Ashby (2008). Cahn (2001) has offered a magisterial survey of “The coming of materials science”. Ranganathan (2003) wrote on alloyed pleasures—an ode to alloying. In the following sections, a few episodes in this epic journey are covered.
1.2 The Coming of Alloys
Native alloys such as tumbaga and electrum are alloys of gold–copper and gold–silver, respectively. When platinum was discovered in 1735, it was compared with silver. Also, mixtures of platinum metals are found to occur in nature. It is an early example of multicomponent high-entropy alloys (HEAs), since platinum is often found as alloys with the other platinum group metals and iron mostly.
Alloying was an accidental discovery. In the primitive fires in the caves, ores of copper got mixed with ores of arsenic, zinc, and tin. The first alloy of copper and arsenic (Arsenical bronze, 3000 BCE) was entirely accidental. A more intentional alloying of tin with copper (tin bronzes in 2500 BCE) gave birth to the Bronze Age, as bronze was superior in its mechanical properties.
The seven metals found in antiquity were gold, copper, silver, iron, lead, tin, and mercury. The eighth metal, zinc, is added because of the unique Indian context but also because the discovery of other metals had to await the advent of the scientific revolution for a few centuries.
It is interesting to mention that intermetallics of copper–tin alloys had been used in ancient time. Mirrors were made of bronzes in different parts of the Old World including India and China, due to their higher hardness which makes it easy in getting mirror finish to reflect like silver. Archaeo-metallurgical investigations by Sharada Srinivasan on vessels from South Indian megaliths of the Nilgiris and Adichanallur (1000–500 BCE) showed that they were of wrought and quenched high-tin beta bronze, ranking among the earliest known artifacts. This is an early application of an intermetallic. When the sulfide ores of copper and nickel were smelted together, it led to copper–nickel alloy in the fourth century in China. Zinc was added in the twelfth century to form silvery and rust-resisting alloy known as paktong (white copper), which was widely used in Europe before stainless steel was invented.
Wrought iron was produced as early as in 1000 BCE and cast iron and cast steel were produced one millennium later. Steel was an accidental alloy of iron with carbon. This is all the more astonishing as carbon was not recognized as an element until recently. This accidental discovery also led to the production of wootz steel in India around 300 BCE. This has been rightly celebrated as the most advanced material of the ancient world, as this steel was used to fashion the Damascus swords. The deciphering of wootz steel by European scientists led to the correlation between structure and properties at first and subsequently between composition, processing, structure, and properties. Figure 1.3 shows that materials hypertetrahedron that links the above four with modeling is necessary to understand the performance of wootz steel. (Ranganathan and Srinivasan, 2006; Srinivasan and Ranganathan, 2014). The facets of the ultrahigh carbon steels, Buchanan furnace, the Fe
C phase diagram, the microstructure of dendrites in the as-cast state and spheroidized cementite in the forged state, the superplastic elongation, and the Damascene marks are emphasized for the strong interconnections among them. It can be regarded as a classic example of the materials tetrahedron but including a fifth vertex of modeling (e.g., CALPHAD method to calculate phase diagram) to make it a hypertetrahedron.
When the first industrial revolution began in later half of 18th century in England, more and more elements were found and produced by humankind. From these “new” elements, numerous metallic materials including engineering and advanced alloys have been developed. They were synthesized with different compositions and produced by various processing routes. Up to now, about 30 alloy systems, each system based on one principal metallic element, have been developed and used for a variety of products (Handbook Committee, 1990).
Several alloys of engineering importance have been developed. Michael Faraday created some of the first alloy steels in his efforts to reproduce the wootz steel from India and is hailed as the father of alloy steel. Aluminum alloys were produced after the commercialization of Hall-Héroult process in the mid 1880s and underwent large progress in precipitation hardenable alloys such as AlCu(Mg) and AlZnMg(Cu) for light and strong airframes in the explosive expansion of the airplane industry during and after World War I (1914–1918). High-speed steel for cutting tools was first produced in the early 1900s. To meet the challenge for even higher cutting speed, cemented or sintered carbides of WC/Co composites were introduced in the 1930s. At the same time, superalloy development began in the United States in the 1930s and was accelerated by the demands of gas turbine technology. Ferritic, austenitic, and martensitic stainless steels were almost simultaneously developed around 1910.
1.3 Special Alloys
Besides the above modern engineering alloys, several special alloy systems with specific compositions, structures and properties have been developed with intensive research in last 50 years. They are intermetallics, quasicrystals, and metallic glasses as introduced in the following sections.
1.3.1 Intermetallics and Quasicrystals
Intermetallic compounds are basically compounds of two or more metallic elements. They are brittle by nature. Besides the ancient intermetallic mirrors made of high-tin bronze mentioned in last section, they have given rise to various novel materials developments in modern time including magnetic AlNiCo alloys and the LaNi5 for nickel metal hydride batteries, and various aluminides, NiAl-, TiAl-, FeAl-based compounds for elevated-temperature light-weight applications in turbine engines.
Another class of intermetallics includes those which demonstrate so called forbidden rotational symmetries (such as 5- and 10-fold rotational symmetries) and quasiperiodic translational symmetry. These were discovered by Shechtman (1984) in 1982 as described in his notebook, when he first observed a 10-fold electron diffraction pattern from a rapidly solidified AlMn alloy. These were christened as quasicrystals. This observation has shaken the beliefs of crystallographers to the extent that the definition of a crystal has been modified in 1990, based on the discovery of this new class of materials. There has been intensive research on these exciting materials in the past three decades, both towards the understanding of the structure and properties of these materials.
1.3.2 Metallic Glasses
The first reported metallic glass was an alloy (Au75Si25) produced at California Institute of Technology by the research group of Pol Duwez (Klement et al., 1960), which was cooled rapidly from the liquid state at a rate of around 106 K/s to avoid crystallization and possess noncrystalline or glass-like structure. Among the different metallic glasses developed later, the soft magnetic metallic glass of iron, nickel, phosphorus, and boron, known as Metglas, was commercialized in early 1980s and is used for low-loss power distribution transformers. In 1974, H.S. Chen first reported that bulk metallic glass rods in diameters ranging from 1–3 mm can be produced in various glassy ternary alloys including PdAuSi, PdAgSi, and PtNiP systems (Chen, 1974). After this discovery, multicomponent glassy alloys based on lanthanum, magnesium, zirconium, palladium, iron, copper, and titanium, etc. with critical cooling rate in the range of 1–100 K/s, comparable to oxide glasses have been developed and researched.
1.4 The Coming of Multicomponent Heas
From the above description of conventional and special alloys, historically over five millennia the alloy design, alloy production, and alloy selection were all based on one principal-element or one-compound concept. This alloy concept has generated numerous practical alloys contributing to civilization and daily life. But, it still limits the degree of freedom in the composition of the alloy and thus restricts the development of special microstructures, properties, and applications. Consequently, materials science and engineering of alloys is not fully explored since those alloys outside this conventional scheme have not been included.
1.4.1 Karl Franz Achard
It should be mentioned that in the late of eighteenth century, a German scientist and also metallurgist Franz Karl Achard had studied the multicomponent equimass alloys with five to seven elements (Smith, 1963). He could be most probably the first one to study multiprincipal-element alloys with five to seven elements. In many ways, he is the predecessor for the researches of Jien-Wei Yeh on HEAs. More than two centuries separate them. In 1788, Achard published a little-known French book “Recherches sur les Propriétés des Alliages Métallique,” the first compilation of data on alloy systems in Berlin. He disclosed the results of a laborious and comprehensive program on over 900 alloy compositions of 11 metals, including iron, copper, tin, lead, zinc, bismuth, antimony, arsenic, silver, cobalt, and platinum. Because of high cost, he studied fewer compositions with silver, cobalt, and platinum. For other elements, he made representative alloys of every possible combination of components up to seven components. Besides many binary, ternary, and quaternary alloys, he made quinary, sexinary, and septenary alloys only in equal proportions in weight. All the alloys were in the as-cast condition and on these he carried out tests for density, hardness, strength, impact resistance, ductility, the resistance to a file, the degree to which the alloy could be polished, and finally the results of exposing a polished surface to dry air, moist air, and moist air with HCl acid fumes, and moist hydrogen sulfide. In this book, he pointed out that the properties of alloys are quite different from those of the pure metals and are unpredictable. Only experiment can instruct us. This book is mainly a report without any discussion. All the experimental results were given in tabular form. Although this book was written in the French language insisted upon by Frederick the Great, but unfortunately it was virtually ignored by metallurgists everywhere. This rare work was brought to light only in 1963 by Professor Cyril Stanley Smith (Smith, 1963).
Toward the end of the twentieth century two entirely independent investigations by Brian Cantor in the United Kingdom and Jien-Wei Yeh in Taiwan made a disruptive break with the classical tradition of alloys. A brand new alloy concept “HEAs” has been proposed and explored and has led to a flurry of excitement. Figure 1.4 gives the number of year-wise journal publications (until 2013) in the area of HEAs.
1.4.2 Brian Cantor
The first work on exploring this brave new world was done in 1981 by Cantor with his student Alain Vincent. They made several equiatomic alloys mixing many different components in equal proportions. In particular, the world record holding multicomponent alloy consisting of 20 different components each at 5% is held by this study. It was noticed that only one alloy with a composition of Fe20Cr20Ni20Mn20Co20 forms a single FCC (face centred cubic), Vincent was an undergraduate project student and the work was only written at that time in his thesis at Sussex University.
After this initial experiment there was a hiatus. Similar studies on a wider range of alloys were repeated with another undergraduate project student, Peter Knight, at Oxford in 1998. He achieved some similar results and some new ones, published his results in a thesis at Oxford. Finally, Isaac Chang repeated the work again in about 2000 at Oxford, and finally published the results in the open literature by presenting at the Rapidly Quenched Metals conference in Bangalore in 2002, which was then published in the journal Material Science and Engineering A in July 2004 (Cantor et al., 2004). In this paper entitled “Microstructural development in equiatomic multicomponent alloys”, several important conclusions were drawn. A five component Fe20Cr20Mn20Ni20Co20 alloy, on melt spinning, forms a single FCC solid solution, which solidifies dendritically. Based on this composition, a wide range of six to nine component alloys by adding other elements such as Cu, Ti, Nb, V, W, Mo, Ta, and Ge exhibit the same majority FCC primary dendritic phase, which can dissolve substantial amounts of other transition metals such as Nb, Ti, and V. More electronegative elements such as Cu and Ge are less stable in the FCC dendrites and are rejected into the interdendritic regions. Besides, alloy containing 20 components, that is, 5 at.% each of Mn, Cr, Fe, Co, Ni, Cu, Ag, W, Mo, Nb, Al, Cd, Sn, Pb, Bi, Zn, Ge, Si, Sb, Mg, and another alloy consisted of 16 elements, that is, 6.25 at.% each of Mn, Cr, Fe, Co, Ni, Cu, Ag, W, Mo, Nb, Al, Cd, Sn, Pb, Zn, and Mg are multiphase, crystalline and brittle both in as-cast condition and after melt spinning. Surprisingly, however, the alloys consisted predominantly of a single FCC primary phase, containing many elements but particularly rich in transition metals, notably Cr, Mn, Fe, Co, and Ni. Finally, the total number of phases is always well below the maximum equilibrium number allowed by the Gibbs phase rule, and even further below the maximum number allowed under non-equilibrium solidification conditions.
It is also important to point out that Cantor came up with another novel idea of equiatomic substitution later, in the early 2000s (Kim et al., 2003a), as a method of exploring metallic glass. These compositions are also in this vast uncharted region of materials space.
1.4.3 Jien-Wei Yeh
J.W. Yeh independently explored the multicomponent alloys world since 1995 (Yeh et al., 2004b; Hsu et al., 2004). Based on his own concept that high mixing entropy factor would play an important effect in reducing the number of phases in such high order alloys and render valuable properties, he supervised a master student K.H. Huang in 1996 to start the research and see the possibility of success in the fabrication and analysis of HEAs. Around 40 equiatomic alloys with five to nine components were prepared by arc melting. Investigations were made on microstructure, hardness, and corrosion resistance of as-cast state and fully annealed state. The alloy design is mainly based on commonly used elements. From those data of around 40 compositions, 20 alloys based on Ti, V, Cr, Fe, Co, Ni, Cu, Mo, Zr, Pd, and Al, with or without 3 at.% B addition were selected as experimental alloys in the MS thesis of Huang in 1996 (Huang, 1996, published as MS thesis of National Tsing Hua Univeristy, Taiwan).
Based on this study, typical dendritic structure was seen in the as-cast structure. All alloys have high hardness level in the range from 590 to 890 HV depending on the composition and fabrication conditions: as-cast or fully annealed. Full annealing treatment in general retained similar hardness level of as-cast state. Higher number of elements increased the hardness but nine-element alloys more or less displayed a small decrease in hardness. Small addition of B has led to some increase in hardness.
After this study, two more studies were made before 2000 on different aspects of HEA and were submitted as MS theses (Lai et al., 1998 and Hsu et al., 2000, all published as MS theses of National Tsing Hua University, Taiwan). During 2001–2003, nine different studies were conducted by Professor Yeh’s group: five studies on HEAs bulk alloy concerning with deformation behavior, wear behavior, and annealing behavior; two studies on HEA thin films deposited by magnetron sputtering; and two on HEA thermal spray coatings (Huang et al., 2001; Chen et al., 2002; Tung et al., 2002; Chen and Lin, 2003; Huang and Yeh, 2003; Hsu et al., 2003; Lin et al., 2003; Tsai et al., 2003a; Tsai et al., 2003b, all published as MS theses of National Tsing Hua University, Taiwan). Until 2013, Professor Yeh has supervised 79 MS theses and 10 Ph.D. theses in this exciting area of HEAs. Besides the supervision by Yeh, a portion of some of these theses on HEAs and related materials were also supervised by his colleagues and collaborators: S.K. Chen, S.J. Lin, T.S. Chin, J.Y. Gan, and T.T. Shun. In the discussion on these trends, high solution hardening due to large lattice distortion and stronger bonding were proposed. All these alloys in general displayed very good corrosion resistance assessed by the weight loss after immersion in four acidic solutions of HCl, H2SO4, HNO3, and HF, each in 0.01 and 1 M, for 24 h. The addition of passive elements and the benefit of low free energy due to high entropy were thought to contribute the corrosion resistance. This study thus led to valuable suggestions about high-entropy effect, lattice distortion effect, and slow diffusion effect.
Yeh had submitted the “HEA concept” paper to Science in January 2003 but finally unaccepted by Science. After this, he submitted the same paper to Advanced Materials and then agreed the transfer to her sister journal, Advanced Engineering Materials for publication. In May 2004, this paper entitled “Nanostructured high-entropy alloys with multiprincipal elements—novel alloy design concepts and outcomes” was published. It becomes the first one to elucidate the concept of HEAs by providing experimental results and related theory (Yeh et al., 2004b). Besides this, another paper entitled “Multi-principal-element alloys with improved oxidation and wear resistance for thermal spray coating” was published in Advanced Engineering Materials in February 2004 (Huang et al., 2004). But the term of HEA was not used in this paper. Two papers entitled “Wear resistance and high-temperature compression strength of FCC CuCoNiCrAl0.5Fe alloy with boron addition” and “Formation of simple crystal structures in solid-solution alloys with multi-principal metallic elements” were published in Metallurgical and Materials Transactions A later in the same year (Hsu et al., 2004; Yeh et al., 2004a). Before the submission of the first of the above paper, Professor Yeh had applied for HEAs patents in Taiwan (December 10, 1998), Japan, United States, and Mainland China.
1.4.4 Srinivasa Ranganathan
Professor S. Ranganathan has also spent a long time to look into such multicomponent alloys unexplored by people. Through the communications and discussions on this unknown field with J.W. Yeh, he published a paper entitled “Alloyed pleasures—multimetallic cocktails” to introduce three new alloy areas: bulk metallic glasses by A. Inoue, superelastic and superplastic alloys (or gum metals) by T. Saito, and HEAs by J.W. Yeh in Current Science in November 2003 (Ranganathan, 2003). This becomes the first open publication in journals on HEAs, which led to the activation of this new field.
In this article, he said that the multicomponent alloys represent a new frontier in metallurgy. They require hyperdimensions to visualize. If we use a coarse mesh of 10 at.% for mapping a binary system, the effort involved in experimental determination of phase diagrams rises steeply. Thus, the effort of experimental determination of a seven component system will be 105 times that of a binary diagram and will alone need as much effort as has been spent over the last 100 years in establishing ~4000 binary and ~8000 ternary diagrams. While the computation of phase diagrams from first principles has made impressive progress in the last decade, the calculation of higher order systems is a daunting task. In this scenario, we have explorers like A. Inoue, T. Saito and J.W. Yeh pointing to exciting new alloys with applications.
1.5 The Scope of This Book
From the open publications by the three initiators mentioned above, HEAs have become an emerging field with many more researcher’s efforts and contributions. In a broad view, many aspects have been explored and researched. Figure 1.5 shows the materials hypertetrahedron for the HEAs, which shows in a nutshell the broad spectrum of research and development that is taking place in this field. Based on this, the content of this book has been designed to cover this broad spectrum. Chapter 2 gives the basic definition of HEA and the compositional notations. Chapter 3 describes the factors affecting the phase selections and gives the parametric approaches to design alloy compositions. Chapter 4 describes the different simulation and modeling methods involved in integrated computational materials engineering (ICME) and material genome initiative (MGI). Chapter 5 describes the different synthesis methods used for HEAs. Chapter 6 describes the formation of solid solutions and their microstructure in different HEA compositions. Chapter 7 describes the intermetallics, interstitial compounds and metallic glasses found in HEAs. Chapters 8 and 9 describe the structural and functional properties, respectively. Finally, Chapter 10 describes property goal pursued, potential advanced applications and future trends. As for the interconnections in the HEAs hypertetrahedron, all chapters also emphasize them with an aim to give comprehensive understanding on HEAs.
