Metallic alloys have been instrumental through the ages in shaping the progress of human civilization. The development of the alloys from ancient to present time initiated from accidents to through the use of well-defined scientific principles. This article provides a snapshot of the alloys development from ancient to present time and the likely future direction.
Materials have played a key role in evolution of humans from prehistoric times. The development, discovery and use of materials are largely responsible for easing their work pressure while continuously contributing to an enhancement in their comfort levels and social development [
The development of materials progressed more scientifically over last two centuries and currently materials can be classified into four categories [
1) Polymers: Polymers are lightweight and corrosion resistant materials used in diverse spectrum of applications where strength and stiffness are not the primary design criteria. Polymers can be transparent and often serve to replace ceramic glasses. Applications of polymers include home appliances, casings of electronic gadgets, stationary items, coatings etc.
2) Ceramics: Ceramics are typically hard and brittle materials with low fracture toughness. They are made of a metallic and non-metallic atoms bonded through ionic, covalent or mix of them. They are good insulators and thermally stable at higher temperatures. They are most commonly used in infrastructural applications, as insulators, fibers and abrasives.
3) Metals: Metals are class of materials that are capable of conducting electricity at a temperature of absolute zero. Generally, they exhibit good combination of strength and ductility and can conduct heat. They can be used at temperatures intermediate between the application domains of polymers and ceramics. They are widely used in practically all applications ranging from aerospace industry to home appliances.
4) Composites: Composites materials are blend of different categories of materials and are capable of exhibiting the properties that cannot be displayed by their constituting counterparts. Typically ceramics are used as reinforcements in metals or plastics to enhance their targeted properties governed by the end application.
As evident, each category of materials has its own unique significance and their application zone that is principally governed by their unique microstructure governed properties. In modern applications, these materials co-exist. As an example in automobiles, we can see different parts made of polymers (interiors), metals (body and engine), composites (fiber reinforced plastics) and ceramics (glasses). The relative dominance of any type of above stated category of materials depends on the spectrum and combination of properties that they can exhibit. For example, ceramics exhibit excellent properties under compression and high temperature and hence they are accordingly used in infrastructural applications and as insulation materials in high temperature environment such as in furnaces. On the other hand, polymers are not able to withstand high temperature and exhibit low modulus and accordingly used where these two service conditions are absent. Polymers are usually light and deformable and hence are used for example in weight-critical applications. As a result of vast difference in properties of metals, polymers and ceramics, composites were developed to bridge the gap between any two categories of materials while exhibiting a combination of properties that can be tailored based on end applications. Composites are therefore used in infrastructural applications from ancient times to modern times in addition to more targeted applications in space, aerospace and automobile sectors. To note that composites are also favored by nature as our bone and wood are classical examples of nature built composites [
1) Compositional control: Alloys irrespective of the primary base metal with diverse combinations of properties are often realized primarily through composition control with classical examples such as stainless steel, had field steel and maraging steels in case of iron based alloys. A method currently and continuously pursued actively.
2) Heat treatment: Heat treatments are often utilized to realize different combination of properties even maintaining the same composition and to enhance certain characteristics of the metallic alloys. Evolution of T4, T6, hardening and tempering are some such examples.
3) Processing control: The interrelation between processing microstructure and mechanical properties is well known. Accordingly, the nature of primary processing (such as casting or powder metallurgy) and secondary processing (such as rolling, forging or extrusion) can result in alloys of the same composition displaying totally different combination of properties.
While the compositional variation was primarily utilized till nineteenth century, the development of characterization tools picked up in early twentieth century with the development of transmission electron microscope and x-ray diffractometer. With the increasing understanding of the structure of metals, phase diagrams started to appear in later part of nineteenth century and in early twentieth century initiating the golden era of alloy development through compositional and heat treatment control (accidental discovery of age hardenable Al alloy Duralumin) [
Metallic alloys were made and used by humans from prehistoric time mostly through experimental trials and without any scientific knowledge. Notable examples are BUTP steel in ancient India, Damascus and Moorish steel in middle ages and Sheffield steel in 14th century. The iron pillar (6000 kg) in New Delhi, India erected in 402 CE is still considered a metallurgical wonder as it has not corroded so far (
that steel is an alloy of iron and carbon and further the concept of heat treatment remained unexplained till end of 19th century [
Improvement and variation in properties were realized principally through judicious composition (
As a result of rapid pace of technological advance in past few decades and emergence and rapid growth in engineering sectors such as automobile and aerospace posed the need to develop higher performance metallic materials due to ever increasing stringent service conditions. Development of nickel based superalloys, damage tolerant titanium alloys and maraging steels are some such
Metal/Metallic Alloys | Applications |
---|---|
Fe/Fe based alloys-Steels | Transportation sector and infrastructural application, biomedical sector. |
Al/Al based alloys | Weight critical applications in transportation sector, sports sector and electronic sector. |
Cu/Cu based alloys (Brasses, bronzes) | The best material for electricity transmission, infrastructural applications, electronic sector, defense applications, marine industry. |
Zn/Zn based alloys | Automobile, construction, shipbuilding, household electrical appliances and galvanizing. |
Ni based alloys | High temperature applications such as in engines and heating elements. They can also be used where shape memory effect is required. |
Ti based alloys | Light weight structures, defense applications, aerospace sector, and biomedical sector. |
Mg based alloys | Weight critical applications in transportation sector, sports sector and electronic sector. |
Sn based alloys | Most commonly used as lead free solders in electronic industry. |
examples of alloys developed in recent decades and used extensively. Last two decades have also seen the emergence of new magnesium based alloys due to their light weight and capability to reduce greenhouse gas emissions [
Type of Alloy | Yield Strength (MPA) | UTS (MPa) | Failure Strain (%) | Density (g/cc) | Reference |
---|---|---|---|---|---|
UNS N10276: Ni superalloy (C276) | 204 2.4 mm thickness, 429˚C | 601 4.8 - 25.4 mm thickness, 538˚C | 56 1.6 - 4.7 mm thickness, 204˚C | 8.89 | [ |
Cartridge Brass (70/30) | 76 - 448 (Depends on temper) | 303 - 896 (Depends on temper) | 66 | 8.53 | [ |
SS304 steel | 215 | 505 | 70 | 8 | [ |
Maraging steel ASTM A 538 | 2617 | 2693 | 6 | 8 | [ |
Mild steel: S275 | 245 - 275 | 410 - 660 | 20 - 22 | 7.85 | [ |
Ti-6Al-4V: Ti alloy | 880 - 920 | 900 - 950 | 5 - 18 | 4.5 | [ |
AA 7075: Al alloy (T6) | 430 - 480 | 510 - 540 | 5 - 11 | 2.81 | [ |
AA 2024: Al alloy | Min. 345 | Min. 427 | 5 | 2.78 | [ |
AZ91: Mg alloy (Die cast alloy) | 160 | 230 | 3 | 1.81 | [ |
AZ31: Mg alloy | 200 | 260 | 15 | 1.77 | [ |
While the research community is still engaged in developing new alloys based on the current fundamental understanding of the interaction between different elements, heat treatments, processing routes and the technological needs (such as light weighting for controlling environmental pollution), innovative efforts were also made in parallel to develop non-crystalline or amorphous metal based alloys.
First reported in 1960 by Klement, Willens and Duwez [
1) Their dimension: One of the dimensions in earlier research pursuits was limited to typically less than 100 microns to ensure high heat transfer. They were produces in forms like powders, films, ribbons and foils with almost no engineering applications.
2) Cost: The cost of the metallic glasses was high due to batch process, limited amount of metals that can be transformed to glassy state besides the cost of primary constituents. For example the first metallic glass reported was Au-Si based [
3) Synthesis method: The methods (splat quenching, twin roller quenching, melt spinning etc.) that were able to create metallic glasses had to ensure a fast cooling rate (105 - 106 K/s) were much beyond the capabilities of conventional processing types such as casting making them almost of no use for common engineering applications.
4) Compositional limitations: In the initial almost two decades, there were limited progress in expanding the number of compositions that can lead to formation of metallic glasses and at times it was perceived only as a scientific curiosity.
In 1990, Inoue and coworkers synthesized multicomponent glassy alloys using transition metals and at low cooling rates (as low as ~10−3 K/s). Commonly known a bulk metallic glasses (BMGs), these alloys due to their millimeter size (>~1-mm) enabled the researchers to investigate their mechanical, physical and chemical properties (
Attempts were made to stipulate the rules that lead to metallic glass formation and following crucial factors were identified:
1) Atomic packing density: Glass forming ability seems to increase with higher
atomic packing density. Higher packing density leads to lower energy and higher stability [
2) Multicomponent system: With higher number of constituent elements (typically more than three elements), the confusion principle prevails indicating that the alloy cannot select viable crystal structures lading to enhanced glass forming ability.
3) Atomic size: The wider the atomic size (>12%) the better is the glass forming ability.
4) The combination of constituents should have negative heat of mixing.
5) The composition has a deep eutectic. This was especially true for simple compositions (binary and ternary) as the compositions near to eutectics have lower melting points and stable liquid phase.
The rules indicated above should be taken as guidelines as the laws for quantitative composition design are still unknown and more efforts are required in this area [
While the experimental investigations have validated the above properties, underlying mechanisms are still not well understood for many of the above properties. The main limitations of metallic glasses include: 1) dimensional limitations, 2) metastable nature, 3) low ductility at low temperatures, and 4) low elastic modulus.
Some of the applications of glassy alloys include: 1) as a soft magnetic material in electronic devices, automobiles and electrical power transmission, 2) as biomedical materials due to their good corrosion resistance in physiological solution [
In recent years, researchers have also utilized metallic glasses as reinforcement in light matrices based on aluminum and magnesium and reported significant improvements in properties [
The improvement was primarily attributed to their metallic nature leading to good interfacial bonding, delayed fracture of reinforcement and fairly uniform distribution in matrix. The results were superior to ceramic particles as ceramic particles tend to exhibit early initiation of particle damage mechanisms resulting in the underutilization of properties enhancement capabilities of reinforcement especially when used in micron and higher length scales.
Conventionally alloys are formulated based on phase diagrams and the atoms of constituent elements are mixed during the liquid or solid state processing. The outcome is a thermodynamically stable solid solution phase and precipitates that are further manipulated through heat treatment and secondary processing for realizing the desired application driven end properties. Composite metals, on the contrary, involve the combination of two metals which may be immiscible [
microwave sintering can be successfully used. For the immiscible system, conventional casting techniques can be conveniently used for bulk production. The composite metal systems have shown improvement in the following properties over their dominant matrix counterpart [
The composite metals can be classified as metastable (Mg-Al) as the processing is controlled to avoid their interaction and thermodynamically stable such as Mg-Ti and Mg-Mo system where the two metals are immiscible and non-reactive (
To note as shown in
1) Deviation from conventional norm of designing alloys and realizing properties that are otherwise not achievable in conventional alloys.
2) Good interfacial bonding between metal servings as matrix with the one serving as reinforcement. The good interfacial bonding leads to superior mechanical properties by delaying the onset of crack nucleation in the presence of stresses.
3) A wide spectrum of properties between metals to design a new series of materials. For example, E = 40 ~ GPa for Mg and E= ~200 GPa for iron allows the variation in elastic modulus that can be realized when combining these two metals.
The properties of some of the composite metals are shown in
The advantages associated with designing composite metals give them multiple degrees of freedom to realize properties that are required by end application. Further, the properties of composite metals can be tailored through the judicious utilization of conventional and new secondary processing techniques such as equi-channel angular processing and friction stir processing.
High entropy alloys (HEAs) are a new advancement in alloy design that is principally based on entropy of mixing (ΔSmix) and enthalpy of mixing (ΔHmix) [
Materials | Reinforcement (Vol.%) | 0.2% YS (MPa) | UTS (MPA) | Ductility (%) | Work of Fracture (J/m3) |
---|---|---|---|---|---|
Mg | - | 100 ± 4 | 258 ± 16 | 7.7 ± 1.2 | 20.2 ± 2.6 |
Mg/5.6TiP | 2.2 | 163 ± 12 | 248 ± 9 | 11.1 ± 1.4 | 25.7 ± 2.9 |
Mg/9.6TiP | 4.0 | 154 ± 10 | 239 ± 5 | 9.5 ± 0.3 | 20.7 ± 1 |
having many elements, HEAS should have entropy of mixing in the range of 11 ≤ ΔSmix ≤ 19.5 J/K mol and enthalpy of mixing in the range of −22 ≤ ΔHmix ≤ 7 kJ/mol [
Typical microstructure of a HEA is shown in
For the initially developed HEAs, some of the extraordinary properties displayed included: 1) high hardness (100 to 1100 Hv), 2) work hardening capacity, 3) wear resistance, 4) high temperature precipitation hardening (600˚C to 1000˚C), and 5) anti-oxidation and anti-corrosion resistance. Selected properties of HEAs exhibiting a wide range of densities are shown in
In view of these properties, some of the cited and potential applications include [
Recently, researchers have also used HEA in the powder form as reinforcement in magnesium matrix [
While HEAs have shown a different path in alloy design and a different spectrum of properties, it rests to be seen whether they can qualify through some of
Alloy | Density (g/cc) | Process | Phase |
---|---|---|---|
NbTiVTa | 11.49 | Arc melting | BCC |
Al26.6Nb23.8Ti25.1V24.5 | 5.59 | Casting | BCC |
Al20Be20Fe10Si15Ti35 | 3.91 | Casting | 3 phases |
Mg43(MnAlZnCu)57 | 2.51 | Induction melting | HCP + Al-Mn icosahedral Quasi-crystal |
Al20Li20Mg10Sc20Ti30 | 2.67 | MA | FCC |
Alloy | Density (g/cc) | Micro-hardness (Hv) | |
---|---|---|---|
As Solidified | homogenised | ||
NbTiVTa | 11.49 | NA | |
Al26.6Nb23.8Ti25.1V24.5 | 5.59 | 448 ± 12 | |
Al20Be20Fe10Si15Ti35 | 3.91 | 911 | |
Mg43(MnAlZnCu)57 | 2.51 | 255 | |
Al20Li20Mg10Sc20Ti30 | 2.67 | 591.4 (as milled) | 499.6 (annealed at 500˚C) |
Alloy | Density (g/cc) | Yield Stress (MPa) | Peak Stress (MPa) | Fracture Strain (%) |
---|---|---|---|---|
NbTiVTa | 11.49 | 1092 | NA | >50 |
Al26.6Nb23.8Ti25.1V24.5 | 5.59 | 1020 | 1318 | 5 |
Mg33(MnAlZnCu)67 | 3.26 | 437 | 437 | 3.41 |
Mg43(MnAlZnCu)57 | 2.51 | 500 | 500 | 3.72 |
Mg45.6(MnAlZnCu)54.4 | 2.30 | 482 | 482 | 4.06 |
the issues such as: 1) economic feasibility, 2) clear superiority over conventional alloys in terms of mechanical properties, and 3) joining capability.
From the early stages of human civilization till now, metal based alloys are constantly being developed even in the case of most traditional and widely used metal such as iron. The pursuit of alloy development will never end and more superior properties or a combination of properties will always be sought governed by ever changing service requirements in the constantly developing and advancing area of engineering applications. The alloy development work will continue as long as the pursuit of engineers and doctors will continue to bring more comfort to human’s life. The rules governing the development of traditional alloys and amorphous alloys are reasonably well established. High entropy alloys are being actively developed over almost 14 years and reasonable success has been made for high density HEAs (ρ ~ 10g/cc). Challenges remain and being addressed to develop light weight HEAs for weight critical applications specially targeted to reduce CO2 emissions. With the extent of activities going on in the area of LWHEAs, it is expected that targeted solutions will be realized in next 5 years. Another window of alloy development that is not yet addressed relates to medium entropy alloys (1.5R ≥ ΔSconf ≥ 1 R). Medium entropy alloys are again multicomponent alloys that are intermediate between traditional alloys (ΔSconf ≤ 1 R) and high entropy alloys (ΔSconf ≥ 1.5 R). A judicious choice of alloying elements based on their capability to constructively interact with each other may lead to development of microstructures capable of demonstrating the properties that may well complement other metal based alloys conventionally used and developed currently. In essence, there are still gaps in alloy development which need to be filled through intelligent use of core fundamentals of materials science and engineering.
The authors would like to acknowledge the Ministry of Education Academic Research Funding (WBS# R-265-000-586-114) for the financial support.
The author declares no conflicts of interest regarding the publication of this paper.
Gupta, M. (2018) Changing Wisdom of Metallic Alloys Development. Materials Sciences and Applications, 9, 1021-1035. https://doi.org/10.4236/msa.2018.913074