Changing Wisdom of Metallic Alloys Development

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.


Introduction
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 [1]. For example, use of stones as tools assisted humans in self-defense and hunting thus securing their lives better while simultaneously providing them with much needed food and clothes in ancient times.
The development of materials progressed more scientifically over last two centuries and currently materials can be classified into four categories [1]. Their salient features are given below: 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 [2] [3]. Metals and alloys form a unique category of materials as they can circumvent disadvantages of ceramics (such as low toughness) and polymers (such as low elastic modulus and high temperature capabilities) and provide various combination of properties enabling their use in a wide spectrum of applications. Over the ages, metal based materials have been developed through:

Traditional Alloys
Metallic alloys were made and used by humans from prehistoric time mostly  systems and some on peritectic systems (Al-Ti system). Table 1 shows some of the commonly used metallic alloys that are widely used in multiple engineering applications. Improvement and variation in properties were realized principally through judicious composition ( Figure 2 & Figure 3), heat treatment control ( Figure 4) and processing ( Figure 5). For example, fundamental understanding of heat treatment allowed the researchers to develop steels with various microstructural features through variation in heat treatments (TTT: Time-temperature-transformation diagrams). Similarly, in the case of non-ferrous materials such as aluminum, precipitation strengthening (such as T4 and T6 conditions) was utilized to realize different properties even by keeping the same composition [8]. The variation in composition in Al alloys, for example, led to 8 series (1xxx to 8xxx) of wrought alloys and similarly different types of steels (plain carbon steels, alloy steels, tool steels, specialty steels etc.). Traditional alloys such as mentioned above are still being used widely in many engineering applications [1] [2] [7]. The effect of secondary processing to realize good strength is perhaps best demonstrated by Samurai swords where thermomechanical processing was very intelligently used ( Figure 5) 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 Table 1. List of Some of the Most Commonly Used Metals in Applications.

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.       (Figure 6). Some of the metallic alloys that were and are used in widespread applications are listed in Table 2 and are arranged in the descending order of their densities. Figure 6. Mg alloy motorcycle engine blocks to minimize fuel consumption through weight saving [11].  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 [23].

Amorphous Alloys/Metallic Glasses
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 (10 5 -10 6 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 (Figure 7) [26]. These results were seen as a quantum leap as the methods at low cooling rate can be scaled up and the dimensions in millimeter length scale can enable these materials to be used in much wider spectrum of engineering applications.
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 [24].

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 [24]. With the synthesis of BMGs, various characterizations studies revealed the following notable properties exhibited by glassy alloys [23] [24] [25] [26]: 1) extremely high strength (~1 -2 GPa) at low temperatures, 2) high elastic strain limit (~1% -2%), 3) high hardness, 4) high flexibility at high temperatures, 5) excellent wear resistance, 6) excellent corrosion resistance due to absence of grains and grain boundaries, and 7) extraordinary formability and superplasticity. While

Composite Metals
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 [28] or miscible [29] with each other. To qualify as composite metal both the metals should be identifiable in their native form following processing with insignificant formation of solid solution or precipitation phases. The choice of element that forms the matrix and the one that form the reinforcement phase can be made based on the target properties that need to be realized. Composite metals can be processed using traditional liquid and solid phase routes. For the miscible metal-metal system, the processing needs to be controlled to avoid reaction between two metals and the techniques such as powder metallurgy method using To note as shown in Figure 9, the length scale of the reinforcement can be chosen from nano-length scale to millimeter length scale and from continuous to interconnected reinforcement [10] [28] [29]. The main advantages of composite metals include: 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 Table 3.
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
High entropy alloys (HEAs) are a new advancement in alloy design that is principally based on entropy of mixing (ΔS mix ) and enthalpy of mixing (ΔH mix ) [30] [31] [32]. HEAs comprise of at least 5 elements each having weight percentage between 5 to 35%. For the likelihood of having a simplified microstructure besides  having many elements, HEAS should have entropy of mixing in the range of 11 ≤ ΔS mix ≤ 19.5 J/K mol and enthalpy of mixing in the range of −22 ≤ ΔH mix ≤ 7 kJ/mol [32]. Initially HEAs were developed for high temperature applications with a density hovering around 10 g/cc and later efforts were made to develop light weight HEAs [33]. Conventional processing techniques such as powder metallurgy and casting routes can be utilized for synthesizing HEAs indicating that industrial production is not a major issue but the challenge is to find the composition that suits the end application. Table 4 provides HEAs processed using different methods and with different densities and crystal structure [33].
Typical microstructure of a HEA is shown in Figure 10.
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 Table 5 and Table 6 [33].
In view of these properties, some of the cited and potential applications include [31] [33]: 1) engine materials for withstanding mechanical properties and corrosion degradation at higher temperatures, 2) nuclear materials with low irradiation damage, 3) marine structures for improved corrosion and erosion resistance, 4) cryogenic applications (liquefied gas storage) e.g. CrMnFeCoNi alloy, 5) in food preservation and cooking ware, 6) to suppress electromagnetic interference, 7) as coatings on tools and machine component due to their high hardness and wear resistance, 8) high temperature applications such as in waste incinerators, 9) transportation sector, 10) biomedical sector, 11) as functional coatings for corrosion and wear resistance, 12) as hydrogen storage materials.
Recently, researchers have also used HEA in the powder form as reinforcement in magnesium matrix [34]. Key beneficial outcomes included: 1) micro-     the issues such as: 1) economic feasibility, 2) clear superiority over conventional alloys in terms of mechanical properties, and 3) joining capability.
M. Gupta

Future Directions in Alloy Development
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.