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Mechanical alloying is a method used by metallurgists to create alloyed metals from powdered materials. The microstructure and physical properties of the finished alloys can be varied to the metallurgist's requirement via selection of alloying elements to the solute metal. Mechanical alloying allows for difficult or normally incompatible system to be created. Mechanical alloying is a method that does not require the melting of the metals or requires casting therefore allowing for a larger range of alloys to be considered and produced.
For steels, the balance metal used is typically iron with small amounts of carbon but additional alloying elements can be added to induce additional physical properties such as corrosion resistance and increased hardness.
Initially there are two stages during the production of mechanically alloying of steels: Crushing or fracturing the metallic powders and sintering. However, during these two steps, the microstructure of the alloy changes vastly which needs to be explored in depth from the beginning of ball milling to the end product after sintering.
Crushing or fracturing of the iron, copper and other alloying powders are typically done with the use of a ball mill or any machinery that can 'grind' powders very finely. A ball mill typically consists of a barrel that rotates horizontally with the metallic powders and a spherical medium used to crush the powders between the rotating barrel and the medium as shown in Figure 1 below:
Figure 1. A typical ball mill in motion (The large dark circles are the grinding mediums) (n.d.)
Iron and carbon are metallic elements that are considerably soft, so they will increase in size when colliding with adjacent iron or carbon particles due to their softness. To prevent excessive nucleation of powders whilst crushing and promote fracture, the ambient temperature whilst crushing is lowered to a temperature below all of the elemental powder's ductile-to-brittle temperature or cryogenic milling. Another method to prevent nucleation is the addition of a Processing Control Agent to separate contact between adjacent iron and carbon particles during collision. This ensures that initial ball-powder-ball collisions will cause iron and carbon to work harden and a reduction in area.
Additional alloying elements added to the ball mill are typically powdered intermetallic compounds that are considerably more brittle and will be crushed further during ball-powder-ball collision.
During the first stages of mechanical alloying, iron and carbon are pressed together during ball-powder-ball collision, forming incoherent interfaces. With continuous collisions a layered combination of iron and carbon are formed but due to additives and the low temperatures, these cold welded particles are constantly crushed by ball-powder-ball collisions. These new, smaller iron-carbon cold-welded particles will then be combined with adjacent iron-carbon cold-welded particles to form new nucleated particles and so on as long as the ball mill is running. Over time, the iron and carbon particles are continually refined in terms of layers and are effectively homogenised.
Change in microstructure during Crushing
The microstructure of the homogenised particles during continuous ball-powder-ball crushing will change according to the fineness of the layered iron and carbon.
â€¢ In the early stages of crushing the interfaces between the iron-carbon can be seen. Additional additives such as chromium will be typically seen in-between iron-carbon interfaces finely spaced.
â€¢ After additional ball-powder-ball crushing the iron-carbon together they form lamellar plates. Kinetic energy produced by the collisions is converted to heat, allowing for diffusion to occur within iron-carbon particles. The additional additives become further refined and eventually act as precipitates in-between the iron-carbon lamellar plates.
â€¢ After extended periods of time, the iron-carbon lamellar plates become more refined and smaller in surface area compared to the rest of the microstructure. The additional additives have homogenously spread throughout the particle, almost supersaturated. The final microstructure is similar to that when heat-treated using an Iron-carbon phase diagram due to the extremely fine mixing of iron and carbon particles that precipitation of carbon is possible.
Due to this constant plastic deformation of iron, carbon and alloying additives, the powder has a higher hardness to that of a melted steel of equivalent composition.
Merging of powder
After sufficient repeated layering and fracturing of iron, carbon and other alloying elements the steel particles are heated (below the steel's melting temperature) and compacted, often called sintering. The final steel alloy can be drawn out to form slabs or other desirable shapes. Temperatures typically used while sintering are below that of the steel powder's melting temperature to retain grain size but allow adjacent steel particles to adhere to each other. The temperature used while sintering can be changed in order to promote recrystalisation and therefore increase grain size of the final product.
Mechanical properties of Mechanical Alloyed Materials
The mechanical alloying of ductile-brittle materials usually results in a fine homogeneous dispersion of brittle phase in the ductile matrix (e.g. Y2O3 found in iron and nickel based alloys). The final powder is characterised by a narrow particle size distribution and uniform composition. Mechanical alloying enables the combination of numerous strengthening factors including oxide dispersion, carbide dispersion, fine grain, high dislocation density and substructure and solid solution strengthening to be present within an alloyed material.
The tensile strength of mechanically alloyed materials is proportional to the square of relative densities. The yield strength of mechanically alloys follows the Hall-Petch relationship. It depends on the inverse of the square root of the grain size. The tensile properties of mechanical alloys depend upon the compact density and grain size. In oxide dispersed materials the dispersed particles influence the room temperature yield strength of the material by preventing grain growth during consolidation. The oxide particles impede dislocation motion, as they will not shear. A fine and even distribution of the oxide particles has a greater effect on tensile strength than just large volume of particles. During mechanical alloying and consolidation the powders are subjected to compressive impact forces and undergo severe plastic deformation (e.g. extrusion, rolling, forging etc.), which will usually require further heat treatment processes to lessen.
The main strengthening factor of mechanical alloys is the grain size. Mechanical alloying allows grain size to be controlled by the powder production process. Dispersed oxide particles harden the matrix material by obstructing the motion of dislocations. Fracture occurs by the dislodging of the dispersed particles from within the matrix. Crack initiation results at the dispersed oxide/matrix interface. Fracture may be trans-granular or inter-granular resulting in mixed mode failure.
Mechanically alloyed dispersion strengthened alloys have enhanced creep resistance and microstructural stability. This is due to the fine scale dispersion of oxides throughout the matrix. The improvement in creep strength in mechanical alloys is due to the dispersed particles interacting with mobile dislocations over a much wider range of temperature when compared to the precipitates present in the conventional alloys.
Thus, the resulting improvement in mechanical properties of mechanically alloyed materials is far greater than can be achieved by conventional methods.
Effect of Alloying Element
The mechanical alloying process can produce metal alloys that have desirable physical properties due to the various elemental powders used in production. The following are some of the more common alloying elements and their effects on the alloy.
Chromium: Added to increase corrosion and oxidation resistance by forming an oxide layer. It also increases hardenability and improves high temperature strength.
Carbon: Used to enhance steel strength. With increasing carbon content, hardness and tensile strength increases. When it combines with chromium it forms chromium carbides.
Nickel: It is a ferrite stabiliser. Nickel does not form carbides as it remains in solution in ferrite. This strengthens and toughens the ferrite phase. Nickel also increases the hardenability and impact strength of steels. Corrosion resistance is also improved by the addition of nickel.
Molybdenum: Is also a ferrite stabiliser. It increases the hardenability of steel and improves the hot and creep strength of low alloy steels at elevated temperatures. It enhances pitting and crevice corrosion resistance in steels and forms abrasion resistant particles.
Titanium: Stabilises the formation of chromium carbides by trapping carbon. It also limits grain growth which improves toughness. It reduces hardenability of chromium steels and prevents the formation of austenite in high chromium steels.
Mechanical Alloying has been used to develop materials of various compositions and to improve the performance of many existing materials. The limits of solubility in solidification can be exceeded and a dispersion of particles (usually oxides) can be introduced uniformly into the materials. Many of the alloys produced have significant commercial application or are candidates for future applications so are produced industrially. The mechanical alloying process is a relatively difficult manufacturing method as variations in processing conditions can completely change the subsequent properties of the alloy. Two alloys of significant commercial importance are oxide dispersion strengthened iron and nickel based alloys.
Mechanically Alloyed Steels:
Steels produced by mechanical alloying have been found to have desirable properties in a variety of applications. This is generally because they have been designed to be oxidisation and corrosion resistant. They also have greater creep resistance when compared to the equivalent cast alloy. MA 956 and MA 957 are two steel alloys that are commercially produced (Fig. 2).
Fig. 2. The nominal compositions (wt %) of mechanically alloyed iron based alloys
MA 956 is a chromium rich, ferritic stainless steel. The high chromium content and presence of aluminium gives MA 956 a high oxidation resistance. This allows for use at high temperatures in corrosive atmospheres. A uniform dispersion of Y2O3 particles in the steel improves creep resistance by allowing aluminium to form Al2O3, which is a protective layer at high temperatures. The alloy is used as sheet material for aircraft and industrial gas turbine combustors and heat exchangers of power generation equipment.
MA 957 is also a ferritic stainless steel. It contains molybdenum rather than aluminium. This alloy is designed for application in the nuclear industry as it is less susceptible to radiation induced void swelling. MA 957 alloy has been used for nuclear fuel cladding material in fast breeder reactors. Conventional austenitic alloys cannot be used due to swelling caused by the high neutron fluxes and the formation of titanium carbides as a result of a higher carbon concentration and temperature. Conventional ferrite steels generally have poor creep strength at the service temperature of 700Â°C. The titanium in this alloy combines with the chromium, molybdenum and iron to form a stable intermetallic phase during aging at 800Â°C, which increases creep strength.
These steels are produced basically to meet application requirements. The steels after mechanical alloying and consolidation have an ultrafine microstructure containing extremely small grains of ferrite. The hardness in this condition is unacceptably high so the steels are usually recrystallised to have a coarse directionally grain structure. This is ideal for elevated temperature applications where creep resistance is desired.
Nickel based alloys:
The most significant advantage of nickel-based alloys is the increased stress rupture properties. Nickel based oxide dispersion and precipitation strengthened MA 6000 contains tungsten and molybdenum, which provides solid solution strengthening. Chromium and aluminium with titanium provides oxidisation resistance. The mechanically alloyed nickel-base alloys are considered for three main application areas - gas turbine vanes, turbine blades and sheets for use in oxidizing/corrosive atmospheres.
Advanced small turbine blades can be machined from a solid bar of MA 6000. The high temperature strength of the alloy allows it to be made without intricate cooling systems. When the stresses are low, the alloy can be used in areas of higher temperature. The complex composition of the alloy provides enough corrosion and oxidation resistance so the blades can be used without coatings. The MA 6000 alloy can maintain a given stress for a much longer time than a conventional alloy for similar turbine blade applications. This is due to the combined strengthening modes in the alloyed material. MA 6000 combines two types of strengthening:
â€¢ Gamma-prime hardening (from its aluminium, titanium, and tantalum content) for intermediate temperature strength.
â€¢ Oxide dispersion strengthening (from the Y203 addition) for strength and stability at very high temperatures.
The oxide particles directly increase high temperature strength by acting as obstacles to dislocation motion. Tungsten and molybdenum improve solid solution strength. Oxidation resistance is due to the aluminium and chromium content of the alloy.
Benefits of the combined strengthening modes in the mechanically alloyed MA 6000 material (Fig. 3):
â€¢ At lower temperatures the strength of MA 6000 is similar to that of the complex directionally solidified alloy DS MAR-M200 + Hf.
â€¢ At intermediate temperatures the strength of MA 6000 is superior to both the cast nickel-base superalloys and thoriated nickel (thorium oxide particles in pure nickel) because the two strengthening mechanisms.
â€¢ At high temperatures the DS MAR-M200+ Hf has lost most of its strength due to growth and dissolution of gamma-prime precipitates. MA 6000 still has high strength due to the oxide dispersion.
Fig. 3. Stress for 1000 hours life to rupture as a function of temperature for alloy MA 6000 and other nickel based alloys.
The use of mechanical alloying has allowed metallurgists to create steel alloys with variable microstructures that were unable to create traditionally. This has led to the improvement of desirable quantities such as creep resistance, tensile strength and fracture toughness.
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