Reinforcing Nickel based alloys and their processing

The alloys of nickel, cobalt and titanium, are the core material of different industries such as aerospace, energy, chemical and medical plants.  It offers excellent mechanical characteristics, corrosion resistance and is bio compatible that make this alloy the best choice for major applications.

The connection between microstructure and mechanical properties of conventionally developed cast cobalt and Nickel based super alloys and directionally solidified and mono crystal casting of these materials has been described.

Preface

The development of aerospace has required regular improvements in the material properties since high speed improves the material heating because of air friction and increased power also raises the engine temperature. Engine skins are developed from alloys of nickel, titanium and aluminum. Steel has been replaced by nickel and cobalt alloys in the aeronautic engines.

Various advanced metals are used in the modern time fir gas turbine and engine in the aerospace industry. The components in the turbines are exposed to various kinds of conditions – high temperatures, corrosive gases, vibrations and high mechanical loads resulted by centrifugal forces. The engine begins, accelerates, decelerates and ceases every time when the engine commences, flies and lands. The repetition of this process damages the alloy’s properties and bends on wire back and forth repeatedly resulting into the metal fatigue.

The main design of an aero-engine remained basically the traditional for over 3 decades. The metallic materials are made to adhere in the high temperatures and big stresses, including engines, replacement of less suitable components and therefore improving service and reliability. Production of aero-engines is the crucial factor for the development of advanced metals and gas turbine engine describes severe conditions that the engines have to face.

The components in the various engine parts have different structural requirements. The vanes and blades in the compressor must have potential to withstand the aerodynamic pressures and rotating blades must prevent the creep damage that extends its ability widely because of the centrifugal force. The discs that keep rotating blades must be able to adhere in the corrosive gases and at the high temperatures unlike to those occurred in compressor. The engine components must have a regular microstructure to keep their properties for prolong service periods.

Super alloys of iron, nickel and cobalt are utilized in these services and are usually used at the temperatures above 800oC like as often in 0.7 of the melting temperature. The metals like cobalt, nickel and iron with regular positions in the periodic table.

The nickel super alloys are widely used. Their perfection is based on the presence of chromium, specifically to provide oxidation resistance and various other elements are present that offer good creep resistance.

Although the alloys of titanium, nickel and cobalt are used in the cast and wrought forms, they are developed by melting and casting process.

Phase & Structure

Nickel based superalloys consist of austenitic face centered cubic matric phase gamma together with various secondary phases. The gamma face centered cubic structure orders intermetallic compound. Their strength is achieved from the hardness with which mono dislocations move through cuboids of gamma phase.

The dislocation moves more easily by the undeformed gamma matrix of superalloy. As the gamma phase is ordered, a single dislocation cannot move through it traditionally and therefore the gamma cuboid in the matrix pin travels dislocations in area, making it harder to deform the component.

The carbides may provide limited reinforcing directly. Direct describes the dispersion hardening and indirect specifies the stabilized grain boundaries against the vast shear. Additionally the elements providing solid solution hardening and improve carbide and gamma development, elements such as boron, zirconium, hafnium, cerium are included to enhance the mechanical and chemical service.

Conventional melting and casting:

To develop a conventional turbine blade, molten metal is poured in the ceramic mold and allowed to solidify. The final result is a fine grained polycrystalline structure were the special grains move randomly. Different super alloys, particularly those with cobalt and iron are air melted by various methods that are performed to stainless steels, however for several nickel based super alloys vacuum induction melting is required as the basic melting process. The vacuum induction melting reducing the extent of interstitial gases such as oxygen and nitrogen, allows bigger and limited levels of aluminum and titanium to be attained and the results in the nominal contagion from slag as compare to air melting.

The nickel and cobalt super alloys with big volume fraction of gamma phase are processed to complex ultimate shapes by investment casting. The investment casting starts with preparation of wide pattern usually wax from pattern die. The service of extensible pattern has been found as a differentiable feature of the investment casting process. The patterns are created on wax runner to develop an assembly that is secured or invested with fine coating of refractory materials. In the vast formation ceramic shell method, the pattern is performed with adjacent layers of refractory powder and full shell has been developed. The wax is eradicated from weld and is blazed to develop strength, molten metal is distributed in the warm mold and soon it is quenched, it is damaged to provide castings that are taken out from the runner systems and finished following the customer requirements. The process alters the molten metal in a single process to precision engineered components with minor material wastage and nominal machining required. It is certainly, the model of approx net form.  

Microstructure characteristics – Basic carbides are primarily developed while cooling of the super alloy melt. By additional cooling to the melt the gamma phase solidifies initially and with reducing temperature more little cuboids are developed in the gamma matrix. The final residual melt solidify as the matrix of double phases like gamma and gamma prime eutectic. The ultimate size of gamma prime precipitate can be evaluated by varying the material cooling rate after the solidification. Nominal cooling offers the smaller gamma prime particles. The super alloy engine components become extremely strong when they are made from the large level of nominal gamma prime particles.

The nickel based super alloys, particularly those comprising of gamma prime phase usually have significantly high strength at the elevated temperatures up to 900oC. The gas turbine components of aero-engine are constructed from heat resistant nickel alloys. They are utilized to develop compressor blades in the components that encounter with air at its highest temperature and pressure levels. The turbine blades in the components face air at its highest temperature and pressure levels. The turbine blades in the regions closest to the combustion region where the exhaust gases are the hottest are made by using nickel super alloys.

The choice and use of super alloys are usually based on their properties.  Taking this factor into account, the aero-engine turbine blade and discs are made from nickel super alloys. However in the nickel based turbine blades, high temperature service causes change in microstructure that results into wide damage of mechanical characteristics. The blocky plate carbides have rectangular edges are observed at the grain boundaries. The number of gamma prime particles widely decreases. The gamma prime denuded area occurs near the grain boundary. With increase in temperature, level of magnitude of gamma prime particles reduces significantly. Eventually this loss is more severe when the greater cooling rate was performed. Creeping resistance decreases with increase in temperature.

Directional and monocrystal solidification

Modern techniques are followed for metal processing to develop the latest alloys. Directional solidification is the most important processing technique. Unlike to the multi-crystalline solidification, the directional solidification involves preheating of mold to a temperature limit similar to the molten metal, the smaller mold part is linked to water cooled chill plate. It is placed in a hot region shielded by insulated heat baffles. The melt is poured into the mold and begins to crystallize in the quenched plate area. The complete mold is then nominally decreased and taken, bottom before and from the warm area. Normally, many small sized individual crystals develop and randomly create at the copper quenched plate combine with enhanced columnar area of grains making perpendicular to the quenched plate.

To develop a mono crystal, a melt is dispensed into a ceramic mold that comprises of a pig tail shaped chooser among the chill plat and upper area of the mold. When the mold is extracted from the heat baffles, the columnar grains begin to develop, however the selector is tight that just a crystal will be developed around it.

As the mold increases the selector, a crystal becomes large in diameter is the single to make into the mold and therefore the final sample will be developed of a single crystal with a featured dendritic structure.

The standard casting is made of many crystals randomly oriented, in directionally solidified blade columnar grains are parallel to the vertical axis of the blade and many grains are significantly reduced, mono crystal blade doesn’t consist of grains. The results of stress-rupture service evaluating the Inconel super metal describe the significant improvement in creeping resistance when the magnitude of grain limits is reduced and is parallel to unaxial stress or actual completely prevented. The enhanced gamma phase during heat processing improves the stress rupture resistance.

The stress rupture service of the mono crystal castings was longer as compare to the stress life of the directionally solidified or specifically the conventionally cast material. This result states extended stress rupturing resistance of the mono crystal that is mentioned as the result of absence of grain limits as weak areas in the structure. The stress rupture life following the single crystal of Inconel castings are the small several times more than the previously analyzed in the evaluation.

Cobalt super alloys

Nickel based super alloys describe drawbacks at the very high temperatures and hence components in the combustion unit that interact with high temperature about 1100oC are usually made from cobalt based alloys.

The cobalt super alloys do not have strength higher than nickel super alloys however they maintain the good strength at the high temperatures. They maintain strength significantly from the dispersion of refractory metal carbides of carbon that try to collect at the grain limits. The carbides matrix strengthens the grain boundaries making alloy stable up to its melting point. In addition of refractory metals and carbides, the cobalt super alloys usually keep high levels of chromium that offers greater corrosion resistance that normally occurs in the presence of warm exhaust gases. Chromium interacts with oxygen to develop a layer of chromium oxide that protects the alloy from being attacked.

Cobalt bas superalloys are slightly harder than nickel based alloys and they are resistant to cracking in the hot shocks opposite to other super alloys. The cobalt based super alloys are fitter for parts that required to be processed or welded like those in the intricate structures of the burning component.

Aerospace and Land turbines: The cobalt based super alloys are fit for offering resistance to high temperature and fatigue in the non-rotating services that have smaller stress levels below the moving components. Hence turbine vanes and other static non-moving components are quickly designed by using cobalt alloys.

These alloys keep smaller coefficient of thermal expansion and improved heat conductivity rather nickel based super alloys therefore cobalt based super alloys that include heat fatigue as the major issue. Offering prolong service life, land based casting components use cobalt base alloys for more severe media rather aircraft engine components.

Medical Plants:  The alloys are used in orthopaedic implants, commonly used as artificial hips and knees. It is usually named ASTM F75 comprising of chromium 29% and molybdenum 6% with carbon concentration is about 0.35%. An addition of nitrogen has offered cobalt alloys to receive high strength with improved ductility even without consisting of corrosion resistance. The cobalt alloy implants are consisting of casting and forging.

Titanium Alloys

The outstanding strength, lightweight, better corrosion resistance offered by titanium and its alloys have made them supreme in the extensive range of industrial applications that require vast levels of reliable service in aerospace, automotive, chemical plants, power units, oil and gas development, sports and medical plants.

Nickel and cobalt super alloys, titanium have smaller density and keep higher strength to weight ratio for temperatures below 500oC. The titanium alloys are weak at temperatures below than half of their melting temperature, superalloys sustain their strength closest to their melting temperatures, unlike to superalloys sustain their strength closest to their melting temperature. In some applications high strength and consistency at high temperatures are not important, and the weight of every component is important. In the gas turbine engines where temperatures and pressures are moderate, the titanium alloys have significant applications.

Important characteristic of titanium alloys is the reversible conversion of crystalline structure from alpha hexagonal close packed to beta body centered cubic structure at temperatures above the specific level as named as transus temperature. This allotropic nature is based on the chemistry of alloy, sustains complex level in the micro-structure and further different reinforcing as compare to various non-ferrous alloys.

Important characteristic of titanium alloys is the reversible conversion of crystalline structure from alpha hexagonal close packed to beta body centered cubic structure at temperatures above a specific level are named as transus temperature. This allotropic nature depends on the chemistry of alloy, sustains complex change in the microstructure and further different reinforcing as compare to the various non-ferrous alloys.

The requirement for application of titanium and its alloys in the various sections of military and domestic services have been increasing in the recent years because of demand for lightweights. The expensive titanium and application of overall shape and techniques are getting a wide interest considering the economic potential of this method in the development of components of complicated shapes. Precision casting is completely developed net shape method as compare to powder metallurgy, superplastic producing and rolling. The major issues in the development of titanium and titanium alloy casting are high melting temperatures, extreme reactivity of melt with solids, liquids and gases at the high temperatures.