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\title{Superalloys: An Introduction with Thermal Analysis}
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             \author[1]{Syed Shehzad  Raza}

             \affil[1]{  Allama Iqbal College of Technology}

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\date{\small \em Received: 10 April 2015 Accepted: 1 May 2015 Published: 15 May 2015}

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\begin{abstract}
        


Nickel based superalloys are commonly used materials in the aero industry and more specifically in the hot section of aero engines. These nickel and nickel iron based superalloys are precipitation strengthened alloys with a face centered cubic gamma matrix. Alloy 718, Allvac 718Plus and Waspaloy have been of great interest in the present study. Alloy 718 is a precipitation strengthened nickel-iron based alloy having gamma double prime phase (Ni3Nb) as a main strengthening phase up to 650 °C. Waspaloy, another precipitation strengthened nickel base superalloy, has a very good strength at temperatures up to ~750 °C whereas Allvac 718Plus is a newly developed nickel based precipitation strengthened superalloy which retains good mechanical properties at up to ~700 °C. These three alloys were investigated in terms of how their respective solidification process reveals upon cooling.Latent heat of soloidification has been estimated for all three alloys. Differential thermal analyses (DTA) have been used to approach the task. It was seen that Waspaloy has the smallest solidification range whereas Allvac 718Plus has the largest solidification interval in comparison.

\end{abstract}


\keywords{superalloys, phase reactions, differential thermal analysis, latent heat of solidification.}

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\let\tabcellsep& 	 	 		 
\section[{Introduction}]{Introduction}\par
superalloy is a type of alloy that retains excellent mechanical properties at elevated temperature (super good properties at elevated temperatures). This includes excellent creep resistance, corrosion and oxidation resistance. The base metal is usually nickel, cobalt or nickel-iron. These alloys are commonly used in the aerospace or gas turbine industry, in parts that requires excellent mechanical properties at elevated temperatures. Superalloys are also utilized in the chemical and petrochemical industries apart from the gas turbine industry. In aircraft gas turbine components these superalloys may find their applications in disks, bolts, shafts, cases, blades, vanes, combustors, afterburners etc (basically anywhere where the temperature is above \textasciitilde 500 °C). 
\section[{a) Applications}]{a) Applications}\par
The major applications of superalloys are categorized below; the bulk of tonnage is used in gas turbines:\par
? In steam turbine power plants bolts, blades, stack gas re-heaters are made from these alloys. ? In reciprocating engines; turbochargers, exhaust valves, hot plugs and valve seat inserts are made from these alloys. ? During metal processing; hot-work tools and dies and casting dies are made from these alloys. ? In medical applications, dentistry uses and prosthetic devices are made from these alloys. ? In space vehicles: aerodynamically heated skins and rocket engine parts are made from these alloys. ? In coal gasification and liquefaction systems they are used in heat exchangers, re-heaters and in piping systems. 
\section[{b) Chemical Composition}]{b) Chemical Composition}\par
The nickel-base superalloys discussed below are considered to be complex because they incorporate as many as a dozen of elements. In addition, deleterious elements such as silicon, phosphorus, sulfur, oxygen, and nitrogen must be controlled through appropriate melting practices. Other trace elements such as selenium, bismuth, and lead, must be held to a very small (ppm) levels in critical parts. Many wrought nickelbase superalloys contain 10 to 20\% Cr, up to about 8\% Al and Ti combined, 5 to 15\% Co, and small amounts of boron, zirconium, magnesium, and carbon. Other common additions are molybdenum, niobium, and tungsten, all of which play dual roles as strengthening solutes and carbide formers. Chromium and aluminum 
\section[{c) Microstructure}]{c) Microstructure}\par
The major phases that may be present in nickelbase alloys are:\par
? Gamma matrix, ?, in which the continuous matrix is an FCC nickel-base nonmagnetic phase that usually contains a high percentage of solid-solution elements such as cobalt, iron, chromium, molybdenum, and tungsten. All nickel-base alloys contain this phase as the matrix. ? Topologically close-packed (TCP) type phases, which are plate-like or needle-like phases such as ?, and ? that may form for some compositions and under certain conditions. These cause lowered rupture strength and ductility. 
\section[{d) Strengthening Mechanisms}]{d) Strengthening Mechanisms}\par
The strengthening mechanisms in superalloys are usually governed by solid solution and/or precipitation strengthening \hyperref[b0]{[1]}. These alloys can be used up to a higher fraction of their melting points than any other commercially available alloy system. Refractory materials have higher melting points than superalloys but don't have the desired characteristics related to its chemistry but also to the primary melting, forming and casting techniques. Heat treatment procedures may considerably improve the properties. Many alloying elements are added to these alloys, all with different purposes, and these may be as many as 14 in some cases \hyperref[b1]{[2]}. 
\section[{e) Phase Transformations}]{e) Phase Transformations}\par
During the solidification process of the present nickel and nickel iron base superalloys, there are three main reactions:\par
1. Liquid ? Gamma 2. Liquid ? Gamma + MC 3. L ? Gamma + Laves (not applicable for Waspaloy)\par
In Alloy 718 the first thing to take place apart from nucleation of nitrides is the Gamma matrix phase to nucleate. During solidification the liquid becomes enriched in niobium and carbon which result in formation of a non-invariant Gamma/MC eutectic reaction proceeding with more enrichment of Nb while depleting C resulting in a final reaction, Liquid through Gamma/Laves eutectic reaction which is not encountered in Waspaloy \hyperref[b2]{[3]}. 
\section[{Experimental Part}]{Experimental Part} 
\section[{II.}]{II.}\par
Test Procedure and Experimental Setup a) Differential Thermal Analysis Setup Thermal analysis is a method used to measure a physical property of a material as a function of its temperature. The obtained measurements are the basis for calculations of thermodynamic properties such as enthalpy and specific heat.\par
Through differential thermal analysis (DTA) studies it is possible to determine phase reactions and solidification phenomenon of alloys. The heat loss to the surrounding and the influence of the thermocouple wires can more or less be neglected \hyperref[b3]{[4]}. 
\section[{b) Experimental Methodology}]{b) Experimental Methodology}\par
DTA experiments have been carried out on Alloy 718, Allvac 718Plus and Waspaloy. Each thermal cycle had a heating, soak and cooling part in which the samples have been heated up to the liquidus temperature, soaked at that temperature and then cooled down to a temperature below the solidus temperature. The starting temperature was set to 25 ºC and 1400 ºC as the maximum soak temperature since all of the present alloys were fully liquid at this temperature.\par
The samples were subjected to thermal cycle with cooling rate of 6 K/min. The soak temperature was 1400 °C for a dwell time of \textasciitilde 300 s. This dwell time was and consequently not as widely used. It should be noted that the superb strength of superalloys are not only selected in accordance with a previous study on Alloy 718 \hyperref[b4]{[5]}.\par
the sample where after the sample was placed in an alumina crucible which was covered by a ceramic lid. Another crucible and thermocouple was used for the graphite reference. The whole sample assembly was covered by a ceramic shielding tube to ensure protection from contamination.\par
At testing, the power was switched on and start mode was initiated. The recording unit was set to heating mode. Argon and water was tapped on before starting experiments. When the temperature reached the maximum limit, after 300s of dwell time, cooling mode was initiated whereas the sample cooled to room temperature. Argon inert gas protection and water cooling were shut off and stop mode was initiated by end of experiment. The recorded data was collected and exported to an excel data sheet. Two different graphs were derived; one graph revealing the reference and furnace temperature and a second one showing their respective differential curves for analyzing the experiments. 
\section[{c) Experimental Analysis}]{c) Experimental Analysis}\par
Thermal analysis is generally carried out to investigate the melting, solidification and phase reactions for different alloys \hyperref[b5]{[6]}. Cooling curves can be used to determine the liquidus, solidus temperatures and also the total time of solidification \hyperref[b6]{[7]}.\par
The cooling curves are generally presented in a temperature and time plot. A change in slope of the curve indicates a phase reaction. Different cooling rates influence phase reaction temperatures and can be analyzed by plotted thermographs \hyperref[b3]{[4]}.\par
Calculation of the heat of fusion for a sample is based on the law of energy. The law of energy gives, for the case when no phase transformation is going on, i.e. before and after the solidification process. The heat of fusion can be estimated by the following equations: dQ l /dt=V? l C p l [dT l /dt] (1) dQ/dt= V[? l (1-f s ) C p l [dT l /dt]] +? s [f s C p s dT s /dt+?H df/dt] (2) [dT s /dt] ? C p s V s + ?H ?V s df/dt= h[T s -T f ]\textbf{(3)}dQ s /dt= dQ l /dt ?V? l C p l [dT l /dt]=V[? l (1-f s ) C p l [dT l /dt] + ? s [f s C p s dT s /dt + ?H df/dt]]\textbf{(4)} 
\section[{d) Determining the Latent Heat of Solidification}]{d) Determining the Latent Heat of Solidification}\par
The latent heat is determined for respective alloys through four different steps, as follows:\par
1. To determine the area underneath the thermograph 2. To determine the cooling rate 3. To determine the temperature difference in between T s and T ref 4. Finally perform the calculation using equation no. \hyperref[b4]{5} above 
\section[{e) Estimation of Latent Heat of Solidification}]{e) Estimation of Latent Heat of Solidification}\par
The heat of fusion means the amount of energy needed to melt a unit mass or a mole of the substance; i.e. the total amount of energy needed to break the bonds between the atoms in a crystal lattice.\par
The heat of fusion values are lower for Alloy 718 and higher for Waspaloy and Allvac 718Plus. The cooling rates used in present investigation have not affected the values. The latent heat value for Alloy 718 is on a lower side as investigated in literature. For Alloy 718 Hasse and Antonsson estimated the value to 170 KJ/Kg \hyperref[b4]{[5]}.The estimated values and the respective intervals are reported in table \hyperref[tab_2]{1}.\par
Before testing, all samples were weighed. The ceramic tube was cut by a high speed steel cutter to fit  
\section[{Global}]{Global} 
\section[{Results and Discussions}]{Results and Discussions}\par
The solidification sequence of the investigated nickel based superalloys starts with a primary precipitation of ?-phase, followed by one or two final reactions. The first reaction to take place after primary ?phase is the formation of MC precipitates which in turn is followed by presumably Laves phase in Alloy 718 and Allvac 718Plus. The evaluation is based on the temperature difference between the sample and the reference, as a function of the temperature of the sample. Figure1 shows the DTA thermograph for Alloy 718 during the cooling sequence as plotted in Microsoft Excel. The differential curve is denominated by the right hand side axis. An abrupt deviation in the slope of the differential curve is around \textasciitilde 1325 ºC and is disclosed at a cooling rate of 6 °C/min. This is most probably associated with the primary nucleation of the ? phase and the start of solidification for Alloy 718.\par
The second noticeable deviation (a peak) is around 1250 ºC for Alloy 718 which corresponds to the precipitation of MC as shown in figure1. The final precipitation event; i.e. the precipitation of Laves phase occurs below 1170 ºC depicted by a solid hump in figure1.   The thermographs for Waspaloy as revealed in figure \hyperref[fig_1]{3} are smoother in comparison with Alloy 718 and Allvac 718Plus. It is therefore not as easy to determine any relevant phase reaction taking place during the cooling sequence. However, apart from the primary ? phase revealed by the main peak in the thermograph a small indication of what is presumable believed to be MC is disclosed in figure \hyperref[fig_1]{3}.  \begin{figure}[htbp]
\noindent\textbf{3}\includegraphics[]{image-2.png}
\caption{\label{fig_1}3 )}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{1}\includegraphics[]{image-3.png}
\caption{\label{fig_3}Figure 1 :}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{2}\includegraphics[]{image-4.png}
\caption{\label{fig_4}Figure 2 :}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{3}\includegraphics[]{image-5.png}
\caption{\label{fig_5}Figure 3 :}\end{figure}
  \begin{figure}[htbp]
\noindent\textbf{1} \par 
\begin{longtable}{P{0.5543478260869565\textwidth}P{0.08869565217391304\textwidth}P{0.12565217391304348\textwidth}P{0.08130434782608696\textwidth}}
.Parameter\tabcellsep Alloy 718\tabcellsep Allvac 718Plus\tabcellsep Waspaloy\\
Latent heat of solidification [kJ/kg]\tabcellsep 152\tabcellsep 346\tabcellsep 227\\
Solidification interval [°C]\tabcellsep \tabcellsep \tabcellsep \end{longtable} \par
 
\caption{\label{tab_2}Table 1 :}\end{figure}
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\subsection[{Acknowledgments}]{Acknowledgments} 			  			  				\begin{bibitemlist}{1}
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\end{document}
