How is the specific heat of an alloy obtained?
The enthalpies and calorific capacities of certain mixtures are tabulated in standard references. If you do not have such data, you can use the following approximation:
  
  Rule 1: For a mixture of gases or liquids, calculate the total change in enthalpy as the sum of its changes in the pure components of the mixture. In fact, the enthalpy changes associated with component mixing are neglected, which is an excellent approximation for gas mixtures and similar liquid mixtures such as pentane and hexane, but is bad for different liquids, such as nitric acid and Water.
  
  Rule 2: For highly diluted solutions of solids or gases in liquids, neglect the change in enthalpy of the solute. This approach is better, as the solution is more diluted.
  
  The calculation of the enthalpy changes for heating or cooling a known composition mixture can often be simplified by estimating the calorific capacity of the mixture as follows:
  \begin{equation}
    C_{p,m}(T) = \sum_i y_i\cdot C_{p,i}(T)
\end{equation}
  
  Where:
  
  $C_{p,m}(T)$ Calorific capacity of the mixture
  
  $y_i$ Mass or mole fraction of the i-th component
  
  $C_{p,i}(T)$ Calorific capacity of the i-th component


Reference:

Richard Mark Felder and Ronald W. Rousseau. Elementary Principles of Chemical Processes. John Wiley &amp; Sons Inc.; 3rd Revised edition. 702 pages.