Discuss any two of the following: (i) The difference between an interstitial solid solution and a substitutional solid solution; (ii) A cooling curve for an alloy; (iii) One example of metal refining in Ireland; (iv) The meaning of the term solvus - Leaving Cert Engineering - Question 4 - 2012

A cooling curve displays the temperature of an alloy as it transitions from the liquid phase to solid state over time. The curve should show a plateau during the solidification process, indicating that while heat is being released, the temperature remains constant until solidification completes. The start point indicates where crystallization begins, and the end point shows when solidification is complete.

Aluminium refining is an important process in Ireland, especially at the Shannon estuary where significant operations occur. The process involves electrolytic reduction of bauxite to produce pure aluminium, which is vital for various industries.

The term solvus refers to the solvus line on a phase diagram, which defines the limits of solid solubility between two components. It indicates temperatures and compositions at which solid solutions can exist, marking the transition from one solid solution to another.

To draw the thermal equilibrium diagram, plot the % of Cadmium in the alloy on the x-axis and the temperature (°C) on the y-axis. Mark the start and end of solidification temperatures for each alloy composition, connecting the points to visually represent the liquidus and solidus lines. The eutectic point should also be indicated where the lowest melting point occurs.

In the thermal equilibrium diagram, the liquidus line separates the liquid and solid phases, while the solidus line indicates where the alloy is completely solid. The regions labeled as 'Liquid', 'Solid Cadmium', and 'Bismuth and Liquid' should indicate the phase behavior for different compositions. The eutectic point represents the composition that solidifies at the lowest temperature, offering insights into the alloy's thermal properties.

A eutectic alloy is a specific mixture of two or more metals that solidifies at a single, unique temperature, lower than the melting points of its individual constituents. At the eutectic composition, the system undergoes a phase change directly from liquid to solid without passing through a pasty phase.

The diagram displays a line defect known as a dislocation. Dislocations are line defects where atoms in the crystal lattice are not in the proper position. This imperfection allows the crystal to deform more easily under stress, impacting mechanical properties such as yield strength.

When a shear force is applied, the dislocation can move through the crystal lattice. This movement facilitates plastic deformation in metals, as atoms in the slip plane shift to relieve stress. The ease of this movement is critical to the ductility and hardness of materials, greatly influencing their mechanical performance.