Why during machining of light weight or soft material the addition of alloying elements provides good surface finish as compared to base material (Pure form of metal).
It makes the material harder and thus less deformable. Thus the effect of the tool on the substrate material is less.
When u start the machining process of a soft material (imagine a plastic rod) it will start plastic deformation on the surface instead of fracture on it so your surface quality decreases! the main reason of that phenomena comes from their lower elastic module so their module have to increase to increase the machinability and It will increase by adding allowing elements, on the other hand by increasing Elastic module your hardness will increase too.
Additions of alloying elements due to diffusion mass transfer will in fact be accompanied by the appearance of a concentration gradient along the diameter of the work piece. Then the effect that is expected in volumes near the surface, for more distant ones, may not be fulfilled. On the other hand, for example for steels, special alloying with sulfur leads to the formation of a low-melting eutectic, which contributes to the improvement of the quality of the surface during turning operations.
On the question under consideration: in contrast to alloying, the issue of local temperature reduction on the surface of the work piece can be considered. Or a certain degree of supercooling of the entire part to such a temperature to compensate for possible heating during the cutting process.
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