I observed in AES depth profile of some binary metal alloys(AB , B as solute), segregation of solute occurs . Is it only surface energy plays the role in driving the segregation ? i e lower surface energy component will segregate to surface.I also read atomic ordering , i e AA,BB,AB bonding inside the bulk and surface also has an influence on segregation. The system tries to attain the lowest free energy segregation occurs.I have seen some sub surface or near surface segregation of solute in some alloys also , how is it happening.Is it because when the segregant reaches the surface the component with more negative free energy for forming oxide(If it is the solvent metal) excludes or push the segregant back beneath the oxide layer and segregation enriched in that subsurface layer ? If any body can explain the predominant mechanisms associated with this and why diffusion and segregation are different process it will be so helpful
A number of factors operate of which you've identified the Gibbs free energy of the individual components. Another factor is chemisorption induced segregation in which the oxygen of the air preferentially pulls one component to the surface and thus enriches the surface in that component. One example in Pd-Au alloys where the surface is enriched in Pd due to the greater affinity of Pd toward oxygen in comparison to the Au - oxygen affinity. See slides 30 and 39 onwards in:
Nov 11th, 2008. Dispersion and nanotechnology http://tinyurl.com/hpywsge
You also have to look at the relative sizes of the components size - gold diffuses quicker in lead than lead in lead (radioactively labeled lead shows the latter). See:
August 13th, 2015 The importance of the measurement of diffusion in 2-phase systems
https://www.brainshark.com/malvern/vu?pi=65736585&b=1&tx=120486&c1=22
ESCA/XPS and Auger are vital in showing the surface (5 - 10 atomic layers at most) is never the same as the bulk for 2 component systems.
Thanks Dr Rawle for the information.So in binary alloys like Sn-Pb solders chances for segregation is more as Pb atoms are bigger? Is this segregation associated with any stress relaxation in thin films?
Hello Sherin I think you have misinterpreted my answer. In any binary system in air the bulk and surface (top 5 - 10 atomic layers) compositions MUST be different - you can should show this by ESCA/XPS. There are no 'chances of segregation' - this segregation is as certain as night follows day. The size of a lead atom is larger than a tin one and thus the lead would tend to diffuse through the grain boundaries and be predicted to be on the surface from that perspective - look at the solid-solid diffusions in the second web link in my first answer above The delta H's/enthalpies for the formation of PbO2 (- 277 kJ/mol) and SnO2 (- 578 kJ/mol) indicate that lead oxide is formed more easily too. On the above bases, one could predict that one would expect Pb to be concentrated at the surface in the same manner as Pd in Pd-Au (carefully examine and study slide 41 in the first web link in my earlier reply). This conjecture would need to supported by experimental work.
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