$(0\mathrm{.}3$p$)$ Figure $2$ shows the binary Fe$-$Ni phase diagram.

a$)$ Which phases are in equilibrium and what are the amounts and compositions of these phases when $T=200C$ and $x_{Fe}=0\mathrm{.}09?$

b$)$ Describe the changes in phase equilibria from $1000C$ down to $200C$ when the nominal composition of the $Fe-Ni$ alloy is $($i$)$ xNi$=0\mathrm{.}49($ii$)$ xNi$=0\mathrm{.}10.$

Figure $2.$ Fe$-$Ni phase diagram.