Considering precipitation hardening in $Al-Cu,$ answer the following questions: $(5$ total points$)$

a$.$ What $2$ characteristics must a system have in order for it to be precipitation strengthened? $(2$ points$)$

b$.$ Why do metastable phases form during precipitation in this system? $(1$ point$)$

c$.$ If we wanted to form the stable $$ precipitate, how might we change our heat treatment $($temperature & time$)?$ How might this impact strength? $(2$ points$)$