Almost all alloying elements (except Al, Co, Si) increases the stability of super cooled austenite and retard both proeutectoid and the pearlitic reaction and then shift TTT curves of start to finish to right or higher timing.

This is due to-

i) Low rate of diffusion of alloying elements in austenite as they are substitutional elements,

ii) Reduced rate of diffusion of carbon as carbide forming elements strongly hold them.

iii) Alloyed solute reduces the rate of allotropic change, i.e. γ→α, by solute drag effect on γ→α interface boundary.

Ni, Mn, Ru, Rh, Pd, Os, Ir, Pt, Cu, Zn, Au these elements stabilize the austenite and depress the position of TTT curve to lower temperatures.

Be, P, Ti, V, Mo, Cr, B, Ta, Nb, Zr these elements on contrast favours ferritic phase and increases eutectoid temperature n moves the TTT curve to high temperature.

Here aluminium cobalt and nickel increases the rate of nucleation and growth of ferritic and perlitic phase which shifts the curve to left.

The effect of alloying elements is less pronounced in bainitic region as the diffusion of only carbon takes place in a very short time after supersaturated ferrite formation by shear during bainitic transformation and there is no need for redistribution of mostly substitutional alloying elements.

Addition of alloying elements lead to a greater separation of their actions and result separate C-curves for pearlitic and bainitic regions.