Phase relations in the system Fe–Ni–Si to 200 GPa and 3900 K and implications for Earth's core

•Phase relations in the system Fe–Ni–Si were investigated to 200 GPa and 3900 K.•An internally resistive heated diamond anvil cell was used.•The fcc–hcp transition boundary has a steeper dP/dT slope than in pure Fe.•The triple point for fcc, hcp, and liquid is placed at 145 GPa and 3750 K.•The melting point of Fe–Ni–Si hcp at 330 GPa lies at 550 K lower than in pure Fe.