"a will of iron" is a common expression in English and is correct and usable in written English.
You can use it to refer to someone who has an unyielding determination and strength of character, such as: "She faced every challenge with a will of iron."
Whiteness has invaded her eyes, muscles have taken over her face, a will of iron behind a face of steel.
Mr Martinez, a man with a will of iron and a disarming grin, complains bitterly of county inefficiency.
Below his litanies of humble thank yous is a will of iron, along with a streak of avarice.
We feel the terrible vulnerability of the frail man with a will of iron; his torments at the hands of grotesque oppressors hurt us too.
You may have nerves of steel but they'll be no match for a will of iron, especially if iron has entered your adversary's soul.
In practice, that would take a will of iron, and the few people who have such willpower rarely get fat in the first place.
Freudian prognoses (the young Adi's witnessing of the primal scene as the source of his father-hate) mingle with cod psychology (his "will of iron" is forged by his early home above a smithy).
All the hallmarks of Violet's character — the implacable cruelty, the shrill self-pity, the wily manipulation and the will of iron — are present and accounted for in Ms. Parsons's superb performance.
He's the man with the will of iron and limbs of elastic, the man who could walk through walls on court and the man who stares defeat in the face and laughs right back at it.
— It takes a will of iron and a warm jacket to carry on as a former president.
You will find a lot of iron on the way, and iron will speed up the process.
