1. When broken, the material has a black, silken fracture.
broken - Participle II,
функция - обстоятельство времени
2. Iron oxide is the major problem when forging steel.
forging - Participle I,
обстоятельство времени
3. The film, made by Stephen Spielberg, is expected to be a great hit.
made - Participle II, определение
expected - Participle II, часть сказ, сказ стоит в форме Present Simple Passive
4. They watched the forest helplessly.
Причастия НЕТ!
watched - глагол-сказуемое в Past Simple
5. Owing to these methods it has become possible to obtain new mechanical properties of the alloy.
Owing - Participle I, обстоятельство
become - Participle II, часть сказуемого, сказуемое стоит в форме Present Perfect Active