Let's analyze the question step by step: ### **Given:** - **Iron-carbon alloy of eutectoid composition** (0.76 wt% C) - **Heat treatment:** 1. Rapidly cooled to 500°C, held for 5 seconds 2. Rapidly cooled to 300°C, held for 10,000 seconds 3. Quenched to room temperature - **Initial condition:** Specimen begins at 760°C (homogeneous austenitic structure) --- ### **Steps for Solution:** #### **1. At 760°C** - The alloy is entirely **austenite** (γ-phase). #### **2. Cooling to 500°C (hold for 5 s)** - At 500°C, we are just above the temperature where **bainite formation** starts (on a TTT diagram). - Holding for only 5 seconds is **not enough time** for significant transformation (neither pearlite nor bainite forms significantly at this short time). #### **3. Cooling to 300°C (hold for 10,000 s)** - At 300°C, **bainite** formation is favored (as per the TTT diagram for eutectoid steel). - Holding for 10,000 s is **more than enough** to complete the transformation of austenite to bainite. #### **4. Quenched to room temperature** - Any remaining austenite would transform to martensite, but since most, if not all, austenite has already transformed to bainite during the long hold at 300°C, there will be almost no martensite. --- ### **Final Microstructure:** - **100% Bainite** (since the austenite completely transforms to bainite during the long hold at 300°C). --- ### **Correct Answer:** > **100% is bainite.** **(Option 4)** --- If you need an explanation or the TTT diagram for this process, let me know!