Question

In the Fe–C system, the invariant reaction \( \text{Liquid} + \delta \rightleftharpoons \gamma \) occurs at \(1493^{\circ}\text{C}\). This reaction is called _____.

• A. eutectic
• B. eutectoid
• C. peritectic
• D. monotectic

Hint:

Mnemonic: \(\underline{\text{Peri}}\)tectic involves a \underline{pair} on the left (liquid + solid) forming a new single solid; eutectic is \(L \to 2\,\text{solids}\); eutectoid is solid \(\to 2\,\text{solids}\); monotectic includes two \emph{liquids}.

Answer 1

The Correct Option is C

Step 1: Identify phase types on each side.
Left: liquid \(L\) + solid \(\delta\) (bcc \(\delta\)-ferrite). Right: solid \(\gamma\) (fcc austenite). Step 2: Match with invariant-reaction definitions.
- Eutectic: \(L \rightarrow \alpha + \beta\) (liquid to two solids).
- Eutectoid: \(\alpha \rightarrow \beta + \gamma\) (solid to two solids).
- Peritectic: \(L + \alpha \rightarrow \beta\) (liquid + solid to a different solid).
- Monotectic: \(L_1 \rightarrow L_2 + \alpha\) (liquid to liquid + solid). Step 3: Conclude.
Since \(L+\delta \Rightarrow \gamma\), it is a peritectic reaction at \(1493^\circ\text{C}\). \[ \boxed{\text{Peritectic reaction}} \]