**Shi-Hao Li, Jing-Ting Li and Wei-Zhong Han \***

Center for Advancing Materials Performance from the Nanoscale (CAMP-Nano), State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, China; lsh4007025@gmail.com (S.-H.L.); lijingting235@sina.com (J.-T.L.)

**\*** Correspondence: wzhanxjtu@mail.xjtu.edu.cn

Received: 6 March 2019; Accepted: 26 March 2019; Published: 28 March 2019

**Abstract:** Helium (He) bubbles are typical radiation defects in structural materials in nuclear reactors after high dose energetic particle irradiation. In the past decades, extensive studies have been conducted to explore the dynamic evolution of He bubbles under various conditions and to investigate He-induced hardening and embrittlement. In this review, we summarize the current understanding of the behavior of He bubbles in metals; overview the mechanisms of He bubble nucleation, growth, and coarsening; introduce the latest methods of He control by using interfaces in nanocrystalline metals and metallic multilayers; analyze the effects of He bubbles on strength and ductility of metals; and point out some remaining questions related to He bubbles that are crucial for design of advanced radiation-tolerant materials.

**Keywords:** helium bubbles; bubble evolution; interfaces; radiation hardening; helium embrittlement
