Grain refinement and morphological control of intermetallic compounds: A comprehensive review

Intermetallic compounds (IMCs) are ordered solid-state compounds formed from chemical reactions between two or more metals exhibiting distinctive crystal arrangements and precise stoichiometric ratios, setting them apart from the matrix of the alloys. In general, IMCs are formed in three configurations: In the form of secondary phase precipitates distributed within the matrix phase, in the form of an IMC alloy, and at the bimetallic interfaces of functionally/transitionally graded structures. However, the IMCs as precipitates in the matrix phase, do not possess many challenges and are often desirable to improve the strength by imparting precipitation hardening. But, in the case of IMC alloys and bimetallic structures, the grain size and morphology of IMCs directly influence the integrity and durability of the developed structure. Given the inherent brittleness of most IMCs, the utilisation of IMCs in critical applications is substantially restricted. In response to this long-standing challenge, there has been extensive research into methods for improving the ductility of IMCs. This review emphasises two key methodologies: solidification-based and non-solidification-based approaches, both aiming to enhance IMC’s mechanical properties either by transitioning large to smaller grain microstructure or dendritic to equiaxed morphology. Solidification-based strategies, including heterogeneous nucleation and external-field-induced morphological alteration like the use of ultrasonic vibration, magnetic, and electric fields, are meticulously evaluated, uncovering research gaps. Non-solidification-based methods like severe plastic deformation and mechanical alloying are critically examined on the suitability of modern manufacturing techniques such as additive manufacturing. Among these, ultrasonic vibration emerges as the most promising for IMCs morphological transformation. Although static magnetic and electric fields exhibit potential, further investigation is required. Despite knowledge gaps, these techniques hold the potential to elevate IMC-containing alloy characteristics. Future research, especially for specific IMC groups and emerging manufacturing processes, is encouraged to propel metallurgical grain refinement or morphological transformation. In addition, the current and emerging application of various IMCs are thoroughly discussed to identify the importance of IMCs in various science and engineering domains. This comprehensive review enhances comprehension of IMC-based grain alteration, paving the way to design advanced materials across various applications.