In Situ Hybrid Aluminum Matrix Composites: A Review of Phase Transformations and Mechanical Aspects Amir Azarniya, Amir Abdollah-zadeh,* Hamid Reza Madaah Hosseini,* and Seeram Ramakrishna The growing industrial needs for the development of strong load-bearing materials by powder metallurgy and casting technologies has led to recent progress in the synthesis of in situ hybrid aluminium matrix composites (AMCs). Unlike their conventional counterparts, this class of engineering materials and their physicomechanical properties are sparsely investigated with no satisfactorily systematic approach and are not reviewed up to now. This is why providing an overview summarizing the formation mechanisms of in situ phases and mechanical properties of hybrid AMCs can systemati- cally guide the research path in this field. The present review strives to categorize hybrid AMCs based on their dominant in situ formed reinforce- ments and draw a comparison between different fabrication methods and used starting materials. Finally, the present challenges and potential solutions are addressed both from scientific and technological aspects. 1. Introduction Over the past few decades, there has been a growing interest in the development of reliable processing routes for the fabrication of Al matrix composites (AMCs) with desirable physicomechanochemical properties for various applications from automotive and aerospace to sports and electronic packaging. [1,2] Both solid and liquid state processes are employed to synthesize these materials among which casting, [3] powder metallurgy, [4] hot pressing (HP), [5] hot extrusion, [6,7] solid-state aluminothermic reaction, [8] friction stir processing (FSP), [9] hot rolling, [10] accumulative roll bonding (ARB), [11] tungsten inert gas (TIG) welding, [12] combustion synthesis, [13,14] and spark plasma sintering (SPS) [15,16] are the most significant. There are two main approaches to fabricate AMCs through above-mentioned methods, i.e., ex situ and in situ routes. The ex situ approach, recognized by the addition of externally prepared particles to the matrix, is conventionally characterized by high- porosity content, large grain size, non- uniform dispersion, particles agglomera- tion, weak interfacial adhesion, and unfavorable interfacial reactions. [16,17] These drawbacks can be mostly avoided by taking the in situ synthesis method in which the reinforcing particles are pro- duced within the matrix through the chemical reactions during the process. [18] The in situ formed phases such as Al 2 O 3 , TiC, TiB 2 , SiC, and Al 3 Ti possess out- standing physicomechanical properties including low-coars- ening rate at elevated temperatures, strong interfacial bonding with Al alloy matrix, excellent load bearing capacity, and satisfactory oxidation and corrosion resistance. [19–23] To benefit from the synergistic effects of different phases in AMCs, hybrid composites have been developed with two or more kinds of in situ particles. Al 2 O 3 , Al 3 Ti, and TiB 2 are the most prevailing in situ formed phases to fabricate Al-based in situ hybrid composites. Initial materials, additives, and alloying elements used for producing the in situ phases, and types of in situ formed phases for Al 2 O 3 -/Al 3 Ti-/TiB 2 - containg hybrid composites are summarized in Table 1. In this paper, the microstructure evolution and reactive mecha- nism of in situ phases formation and their mechanical properties are separately discussed for each system in full details. 2. Hybrid Al 3 Ti-Containing AMCs 2.1. Al–Ti System 2.1.1. Al 3 Ti Formation Mechanism The Al–Ti system is one of the most promising and advantageous systems for the fabrication of Al–Al 3 Ti composites by using liquid-sate or powder metallurgy routes. [4] In the Al–Ti solid-state system, the reactive diffusion between Al and Ti is responsible for Al 3 Ti particles formation as the most Am. Azarniya, Prof. A. Abdollah-Zadeh Department of Materials Engineering Tarbiat Modares University P.O. Box: 14115-143, Tehran, Iran E-mail: zadeh@modares.ac.ir Ab. Azarniya, Prof. H. R. Madaah Hosseini Department of Materials Science and Engineering Sharif University of Technology P.O. Box 11155-9466, Azadi Avenue, Tehran, Iran Prof. S. Ramakrishna Center for Nanofibers and Nanotechnology and Department of Mechanical Engineering National University of Singapore Singapore 117576, Singapore DOI: 10.1002/adem.201801269 www.aem-journal.com REVIEW Adv. Eng. Mater. 2018, 1801269 © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1801269 (1 of 40)