Formation of amorphous alumina during sintering of nanocrystalline B2 aluminides Niraj Chawake a , N.T.B.N. Koundinya a , Sanjay Kashyap b , Ajeet K. Srivastav a,1 , Devinder Yadav a , R.A. Mondal c,2 , Ravi Sankar Kottada a, ⁎ a Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600036, India b Department of Materials Engineering, Indian Institute of Science, Bengaluru 560012, India c Department of Physics, Indian Institute of Technology Madras, Chennai 600036, India abstract article info Article history: Received 6 July 2016 Received in revised form 2 August 2016 Accepted 4 August 2016 Available online 05 August 2016 Nanocrystalline B2 aluminide (FeAl and NiAl) powders were synthesized using high energy ball milling and were consolidated using three different sintering methods: conventional pressureless sintering, microwave sintering and spark plasma sintering. The particle-particle contacts after sintering were found to have compositional gra- dient from interior to surface leading to formation of an Al-O rich phase. This Al-O rich phase formation is primar- ily attributed to oxygen pick-up during handling of powders and during sintering, leading to formation of amorphous alumina. The formation of amorphous phase is explained by examining the defect concentrations and their interactions. The composite developed with amorphous alumina as a second phase in an ordered B2 matrix can be of great interest for potential structural applications. © 2016 Elsevier Inc. All rights reserved. Keywords: Intermetallics (aluminides) Powder metallurgy Sintering Mechanical alloying Microstructure Microscopy 1. Introduction High energy ball milling is one of the robust techniques to synthesize aluminides or intermetallics using elemental powders [1]. In spite of its various advantages, the clean handling of ball milled powders is chal- lenging [2]. The refinement of powder during mechanical alloying in- duces increase in surface activity due to an increase in its surface area. Adsorption of gases, mainly oxygen, may occur during powder handling [3]. Consequently, oxides may form on powder surfaces during further processing of powders [4]. The ball milled powders are often sintered to obtain dense pellets for evaluation of their physical or mechanical properties. Although, grain growth is inevitable during sintering [5], fully dense nanocrystalline pellets can be obtained using the shorter high temperature exposure typical of spark plasma sintering (SPS) [6]. In spite of many advantages of SPS, the high electric current, high heating rate and localized heating mechanisms may lead to local varia- tions of stoichiometry or the appearance of different phases at particle- particle contacts [7,8,9,10,11]. Such second phases on contact surfaces should not be neglected since even minute fraction of second phase along grain boundaries or triple junctions in polycrystalline material can significantly affect the overall bulk mechanical behavior [12,13]. There have been numerous studies on the formation of FeAl and NiAl using high energy ball milling. Most of these studies aimed at address- ing microstructural, phase or morphological evolution during milling [14–28]. Very few of these studies have mentioned/discussed about the contamination caused, majorly of the oxygen pickup during the pro- cessing of powders [14,16,20,25]. Also, in majority of these studies, XRD analysis is primarily used for investigations, and thus detection of min- ute fraction of other phase was not feasible. However, in all these stud- ies, the properties of aluminides were the main focus and thus very little attention was devoted to the formation of second phases/contamina- tion. The Table 1 presents the various reports from the literature dealing with synthesis of these aluminide powders. The major observations pertaining to the second phase/contamination are also presented in this Table. Apart from the studies mentioned in Table 1, there have been few studies dealing with sintering of NiAl and FeAl powders reporting the formation of alumina (Table 2), but without adequately explaining the reasons for its formation. There are also other studies which mention about formation of alumina/oxides on surfaces of aluminides but the formation mechanisms involve electrochemistry [34] [35]. However, a closer look at the Table 2 suggests that the majority of the observations were made during SPS studies except in a couple of other sintering methods. Thus, it is not clear whether it is an SPS related phenomenon (electric current effects/high heating rates/localized heating etc.) [7,9] Materials Characterization 119 (2016) 186–194 ⁎ Corresponding author. E-mail addresses: ravi.sankar@iitm.ac.in, raviskottada@gmail.com (R.S. Kottada). 1 Department of Metallurgical and Materials Engineering, Visvesvaraya National Institute of Technology, Nagpur 440,010, India. 2 Department of Physics, Hindustan University, Chennai, 603,103, India. http://dx.doi.org/10.1016/j.matchar.2016.08.004 1044-5803/© 2016 Elsevier Inc. All rights reserved. Contents lists available at ScienceDirect Materials Characterization journal homepage: www.elsevier.com/locate/matchar