1 Introduction and historical background
There is a demand for materials featuring high strength at temperatures at which conventional heat-resistant materials cannot satisfactorily performfor future aircraft and energy applications. In recent yearsintermetallic compounds with intermediate characteristics between ceramics and metals have attracted attention as structural materials for high temperature service [1]. An intermetallican abbreviated term which has come into universal useis an ordered compound between two or more metalsin simple atomic proportions [2]. The name intermetallic refers to the bonding between the constituent elements of a material system. Specificallythese substances are described to have “mixed bonding”being partially covalentpartially ionicand partially metallic [3]. The crystal structures of intermertallics are different from those of their constituents. This definition includes intermetallic phases and ordered alloys [4]. Intermetallics fall into two large families: weakly ordered (reversible) ones like Cu3Au or CuZn which become disordered on heating before they meltand strongly ordered (permanent) ones like NiAlTiAlor Nb3Al which remain ordered up to the melting temperature [2]. Intermetallics are a fascinating group of materialswhich attract attention from the viewpoints of fundamentals as well as applications [4–6]. For last few decadestremendous world-wide efforts in intermetallics have been seenwhich have largely focused on aluminides with some smaller efforts on silicides. The primary goals were high-temperature or power-generation applicationssuch as aeroengineshigh-temperature edges on aircraft wings and rocket finsautomobile engine valvesturbochargers and so on. Important properties for these applications are the ability to withstand high temperature and aggressive oxidising or corrosive environmentsas well as low specific weight or inertia [7]. Like ceramicshoweverthe greatest disadvantage of intermetallics is their low ductilityparticularly at low and intermediate temperatures. The reasons for the lack of ductility vary from compound to compoundbut include: (1) a limited number of easy deformation modes to satisfy the von Mises criterion(2) operation of dislocations with large slip vectors(3) restricted cross-slip(4) difficulty of transmitting slip across grain boundaries(5) intrinsic grain boundary weakness… etc. [8].
The use of intermetallic compound can be early found in the history. In prehistoric timeshumans used the ordered alloys nature provided (meteoritic Fe
Ni for toolsand native AuCu for jewellery and fish hooks). Afterwardsfrom ancient to modern-timesalloys compositions and processing techniques were optimized for particular applications without the realization that intermetallic compounds were responsible of such successes [9]. The first ordered intermetallic compound was identified in Russia in 1916by Kumakov and co-workers on the system AuCu [10,11]although the concept of long-range order was only clarified by x-ray diffractionin Swedenten years later. Initiallysuch materials were studied only by chemists trying to understand the basis of combination between metals; latermetallurgists began to be interested in their propertiesfirst magnetic ones and later mechanical properties [12]. Serious research on high temperature intermetallics began in the early 1950”'s and increased significantly around 1970 because of their perceived potential in aerospace. With weight saving being a key requirementearly work concentrated on the aluminide intermetallics based on nickel and titanium. Subsequently intermetallics such as Fe3Al have been developed because of their potential benefits in replacing steels in various high temperature applications [13]. Intermetallic-based materials have been exploited in a number of applications not only their mechanical properties but also their chemicalmagneticoptical and semiconducting. The initial references in the literature date of the beginning of last century but the inflexion point was marked by the experiments carried out by Aoki and Izumi [14] who discovered that small additions of boron to Ni3Al improved significantly its poor ductility [15]. According to the studiesthere are estimated about 11,000 distinct binary intermetallic compoundsmost of which are known only through phase diagrams and crystallographic studies but there is no knowledge on the properties they can offer. 500,000 trues ternary intermetallics are calculatedonly 3% of which are known to existand for only an infinitesimally tiny fraction of these do we have any knowledge of their properties. For the likely 10 × 106 quaternary intermetallic compound< 0.01% are even known [9]. Presently they are subject to intensive research in all developed countries around the world. This research is concentrated in universities and specialized research centers particularly in the USACanadaJapanGermanyFrance and Switzerland. The research of these intermetallic alloys is supported particularly by aviation and automobile industry companies [16,17].
Transition metal (TM) intermetallicsespecially TM aluminides like: TiAlNiAlFeAl and Fe3Al have unique propertiese.g.high melting pointsenhanced oxidation resistancerelatively low densityand can be used as soft magnetic materials [18–23]. They have been known and studied since the late nineteenth century. Initiallyinterest in these materials was confined to chemistswho had much difficulty in understanding how metals could combine at all among themselvesoften in several different proportions for the same pairsince this generally seemed to be at variance with elementary notions of valency [24]. Early TM aluminides have fcc-based crystal structure in contrast to the bcc-based crystal structure of late transition metal alloys. Due to the strongly attractive chemical bonding between the bi-metallic speciesthey are ordered and have stoichiometry. Howeverthe energy of interatomic bonds differs from the early TM (TiAlVAl) to the late TM alloys (CoAlNiAl and FeAl) [23]. Conventional processing techniques used to produce intermetallic compounds are generally through a combination of meltingcastingpowder grindingand consolidation by hot pressing. Howeverthese techniques such as melting and casting methods are inapplicable to the fabrication of many intermetallic alloys due tofor examplea large difference between the melting points of constituent elements [25].
For several years mechanical alloying (MA) and mechanical milling (MM) have been known to be a promising route for the processing of structural intermetallics such as iron-nickel- and titanium- aluminides. This is due to many beneficial properties i.e. reliablecheaphigh flexiblility in the selection of synthesis processes and different types of products can be attainedsimplicityeasy to operate and capable of synthesizing explosive materials…etc. [26–28]. The first attempt of producing intermetallics by MA was made by Ivanov [29] for the AlNi system. MA is a promising technique to synthesize a variety of equilibrium and nonequilibrium alloy phases starting from blended elemental or prealloyed materials including metalsceramicspolymers and composites. It is a widely used technique that allows the manufacturing of amorphous materialsextended solid solutions, solid solutions of immiscible systemsnanocrystalline materials and quasicrystalline materials. MA is actually a high-energy ball milling process for producing new nanostructured and metastable materials. In this processmechanical energyrather than thermalchemicalelectricor other common forms of energyis used to create phase transformations and chemical reactions at very low temperatures [30]. In MAthe powder particles are trapped between grinding balls and vial wall experiences severe deformationfracture and cold welding. Due to intense deformation and fracture during particle refinement in MAseveral defects such as dislocationsvacancies and stacking faults are induced into the particles resulting increment in free energy of the system [31]. On the other handit has been reported that MA can improve significantly the room temperature ductility of intermetallics because it can achieve: (i) reduction in grain size(ii) disordering of the latticeto improve the dislocation motion (superdislocations do not exist in disordered lattices and therefore only single dislocations need to move for deformation to occur)and (iii) modifying the crystal structure of the phase into a more symmetrice.g.cubic one [31]. The types of intermetallics synthesized by MA include both quasicrystalline and crystalline intermetallic phases. Both equilibrium and metastable phases have been synthesized in the latter category and these also include the disordered and ordered phases [31]. Reasons for the formation of ordered intermetallics have not been investigated in detailit may be assumed that a phase will exist either in the ordered or disordered condition depending upon the balance between atomic disordering introduced by MA and the thermally activated reordering. The reordering is caused by the difference in energy between the ordered and disordered states. Thusif this difference in energy is smallthe alloy will exist in the disordered state whereas if it is large the alloy will be in the ordered state [32]. It has been reported by Eckert et al. [33] that the nature of the phase formed in quasicrystals was different depending on the milling intensity. At very high milling intensitya crystalline intermetallic phase formed while at very low intensityan amorphous phase formed. A quasicrystalline phase formed at intermediate intensities. A number of intermetallics in metallic alloy systems have been synthesized. These include aluminidessilicides and other intermetallics [34]. A novel solid-liquid reaction ball milling technique based on the coupling effect of both mechanochemistry and thermochemistry was developed. Many types of binary intermetallic compound powders were fabricated successfully and most of them are single-phase nanoparticles. Compared with traditional mechanical alloying (MAi.e. high energy ball milling) processesthis mechanochemistry approach shows the advantages in finer particle sizehigher purity of productionfaster reactivity speed and different micro-reaction mechanism. A number of ternary intermetallics of Al-Cu-X (X = FeCoNi) alloy systems such as Al7Cu2FeAl13Cu4Fe3Al65Cu20Fe15Al65Co15Cu20Al69Co25Cu6Al17Cu4Ni and Al0.28Cu0.69Ni have been successfully preparedas reported by Refs [35–38].
It has been long known that partially ordered phases are stronger than those wholly disordered or fully ordered. Thusit is of interest to study the mechanical behavior of materials in various states of partial order [32]. Disordering phenomena of ordered alloys have also been studied to understand the mechanism of disordering and also to produce the disordered material that has a higher ductility/formability than the ordered alloys [33]. Mechanical milling (MM) has emerged as a powerful method of producing disorder in intermetallic alloys. Indeedwhen an ordered intermetallic undergoes heavy deformationone or more of the following polymorphous transformations will occur [39–41]: (a) it will undergo grain refinement down to the nanocrystalline regime(b) chemical long-range order may disappear as signaled by the disappearance of superlattice reflections(c) chemical short-range order (CSRO) may be decreased toward that of a random solid solution [42]CSRO is a measure for the order in the surrounding of an atomi.e.it indicates how strongly neighbouring atoms are correlated(d) the basic topological order and fundamental unit cell of the crystal may change such as in a b.c.c.–f.c.c. transformation [43](e) long-range crystalline topological order may crumble altogether into an amorphous phase [44,45]. The progress of disordering has been monitored by several techniques including X-ray diffraction techniques to measure the lattice parameter and long-range order parametermeasurement of superconducting transition temperature and magnetic susceptibility (if the compound is superconducting in the initial state)Mössbauer techniquesdiffierential scanning calorimetry… etc. [29].
This review is in many aspects different from other papers written on this topic [11,30–32,46]. Indeedas the title indicatesthe current paper emphasizes on the various aspects of synthesis and disordering of B2 TM-Al (TM = FeNiCo) intermetallic alloys by high energy ball milling. A general overview on B2 intermetallics is presentedthen a detailed presentation concerning FeAlNiAl and CoAl intermetallic alloysas well as their properties and synthesis or disordering routesare provided.
