Application of SPS consolidation and its influence on the properties of the FeAl20Si20 alloys prepared by mechanical alloying

Abstract

The FeAl20Si20 alloy was prepared by a combination of short-term mechanical alloying and spark plasma sintering. The processing parameters either of the mechanical alloying or spark plasma sintering were optimized to yield the maximal mechanical properties of the alloy. For the mechanical alloying, two amounts of powders batch (5 g or 20 g) were compared. The spark plasma sintering regimes combined pre-pressing prior heating and vice versa, direct and pulse current flow. The MA + SPS alloys exhibited ultrafine-grained microstructure composed of FeSi, Fe₃Si and Fe₃Al₂Si₃ phases (with an average crystallite size of approximately 30 nm) with a presence of randomly distributed Al₂O₃ particles (with diameters ranging from 5 to 100 nm). The FeSi, Fe₃Si phases were supersaturated by Al, which resulted in an increase of lattice parameters. The hardness of the compact alloys reached up to approximately 1100 HV 0.1 for both the powder batches. The 20 g samples showed a standard deviation nearly half the of 5 g powder batches and the 20 g prepared by a regime combining pre-pressing prior heating up to consolidation temperature using pulse current flow resulted in the highest compressive strength of 2008 MPa. Combination of pre-pressing prior heating-up also reduced the increase of the Fe₃Al₂Si₃ phase weight fraction especially in the 5 g alloys that otherwise had a tendency to microstructural coarsening.

Introduction

Aluminides of transition metals, especially of iron, titanium or nickel are intensively studied as a potential substitute for nickel-based superalloys, heat-resistant steels or cobalt alloys. These intermetallic alloys are usually employed such as materials for aeroplane engine parts and other high-temperature components. However, aluminides are frequently alloyed by some of the critical raw materials (CRMs) [1] such as tungsten, niobium, cobalt or chromium to improve the high-temperature strength or room temperature ductility. Therefore, ongoing research is currently focused on the CRM-free materials that can offer comparable or even better properties than the alloys used nowadays. Iron aluminides are very promising candidates among the investigated materials because they exhibit excellent properties even if they do not contain these critical raw elements.

Iron aluminides are generally of interest due to their low density, low production cost, good mechanical properties and corrosion resistance, mainly in oxidising and sulfidizing environments [[2], [3], [4], [5], [6], [7], [8]]. It is known that oxidation resistance is caused by the formation of a protective Al₂O₃ layer on the surface and hence it increases with the concentration of aluminium [7,9]. However, oxidation resistance is limited by temperature as the protective oxide layer loses its protective effects above 700 °C. It is caused by the formation of porous Fe₂O₃ in the layer which leads to the rapid oxidation of the material beneath this layer [10]. Another significant feature of iron aluminides (namely Fe₃Al) is high magnetic permeability allowing its application as soft magnetic materials [2,[11], [12], [13], [14], [15]]. Anomalous strengthening in a certain temperature range is also typical for this system as it has been reported by George et al. [16]. In spite of the all above-mentioned properties, the Fe–Al-based alloys have not been applied widely. It is caused by their low ductility at ambient temperature resulting in a brittle fracture that complicates their broader utilisation [3,12,16]. The binary phase diagram of Fe–Al system consists of several intermetallic compounds: Fe₃Al, FeAl, FeAl₂, Fe₂Al₅ and FeAl₃ [2] where the FeAl and Fe₃Al compounds have been the most investigated in recent years. The Fe₃Al phase has ordered D03 structure below 550 °C and the ordered B2 structure in a temperature range from 550 to 750 °C [17]. Above the temperature of 700 °C, the Fe₃Al reduces its oxidation resistance due to the previously mentioned effects as well as lowering its strength due to the order-disorder phase transformation [10].

Negative properties can be eliminated by alloying of the Fe–Al alloy by other elements like chromium and/or silicon. For example, the addition of chromium increases the ductility and strength of the Fe–Al-based alloys due to solute strengthening [[18], [19], [20]]. Furthermore, silicon increases the hardness of alloyed material due to a formation of silicides, which presence improves electrical and thermal conductivity as well as microstructural stability against grain coarsening [12,17,21]. Partial substitution of aluminium by silicon also affected magnetic properties positively [5] and by this time, a mixture of Fe₃Si and Fe₃Al alloys in a ratio of almost 2:3 called Sendust has been used as a material for heads in magnetic recorders [[22], [23], [24]]. It was also proven that the addition of silicon prevents the disordering in Fe–Al system [21,25]. However, the addition of silicon into the Fe–Al alloy increases the alloy brittleness due to the formation of extremely hard but also brittle FeSi or Fe₃Si phases [26]. For this reason, Fe–Al–Si alloys prepared by various techniques have been studied widely [13,[27], [28], [29], [30], [31]]. Nowadays, the FeAlSi-based alloys are used in low-frequency devices including filter inductors and transformers [28] or are considered as protective layers prepared by laser cladding to cover and protect the base material against high-temperature reactions and significantly increase wear resistance [32]. On the other hand, the Fe–Al–Si ternary system is difficult to be characterised because of its complexity which manifests itself by a large number of coexisting phases along a small composition range [14,29].

The preparation of the Fe–Al-based alloys via powder metallurgy seems to be a promising method of how to improve negative properties of the Fe–Al-based alloys and to avoid all limitations of cast-metallurgy processes. This process suppresses the undesirable grain growth and mitigates the problems with the heterogeneity of the chemical composition within the material [33]. Mechanical alloying (MA) as one of the representatives of powder metallurgy processes is used to synthesise equilibrium, non-equilibrium or amorphous materials from pure powders during alloying [34,35]. The materials exhibit better mechanical properties compared to its as-cast counterparts because of processes during the MA that combines cold welding with limited diffusion of elements, deformation strengthening, and continuous fracturing of the newly developing phases [35,36]. This leads to a significant microstructural refinement in the means of grain size reduction and good homogeneity within the alloy [34,35]. The temperature increase during MA can promote phase transformations from ordered to disordered state [37]. Depending on the MA process parameters a nanostructured powder can be obtained [38]. In general, the methods suitable for consolidation of prepared powders are hot-extrusion, hot-isostatic pressing (HIP) and spark plasma sintering (SPS) [35,[39], [40], [41]]. The main problem lies in the elevated temperature of these processes which supports the recrystallisation of alloy decreasing some of its properties [41]. From this point of view, the SPS seems to be the most suitable consolidation method since it allows to compact alloys at lower temperatures compared to the rest of previously mentioned techniques with the total duration of the process in a range of a few minutes. Unfortunately, studies devoting to an optimisation of processing parameters of Fe–Al or Fe–Al–Si preparation via MA + SPS have not been published yet.

In this work, the FeAl20Si20 (wt.%) alloy has been prepared by a combination of short-term ultrahigh-energy MA [42] and consequently consolidated by an SPS technique. The aim was to describe the influence of the amount of powder batch per each MA as well the influence of the SPS conditions on microstructure and mechanical properties of the FeAl20Si20 alloys prepared by non-equilibrium conditions. As this material is expected to have the brittle room-temperature behaviour, any cracks, voids and other sintering defects would result in a rapid decrease of room-temperature strength [43] and even small variations in SPS parameters could affect the materials’ properties strongly. This study can thus provide a general background for sintering of this group of materials, which comprise many intermetallics.