Synthesis and Characterization of Mechanically Alloyed Nanostructured Al-Fe

Abstract

Elemental powders of Al and Fe with an atomic composition of 50 % each were mechanically alloyed in a high energetic planetary ball mill. Tungsten carbide milling media (MM) were utilized for ball milling. The milling vial was filled with toluene liquid to prevent contamination of the milled products from the milling media and atmosphere. Ball milling duration extended upto 185 hours; characterization of the milled powder was carried out by X-ray diffractometer (XRD), differential scanning calorimetry (DSC), vibrating sample magnetometry (VSM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The major objectives of characterization were to investigate phase transformation, morphology and topography, structural changes, determination of particle size and magnetic properties of the ball milled powders. Crystallite size was reduced to 86 nm at the end of one hour of ball milling and to 12 nm at the end of 65 hours of milling time. Simultaneously, mechanical alloying (MA) has occurred with the formation of Fe rich BCC Fe (Al) solid solution whose lattice parameter was 0.29316 nm at the end of 185 hours of ball milling duration. During the course of ball milling, slight amorphisation was observed at higher duration of ball milling time (> 125 hours). Heating the ball milled powders in the calorimeter showed the transition to ordering from a near ideal solid solution. The decrease (122 to 76.5 emu/gm) in saturation magnetization (MS) was due to progressive alloying of Al in Fe, moreover, powder samples ball milled for ≤ 20 hours of milling duration exhibited soft magnetic properties (coercivity, HC ≤ 125 Oe) and the remaining ones were magnetically hard (HC >125 Oe). Transition to intermetallic compound from solid solution has induced paramagnetic behavior when ball milled powders were heated beyond 520 0C. Further, ball milled powder (185 hours of milling time) was compacted, sintered and annealed, due to which annealed compact started exhibiting a fully ordered (long range order, LRO, parameter ‘S’ of 0.7) structure. Such an ordered compact was subjected to plastic deformation by various compressive stresses for a very short duration of time. The highly ordered compact transformed into a fully disordered (LRO, ‘S’ = 0.20) structure when plastically deformed with stresses ≥ 4 GPa. Increase in compression stresses caused crystallite size refinement, increased hardness and MS in the deformed compact. In another set of experiments, the role of milling media (MM) was investigated. Elemental powders of Al and Fe with an atomic % of 50 each were ball milled in high energetic planetary ball mill utilizing stainless steel MM. The milling vial was filled with argon inert gas to avoid atmospheric and MM contamination of the milled products. Milling was carried out for a maximum duration of 400 hours. It was inferred that Longer duration of milling was required to achieve MA, crystallite size refinement and amorphisation to certain extent using stainless steel MM compared to that of ball milling with tungsten carbide MM. Metastable amorphous solid solution of Fe (Al) was found to be formed at the end of 400 hours of milling time which has alattice parameter of 0.2934 nm, crystallite size refinement to the tune of 5 nm and lattice strain of 2.215%. Equilibrium phases (AlFe/AlFe3 intermetallics) have precipitated when metastable amorphous solid solution was thermally activated in the temperature range of 500 0 to 700 0C. Ball milled powders exhibited soft magnetic properties with the sole exception of powder milled for 300 hours which behaved as a hard magnetic material. MS of 400 hour ball milled powder was 21.37 emu/gm. Ferromagnetism at room temperature decreased notably with increase in temperature reaching a paramagnetic phase in the temperature range of 620 0 to 720 0C for most of the ball milled powders. Transmission electron microscopy (TEM) with selected area diffraction pattern (SADP) images has authenticated the XRD results of indexing, the presence of amorphous phase and nanocrystalline domain which appeared to be less than 20 nm. Further, ball milled powders which have been mechanically alloyed using stainless steel MM was consolidated to bulk pellet form using equal channel angular pressing with back pressure (ECAPBP). Such pellets have been characterized by XRD, optical microscopy (OM), SEM, nanoindentation, TEM, DSC, VSM and texture analysis. Slight increase in crystallite size (13.4 nm) occurrred as a result of elevated temperature consolidation; though back pressure assisted ECAP-BP consolidation has decreased the diffusion coefficient thereby restricting the grain boundary mobility. Depending on the consolidation temperature and back pressure, either transitional (not fully ordered) AlFe alloy or a combination of AlFe/AlFe3 intermetallics coexisted in a few pellets. Pellets consolidated at higher temperature (450 0C) and back pressure (480 MPa) possessed better densification compared to those pellets processed at lower temperatures and back pressures. Metallurgical events like grain refinement, nucleation of nanopores, and precipitation of second phase particles were noted from SEM images of the pellets. Mechanical properties at the nano level measured by load induced nanoindentation are dominated by the local phase composition rather than the bulk mechanical properties of the pellets. Intermetallic phases provided greater nanohardness and elastic modulus values than those regions where transitional alloy and elemental phases were present. In addition, finer crystallite sizes in the nano level in the pellets promote higher nanohardness and elastic modulus. Comparision between the magnetic parameters of ball milled powders and ECAP-BP consolidated pellets, MS of the pellets is lower while HC is higher with most of the pellets possessing hard magnetic properties. Steep increase in LRO ‘S’ (0.45 to 0.85) and similar decrease in lattice parameter (0.28987 nm to 0.2882 nm) beyond 450 0C caused a transition to paramagnetic (≤ 2 emu/gm) behavior in the pellets. In addition to ECAP-BP consolidation, powders ball milled in stainless steel MM were consolidated by compaction followed by sintering in a nitrogen atmosphere. Sintered compacts exhibited coarser crystallites (120 nm) and reduced lattice strain (0.232 %) compared to ball milled powders and ECAP-BP consolidated pellets. Larger lattice parameter (0.2949 nm) of AlFe intermetallic is noted in addition to the presence of AlN and traces of Al, Fe and Fe4N phases in the sinteredcompacts. However, stronger texture (Maximum Intensity of 703.500) coupled with the presence of AlFe intermetallic and AlN phases caused sintered compacts to possess higher mechanical properties than the ECAP-BP processed pellets. In another set of experiments, elemental powders of Al and Fe (initial composition of 90 atomic % and 10 atomic % respectively) were ball milled in argon atmosphere using stainless steel milling media. Milling caused Al lattice dilation to the tune of 0.4088 nm forming Al rich solid solution (Al (Fe)) resulting in crystallite size refinement to as low as 6 nm. Quantitatively, 9 % of unalloyed Al was present after 400 hours of ball milling duration. Due to amorphous structure, distinct glass transition temperature prior to crystallisation and a narrow supercooled liquid like region could be observed in all the powder samples. Hard magnetic properties (HC ≥ 294.10 Oe) with low MS (≤ 8.432 emu/gm) were observed in all the ball milled powders. Appearance of grain boundaries, dislocations and amorphous structure was evident from the TEM analysis. Further, the ball milled powders (Al90-Fe10 category) were consolidated by ECAP-BP as well as by compaction and sintering. Metastable Al rich solid solution transformed into a stable Al76Fe24 compound of monoclinic structure in all the pellets due to consolidation of the ball milled powders. DSC analysis indicated the slight amorphous structure in the pellets. Pellets exhibited soft magnetic (HC≤ 107.63 Oe) properties with a very low paramagnetism MS (≤ 0.6582 emu/gm). Pellets with excellent bonding between the particles were produced with a low back pressure (300 MPa) and temperature (450 0C), which is much lower than the sintering temperatures. Nanosized grain boundaries, amorphous and Al76Fe24 compound phases were distinctly visible in the SEMFEG micrographs of the pellets. Sintered compacts exhibited crystalline phases of Al76Fe24 compound, AlN and unalloyed Al. Larger quantity (74 %) of Al76Fe24 crystallized in the 400-h sintered compact, although the lattice dilation of Al in the 250-h sintered compact was to the tune of 0.4109 nm. Maximum size of the crystallites in the sintered compact was 135 nm, which was much larger than that of ECAP-BP consolidated pellets (D = 11 nm). Among all the consolidated materials of both the categories (Al50-Fe50 and Al90-Fe10), 400-h sintered compact of Al90-Fe10 system possessed strongest texture (Maximum intensity of 6556.00), coupled with highest nanohardness and elastic modulus caused by larger quantities of Al76Fe24 intermetallic and AlN phases. The present investigation unambiguously brings out the fact that mechanical alloying followed by consolidation in different modes (ECAP-BP and sintering) and conditions produces nanomaterials with spectrum of structure and properties from which a choice of industrial product can be developed.