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Journal of Nanomaterials
Volume 2012 (2012), Article ID 814915, 10 pages
Consolidation of Amorphous Powders by Hot Pressing
1Department of Material Engineering, Nanotechnology and Advanced Material Institute, Isfahan University of Technology (IUT), Isfahan 84156-83111, Iran
2Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea
Received 13 May 2012; Accepted 8 July 2012
Academic Editor: Mohammad H. Maneshian
Copyright © 2012 M. Tavoosi et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
The current study investigates the feasibility of fabricating amorphous Al80Fe10Ti5Ni5 powders by mechanical alloying and consolidating them into bulk samples by a hot-pressing technique. As-milled and hot-pressed samples were examined by X-ray diffraction, scanning electron microscopy, transition electron microscopy, and differential scanning calorimetry. The results showed that milling of Al80Fe10Ti5Ni5 powder for 40 h and hot pressing at 550°C under 600 MPa led to a fully dense bulk sample. During consolidation, an AlTi intermetallic phase with average crystallite size of 10 nm precipitates in the amorphous matrix.
Alloys with Al content of 80 to 90 at.% have attracted considerable attention due to their combination of good ductility and high strength [1–7]. Among the various aluminum alloys, Al-Fe systems are of technological interest due to their advantageous properties, including high specific strength, high specific stiffness, good strength at intermediate temperatures, and excellent corrosion resistance at elevated temperatures under oxidizing, carburizing, and sulfidizing atmospheres [8–12]. A new class of Al-based alloys, which has received a great deal of attention in recent years, is amorphous as well as nanocrystalline [13–23].
Indeed, amorphous alloys exhibit several superior properties that cannot be obtained in crystalline materials. In general, bulk amorphous alloys can be fabricated by two main processes: direct solidification from the melt and consolidation of an amorphous powder or ribbons. Generally, very high cooling rates are required for the formation of an amorphous phase from the liquid state and only a few amorphous alloys with high glass-forming ability can be applied to the fabrication of bulk amorphous alloys by a direct solidification process. Due to the requirement of a high cooling rate, amorphous alloys have largely been fabricated in the form of powders, ribbons, and wires with small thickness or diameter [14–17], and hence the application of amorphous alloys as a structural material has been limited.
The glass-forming ability of Al-based alloys is so low and there is not any report about formation of bulk amorphous structure for these alloys by the use of direct solidification methods [11–13]. Nevertheless, a great number of amorphous alloys in Al-based alloys have been synthesized by various preparation methods of rapid quenching from liquid or vapor and solid-state reactions . However, the maximum thicknesses of the resulting amorphous alloys have usually been limited. The limitation of the maximum sample thickness has prevented a wide extension of application fields of amorphous alloys. Consequently, great efforts have been devoted to preparation of a bulk amorphous alloy from amorphous alloy powder by using various techniques of warm pressing, warm extrusion, explosive compaction, hot pressing, and spark plasma sintering [13, 24]. The powder metallurgy process promotes high strength bonding between particles and a microstructure similar to that of a wrought product [25–30].
The purpose of this study is the investigation of the feasibility of fabricating amorphous Al80Fe10Ti5Ni5 powders by mechanical alloying and consolidating them into bulk samples by a hot-pressing technique.
2. Experimental Procedure
Commercial elemental powders of Al (99%), Fe (99.9%), Ti (99%), and Ni (99.99%) were used as raw materials. The mixture of elemental powders with a composition of Al80Fe10Ti5Ni5 (in atomic percentage) was mechanically alloyed in a planetary ball mill under an argon atmosphere in a steel container at room temperature. A rotation speed of 250 rpm and a ball to powder ratio of 10 : 1 were employed. 1 wt% stearic acid powder supplied by Merck was used as the process control agent (PCA).
The produced amorphous powders were consolidated in a vacuum hot-pressing machine to prepare bulk amorphous discs with 10 mm diameter and 1 mm thickness. Hot pressing was performed at different temperatures and pressures. The hot-pressing chamber was evacuated and pressure was applied during the entire consolidating process.
The as-milled powders and consolidated samples were characterized by X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), transition electron microscopy (TEM), and differential scanning calorimetry. The XRD analysis was performed using a Philips diffractometer (40 kV) with Cu Kα radiation (λ = 0.15406 nm). The XRD patterns were recorded in the 2θ range of 30–70° (step size 0.03° and time per step 1 s). The morphology, fracture surface, and cross-sections of prepared samples were examined using SEM. The microstructure of the produced amorphous powder was investigated by TEM and a selected area diffraction (SAD) pattern analysis carried out at an accelerating voltage of 200 kV resolution of 0.19 nm. A differential thermal analysis (DTA) was conducted to study the thermal stability of the produced amorphous alloy using a Reometric STA 1500 differential thermal analyzer. The samples were placed in Al2O3 pans and heated in a dynamic Ar atmosphere up to 1200°C at a constant heating rate of 40°C/min.
3. Results and Discussion
3.1. Production of Al80Fe10Ti5Ni5 Amorphous Phase
The XRD patterns of the Al-10%Fe-5%Ni-5%Ti powder mixture after various milling times are shown in Figure 1. In the early stage of milling, broadening of Al, Fe, Ni, and Ti peaks accompanied by a remarkable decrease in their intensities occurred as a result of refinement of crystallite size and enhancement of lattice strain. Increasing milling time up to 10 h led to the disappearance of Fe, Ti, and Ni peaks. This may be due to dissolution of these elements in the Al lattice. By increasing the milling time, two narrow peaks corresponding to the decagonal phase (D-phase) are observed in the XRD patterns. Analysis of the XRD patterns in Figure 1 reveals that the D-phase, which formed from the solid solution during milling, could be amorphized by further milling (after 40 h) and there is no significant change as the milling time is prolonged to 100 h. A TEM micrograph and SAD pattern of powders ball milled for 40 h are also presented in Figure 2. As can be seen in this figure, the SAD pattern of this sample consists of only amorphous halo and there is no evidence of the presence of any crystalline phase, which is in agreement with the XRD result. This indicates that the product of mechanical alloying of the Al-10%Fe-5%Ni-5%Ti powder mixture is an amorphous phase.
SEM cross-sectional micrographs of powder particles after different milling times are presented in Figure 3. According to the micrographs, in the early stages of mechanical alloying (MA), the ductile components are flattened to platelet shapes, cold welded together, and form a composite lamellar structure of the constituent metals (Figures 3(a) and 3(b)). With increasing MA time, the composite powder particles are work hardened and fragmented. With further milling (Figures 3(c) and 3(d)), the interlamellar spacing decreases and true alloying at the atomic level occurs, resulting in the formation of solid solutions, intermetallics, or even amorphous phases. In this stage, the interlayer spacing disappears or becomes so fine that it is no longer visible using SEM (Figure 3(e)). The morphology of Al80Fe10Ti5Ni5 powders after 40 h of ball milling is also shown in Figure 4. As seen here, the powders milled for 40 h are spherical and have an average diameter of 2 μm.
3.2. Thermal Behavior of Produced Amorphous Powder
As noted in the previous section, milling the mixed powder for 40 h led to the formation of the Al80Fe10Ti5Ni5 amorphous phase. The mechanically milled microstructure is in a metastable state and considerable microstructural changes can occur upon heating the milled powder. In order to study the thermal stability of the produced amorphous phase, the sample was examined by DSC under continuous heating conditions. Figure 5 shows the DSC heating traces of the Al80Fe10Ti5Ni5 amorphous alloy at a constant heating rate of 20°C/min. As seen in this figure, two peaks appear in the DSC curve (an exothermic peak at 950°C and an endothermic peak at 1100°C). To analyze the crystallization process responsible for the exothermic peak, the as-blended sample was annealed under an Ar atmosphere at 1000°C for 20 min. The XRD pattern and a TEM micrograph of the Al80Fe10Ti5Ni5 amorphous phase after annealing at 1000°C are presented in Figures 6 and 7, respectively. As can be seen here, the annealed sample consists mainly of an Al13 (Fe,Ni)4  and Al3Ti  intermetallic phases. Therefore, the exothermic peak in Figure 5 is attributed to precipitation of these phases from the amorphous phase. Meanwhile, the endothermic peak at 1100°C in Figure 5 is for the melting of the produced alloy. These results indicate that the total transformation sequence of the Al80Fe10Ti5Ni5 amorphous alloy is a one-stage process in the temperature range from 930°C to 980°C.
In fact, according to the DSC results, the crystallization in Al80Fe10Ti5Ni5 proceeds in the same manner as in the Al80Fe10Ti10  system and is different from that in Al80Fe20  and Al83Fe17  amorphous systems. In contrast to the Al80Fe10Ti5Ni5 amorphous alloy (which exhibits one-stage crystallization on heating), the Al80Fe20 and Al83Fe17 amorphous phases exhibit three-stage crystallization during heating.
3.3. Consolidation of the Ball-Milled Amorphous Powders
Fully amorphous powder particles with high crystallization temperature (950°C) in Al80Fe10Ti5Ni5 alloy provide good conditions for the fabrication of a bulk amorphous material. In this study, the as-milled powders were consolidated by hot pressing into a disk shape. To optimize the condensation parameter, the ball-milled amorphous powders were hot pressed at different temperatures (400, 450, 500, and 550°C) under various pressures (200, 300, 400, 500, and 600 MPa) for 30 min. In order to investigate the condensed samples, the fracture surface morphology after compression tests was examined using SEM. Fracture surface photographs of samples hot pressed at different temperatures under a constant pressure of 600 MPa and under different pressure values at a constant temperature of 550°C are shown in Figures 8 and 9, respectively. As seen in these figures, the trace of the interparticle boundaries between the powders increases and the porosity in the hot pressed samples decreases as the pressing temperature and pressure increase. These results indicate that the optimum temperature and pressure for consolidation of the amorphous powders via the hot-pressing method are 550°C and 600 MPa, respectively, in our experimental conditions. The polished cross-sectional view of the consolidated sample at 550°C under 600 MPa in Figure 10 shows no remaining pores.
Figure 11 shows the XRD pattern of the consolidated at 550°C under 600 MPa. From the peaks corresponding to AlTi, it can be concluded that the amorphous phase does not remain after consolidation at temperature of 550°C for 30 min. The presence of the crystalline phase in the amorphous matrix in the consolidated bulk sample can be further confirmed by TEM and elemental maps. The TEM micrograph and elemental map of the hot-pressed sample are shown in Figures 12 and 13, respectively, where limited nanocrystallization (formation of AlTi with average size of 10 nm in the amorphous matrix) occurred during the consolidation processes and the produced bulk material is not fully amorphous.
3.4. Thermal Behavior of Produced Bulk Material
In order to investigate the thermal behavior and microstructural changes of the consolidated bulk amorphous alloy (below crystallization temperature), pressed samples were annealed at several temperatures for 3 h. XRD patterns of as-hot pressed and annealed samples at 550°C, 600°C, 650°C, and 700°C are presented in Figure 14. During the consolidation of the amorphous powders at 550°C for 30 min, the AlTi phase precipitates in the amorphous matrix. Analysis of the XRD patterns in Figure 14 reveals that by annealing the samples at temperature above 550°C (below crystallization temperature), the AlTi diffraction peaks disappeared and several peaks corresponding to Al13(Fe,Ni)4 and Al3Ti intermetallic phases appeared. By increasing the annealing temperature, the intensity of Al13(Fe,Ni)4 and Al3Ti peaks in the XRD patterns increases as a result of increasing the crystalline size and the fraction of this phase during annealing.
In the present work, we fabricated amorphous Al80Fe10Ti5Ni5 powders by milling elemental powder mixtures for 40 h. The crystallization process of this amorphous alloy is a one-stage mode of the Al13(Fe,Ti)4 and Al3Ti intermetallic compounds. The results showed that the as-milled amorphous Al80Fe10Ti5Ni5 powders were consolidated successfully into bulk metallic glasses by a hot pressing technique. The temperature and pressure for successful condensation of amorphous powders in the hot pressing method are 550°C and 600 MPa, respectively. During the consolidation, the amorphous phase does not remain and an AlTi intermetallic phase precipitates in the amorphous matrix.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST, no. 2010-0026981). H. S. Kim acknowledges the support of a grant (B551179-08-01-00) from the cooperative research project “Development of common manufacturing technology for high functional susceptor” funded by the Ministry of Knowledge Economy, Republic of Korea.
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