Consolidation of Amorphous Al 80 Fe 10 Ti 5 Ni 5 Powders by Hot Pressing

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 Xray 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.

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][15][16][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][12][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 [13].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][26][27][28][29][30].The purpose of this study is the investigation of the feasibility of fabricating amorphous Al 80 Fe 10 Ti 5 Ni 5 powders by mechanical alloying and consolidating them into bulk samples by a hot-pressing technique.

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 Al 80 Fe 10 Ti 5 Ni 5 (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 Al 2 O 3 pans and heated in a dynamic Ar atmosphere up to 1200 • C at a constant heating rate of 40 • C/min.As seen here, the powders milled for 40 h are spherical and have an average diameter of 2 μm.

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 Al 80 Fe 10 Ti 5 Ni 5 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  [11] system and is different from that in Al 80 Fe 20 [23] and Al 83 Fe 17 [10] amorphous systems.In contrast to the Al 80 Fe 10 Ti 5 Ni 5 amorphous alloy (which exhibits onestage crystallization on heating), the Al 80 Fe 20 and Al 83 Fe 17 amorphous phases exhibit three-stage crystallization during heating.

Consolidation of the Ball-Milled Amorphous Powders.
Fully amorphous powder particles with high crystallization temperature (950 • C) in Al 80 Fe 10 Ti 5 Ni 5 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    the consolidation processes and the produced bulk material is not fully amorphous.

Conclusions
In 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.

Figure 10 :Figure 11 :
Figure 10: Polished cross-sectional view of sample hot pressed at 550 • C under 600 MPa.

Figure 12 :
Figure 12: TEM image of sample hot pressed at 550 • C under 600 MPa.