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Print version ISSN 1516-1439
Mat. Res. vol.15 no.5 São Carlos Sept./Oct. 2012 Epub Aug 09, 2012
Ana Karla MelleI; Mauricio Mhirdaui PeresII,*; Claudemiro BolfariniI; Walter José BottaI; Alberto Moreira Jorge JuniorI; Claudio Shyinti KiminamiI
IScience and Materials Engineering Post Graduation Program, Department of Materials Engineering, Federal University of São Carlos - UFSCar, Rod. Washington Luiz, Km 235, CEP 13565-905, São Carlos, SP, Brazil
IIFederal University of Itajubá - UNIFEI, Rua São Paulo, 377, CEP 37500 903, Itabira, MG, Brazil
The amorphous Cu46Zr42Al7Y5 alloy presents large supercooled liquid region (ΔTX = 100 K), with a viscosity of about 106 N.s/m2 where the material can flow as a liquid, making it possible an easy deformation in this temperature region. The aim of this work was to analyze processing routes to produce bulks of metallic glasses. Two kinds of materials were used: amorphous powders and ribbons, both were consolidated by hot extrusion in temperatures inside the range between Tg and Tx, with a ram speed of 1 mm/min and extrusion ratio of 3 : 1. Analysis of X-Ray Diffratometry (XRD), Differential Scanning Calorimetry (DSC) and Scanning Electron Microscopy (SEM), revealed that the proposed consolidation routes were effective to produce large bulks of amorphous materials, even with the strong decreasing of ΔTX observed after deformation by milling and during extrusion.
Keywords: amorphous copper alloys, hot extrusion, metallic glass
Bulk metallic glasses (BMG) of copper based alloys show a scientific and engineering importance due to its higher strength and ductility1-3. Strength of 2265 MPa and ductility up to 18% are reported for this alloy at room temperature4. Among the binary copper alloys, the Cu-Zr system has the highest glass-forming ability (GFA), among which Cu46Zr54 has a critical casting thickness up to 2 mm5-7. The presence of aluminum, in the ternary Cu50Zr45Al5alloy, increases the GFA and the critical casting thickness can be increased up to 3 mm. This alloy shows a large supercooled liquid region (ΔTx = Tx - Tg), ΔTx = 72 K, and a high reduced glass transition temperature (Trg = Tg/Tl), Trg = 0,61. Some quaternary Cu-based alloys, such as those belonging to the Cu-Zr-Al-(Nb, Y, Gd)[9,1,10] and to the Cu-Zr-Hf-Al systems, presented a higher GFA. Cu-Zr-Al-Y system, have been shown unusual high GFA Cu46Zr42Al7Y5 alloys have shown critical casting thickness up to 1 cm and a ΔTx of around 100 K. Large ΔTx is an important feature because when metallic glasses are deformed inside this temperature region they exhibit homogeneous deformation with significant plasticity11 without crystallization. Considering these aspects, large BMGs can be produced by consolidation from amorphous powders12 or directly from amorphous ribbons. In this article, such kinds of materials were consolidated by extrusion; in temperatures inside the supercooled liquid region to compare their microstructural characteristics and thermal behaviors.
Ingots of the Cu46Zr42Al7Y5 alloy were prepared by arc melting under a titanium-gettered high-purity argon atmosphere. The ingots were produced from high purity elements Cu (99.99+%), Zr (99.5%), Al (99.99+%) and Zr (99.9%) after ultrasonically cleaning and mixing. To ensure compositional homogeneity, the ingots were melted several times. The alloys were then remelted in a melt spinning equipment to produce ribbons. For the first route, the ribbons were ball milled with ball-to-powder ratio of 20 : 1, rotation speeds of 150 rpm (during 1 hour) and 250 rpm (in the subsequent 8 hours), resulting in amorphous powders with mean diameter of about d50 = 106 µm. For the second route, small chips of 3 mm length were cut from the ribbons. Cylindrical pre-forms of the powders and of the ribbon chips were obtained by cold pressing at room temperature with uniaxial pressure of 1 GPa and then hot extruded in temperatures between Tg and Tx (671, 683 and 688 K), ram speed of 1 mm/min and extrusion ratio of 3 : 1, producing samples with a diameter of 4.5 mm. The structural characterization was performed by X-ray diffraction (DRX) in a Siemens D5005 diffractometer with CuKα radiation, and scanning electron microscopy (SEM) in a FEI-XL 30 FEG. The glass transition (Tg) and crystallization (Tx) temperatures were determined by differential scanning Calorimetry and scanned using a Netzsch DSC 200F3 Maia at a rate of 0.33 K/s. The mechanical properties were analysed by using Vickers hardness test.
3. Results and Discussion
Figure 2 shows the XRD patterns of the ribbon, powder and extruded samples. All of them exhibit only characteristic amorphous peak without any detectable crystalline phases. This indicates that a single amorphous phase was kept after milling and after consolidation processes.
Figure 3 shows typical crystallization DSC curves. Each of the traces exhibits endothermic event, characteristic of the glass transition, and a distinct undercooled liquid region, followed by one or two exothermic events, characteristic of crystallization processes. As also observed by other authors13, the presence of two crystallization peaks for the powders indicates differences between the routes concerning the type and magnitude of the phases and/or the relative thermodynamic instabilities of the different phase and phase combinations due to the introduction of mechanical energy into the system, during the milling process. The exothermic peak seen before the glass transition is the result of annihilation of excess free volume. Hence, it is possible to estimate the relative changes (due either to heat treatments or to deformation) in the free volume by monitoring its intensity (or enthalpy). From Figure 3 it is also possible to observe that, even with deformation, the annihilation of free volume in all events involving powders was higher than for the ribbons. Really, the as-received ribbon had the highest excess of free volume. The free volume was also decreased after the ribbon extrusion. Probably this was due to the annealing produced or during the extrusion process or during the milling in the case of the powders. However, considering only the extrusion process, the reduction of free volume was more intense for the ribbons than for the powders that appeared to be kept constant.
Table 1 shows the obtained values for Tg, Tx and ΔTx. As the expected, the results show a large supercooled liquid region for the ribbon (ΔTx = 98 K) and for the powder (ΔTx = 77 K), in agreement with the results reported by Xu. By comparing the thermal behavior of the ribbon and the powder it is possible to observe that there is no significant difference between Tg's values (663 K for ribbon and 664 K for powder) and considerable differences between Tx's and, consequently, between ΔTx's, with the smallest values presented for the extruded amorphous powder. Comparing powder and ribbon with extruded samples, it can be noted that the extrusion processes caused more significant variation in Tg values. For extruded samples using ribbons, Tg was increased in about 14 K and Tx was decreased in about 7 K, reducing ΔTx from 98 K to 77 K. For sample obtained through powder consolidation, the Tg increasing was 23 K and the Tx decreasing was 4 K, decreasing ΔTx in more than 27 K. This behavior can be explained by the evolution of the free volume during the processing routes. There are three processes that can change the local free volume concentration: diffusion, annihilation, and generation. The diffusion of free volume is analogous to the diffusion of vacancies in crystalline materials. The free volume is redistributed by diffusion until it is spatially uniform. In crystalline materials, vacancies can annihilate at certain locations, such as grain boundaries and dislocations, where the structural requirement of crystalline translational symmetry is relaxed. In metallic glasses, this requirement does not exist and free volume can annihilate at any position simply by the atomic rearrangement. The annihilation of free volume decreases the total free volume and the metallic glasses become denser after annihilation. As discussed before, after milling or after annealing during the extrusion process, there was a reduction of free volume, leading to more dense materials and so advancing the critical temperature of crystallization
Figure 4 shows SEM images of the extruded samples. In agreement with other authors11, the consolidated amorphous powder and ribbons with large ΔTx were deformed inside this temperature region exhibiting homogeneous deformation with significant plasticity. So is expected that can be produced by hot consolidation some fragmented materials with large BMGs, as the consolidation of amorphous powder12. By comparing powder and ribbon consolidation, it is possible to observe that the extruded ribbons were not fully consolidated (Figures 4a-c) and that its porosity decreased with the increasing of the temperature. Fully consolidation was achieved for the extruded powders (Figure 4d), probably due to the greater surface area, which produced more contact points between particles.
The results obtained from the hardness tests showed that it was obtained 623 HV for the extruded powders and 508 HV for the extruded ribbons at the same extrusion temperature that directly reflects the achieved porosity in such condition. Also, due to the highest amount of free volume in the ribbons, the reduction of hardness in such condition can suggest that nanovoids could be formed, due to the coalescence of the excess free volume during plastic deformation, and this could also be the reason for the observed lower hardness presented by the ribbons.
In summary, bulk metallic glasses of Cu46Zr42Al7Y5 were produced by extrusion process, using amorphous powder and ribbons. The more important results are:
The extrusion parameters (temperature and ram speed) led to an adequate processing window, resulting in fully extruded amorphous samples;
The milling method promoted strong decreasing of Tx probably due to the annealing produced during the milling process;
The microstructure of extruded powders showed lower porosity with better bonding than the extruded ribbons; and
The hardness value was higher for the ingots produced by the powder extrusion (623 HV) than for the ingots produced by the ribbons extrusion (508 HV) due to the higher porosity and to the generation of nanovoids produced by the coalescence of the excess free volume in the ribbons.
This work was supported by FAPESP, CAPES, CNPq and FAPEMIG.
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Received: December 8, 2011; Revised: June 6, 2012