Hot Consolidation of Partially Amorphous Cu-Ti Based Alloy: a Comparison Between Hot Extrusion and Hot Compaction by Sintering

Cu-Ti based bulk metallic glasses (BMG) alloys have attracted interest due to their superior mechanical properties and relatively lower costs1-4. However, the lack of ductility has limited the applications. To overcome this problem, composites of embedded nanocrystals in the Cu-Ti glassy matrix are being developed.5, 6. Furthermore, normal casting routes present the issue of size limitation, in which the largest obtained thickness was around 5mm3. The route of solid state amorphization followed by powder consolidation appears to be an attractive route to produce sized BMGs in alloy systems with marginal glass forming ability, as is the case of Cu-Ti-based alloys. Processes of amorphization by mechanical alloying have been described in Cu-Ti-Zr-Ni systems7-9. Consolidation of amorphous powder can be achieved by ultra-high pressure (UHP), by taking advantage of the decrease in viscosity of the supercooled-liquid region10, sintering-spark plasma (SPS)11-13, equal channel angular extrusion (ECAE)14 and extrusion15. According to preliminary tests16 , the Cu36Ti34Zr22Ni8 alloy is a strong candidate to form metallic glass. It has been shown16 that this alloy presents amorphous phase formation when processed by melt spinning showing supercooled-liquid region (ΔTx=Tx–Tg), ΔTX = 67K 16. The Assuming that Cu and Ni are similar, Cu36Ti34Zr22Ni8 composition is located near to deep eutectic in a Cu-Ti-Zr ternary phase diagram, which decreases the probability of intermetallic phases precipitation17. Therefore, in the present paper, the Cu36Ti34Zr22Ni8 alloy was chosen to produce an amorphous structure by ball milling followed by hot compaction and sintering.


Introduction
Cu-Ti based bulk metallic glasses (BMG) alloys have attracted interest due to their superior mechanical properties and relatively lower costs [1][2][3][4] .However, the lack of ductility has limited the applications.To overcome this problem, composites of embedded nanocrystals in the Cu-Ti glassy matrix are being developed. 5,6 Furthermore, normal casting routes present the issue of size limitation, in which the largest obtained thickness was around 5mm 3 .The route of solid state amorphization followed by powder consolidation appears to be an attractive route to produce sized BMGs in alloy systems with marginal glass forming ability, as is the case of Cu-Ti-based alloys.Processes of amorphization by mechanical alloying have been described in Cu-Ti-Zr-Ni systems [7][8][9] .Consolidation of amorphous powder can be achieved by ultra-high pressure (UHP), by taking advantage of the decrease in viscosity of the supercooled-liquid region 10 , sintering-spark plasma (SPS) [11][12][13] , equal channel angular extrusion (ECAE) 14 and extrusion 15 .According to preliminary tests 16 , the Cu 36 Ti 34 Zr 22 Ni 8 alloy is a strong candidate to form metallic glass.It has been shown 16 that this alloy presents amorphous phase formation when processed by melt spinning showing supercooled-liquid region (ΔT x =T x -T g ), ΔT X = 67K 16 . Te Assuming that Cu and Ni are similar, Cu 36 Ti 34 Zr 22 Ni 8 composition is located near to deep eutectic in a Cu-Ti-Zr ternary phase diagram, which decreases the probability of intermetallic phases precipitation 17 .Therefore, in the present paper, the Cu 36 Ti 34 Zr 22 Ni 8 alloy was chosen to produce an amorphous structure by ball milling followed by hot compaction and sintering.

Experimental
Cu 36 Ti 34 Zr 22 Ni 8 alloy ingots were prepared by arc-melting of high purity constituent elements (i.e., Cu (99,99%), Ti (99,99%), and Zr (99,50%), Ni (99,99%)) in a high-purity argon gas atmosphere and using Ti-gettering.All the ingots were re-melted at least 4 times to obtain a homogeneous master alloy.The ingots were crushed in a hammer-forging die before milling.After crushing, the material was loaded together with hardened-steel balls in a hardened steel vial to perform milling with a ball-to-powder ratio of 30:1.The ball milling process was performed in a Zoz Simoloyer with water cooling temperature control, at 900 rpm, under purified argon gas atmosphere during 8 and 16 hours.The obtained powders were cold compacted at room temperature under 1GPa of uniaxial pressure.Finally, samples with a diameter of 4.5 mm were produced by hot-extrusion, which was done inside the supercooled liquid region at 736 K, with ram speed of 1 mm/min and an extrusion ratio of 3:1.The structural characterizations of powder and extruded samples were performed by X-ray diffraction (DRX) in a Siemens D5005 diffractometer with CuKα radiation, and transmission electron microscopy (TEM) in a Tecnai F20 Philips.The glass transition (T g ) and crystallization (T x ) temperatures were determined by differential scanning calorimetry in a Netzsch DSC 200F3 Maia at a rate of 0.67 K/s.The mechanical properties were analyzed by using a Omnimet MHT Buehler -Wilson Hardness Tukon 1102 system with Vickers indenter loaded by 500 g.The system is automatic and applies the load during a time following ASTM E384 18 , and ISO 6507-1 19 standards.

Results and Discussion
Figure 1 shows the comparison between the XRD patterns of ribbon 16 , 8h and 16h ball-milled powders, and crystalline powders.This comparison shows that ball-milled powders did not present well-defined crystalline peaks, and that both peak-broadening and peak-positions are not matched to the ones for the ribbon.This can be an indication that the mechanism of amorphization is not the same.It has been reported that during ball-milling of pre-alloyed powders, the amorphization is thought to occur by the accumulation of structural defects such as vacancies, dislocations, grain boundaries, and anti-phase boundaries 20 .In comparison with amorphous alloys obtained by rapid solidification, the amorphization through reactions in the solid state has, in principle, the advantage to lead to techniques for producing bulky amorphous materials 21 .Moreover, due to the processing characteristics, it is reasonable to imagine that the physical properties, such as Tg and Tx of an amorphous alloy produced by the solid state reaction during milling, may be different from those of the amorphous alloy prepared by rapid cooling 21 .
In accordance with what was expected from the phase diagram, the unique phase precipitated in crystalline powder is Cu 2 TiZr and the solid solution of Cu with Ni shifted the peaks slightly to right.If one compares the ball-milled powder with the crystalline ones, the small peaks appearing at about 30, 35 and 63 degrees (the more intense) can be assigned as being the zirconium oxide, produced during milling.Due to the presence of higher intense peaks in the crystalline powders, ZrO does not appear in the spectrum and this is an indication that ZrO comprises less than 5% of the whole matrix.
Figure 2 presents the DSC curves of ball-milled powder in comparison with the ribbon, before correction of internal relaxation.The DSC curves confirm the formation of a totally different amorphous phase in the powders, which presented a higher glass transition temperature (T g ) and onset of the primary crystallization (T x ) than the ribbons.Moreover the T g and T x for ribbons are respectively about 100 K and 72 K lower than the ones for ball-milled powders.This will reflect in a larger supercooled liquid region for ribbons, ΔT x = 67K, when compared with the ball-milled powders, i.e., ΔT x = 39K and ΔT x = 36K, respectively for 16 and 8 hours.Even with lower ΔTx, the highest Tx observed for the powders means highest thermal stability 22 , which may be good for final mechanical applications.However, this will also create difficulties for further powder hot-consolidation due to the reduction of the window processing.
Figure 3 shows the bright field TEM images (BF-TEM) and selected area electron diffraction patterns (SAEDP) for both 8h and 16h ball-milled powders.From these images, it is not possible to observe the presence of nanocrystals, because they are BF images whose contrast can be influenced by differences in thickness.However, the SAEDP of powders milled till 8h is showing the presence of an almost continuous ring formed by small white spots inside a diffused halo, indicating the presence of very small crystals inside of an   amorphous structure.The SAEDPs for powders milled for 16 h is showing a broader diffraction halo, when compared with 8 h of milling, as well as continuous outer rings formed by very small spots.The indexation of these outer halos indicated the presence of ZrO, which increased with the milling time.The increased broadening and the absence of in-halo spots indicate that the sample is totally or, at least for the BF condition, "electron-diffraction" amorphous.
These nanocrystals can be better observed in Figure 4 which presents images of high resolution transmission electron microscopy (HRTEM) of the same regions presented in Figure 3. Figure 4a clearly shows cross-grating of Moiré lattice fringes, which are highlighted by red dashed circles and zoomed in the lower right inset, also confirmed by the Fast Fourier Transform (FFT) in the upper right inset, which presents a very high amount of spots.Even considering that they were barely observed, these lattice fringes indicate the precipitation of nanocrystals within the amorphous matrix 23 .The sizes of these nanocrystals were estimated to be about 20 nm. Figure 4b does not show clear evidence of nanocrystallization and the FFT presented in the inset clearly shows a pure amorphous state, which indicates that the effect of additional milling time resulted in a greater amorphous amount.
Moreover, the reinforced composite BMG/nanocrystals have been proven effective in improving their mechanical properties 5 .Even the extrusion process has been done in the supercooled liquid region, just a small part of the sample could be fully extruded (Figure 5a).This can be explained by effective nanocrystallization during the passage in the conical region (partially extruded), where the material is subjected to an intense shearing and locally generates heat 24,25 .This can induce localized nanocrystallization which increases the viscosity.In this context, the pressure needed to continue the process became too high for the pressure machine and it was impossible to continue the process.Further, the extruded part presented brittle behavior, and it was not possible to cut the sample for further preparation.Nevertheless, the previous part of the sample was intensely hot compacted by simple sinterization (Figure 5b) and the amorphous matrix was preserved.This can be observed in Figure 6a where the XRD pattern shows that the amorphous matrix is maintained inside the hot consolidated sample, but there was also a noticeable nanocrystallization indicated by an evolution of the intensity of crystalline peaks.The maintenance of amorphous matrix is also confirmed by the DSC curve presented in Figure 6b which shows crystallization and an increase of both T g and T x temperatures, resulting in a ΔT x of 40K.
Figure 7 shows images of BF-TEM, SAEDP in the inset of Figure 7a, and HRTEM of 16h ball-milled hot consolidated sample (Figure 7b).As was the case in Figure 3, from Figure 7a it is not possible to observe the presence of nanocrystals, but, differently from SAEDPs of Figure 3, the SAEDP of Figure 7 shows the presence of a continuous ring formed by small white spots inside a diffused halo.This means that crystals with very small sizes are embedded in the amorphous matrix and that these sizes are even smaller than the ones in Figure 3.These  The hardness test results show that the hot consolidated sample reaches HV 768.If one can assume that the yield strength (ρ) is ρ≈HV/3,6 14,26 this will give ρ ≈ 2765 MPa.These values can be considered relatively high when compared with others billets obtained in a consolidation process of Cu-Ti based alloys 13 .
From these results, it is apparent that traditional powder metallurgy followed by sinterization can be successfully used to produce sized BMGs or, at least, composites of BMG matrix and nanocrystals which can produce materials with enhanced mechanical properties.

Summary and Conclusions
Amorphous/crystalline nanocomposite powders were produced by high energy ball-milling and consolidated during hot extrusion process.Microstructural, thermal and mechanical investigations were carried out.
The microstructural and thermal analysis revealed the different mechanism of amorphization of the powders when compared with ribbons.Powders milled for 16h presented highest thermal stability indicated by highest crystallization temperature.
The amorphous matrix with embedded nanocrystals microstructure was maintained during extrusion.The hot consolidated samples presented a hardness value of 768 HV which is relatively higher than for other Cu-Ti based billets produced by similar consolidation process.
Traditional powder metallurgy followed by sinterization is a promising route to produce sized BMGs with enhanced mechanical properties.