Fabrication of Mg-based bulk metallic glasses by pressure die casting method

Transcription

1 Indian Journal of Engineering & Materials Sciences Vol. 21, June 2014, pp Fabrication of Mg-based bulk metallic glasses by pressure die casting method A Borowski*, A Guwer, A Gawlas-Mucha, R Babilas & R Nowosielski Institute of Engineering Materials and Biomaterials, Faculty of Mechanical Engineering, Silesian University of Technology, Konarskiego Street 18A, Gliwice, Poland Received 1 July 2013; accepted 11 February 2014 The studied Mg-Cu-Y ternary alloy system exhibits high glass forming ability and investigations on such system have made great progress in study of Mg-based alloys since 1990 s. Bulk amorphous Mg 65 Cu 25 Y 10 alloy is obtained with maximum 10 mm thickness by high pressure die-casting. Mg-based bulk metallic glasses have much higher tensile, fracture strength and Vickers hardness and much lower Young s modulus in contrast to crystalline magnesium alloys. A three-stage process for the preparation of metallic glasses allows the production elements in the form of rods with diameter of 2 mm and 1.5 mm. The final stage of fabrication of Mg 65 Cu 25 Y 10 BMG is pressure die casting method into a water-cooled copper mold under a protective gas. The amorphous of the Mg 65 Cu 25 Y 10 rods are examined by using X-ray diffraction and differential thermal analysis. The fracture surface of Mg 65 Cu 25 Y 10 amorphous rod in as-cast state is tested by scanning electron microscopy with energy dispersive X-ray analysis. Keywords: Metallic glasses, Pressure die casting, Amorphous structure, Mg-based alloys Metallic glasses are the noncrystalline solid material formed by continuous cooling from the liquid state. The first metallic glass was produced in 1960 by Duwez by rapid quenching of a Au 80 Si 20 liquid 1. Further studies in different research centers have resulted in 1974 to discovered the first bulk metallic glass, which was the ternary Pd-Cu-Si alloy 2. Bulk metallic glass is a noncristalline solid material with a critical casting thickness more than 1 mm. In the next years have been studied lanthanide, magnesium, zirconium, and iron-based alloys. The critical thickness growing up and for Pd-based alloys 3 reached a 72 mm. Figure 1 shows the critical casting thickness as a function of the year the corresponding alloy was developed, with marked in red alloy Mg-Cu-Y. The critical casting thickness increased by more than three orders of magnitude in the last 40 years. A fit to the data shows that it tends to increase by one order of magnitude approximately every 12 years. If such a trend were indeed true, bulk metallic glass compositions may be found in the next 10 or 20 years that are, similar to oxide glasses. In 2005 Takeuchi and Inoue5 proposed a classification of bulk metallic glasses (Fig. 2). According to their classification Mg-based glasses represent a fourth group of BMGs. The new Mg-based amorphous alloys with high tensile strength and good ductility were first found in *Corresponding author ( artur.borowski@polsl.pl) Fig. 1 Critical casting thickness for glass formation of chosen alloy systems as a function of their discovery year 4 Fig. 2 Classification of bulk metallic glasses into seven groups [IIA: Alkaline Metal; ETM: Early Transition Metal (IIIA-VIIA); Ln: Lanthanide Metal; LTM: Late Transition Metal (VII-VIIB); BM: IIIB-IVB Metal (In, Sn,Ti, Pb)]5

2 260 INDIAN J. ENG. MATER. SCI., JUNE Earlier the glass formation of Mg-based alloys had been limited to Mg-Zn and Mg-Cu binary systems 6,7. In 1991, Inoue 8 got succeessful finding new Mgbased amorphous alloys; fabricated bulk metallic glass with a diameter of 4 mm by injection casting the Mg 65 Cu 25 Y 10 alloy into a copper mold. The next year, Inoue s group increased of diameter to 7 mm by use of high-pressure die casting method 9. Their research opened the door to design new groups of light alloys based bulk metallic glasses. Mg-Cu-Y ternary alloy system exhibits higher glass forming ability and investigation on such system have made great progress in study of Mg-based alloys since 1990 s. Figure 3 shows liquidus diagram of ternary Mg-Cu-Y alloy with marked reporting composition points with fully amorphous sample. Figure 3 shows arrows on the lines, which indicated the directions of decreasing temperature. There are seven ternary eutectic (E1 to E7) points, seven ternary quasi-peritectic (U1 to U7) points and seven maximum (m1 to m7) points present in this system. On this figure ternary Cu-Mg-Y diagram reporting composition points with fully amorphous samples Mg-based alloys are very attractive materials for many applications as new light-weight structural materials. Mg 65 Cu 25 Y 10 bulk metallic glasses have high tensile stress about 800 MPa, and fracture strength is about 1200 MPa at the room temperature, and Vicker s hardness reaches a value of about 200 HV. Density of Mg 65 Cu 25 Y 10 bulk metallic glasses 7,8,17,19 is about 3.2 g/cm 3. Unfortunately, the brittleness of the room temperature of Mg-based bulk metallic glasses causes some difficulties restrictive use of these materials. The plastic deformation of a metallic glass is localised within narrow shear bands, followed by the rapid propagation of these bands leading to sudden fracture 8. Among the Mg alloys discovered so far, Mg-Cu-Y and specially Mg 65 Cu 25 Y 10 has the largest supercooled liquid region ( T x =T x -T g 33 K;). Crystallization (T x ) and glass transition temperature (T g ) for Mg 65 Cu 25 Y 10 has a value of 463 K, and 430 K 6,20. Holds the sample of Mg 65 Cu 25 Y 10 bulk metallic glass at a temperature above glass transition temperature (T g ) caused a crystallization process with formation of very stable nanocrystalline Mg 2 Cu phase 12. Fig 3 Liquidus diagram of ternary Mg-Cu-Y with marked reporting composition points where fully amorphous sample was reported in literature 8-20

3 BOROWSKI et al.: FABRICATION OF MG-BASED BULK METALLIC GLASSES BY PRESSURE DIE CASTING 261 Experimental Procedure Ternary bulk metallic glass Mg 65 Cu 25 Y 10 was prepared. Pre-alloy was prepared with pure elements copper (99.95%) and yttrium (99.99%) in quartz crucible with 29 mm inner diameter by induction melting method under argon atmosphere. The cooled Cu-Y alloy is crushed and is added to alloy the pre-weighed the amount pure magnesium (99.99%). Master alloy are re-melted in the same manner as earlier, but at a lower temperature. The schematic diagram of induction melting Mg-Cu-Y alloys in quartz crucible is shown in Fig. 4. Studied samples of Mg 65 Cu 25 Y 10 bulk metallic glasses were manufactured by the pressure die casting method in form of rods (Fig. 5). The pressure Fig. 4 Schematic illustration of induction melting Mg-Cu-Y alloys in quartz crucible die casting technique is the method of casting a molten alloy ingot into water cold copper mould under protective gas pressure. Injection pressure of molten alloy in to mold is adjustable between 0.2 and 0.5 MPa and depending on the quantity of material cast. The master alloy was induction melted in a quartz crucible with 16 mm inner diameter and 1 mm diameter hole in the bottom of the crucible (Fig. 6) and cast into a water-cooled copper mold (Fig. 7) under a protective gas pressure to produce rod with diameters of 1.5 and 2 mm (Fig. 8). The differential thermal analysis (DTA) was used to determine melting temperature (T m ) and liquidus temperature (T l ). The heating rate was 6 K/min. Structure nalysis of the amples was carried outusing X-ray diffractometer (XRD) with Cu Kα radiation by XRD X Pert Pro PANalytical on the surface of the rod with a diameter of 2 mm. The data of diffraction lines were recorded by step-scanning method in 2θ range of The fracture morphology of studied Mg 65 Cu 25 Y 10 bulk metallic glass was analyzed using the scanning electron microscopy (SEM Supra 25) with magnification up to on fracture surface of rods with diameter of 2 mm. The scanning electron microscope is equipped with an energy dispersive spectrometer (EDX) provides surface chemical analysis of the field of view, linear or spot. Results and Discussion Figrure 9 show a DTA curves for pre alloy of Mg 65 Cu 25 Y 10 with heating rate 6 K/min endothermic Fig. 5 Schematic diagram of the pressure die casting equipment Fig. 6 Quartz crucible for die casting

4 262 INDIAN J. ENG. MATER. SCI., JUNE 2014 Fig. 10 X-ray diffraction pattern of Mg 65 Cu 25 Y 10 bulk metallic glassy rod in as-cast state with diameter of 2 mm Fig. 7 Schematic diagram of water-cooled copper mold Fig. 8 Outer morphology of as-cast glassy Mg 65 Cu 25 Y 10 bulk metallic glass as rod with diameter of 1.5 and 2 mm Fig. 9 DTA curve of Mg 65 Cu 25 Y 10 as master alloy (heating rate 6 K/min) Fig.11 SEM micrographs of the fracture surface of Mg 5 Cu 25 Y 10 amorphous rod in as-cast state with diameter of 2 mm

5 BOROWSKI et al.: FABRICATION OF MG-BASED BULK METALLIC GLASSES BY PRESSURE DIE CASTING 263 Fig.12 SEM micrographs of Mg 65 Cu 25 Y 10 amorphous rod in as-cast state with marked area for which energy dispersive X-ray analysis (EDX) was performed peak observed on DTA curve allowed to determine the value of melting temperature (T m ) 719 K and liquidus temperature (T l ) 791 K. X-ray diffraction analysis have revealed that the as-cast rod was amorphous (Fig. 10). The diffraction pattern shows only a single broad diffraction halo with the 2θ range of 35º-45º from the amorphous phase. The fracture surface appears to consist of small fracture zones, which leads to breaking of the samples into parts. Figure 11 shows SEM micrographs of tested different rod with diameter of 2 mm in as-cast state at different magnifications. The presented fractures could be classified as mixed type with indicated river and smooth fractures. The river patterns are characteristic for metallic glassy alloys. Indicating of the smooth region could be probably resulted from shear sliding and the distinctly developed shear bands on the fracture surface Figure 12 shows microanalysis of Mg 65 Cu 25 Y 10 amorphous rod with diameter of 2 mm in as-cast state from selected area of the fracture. Energy dispersive X-ray analysis EDX shows existence of magnesium, copper and yttrium elements in studied sample. The chemical composition analysis was only a qualitative test and confirmed existing of main elements in alloy. Conclusions The following conclusions can be drawn from the investigations on the samples of the Mg 65 Cu 25 Y 10 metallic glass: (i) The X-ray diffraction investigations have revealed that the studied as-cast rod was amorphous. (ii) The liquidus temperature assumed as the end temperature of the melting isotherm on the DTA reached, a value of 791 K for master alloy ingot. (iii) The presented fractures could be classified as mixed fracture with indicated river fractures, which as characteristic for bulk metallic glassy alloys. (iv) The success in preparation of the studied Mgbased bulk metallic glass in form of the rods is important for many practical applications like light-weight structural materials or biomaterial. (v) Subsequent attempt on present equipments to die casting allows to produce rods with a diameter of 4 mm (after change of copper mould) of ternary bulk metallic glasses such as Mg-Ca-Cu or Ca- Mg-Zn. References 1 Klement W, Willens R H & Duwez P, Nature, (187) (1960) Chen H S, Acta Metall, (22) (1974) Inoue A, Nishiyama N & Kimura H, Mater Trans, (38) (1997)

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