Chapter 3 Experimental procedures
3.3 Property measurements and analyses
3.3.1 X-ray diffraction
The amorphous nature of the Au-based BMG was examined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The SIEMENS D5000 X-ray Diffractometer with Cu Kα radiation (λ = 1.5406 Å) at 40 kV and 30 mA is utilized. The range of the diffraction angle 2θ is within 20o to 80o scanning rate of 0.1ο per four seconds.
3.3.2 Qualitative and Quantitative constituent analysis
To identify the constituent components and confirm the composition percentage of the bulk metallic glasses, the specimens are characterized with a scanning electron microscope (SEM) with energy dispersive X-ray spectrometer (EDS). The cross-sectional surface of alloys sliced from bulk metallic glass rods is selected to examine the quantity of the designed compositions by EDS.
3.3.3 DSC thermal analysis
Glass forming ability parameters associated with the glass transition temperature (Tg), the crystallization temperature (Tx), the supercooled liquid region (ΔTx), the solidus temperature (Tm) and the liquidus temperature (Tl) are determined by differential scanning calorimeter (DSC). In this study, the thermal behavior of Au49Ag5.5Pd2.3Cu26.9Si16.3 bulk metallic glasses is analyzed using a SETARAM DSC131 differential scanning calorimeter under an argon
atmosphere, at the constant heating rate of 40 K/min and the heating range from 300 to 700 K.
The pre-presence air in the DSC chamber is flushed by pure Ar and it is keep the positive Ar pressure while heating.
3.3.4 Density measurement
The well-know principle of Archimedes is used as an appropriate means to measure the density of amorphous alloys. In this technique, the specimen contained in a vessel is weighted in air and gets a value of mass W1. Then, the specimen is weighted in a liquid of known density β and gets a value of mass W2. Therefore, the density of amorphous alloy can be evaluated by the following equation :
β
= −
ρ W W /
W
2 1
1
. (7)The atomic configurations of amorphous alloys are different from those for the crystalline alloys.
3.3.5 TMA analysis
The relationship among temperature, relative displacement, effective viscosity, and effective linear expansion coefficient were measured using a thermomechanical analyzer (TMA, Perkin Elmer Diamond) under the isochronal (or non-isothermal) condition at a heating rate of 10 K/min and a fixed compression load of 50 mN. The load was applied by a tip with a diameter of 3.8 mm. Temperature was calibrated by using pure In and Zn samples as standards. The diameter of the specimen is 3 mm with aspect ratio around 2. Displacement
data under various pressures and temperatures were collected simultaneously upon heating.
3.3.6 Micro-hardness testing
The sample hardness testing was conducted using a SHIMADZU HMV-2000 Vicker’s micro-hardness tester. In this testing, the parameter was carried out using a load of 200-500 g with 10 second duration, and the hardness value of each sample was average from 10 datum points.
3.3.7 Macro-compression testing
The compressive strength, elastic modulus and compressive strain were evaluated by using an Instron 5582 universal testing machine and the cylindrical specimens fabricated by the Cu mold casting method with a diameter of about 2 mm and a height of 4 mm. The compression specimens were sliced from Au49Ag5.5Pd2.3Cu26.9Si16.3 BMG rods.
All the surfaces are ground by silicon carbide abrasive papers from #1200 to # 2000 with water and polished. Before the compression specimen was fixed into the crossheads of the Instron machine, BN plate used as lubricant to decrease friction between the test samples and compression platens. The fracture surface morphologies were examined by SEM.
Ambient temperature compression tests were conducted on the specimens at different strain rates of 5x10-5,1x10-4,5x10-4,and 1x10-3 s-1.
3.3.8 Micro-compression testing
At room temperature, micro-compression tests were performed with an MTS
nanoindenter XP under the Continuous Stiffness Measurement (CSM) mode using a flat punch, which was machined out of a standard Berkovich indenter also by FIB. The projected area of the tip of the punch is an equilateral triangle of 13.5 μm, as shown in Figure 3.2. In order to check the alignment and calibrate the distance, the pre-compressed experiment to 1.5 μm depth was conducted in locations away from the pillar sample; the distance between the indented position and the pillar was within one hundred microns, as shown in Figure 3.3. A perfect equilateral triangle impression is considered the system is well aligned. Prescribed displacement rates of 0.25-500 nm/s, corresponding to the initial strain rates of 6x10-5–6x10-2 s-1, were used to deform the pillars. This work was performed by Dr. C.J. Lee.
3.3.9 Microstructure examination
The fracture surface morphologies of BMG specimens after macro-compression, and micro-compression testing, and hot embossing were examined by JEOL JSM-6400 scanning electron microscopy (SEM). The different fracture features after compression testing would also be examined in order to study the deformation mechanisms for BMGs.
3.3.10 Hot embossing of micro-lens and V-groove
To know the formability of the Au-based BMG, hot embossing of micro-lens array and V-groove were performed. Originally, the viscous forming was intended to be conducted by hot pressing at low loads. Transforming into the compressive stress, the forming stress should be in the 100 to 1000 kPa range. However, the oil hydraulic plate system provides large embossing load from 20 kg to 400 kg for the hot embossing process. The Au-based BMGs were tested under the applied load of 27 MPa, 62 MPa, and 156 MPa, respectively, for micro-lens array for a duration of 10 minutes. For the case of V-groove, the Au-based BMGs
were tested under the loads of 62 MPa, 137 MPa, and 156 MPa for durations of 1, 5, and 10 minutes. Figure 3.4 shows the hot embossing set-up of the oil hydraulic system. Figure 3.5 illustrates the Ni–Co mold with gapless hexagonal micro-lens array [88]. Figure 3.6 shows the profile of V-groove mold.
3.3.11 Surface morphologies
In order to see the surface morphologies of the Au-based BMG after hot embossing testing, the α step system and atomic force microscope (AFM) were utilized. Since the height difference of V-groove mold is over limitation of AFM, in the case, the samples could only be observed by the α step. For micro-lens array, both AFM and α step were in use.