Figure 4.3 shows the selected area diffraction (SAD) pattern of the Au49Ag5.5Pd2.3Cu26.9Si16.3 alloy with 300 keV and 150 cm camera length. For the fully amorphous material, there should be concentric diffuse rings or halos without any discrete spots which stand for the crystalline phase. From the observation of Figure 4.3 , that shows the material is not fully amorphous.
4.5 DSC analyses
The thermal properties of the Au49Ag5.5Pd2.3Cu26.9Si16.3 BMGs were examined by DSC with the heating rate of 0.67 K/s (40 K/min), as shown in Figure 4.4. The crystallization exothermic reactions for the Au49Ag5.5Pd2.3Cu26.9Si16.3 BMGs in the DSC curves appear as one single peak, indicating one major phase was induced during DSC heating. The Au-Cu binary diagram is shown in Figure 4.5. The glass transition temperature (Tg), the onset crystallization temperature (Tx), the solidus temperature (Tm) and the liquidus temperature (Tl) are all marked by arrows in the DSC traces. From the plot, it exhibits distinct Tg and Tx, followed by a wide supercooled liquid region and then the exothermic reactions due to crystallization followed by the endothermic reactions due to melt. The Tg, Tx, Tm and Tl of the Au49Ag5.5Pd2.3Cu26.9Si16.3 BMGs at a heating rate of 0.67 K/s are 400, 450, 625 and 646K, respectively. The values of ΔΤx and ΔΤm are 50 and 21 K. The Trg, γ, and γm values are parameters to estimate the glass forming ability. From the DSC plot, we can get the Trg, γ, and γm values for the Au49Ag5.5Pd2.3Cu26.9Si16.3 BMGs at the heating rate of 0.67 K/s are 0.619, 0.430 and 0.774, respectively. Figure 4.6 shows the thermal properties with the heating rate of 0.167 K/s (10 K/min). The Tg and Tx are 403 and 442 K, respectively.
4.6 Density measurement
The simplest method to detect whether the volume is changed is to measure the density.
The Au-based BMG has the density value of 13.5 g/cm3. The value is smaller than the pure gold (19 g/cm3), since the Au-based contains many other light elements and free volumes.
4.7 Micro-hardness testing
The hardness of the injection cast Au49Ag5.5Pd2.3Cu26.9Si16.3 BMGs are measured by using the HMV-2000 micro-hardness tester. Ten positions are randomly chosen for each samples, then the Vickers hardness (Hv) are obtained by indenting each sample at the load of 200 g and with the duration time of 10 second. The value of micro-hardness is about 348± 10 which is much higher than ordinary gold alloys.
4.8 Macro-compression testing
The compressive mechanical properties at room temperature of the injection cast Au49Ag5.5Pd2.3Cu26.9Si16.3 BMGs were measured by using an Instron 5582 type machine.
Compression tests are conducted on specimens with various strain rates 5x10-5,1x10-4,5x10-4, and 1x10-3 s-1 with an h/d ratio of ~2.
The engineering compressive stress-strain curves for the injection cast Au49Ag5.5Pd2.3Cu26.9Si16.3 BMGs are shown in Figure 4.7. Table 4.2 shows the summary of the macro-compressive of the Au-based BMGs at different strain rates. The compressive fracture strength and fracture elongation at the strain rate of 5x10-5 s-1 are ~1003 MPa and
~1.85 %. At the strain rate of 1x10-4 s-1, the values are ~1122 MPa and ~1.97 %, respectively.
For the strain rate of 5x10-4 s-1, they are ~933 MPa and ~1.59 %. And the compressive
fracture strength and fracture elongation at the higher strain rate of 1x10-3 s-1 are ~827 MPa and ~2.00 %. The highest value of compressive fracture strength is the one at a strain rate of 1x10-4 s-1, and the highest fracture strain is the one at a strain rate of 1x10-5 s-1.
4.9 Compressive fracture characteristics
The outer appearance and fracture surface morphologies of the Au-based BMG are examined after compression testing by using SEM, as shown in Figures 4.8-4.21. The angle of the fracture plane inclination for the specimen at a strain rate of 5 x 10-5 s-1 is 38.4o, as shown in Figure 4.8. And Figure 4.9 illustrates the fracture surface morphology for the specimen after compression testing; the fracture surface reveals the features of the vein-like patterns. In an enlarged micrograph as shown in Figure 4.9, there shows the so called river-like morphology which appears in the form of islands surrounded by vein-like patterns and intermittent smooth regions that sometimes contain fine striations. The characteristic of vein-like patterns is due to a local change of viscosity in the fracture along shear band in metallic glasses. That shows the evidence for shear band propagation during compression testing.
Figure 4.11 illustrates that the angle of the fracture plane inclination is 38.8o for the specimen at a strain rate of 1x10-4 s-1. Figure 4.12 and Figure 4.13 illustrates the fracture surface morphology for the specimen after compression testing; the fracture surface reveals the vein-like patterns.
Figure 4.14 illustrates that the angle of the fracture plane inclination is 41.0o for the sample loaded at a strain rate of 5 x 10-4 s-1. Figure 4.15 shows the fracture surface morphology for the specimen after compression testing. The fracture surface reveals the
typical vein-like patterns. Furthermore, in an enlarged magnification, Figure 4.16 shows that the fracture surface for specimen consisting some spots around the vein-like patterns, those spots are another evidence to support the fact that this Au-based BMG is not fully amorphous.
Figure 4.17 shows that the angle of the fracture plane inclination is 39.2o for the specimen at a strain rate of 1x10-3 s-1. Figure 4.18 illustrates an enlarged micrograph of the outer appearance fracture plane of the Au-based BMG with a strain rate 1x10-3 s-1. Figure 4.19 is the micrograph of the side view. Figure 4.20 and Figure 4.21 show the fracture surface of Au-based BMG with strain rate 1x10-3 s-1. As shown in the pictures, the vein-like characteristics are denser than the others, indicating that the propagation of shear band is more intense and complicated. And that meets the results of Figure 4.7, the specimen with higher engineering strain, the more ductile one, is the one with more intense vein-like patterns.
4.10 Micro-compression testing
The micro-compression experiments were performed with an MTS nanoindenter XP under the Continuous Stiffness Measurement (CSM) mode using a flat punch. The load-displacement data are presented in Figures 4.22 and 4.23. Traditionally, the curves are converted to the engineering stress and strain curves, with the assumption that the sample is uniformly deformed. But the assumption is violated since a bulk metallic glass is deformed by the highly localized shear bands. Therefore, we present both the load-displacement data and the “apparent” stress-strain data.
The yield strengths of the 3.8 and 1 μm pillars range from 1629 to 1580 MPa, which are
much higher than that for the bulk Au49Ag5.5Pd2.3Cu26.9Si16.3 samples (900-1100 MPa). These data are summarized in Table 4.3. From Table 4.3 it is obvious that average strength of the 1 μm pillar (1849 MPa) is higher than that of the 3.8 μm pillar (1693 MPa). The improved strength is a natural consequence of decreasing defect population and, thus, reducing probability of shear band initiation in smaller samples.
Figures 4.24 and 4.25 indicate the displacement occurred almost at a sudden, which means that the strain does not happen in a gradual way but in the form of burst, even the experiments were performed under strain rate control conditions. Every strain burst event, independent of the strain rate, proceeds within approximately one second. The strain burst illustrates a rapid propagation of localized shear band.
Figure 4.26 shows the morphology of compressed pillar samples. Shear band is anticipated to initiate from the corner of contact between specimen and compression punch due to the sample experiences the maximum stress, not only because it has the minimum cross section as a result of sample taper, but also due to the large constraint caused by the friction between the test specimen and punch. A finer shear band spacing was seen to form at a faster strain rate, as evident from Figure 4.22. The 1 μm pillars has less shear bands than the larger one. This implies that as the size decreases, it needs more energy to form shear bands, further enhancing the yield strength. It seems the shear band number would increase with increasing specimen size and strain rate.
4.11 TMA Analysis
Figure 4.27 illustrates the typical TMA and differential thermo-mechanical analyzer (DTMA) curves measured at a stress level of 7.1 kPa on Au-based BMG. The DTMA curve
is obtained from the derivative of the displacement with respect to time. The onset temperature for the viscous flow (Tonset), the semi-steady-state viscous flow temperature (Tvs), and the finish temperature for the viscous flow (Tfinish), are marked on the TMA or DTMA curve. The values of Tonset, Tvs, and Tfinish are 436 K, 456 K, and 473 K, respectively. The Tg
and Tx measured from the DSC curves without loading are 400 and 450 K, respectively.
Those differ from the Tonset and Tfinish obtained from TMA under loading.
Figure 4.28 shows the viscosity data on the Au-based and Mg-based BMG. The viscosity of the Au-based BMG is ranged from 108 to 1011 Pa.s. At first, the curve decreases with increasing temperature and reach a minimum point when the temperature is near the supercooled liquid region. Above the temperature of Tvs, crystallization starts to occur and viscosity increases.
4.12 Hot embossing of V-groove on Au-based BMG
Since the semi-steady-state viscous flow temperature (Tvs) of the Au-based BMG is around 183oC (456 K), the temperature of the hot embossing experiment is set at 177oC (450 K). Figures 4.29, 4.30, and 4.31 show the replicated patterns, V-groove, on the Au-based BMG materials by optical microscopy (OM), formed at 177oC and 137 MPa for 1 minute, 5 minutes, and 10 minutes, respectively. By the observation of the OM pictures, with the same experimental temperature and pressure, the specimen with longer forming time, 10 minutes, has a longer interspacing between two bands, 153 μm, as shown in Figure 4.31. This could be explained that longer duration time enables the viscous flow of the BMG material to fill up the Ni–Co mold more completely. As a result, better surface quality and formability can be achieved. The value of band interspacing of the specimen at 177oC and 137 MPa for 1 minute is 143 μm. And the value of band distance of the specimen at 177oC and 137 MPa for 5
minutes is 148 μm. The data are all listed in Table 4.3.
Figure 4.32 illustrates the replicated patterns, V-groove, on the Au-based BMG materials by OM at 177oC and 62 MPa for 10 minutes. And Figure 4.33 shows the replicated patterns, V-groove, on the Au-based BMG materials by OM at 177oC and 156 MPa for 10 minutes. To know the exact difference of the specimens under different pressures, the α step system was utilized. Figure 4.34 shows clearly the heights of specimens under 62 MPa, 137 MPa, and 156 MPa, respectively. The average height of the specimen at 177oC and 62 MPa for 10 minutes is 11 μm. The average height of the specimen at 177oC and 137 MPa for 10 minutes is 45 μm. And the average height of the specimen at 177oC and 156 MPa for 10 minutes is 84 μm. All the data and mold information are all listed in Table 4.4 as well. Figure 4.35 shows The morphological curves of V-groove imprinted Au-based BMG at 177oC and 137 MPa for 1, 5, 10 min, respectively, by the α step.
4.13 Hot embossing of micro-lens array on Au-based BMG
For hot embossing of micro-lens array, Figure 4.36 shows replicated patterns on Au-based BMG materials imprinted at 177oC and 27 MPa for 10 minutes. Figure 4.37 shows replicated patterns on the Au-based BMG materials imprinted at 177oC and 62 MPa for 10 minutes. Figure 4.38 shows replicated patterns on Au-based BMG materials imprinted at 177oC and 156 MPa for 10 minutes. Figure 4.39 illustrates the morphological curves of mold and micro-lens array on Au-based BMG at 177oC and 27 MPa, 62 MPa, and 156 MPa, respectively, for 10 minutes by α step. The height and width of individual specimen under different pressure and mold information are all listed in Table 4.5. Figure 4.40 shows the replicated micro-lens array patterns on the Au-based BMG materials at 177oC and 156 MPa for 10 minutes by SEM
Before the embossing process, there are some marks on the surface of Au-based BMG , which is produced during its manufacturing process. To eliminate that, the surface was polished with abrasive papers from No. 1200 to No. 2000. However, parallel bands and marks on the surface can still be observed clearly in all the pictures no matter by OM or SEM.