Effect of Inoculation on Phase Formation and Indentation Hardness Behaviour of Zr47.5Cu45.5Al5Co2 and Zr65Cu15Al10Ni10 Bulk Metallic Glass Matrix Composites ()
Cite this paper
Bulk metallic glass matrix composites    have emerged as a potential material of future  bearing superior properties of strength  , hardness      and elastic strain limit    which place them in an unique position in structural material family   . Various applications have been proposed which make use of their superior properties  . These include, targets of high speed moving projectiles  (such as wipple shield of international space station    ), drill bits, parts of earth moving machinery  , shape memory alloys   , and parts in cryogenic applications (such as gears of outer and deep space missions)    . Various studies have been reported which shed light on different aspects of their manufacturing    and microstructural development     . These include observations under abrasion corrected transmission electron microscopy  , synchrotron light     and in-situ studies     but none have been made on the detailed use of more recent and advanced electron back scattered diffraction. Only three notable studies have been recently reported    but they are only qualitative and are limited by mere generalised phase identification. This is a new and unique technique which has been reported as complimentary to transmission electron microscopy   . It has unique ability to identify  and map crystallinity in materials     . It not only can generate diffraction patterns (kikuchi lines) but can efficiency map and compare presence of different phases (of distinct crystal structure)  in a bulk of material with existing crystal structures in international crystallographic diffraction database. This is a very good aspect which can be used efficiency to determine different properties of material   . With the help of energy dispersive X-ray spectroscopy (EDS) detector, it can also generate map of crystal structure of individual elements which can help identify their nature and microstructural features (e.g. primary, secondary and tertiary dendrite arm spacing)   . It also generates pole figures and grain size histograms which again can be used to determine mechanical properties of material. In present study, which is part one of two-part study, author aims to bridge this gap. A detailed secondary electron and back scatter electron imaging study of as cast inoculated wedge shape samples at three positions of wedge (tip, middle and widest portion), and secondary electron imaging study of indents produced by microhardness testing is carried out whose results are presented. Emphasis is laid on explaining phase formation, evolution and plasticity of as cast inoculated Zr based bulk metallic glass matrix composites. These studies are aimed at explaining and understanding phenomena of nucleation and growth in these alloys as a function of percentage of inoculant and cooling rate while later varies with change of point of observation along wedge  . This is first of its kind of study in this field which make use of carefully selected inoculants   to promote crystallinity, phase formation and increase toughness.
2. Experimental Procedure
Metallographic sample preparation: Two types of Zr based bulk metallic glass matrix composites namely Zr47.5Cu45.5Al5Co2 and Zr65Cu15Al10Ni10 are produced by vacuum arc melting button furnace and suction casting. These are subsequently cut by Struers abrasive cut off wheel with coolant flow. Then they are mounted in 25 grams Stycast epoxy resin with 25 grams Buehler conductive filler. Mounts are cured at room temperature for 8 hours. Once cured, they are placed in Thermolyne Type 48,000 furnace at 50˚C for 2 hours to harden final epoxy. Cured mounts were subsequently subjected to grinding and polishing. Grinding was done using silicon carbide papers. Manual grinding was done starting with papers of 120 to 240 grit with about 20 - 30 seconds on each grit. After this grinding was done on 400, 600, 800 and 1200 grit papers and then, polishing was performed. It was done using 9 µm, 3 µm and 1 µm Diamond Duo polishing solution using a Struers automatic rotating disc polisher for 5 minutes on each grit.“Plan” wheel was used for 9 µm polishing, “Dac” wheel for 3 µm polishing, and “Nap” wheel for 1 µm polishing. Final polishing was done by employing 0.04 µm colloidal silica solution for 5 minutes on “Chem” wheel. Secondary and back scatter electron microscopy: Secondary and back scatter electron microscopy of as cast inoculated samples was performed on Amray 1810 SEM. Microscope was equipped with Lanthanum Boride (LaB6) filament and operates at maximum cathode voltage of 30 KV. Sample was placed in chamber at a vacuum of 10−6 mbar created by diffusion pump connected to it. Secondary electron imaging was performed by collection of electrons at secondary electron (Everhart Thornley type) collecting detector. Back scatter electron imaging was carried out by manipulation of operating voltage and use of pseudo voltage controller. This voltage controller shifts the voltage such that deeper penetration is achieved, and scattered electrons give information about compositional contrast. Working distance was maintained at 25 mm. Indentation microhardness testing: Indentation hardness was performed on Shimadzu HMV―2T microhardness tester equipped with digital camera and operated via Computer Assisted Measurement System (C.A.M.S) software from Newage testing instruments. ASTM standard E384 was used for Vickers hardness testing while ASTM E140 was used for conversion and measurement of readings of test. Load applied was 500 grams for 15 seconds using diamond Vickers indenter. Impressions were manually read by adjusting cross wires and hardness values were obtained by computation of formula in software. After measuring hardness, indents were imaged using thermally assisted schottkey type field emission gun operated high vacuum FEI scanning electron microscope at Institute of Materials Science, University of Connecticut, Storrs.
3. Results and Discussion
3.1. Secondary Electron and Back Scatter Electron Imaging
Secondary electron and back scatter electron imaging  is carried out to determine phase formation and evolution with the change of percentage of inoculants. Images can be grouped into categories based on how the evolution is observed? Below, a detailed tale of their evolution is described in both type of model alloys (that is Zr47.5Cu45.5Al5Co2 and Zr65Cu15Al10Ni10). Emphasis is laid on explaining type, and morphology (size and shape of CuZr B2 phase in Zr47.5Cu45.5Al5Co2, β-Zr and Cu2Zr  and Zr2Cu  in Zr65Cu15Al10Ni10). A little light is also shed on explaining topography of microstructures. A considerable change in microstructure is observed as depicted by increased percentage (volume fraction) of crystal phase evolving out of liquid in background of glassy matrix. A change in morphology (size and shape) is also observed which can be attributed to inoculation treatment (availability of predominant heterogenous nucleation sites, nucleation at them and growth), cooling rate (amount/quantity of heat extracted), rate of heat transfer, diffusion, point of observation and mode of imaging (secondary electron or back scatter electron). Distribution of phases in bulk volume is also an important parameter and effort is made to explain its importance as well. All these parameters are shown to bear importance in explaining the overall crystallinity and increased toughness in these metal matrix composites.
Zr47.5Cu45.5Al5Co2 Bulk Metallic Glass Matrix Composites
Although, six phases have been identified and reported in literature in this type of alloy, only two (namely simple cubic spheroidal CuZr B2  , and brittle Al2Zr fcc  ) have been predominantly explored and will be reported here as well. Others have been reported to have similar or near similar structure to simple cubic, fcc or bcc but exist as off shoots of main features (spheroid or dendrite).
Evolution of microstructure (phase formation and development) is reported as a function of percentage of inoculant added. Secondary and back scatter electron imaging is performed on all samples in all three areas of sample (i.e. tip, middle and wider portion of wedge). It is hypothesized and observed experimentally that size, shape, morphology and count (number density and volume fraction) of phases changes with percentage of inoculant. With increasing percentage, a tendency of decrease in size (finer grain size) while increase in number density is observed and reported. This is due to availability of more nucleation sites and quick solidification rate which tends to fill up liquid space more rapidly. This is also in direct agreement with tendency of inoculants to provide sites of heterogeneous nucleation in parallel to homogeneous nucleation thus promoting nucleation and growth of primary phase precipitates.
Each figure is explained step by step below;
Figures 1(a)-(d) represents base alloy (native alloy) with zero percentage of inoculants. This is Zr47.5Cu45.5Al5Co2 alloy which is predominantly reported to carry CuZr B2 phase     -  . This appears as spheroids nucleating from background of rapidly cooling glassy liquid. In secondary electron images, this can be observed as light regions while glassy matrix is appearing as dark grey areas present in all interdendritic space. Back scatter imaging (Figure 1(d)), generates images based on compositional contrast in which spheroids are imaged relative to glassy matrix based on how their compositional differ. This can easily be seen spread all across the volume of sample. At tip, almost no crystals appear because of presence of highest possible cooling rate there which suppresses kinetics while as we move away from tip to wider portion of wedge, an
(a) (b) (c) (d)
Figure 1. (a)-(d): Zr47.5Cu45.5Al5Co2, inoculant = zero%.
increased percentage of crystallinity tends to appear. In this particular sample which does not have any percentage of inoculant, this phenomenon can be directly linked with cooling rate and rate of heat transfer. Crystals tends to get finer and smaller as area of interest move away from tip. Considerable percentage of crystals tends to appear at a point 2/3rd from tip and this prevails till widest portion.
As percentage of inoculant increase from zero to 0.25% (Figures 2(a)-(f)), crystallinity starts appearing in this alloy. This is depicted by observation of small crystals all throughout the volume of material. This is bench mark of central point of nucleation and growth observed in this alloy. Effect of inoculation shows its effect in the form of appearance of crystals even at tip where there is maximum cooling rate. These crystals have small size while they are uniformly distributed throughout the volume. Their small size is manifestation of high cooling rate observed in this region which suppresses growth. They tend to appear as spheroids and tend to adapt spheroid to plate like morphology with higher aspect ratio. Glass again tends to appear as continuous network in three dimensional interdendritic space. This network appears as dark areas in secondary electron images. As the area of interest moves away from tip to middle portion of wedge, a growth phenomenon is observed. Size of crystals tends to become large while their morphology tends to develop into sphere from spheroid (which is most widely reported morphology of this phase). A continuous long dendrite
(a) (b) (c) (d) (e) (f)
Figure 2. (a)-(f): Zr47.5Cu45.5Al5Co2, inoculant = 0.25%.
is also observed on the left side of micrograph which is proposed to have fcc crystal structure  (as revealed by EBSD study described elsewhere). Further, as the area of interest moves towards the widest portion of wedge, more prominent growth of earlier nucleated crystals is observed. Size of crystals tends to become large while they tend to coalesce into a large mass with grains diffusing into each other. This is typical manifestation of growth phenomena observed in widest region of wedge. Back scatter image (Figure 2(f)) also shows same phenomena in terms of compositional contrast.
As the percentage of inoculant increases to 0.5% (Figure 3(a)-(f)), more prominent crystals tend to appear in all regions of wedge. First, it is manifested by well-developed spheroids which are larger in size as compared to crystals appearing in same region at 0.25% inoculant. Large size and well-developed
(a) (b) (c) (d)
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