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Sci. Technol. Adv. Mater. 15 (2014) 035011
Refined dendrites
Deformed Region
(nugget zone)
Figure 1. Schematic of friction stir processing (FSP). The underlying
material is subjected to high strain during FSP resulting in
fragmentation and homogeneous distribution of the dendrites. The
inset shows the actual image of the nugget zone of friction stir
processed metallic glass. The highly strained material in the nugget
zone comprises fine dendrites compared to elongated ones in the un-
deformed region. All samples for thermal analysis and nano-
indentation were taken from the center of the nugget zone to
eliminate any non-uniformity and edge effects.
In this paper, we report on the structural changes in an
in situ ductile-phase reinforced metallic glass composite
(Ti48Zr20V12Cu5Be15) after friction stir processing. This
composite has one of the highest specific strength of all
known materials (>300 MPa cm-3 g-1) as well as high room-
temperature tensile ductility [1]. FSP was done with two
different conditions of 500 rpm and 900 rpm tool rotational
speeds. FSP has several important consequences-it leads to
refinement and more uniform distribution of the ductile den-
drite phase. In addition, there is change in hardness and
modulus of the amorphous matrix as well as the in situ
crystalline phase. We discuss a mechanism to account for the
observed change in mechanical and thermophysical behavior
based on shear band interaction with the crystalline phase.
Our approach offers a novel strategy in microstructural design
using FSP for a wide range of metallic glass compositions.
This is potentially transformative in the control of length scale
and distribution of crystalline phase in a metallic glass
composite.
2. Experimental procedures
The material used in the current study is a titanium-based
metallic glass composite, Ti48Zr20V12Cu5Be15. This compo-
site has nearly 47% volume fraction of ductile dendritic phase
distributed in the amorphous matrix. FSP was performed on a
computer numerical control vertical milling machine. The
FSP tool used was pin-less with shoulder diameter of 10 mm.
The FSP parameters comprise two different tool rotational
speeds of 500 rpm and 900 rpm, and a plunge depth of
0.3 mm. A schematic for FSP of the composite is shown in
figure 1. The elongated and inter-connected dendrites in the
]1
FSP Tool
Elongated dendrites
3. Results and discussion
Back-scattered SEM images of the as-cast metallic glass
composite, composite friction stir processed at 500 rpm (FSP
500) and 900 rpm (FSP 900) are shown in figures 2(a)-(f).
The as-cast microstructure consists of 47 vol % of body
centered cubic dendritic phase with composition
Ti66V19Zr14Cu1 (excluding Be, which cannot be measured by
energy-dispersive x-ray spectroscopy but is known to be
< 3 at.%), and 53 vol % of amorphous matrix with approx-
imate composition Ti32Zr25V5Cu10Be28 [1]. Using ImageJ
software, the average size of the dendritic phase was found to
be nearly 24 pm. The dendrites get fragmented during FSP as
shown in figures 2(c)-(f). The back-scattered SEM image of
the FSP 900 specimen cross-section is shown in figure 3(a).
The size distribution of the dendrites at increasing depth from
the surface along the specimen cross-section is given in
figure 3(b). The figure shows an average size modeled using
an ellipse fit, the perimeter as well as the dendrite circularity.
It is seen that the dendrites are finer near the top surface of the
specimen with an average size of nearly 4.5 um. The dendrite
size increases almost linearly with increasing depth upto
375 pm, which corresponds to the plunge depth of the FSP
tool. The peak temperature for FSP 500 and FSP 900 speci-
mens was measured to be nearly 300 C and 450 C,
respectively. The solidus temperature of the composite
(~682 C) [1] is significantly higher compared to the peak
temperatures reached during processing. Therefore, the
mechanism for dendrite refinement is likely to be mechanical
fragmentation from the high-strain deformation process rather
than dendrite dissolution. The degree of strain increases with
the increase in FSP tool rotational speed, which explains the
greater refinement of the dendrites at 900 rpm. The strain rate
2
starting material get refined and the dendrite circularity (C)
increases after FSP, where C = 4rcA/P2, A is the area, and P is
the perimeter. An image of the nugget zone for the processed
metallic glass composite is shown in the inset of figure 1.
Temperature of the surface during FSP was measured using a
K-type thermocouple. Microstructural studies were performed
using scanning electron microscopy (SEM). To measure the
grain size of the crystalline dendritic phase, samples were
etched using Kroll's reagent and the microstructure was
observed using SEM. Differential scanning calorimetry
(DSC) was used to determine glass transition temperature
(Tg), crystallization temperature (Tx), and enthalpy changes.
The hardness and modulus were obtained using nano-inden-
tation on the top surface of all the specimens. The reported
values for hardness and modulus are the average of ten
readings for each specimen. Testing was done at a peak load
of 10 mN using a standard Berkovich tip. All the samples
obtained for the DSC and nano-indentation test were taken
from the center of the nugget zone to eliminate any non-
uniformity and edge effects. High-resolution transmission
electron microscopy (HRTEM) was used to analyze the
structure of as-cast and the processed samples.
H S Arora et al
