Supporting Materials of the Article Entitled
“Superior Tensile Ductility in Bulk Metallic Glass with Gradient Amorphous
Structures”
Q. Wang1*, Y. Yang2, H. Jiang3, C. T. Liu2, H. H. Ruan3 and J. Lu2*
1
Laboratory for Microstructures, Institute of Materials Science, Shanghai University,
Shanghai 200072, China
2
Centre for Advanced Structural Materials, Department of Mechanical and
Biomedical Engineering, City University of Hong Kong, Tat Chee Avenue, Hong
Kong
3
Department of Mechanical Engineering, The Hong Kong Polytechnic University,
Hong Kong
1. Measurement of Tensile Strain of as-Cast and as-Treated BMGs with Digital
Image Correlation (DIC)
Tensile testing was carried out on a MTS™ Insight Electromechanical Tester
(MTS, Eden Prairie, MN, USA). Tensile specimens with a 3-mm gauge length,
0.4-mm gauge thickness and 1-mm gauge width were wire cut from the as-cast and
SMATed BMG plates, respectively, prior to the tensile tests. These BMG tensile
samples were first preinstalled into a set of home-made jigs, which were then clamped
to the MTS jaws for fixation. As the BMG tensile samples were too small for
clamping an extensometer, the displacement or strain data were collected using an
optical method. A series of pictures recording the deformation history of the BMG
sample were first captured with a fast-rate camera and then analyzed using the
commercial package Vic-3D™ (Correlated Solutions Inc., Columbia, SC, USA),
which was developed based on the well-established digital image correlation (DIC)
algorithm to extract the full-field displacements and strains of a deformed specimen1,2.
Physically, the accuracy of our DIC-based strain measurement relies on the optical
magnification and the resolution of the imaging sensor. For our case, the camera was
set to an optical magnification of 1with a working distance of 65mm, which led to
the correspondence of one pixel size with an area of 4.4m4.4m. Since the
interpolation schemes and proper affine functions employed in the DIC algorithm
enables an accuracy of 0.05 pixel, the typical displacement resolution in our
measurement is ~0.22 m, corresponding to a strain resolution of ~73 micro-strains
for a sample with a gauge length of 3mm, which is sufficient for our experiment as
the yield/fracture strain of the BMG sample is on the order of 10,000 micro-strains.
Furthermore, a direct-current (DC) regulated fiber optic illuminator was used to
provide
stable
and
even
illumination
of
the
sample
surface.
Figure S1 The displacement fields along the tensile direction overlaying the as-cast (a)
and 25min-SMATed (b) BMG tensile sample at different levels of extension.
In the tensile experiment, a strain rate of 0.01% per second was used and images
were captured 1 frame per second. Figures S1 (a) and (b) show the typical
displacement fields along the tensile direction, which were extracted by DIC at
different levels of sample extension, overlaying the corresponding as-cast and
25min-SMATed samples under deformation, respectively. Within our expectation, it
can be seen that the displacement fields along the tensile direction exhibit a linear
distribution and their gradients then give rise to the overall mechanical strain
experienced by the stretched BMG sample.
2. Achieving superior tensile ductility in a Zr55 Cu30Ni5Al10 BMG through SMAT
Alloy ingots with a nominal composition of Zr55Cu30Ni5Al10 were prepared by
arc-melting mixtures of pure metals (with the purity of above 99.9%) in Ti-gettered
argon atmosphere. Each ingot was melted several times to obtain chemical
homogeneity and finally cast into a water-cooled copper mold to obtain a rectangular
plate with the dimension of ~80101 mm. The thickness is further reduced to 0.4
mm by grinding and mechanical polishing for mechanical characterizations such as
tensile test and residual stress measurements.
In the present study, the gradation of surface glassy structure and optimization of
the pre-generated residual stress profile in the Zr based BMG plate (0.4mm by 10mm
by 60mm in size) is performed by using surface mechanical attrition treatment (SMAT)
with 512 stainless steel balls of 2mm (or 3mm) in diameter. In the SMAT processing,
multi-directionally flying shots, which are activated by an ultrasonic vibration
generator at a high frequency of 20k Hz, impact on the surface of a material
repeatedly. Consequently, the severe surface plastic deformation is expected to alter
the local material structure and residual stress profiles. The SMAT processing was
lasted for 40, 60 minutes, respectively. Note that the process is stopped every 1.5
minutes in order to minimize the effects of overall temperature rising. For one of
60min-SMATed samples, a two-stage treatment route were adopted: 3mm- ball was
used for 30min and then 2mm-ball for the remaining 30min. The amorphous nature of
Zr based BMG plates before and after SMAT were confirmed by X-ray diffraction
(XRD) using a diffractometer with Co-Kradiation, showing diffuse maxima without
Brag diffraction peaks from crystal phases (See Fig S2 (a)).
Fig. S2 Comparison of the experimental results obtained from the as-cast and
SMATed 0.4-mm thick Zr55Cu30Ni5Al10 metallic glass. (a) the X-ray diffraction
patterns confirming the amorphous structure of all BMG samples before and after the
SMAT processes, (b) the variation of the residual stress distribution along the sample
thickness with the SMAT time, (c) The room-temperature true stress-strain curves
obtained at the strain rate of 110-4 s-1 (marks indicate where the stress-curve curves
start to depart from a linear response) (d) The experimentally obtained yield/fracture
strengths as a function of the BMG tensile ductility showing agreement with the
theory.
Figure S2 (b) shows the distributions of residual stress along thickness direction
in the as-cast and SMATed Zr based BMG, which were also measured by using classic
hole-drilling method. Compared to the SMATed Cu46Zr47Al7, the treated Zr-based
BMGs exhibits similar residual stress distributions with a compressive stress
maximum ranging from -210 to -430 MPa at 50 ~60 m depths. Note that after SMAT
for 40min, the compressive stress maximum peaks at -320MPa, but further increasing
of SMAT time up to 60min leads to its reduction to -210 MPa, implying that the
treated Zr based BMG might have also undergone intense structural evolution with
the severe surface plastic deformation caused by the SMAT process. The greater
magnitude of compressive residual stress peak for the Zr based BMG subjected to two
–stage treatment may be associated with the alternative use of 3mm and 2mm
stainless steel ball, which could lead to a more pronounced graded glassy structure
with larger volume fraction of liquid-like regions introduced into in the surface layers
of the treated BMG.
Figure S2 (c) displays the true stress-strain curves obtained from the as-cast and
SMATed BMG samples using the real-time image correlation technique. It can be
clearly seen that as-cast Zr55Cu30Al10Ni5 samples possesses a very limited plastic
strain (<0.014%) prior to catastrophic fracture, which is common for many metallic
glasses tested in tension at room temperature. However, similar to the Cu-Zr-Al BMG,
the SMATed samples exhibit, more or less, a certain degree of strain hardening and
tensile ductility. It is worthwhile to note that the measured tensile ductility attains
about 3.6% (~4%) for the 60min-SMATed sample obtained through a two-step
treatment approach. Moreover, compared to the Cu-Zr-Al BMG, the variation of
strength with tensile ductility for Zr- based BMG also follows quite a similar rule(see
Fig.2S(d)): as a result of strain hardening, the fracture strength of the SMATed BMG
apparently rises with the tensile ductility; however, their yield strength declines as the
fracture strength increases. Additionally, all of the Zr-based BMG samples failed via
shear cracking during the room-temperature tensile tests.
References:
1
Huang, Y. H., Liu, L., Yeung, T. W. & Hung, Y. Y. Real-time monitoring of clamping force of a
bolted joint by use of automatic digital image correlation. Optics & Laser Technology 41,
408-414 (2009).
2
Ait-Amokhtar, H., Vacher, P. & Boudrahem, S. Kinematics fields and spatial activity of
Portevin–Le Chatelier bands using the digital image correlation method. Acta Mater. 54,
4365-4371 (2006).