Electronic Material Properties - und Geowissenschaften ...

The glass transition temperature (Tg) of Cu50Zr50 and Cu47.5Zr47.5Al5 2 mm∅ rods are
estimated to be 671 K and 698 K, respectively. The supercooled liquid region (∆Tx) for
Cu50Zr50 and Cu47.5Zr47.5Al5 is 46 K and 74 K, respectively. This proves that addition of Al
increases the glass-forming ability of binary Cu50Zr50. The resulting Cu47.5Zr47.5Al5 glass
exhibits high strength (2265 MPa) together with a large room temperature ductility of up to
18%, as depicted in Fig. 2. After yielding a strong increase in the flow stress is observed
during deformation indicating a “work-hardening” like behavior.
In recent years, a large number of high strength and ductile Ti-base nanocomposites with
bimodal distribution of micrometer-sized primary bcc β-Ti dendrites / primary FeTi(Co)
phase distributed in a nano-/ultrafine eutectic matrix has been reported. Similar properties
have been achieved by tailoring the eutectic structure without any micrometer-sized
toughening phase. For example, (Ti0.705Fe0.295)100-x Snx (x = 0 and 3.85) ultrafine eutectics
were prepared by slow cooling from the melt through cold crucible casting into 6 mm
diameter rods. The microstructure of both alloys exhibits an eutectic consisting of A2 (β-Ti)
and B2 (FeTi) phases. Ti67.78Fe28.36Sn3.85 shows pronounced colony boundaries and
equiaxed colonies with a cell size of 50 – 10 µm (Fig. 3). The growth of the FeTi phase is
rather parallel at the center of the colony with an interlamellar spacing (λ) of 300 nm, which
is more refined than the binary Ti70.5Fe29.5 eutectic (λ = 500 nm). At the colony boundary
the growth of the FeTi phase becomes restricted in the longitudinal direction and becomes
coarser. TEM investigations suggest that both alloys exhibit the same orientation
relationship ([110]β-Ti || [110]FeTi) and [200]β-Ti || [200]FeTi) between the A2 and B2 structures
(inset to Fig. 3). XRD analysis revealed that there is an increase in the difference between
the lattice parameter values (δ) of the A2 and B2 structures from 0.017 nm (Ti70.5Fe29.5) to
0.028 nm (Ti67.78Fe28.36Sn3.85).
Ti67.78Fe28.36Sn3.85 exhibits a significantly improved ductility reaching a fracture strain of εf =
9.6 % compared with εf = 2.6% for Ti70.5Fe29.5. The fracture strength is σ max = 1935 MPa
for Ti70.5Fe29.5 and σ max = 2260 MPa for Ti67.78Fe28.36Sn3.85 (Fig. 4). Possibly a higher lattice
mismatch in the ternary alloy (δ = 0.028 nm) between the A2 and B2 structures introduces
coherency strains at the interface, which may be a favorable condition to absorb the
dislocations emitted from the β-Ti(Fe,Sn) phase during deformation. This, in turn, allows to
emit dislocations or activates slip in the FeTi phase providing better slip transfer across the
interface.
Fig. 3: SEM (secondary electron) image of the
Ti67.79Fe28.36Sn3.85 nano-/ultrafine eutectic. Inset:
SAED pattern showing diffractions from A2 (β-Ti)
and B2 (FeTi) structures.
Fig. 4: Compressive engineering stress-strain
curves of Ti70.5Fe29.5 and Ti67.79Fe28.36Sn3.85
nano-/ultrafine eutectics.
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