Keywords
Molybdenum-Rhenium alloys; Pillar compression; Mechanical properties;
Orientation-dependent
Introduction
Molybdenum (Mo) and molybdenum alloys have stable body-centered cubic
(BCC) structure at a large temperature range, which makes Mo alloys wide
application in high temperature environment [1,2].
However, high ductile to brittle transition temperature (DBTT) leads
poor ductility and processing ability at room temperature in most Mo
alloys [3]. One of effective methods to improve
processing ability is adding Re to Mo alloys[4,5]. Leichtfried etal justified the Charpy
ductileto-brittle transition temperature decreased monotonically with
increasing rhenium concentration [6]. Combining
the cost and strengthening effect of Re element, Re contents in
commercial molybdenum alloys are often set as 5wt.%, 14 wt.% and 42
wt.%, respectively [6].
In most cases, thermal mechanical processes are necessary in Mo alloys
processes [7,8], including thermal rolling,
forging, etal. On one hand, thermal mechanical processes could
effectively enhance ductility and facilitate proceeding ability[9]. On the other hand, textures with specific
orientations occur during thermal mechanical processes[10].
Textures have large effects on the mechanical properties of Mo alloys[9,10]. According to Xing’s results, two grain
orientations of < 111 > fiber &
< 100 > fiber and < 100 >
fiber & < 110 > fiber are formed in the 47% and
95% rolled molybdenum sheets, respectively [10].
In Lobanov’s results, the molybdenum texture consisted of a set of
stable <110> orientations[11]. Brittle characteristics show typical
cleavage failure in Mo alloys with specific textures. However, It is
hard to evaluate individually how textures effect the mechanical
properties of Mo-Re alloys via traditional test methods. Here, we
propose to clarify this problem via micro-pillar tests.
A lot of researches have been made to investigate orientation-dependent
mechanical properties of Mo [12-16]. Schneider
etal have verified that small-scale Mo pillars exhibit a strain rate
sensitivity similar to bulk Mo, suggesting Mo pillars could evaluate
mechanical properties of bulk Mo well [12]. Kim
etal have studied that molybdenum nano-pillars exhibit
tension-compression asymmetry on [001] and [011]
oriented samples [14,15]. Size-dependent
mechanical responses in Mo are likely due to the relative shortage of
screw dislocations [13]. Mechanical properties in
micro-pillars are similar to bulk Mo as the diameter of pillars larger
than 2μm [16]. In this research, micro-pillars
with orientation [100] and [110] are prepared via FIB methods,
respectively, to investigate the effects of textures on mechanical
responses of Mo-Re alloys.
Experiments
High purity molybdenum powers and rhenium powers were used to prepare
Mo-Re alloys. These powers processed via mixing, pressing and sintering
to form as sintered Mo-Re alloys. The contents of Re were designed as
5wt.%, 14wt.%, and 42wt.%. These Mo-Re alloys are abbreviated as
Mo-5Re, Mo-14Re, and Mo-42Re, respectively. Thermal extrusion with ratio
4:1 was given to reduce geometrical size. Then, thermal annealing at
1100℃ was conducted to eliminate processed stress.
The as-annealed samples were firstly mechanically grinded and polished
to obtain grain sizes under optical microscope. Secondly, Electron
Backscatter Diffraction (EBSD JSM-7200F) was used to characterize the
grains with [100] and [110] orientations in all samples. Then,
pillars with diameter 2μm and height-diameter ratio 2:1 were compared
via focused-ion-beam (FIB). Finally, compression tests were conducted at
room temperature under nano-indentation with a flat indenter. The
diameter of indenter is 20μm that is much larger than the diameter of
sample. The strain rate and final strain were set as
1.67×10-3s-1 and 25%, respectively
for all tests. At least two samples were tested in same experimental
condition to ensure the results.
The surfaces of compressive samples were characterized via scanning
electron microscope (SEM). Microstructural details after deformation
were characterized by transmission electron
microscopy (TEM, JEOL JEM-2100F)
at 200KV. The samples for TEM tests were prepared via FIB methods. The
distribution of composed elements is detected by energy dispersive
spectrometer (EDS) operating at TEM.
Experimental results
Figure 1 shows the metallographs for as-annealed Mo, Mo-5Re, Mo-14Re,
and Mo-42Re. Recrystallization occurs in Mo and Mo-5Re because small and
round grains forming in the grain boundaries, as shown in Figure 1a and
1b. It is noted that the grain sizes in Mo and Mo-5Re are similar. Thus,
the addition of 5wt.% Re has little effect on grain growth. The
processed stresses in Mo-14Re and Mo-42Re have not been totally
released, which may be contributed to the enhanced recrystallization
temperature. The grain size in Mo-42Re is smaller than in Mo-14Re. In
short, grain sizes in all samples are several tens micrometers that is
suitable to pillar preparation.