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.