Microstructure Evolution and Mechanical Properties of a Directionally Solidified Nb-TiSi-Cr-Al-Hf-Dy Alloy

The Nb-Ti-Si-Cr-Al-Hf-Dy alloy was prepared by conventionally casting and then directionally solidified (DS) with the optical floating zone melting technology. Microstructural evolution and mechanical properties of the as cast (AC) and DS alloys were studied by XRD, SEM, TEM, compression and bending tests. The results exhibit that -(Nb,Ti)5Si3, (Ti,Nb)5Si3 and (Nb,Ti)ss phases formed the microstructure of the AC alloy, with precipitates of Cr2Nb, Dy2O3 and (Nb,Ti)3Si along phase or grain boundaries. The DS alignes (Nb,Ti)ss and -(Nb,Ti)5Si3 lamellae parallel to the growth direction and refines the eutectic cell, but coarsens the primary -(Nb,Ti)5Si3 phase. The DS alloy exhibits strong orientation of (310)(Nb,Ti)5Si3 and (110)(Nb,Ti)ss along the growth direction. TEM observation reveals that (Nb,Ti)ss and -(Nb,Ti)5Si3 phases have an orientation relationship of [110](Nb,Ti)ss // [310] -(Nb,Ti)5Si3 and (002)(Nb,Ti)ss // (002) -(Nb,Ti)5Si3. Compared with the AC alloy, the DS increases the mechanical properties of the alloy obviously and the alloy DS at 8 mm/h exhibits the best mechanical properties.


Introduction
Intermetallic compounds have been paid much attention recently, due to their excellent properties in some aspects [1][2][3].As one type of high-temperature intermetallic compound, niobium silicide possesses many superiorities, such as excellent high-temperature creep strength and high melting point (about 2793 K) [4,5].Therefore, the niobium silicide has been thought as a potential material to apply in super high-temperature environment.However, the poor oxidation resistance and undesired room-temperature fracture toughness restrict its industrial application.To solve these problems, many methods have been applied and some niobium silicide based alloys have been developed [6][7].Among these alloys, the Nb-Ti-Si-Cr-Al-Hf alloy is the most promising one which makes use of the Nb solid solution (Nbss) ductile phase as the matrix and the Nb 5 Si 3 as strengthening phase [8].Due to the relative low high-temperature strength of Nbss, the massive presence of Nbss would be detrimental to the high-temperature property.Then, to take full use of the strength of Nb 5 Si 3 and ductility of Nbss, the Nbss and Nb 5 Si 3 phases should be arranged as an alternative lamellar structure.Moreover the precipitation of NbCr 2 Laves phase is also detrimental to the fracture toughness.Therefore, the optimization on the morphology and distribution of these stiffness phases is important, which could help to increase the mechanical property.Based on recent researches [9][10][11], the rare earth element could scavenge impurities and refine grain boundary, which could increase the fracture toughness further.Therefore, in the present research trace Dy has been added in the Nb-Ti-Si-Cr-Al-Hf alloy.In order to obtain an Nbss/Nb 5 Si 3 alternative lamellar structure, the Nb-Ti-Si-Cr-Al-Hf-Dy alloy has been directionally solidified (DS) with the optical floating zone melting technique.Microstructure evolution and mechanical property improvement of the DS alloy are studied.

Experimental
The alloy vacuum arc melting technology was employed to fabricate the alloy with nominal composition of Nb-22Ti-16Si-7Cr-3Al-2Hf-0.1Dy (at.%) from pure metal niobium (99.9%), titanium (99.9%), silicon (99.9%), chromium (99.9), aluminum (99.9%), hafnium (99.8%) and dysprosium (99.8%).A part of the obtained alloy ingot was studied at the as cast (AC) state and the other was left for the DS process.The electro-discharge machining (EDM) was applied to cut the DS rods from the alloy ingot and their dimensions are ĭ8 mm × 80 mm.The DS process is carried out on the optical floating zone furnace (OFZ), which adopted the high-energy visible light to remelt the alloy rod and synchronous rotation to guarantee the uniform and orderly melting.Three growth rates of 5, 8 and 11 mm/h were applied on the present the DS alloy.
Microstructural characterisations of the AC and DS alloys were performed by Phenom™ Pro scanning electron microscopy (SEM).The Bruker D8 X-ray diffraction (XRD) with a Cu KĮradiation was employed to analyze the phases at 40 kV and 40 mA.Constitute phase composition in the alloy was analyzed by the EPMA-1610 electronic probe microanalysis.The specimen for transmission electron microscope (TEM) characterization was cut from the AC and DS alloys with thickness of 0.4 mm.Then the specimen was polished to 30 ȝm and shaped into ĳ3 mm in size followed by twin-jet electropolishing.TEM analyses were carried out on the JEOL-2100 high-resolution transmission electron microscope (HRTEM).Compression sample (4×4×6 mm 3 ) was cut from the AC and DS alloys and mechanical grounded by 800-grit SiC abrasive.Gleeble 3800 was employed to carry out the compression test at strain rate of 2×10 -3 /s.Fracture toughness was obtained by the three-point bending test.The bending test specimens were cut along the DS growth direction to get the lamella structure perpendicular to the force direction.The size of the bending test specimen is 3 mm × 6 mm × 30 mm with a 3 mm notch in middle.

Results and Discussion
Backscattered electron SEM micrographs of the AC Nb-Ti-Si-Cr-Al-Hf-Dy alloy are presented in Fig. 1 (a) and (b).It could be found that the microstructure of the AC alloy primarily comprises four kinds of phases.According to recent research [8], the gray-white matrix should be (Nb,Ti)ss phase and gray phase should be Į-(Nb,Ti) 5 Si 3 .The (Nb,Ti)ss and Į-(Nb,Ti) 5 Si 3 phases form the eutectic structure with the embellishment with of black (Ti,Nb) 5 Si 3 phase and some white particles.Moreover, the microstructure shows coarse Į-(Nb,Ti) 5 Si 3 dendrite.The black (Ti,Nb) 5 Si 3 phases prefer to precipitate along the eutectic cell boundary and form the semi-continuously necklace shape.White particles prefer to precipitate along phase and grain boundary, but its distribution is random.XRD analysis on the AC alloy exhibits that the (Ti,Nb) 5 Si 3 , Į-(Nb,Ti) 5 Si 3 and (Nb,Ti)ss phases are main constituents, see Fig. 1 (c).XRD analysis shows the Į-(Nb,Ti) 5 Si 3 phase possesses the I4/mcm crystal structure (Cr 5 B 3 type) and its amount is higher than that of the (Ti,Nb) 5 Si 3 phase.Additionally, the phases with different crystal orientation exhibit homogenisation tendency in the AC alloy.TEM analysis and selected area diffraction pattern (SADP) on the white phase confirm it to be Dy 2 O 3 oxide, which owns a hexagonal crystal structure (a=b= 3.82 nm, c=6.11 nm) and the space group of P-3m1, as presented in Fig. 1 (d).The formation of this kind of oxide should be attributed to the activity of the rare earth Dy, which could capture the oxygen atom to form the oxide.Such behaviour is helpful to improve the adhesion of grain boundary.Further TEM observation also reveals the small Cr 2 Nb phase along the Į-(Nb,Ti) 5 Si 3 /(Nb,Ti)ss phase interface, as presented in Fig. 1 (e).The reveals that it possesses the C14 crystal structure.Due to the high stiffness of this kind of phase, its precipitation along grain boundary would result in stress concentration and microcrack propagation, which is detrimental to the fracture toughness.Because the formation of Cr 2 Nb phase is related to the element segregation, the change of solidification mode could help to eliminate this kind of phase.In addition, it is still found that there are some metastable (Nb,Ti) 3 Si phases along the eutectic cell boundaries, as shown in Fig. 1 (f).Moreover, an orientation relationship between the (Nb,Ti)ss phase and the (Nb,Ti) 3 Si phase can be described as: [112] (Nb,Ti)ss // [001] (Nb,Ti)3Si and (110) (Nb,Ti)ss // (110) (Nb,Ti)3Si .The presence of this kind of phase should be ascribed to the solidification rate and element segregation.SEM observations on the microstructure of the Nb-Ti-Si-Cr-Al-Hf-Dy alloy DS at different growth rate are exhibited in Fig. 2. Clearly, the DS process has changed the microstructure of the alloy greatly.Different from the AC alloy, the DS alloy mainly exhibits the Į-(Nb,Ti) 5 Si 3 /(Nb,Ti)ss alternative lamellar structure, as shown in Fig. 2 (a)-(f).The DS also make the alloy possesses smaller eutectic cells but coarser primary Į-(Nb,Ti) 5 Si 3 .Moreover, the amount of (Ti,Nb) 5 Si 3 has increased obviously, especially at lower growth rate.When the growth rate increases, the amount of primary Į-(Nb,Ti) 5 Si 3 increases gradually, but its size becomes smaller.It is interesting that the (Ti,Nb) 5 Si 3 participates in the eutectic structure formation at 5 mm/h but just segregates along eutectic cell boundary at 8 and 11 mm/h.Furthermore, the amount of (Ti,Nb) 5 Si 3 seems to decrease, with growth rate increase.In addition, the distribution of Dy 2 O 3 particle becomes more uniform.With the increase of the growth rate, it can be observed that the microstructure become finer and the coarse Į-(Nb,Ti) 5 Si 3 phase tends to decrease.Further observation shows that the interphase spacing and the intercellular boundary width reduce obviously.When the growth rate increase to 11 mm/h, the fine Į-(Nb,Ti) 5 Si 3 /(Nb,Ti)ss eutectic structure forms along the coarse Į-(Nb,Ti) 5 Si 3 and (Nb,Ti)ss lamellae.However, the formation of discontinuous silicide and transversal dendrites would destroy the lamellae structure and deteriorate the mechanical property.Among all DS alloys, the one DS at 5 and 8 mm/h has the desired microstructure.Thought the microstructures of the DS alloys have some difference, they almost have the same tendency.Therefore, the DS alloy at 8 mm/h was chosen to perform the further characterization.XRD analysis on the DS alloy is presented in Fig. 3 (a).The section vertical to the growth direction is chosen as the tested surface.Compared with the XRD of AC alloy, the DS alloy exhibits strong orientation of (310) Į-(Nb,Ti)5Si3 and ( 110 To evaluate the influence of DS process on the mechanical properties of the Nb-Ti-Si-Cr-Al-Hf-Dy alloy, bending and compression tests were carried out at room temperature.The results of the fracture toughness and yield strength of the AC and DS alloys at room temperature are given in Table 1.Clearly, the DS process improves the mechanical properties of the alloy obviously.The fracture toughness and yield strength of the DS alloys are all higher than those of the AC alloy, especially the fracture toughness.Though the average value of the fracture toughness has a little fluctuation with DS growth rate, it still almost 30% higher than that of the AC alloy.The fracture toughness reach the highest value in the alloy DS at 8 mm/h.Similar with the variation of fracture toughness, the DS alloy obtains the highest yield strength at growth rate of 8 mm/h.The improvement of the fracture toughness and yield strength should be ascribed to the microstructure optimization, because the morphology of the ductile phase could influence the mechanical property greatly [12,13].As described above, the DS process aligns the (Nb,Ti)ss and Į-(Nb,Ti) 5 Si 3 lamellae alternately, which could make full use of the ductility of the (Nb,Ti)ss phase.The (Nb,Ti)ss lamellae could provide more opportunities for crack bridging and crack deflection and then increase the fracture toughness [14].However the increased (Ti,Nb) 5 Si 3 and coarse primary Į-(Nb,Ti) 5 Si 3 could contribute to the strength improvement of the alloy.Therefore the DS alloy possesses better mechanical properties than the AC alloy.Due to the optimized microstructure, the alloy DS at 8 mm/h has the best mechanical properties.
(4) The DS alloy possesses the better mechanical properties than the AC alloy.The alloy DS at 8 mm/h exhibits the best mechanical properties.

Fig. 1 (
Fig.1 (a) Backscattered SEM micrograph of the AC alloy, (b) Precipitates along the eutectic cell boundary, (c) X-ray diffraction pattern of the AC alloy, (d) TEM micrograph of the Dy 2 O 3 precipitate (Inset image exhibiting the SADP of the Dy 2 O 3 precipitate), (e) Morphology of the Cr 2 Nb phase along (Nb,Ti)ss/Į-(Nb,Ti) 5 Si 3 interface (Inset image exhibiting the SADP of the Cr 2 Nb phase), (f) TEM micrograph of the (Nb,Ti) 3 Si phase (Inset image exhibiting the composite SADP of the (Nb,Ti)ss and (Nb,Ti) 3 Si phases)

Fig. 2
Fig.2 SEM image of the transverse microstructure (a, c, e) and longitudinal microstructure (b, d, f) of the Nb-Ti-Si-Cr-Al-Hf-Dy alloy DS at growth rate of 5 mm/h (a, b), 8 mm/h (c, d), 11 mm/h (e, f)

Table 1
Fracture Toughness And Yield Strength Of The Ac And Ds Alloys