Lina Jia

Microstructure, Mechanical Properties and Oxidation Resistance of Nb-22Ti-14Si-2Hf-2Al-xCr Alloys

Abstract Nb-22Ti-14Si-2Hf-2Al-xCr (x=2, 6, 10, 14, 17 at%) alloys are prepared by arc-melting under argon atmosphere. Microstructural characteristics, mechanical properties and oxidation resistance of the arc-melted alloys are investigated. At 2 at% Cr content, the microstructure is composed of Nbss, Nb3Si and a small quantity of Nb5Si3, when the Cr contents increase, Nb3Si disappears. For the high Cr content (x ≥ 10 at% alloys, besides the Nbss and Nb5Si3, Cr2Nb is also detected. With the increase of Cr content, the volume fractions of Cr2Nb and Nb5Si3 increase, while that of Nbss increases firstly and then begins to degrade when the Cr content is higher than 6 at%. For the alloy with 2 at% Cr, the room temperature fracture toughness is about 14.5 MPa·m1/2, but badly decreases to about 8.5 MPa·m1/2, when the Cr contents increase. Vickers hardness of Nbss tends to increase linearly from about 400 to 500, while that of silicides is not sensitive to Cr contents, about 950. The appearance of Cr2Nb phase significantly improves the high temperature oxidation resistance of the alloys with high Cr contents. The isothermal oxidation tests show that the oxidation kinetics of the alloys with various Cr contents follows parabolic oxidation kinetics.

Microstructure and room temperature mechanical properties of hypereutectic Nb–Si based alloy processed by directional solidification

Abstract In this investigation, Nb–16Si–24Ti–4Cr–2Al–2Hf alloy was directionally solidified and then heat treated, and the microstructure, fracture toughness KQ and tensile strength σb of the alloy were studied. The directionally solidified microstructure consisted of primary Nb5Si3 blocks and NbSS+Nb5Si3 eutectic cells. The cross section of eutectic cells changed from the square morphology to the nearly round morphology with the increasing withdrawal rate. During heat treatment, the boundaries of the round eutectic cells were eliminated as the NbSS connected to be a net-like structure. The values of KQ and σb increased with the increase of withdrawal rate. The toughening from the crack deflection of continuous Nbss network was significant and refinement of the microstructure improved the tensile strength in heat treated samples.

Microstructure evolution of directionally solidified Nb–Si alloy

Abstract By taking the method of liquid–metal cooled directional solidification, alloys with a nominal composition of Nb–14Si–24Ti–10Cr–2Al–2Hf (at-%) were prepared under different conditions. Alloys were initially directional solidified with different withdrawal rates (Ru200a=u200a1·2, 6, 18 mm min−1) at 1750°C and subsequently heat treated at 1450°C for 10 h. These processes aimed to investigate the microstructure of directionally solidified (DS) and heat treated (HT) alloys by XRD, SEM, and EDS. The microstructure of DS alloy was composed of (Nb,Ti)SS, (Nb,Ti)5Si3, and Laves phase Cr2Nb, and the former two components formed (Nb,Ti)SS+(Nb,Ti)5Si3 eutectics. In addition, (Nb,Ti)5Si3 laths only presented in DS1·2 alloy. With the increasing withdrawal rates, the microstructure of alloy altered from hypereutectic into pseudo-eutectic, accompanied with the eutectic morphology transformation from petaloid into coupled. Also, the dimension of constituent phases reduced. However, after heat treatment, the constituent phases did not change. The petaloid morphology of eutectics in DS specimens disappeared and coupled eutectic transferred into network. The block or needle-like Cr2Nb gathered along the boundary between (Nb,Ti)5Si3 and (Nb,Ti)SS, and the overall alloy composition became homogenisation.

Room temperature mechanical strength and fracture behaviour of Nb–Ti–Si–Cr based alloy

Abstract The effect of heat treatment on the mechanical strength of a three-phase NbSS/Nb5Si3/Cr2Nb alloy with composition Nb–22Ti–12Si–14Cr–2Al–2Hf was investigated. This included the evaluation of microstructure, room temperature mechanical strength and the fracture behaviours. The microstructures of heat treated samples were composed of NbSS, Nb5Si3 and Cr2Nb, and the phase size changed under different heat treatment schemes. The results showed that the microstructure composed of large NbSS and fine Nb5Si3 exhibited best mechanical strength due to the deformation of NbSS in these samples. Moreover, fractographic analysis indicated that the Nb5Si3 and Cr2Nb fractured in cleavage manner in all heat treated samples.

Microstructure optimization of directionally solidified hypereutectic Nb–Si alloy

Abstract Nb–16Si–24Ti–10Cr–2Al–2Hf alloy was directionally solidified with withdrawal rates of 1.2, 6, 18, 36 and 50 mm/min and then heat treated at 1400, 1450 and 1500 °C with withdrawal rate of 50 mm/min for 10 h. The effects of withdrawal rate and heat treatment temperature on the microstructure were studied. The microstructure of directionally solidified alloy was composed of the primary Nb 5 Si 3 , Nbss/Nb 5 Si 3 eutectic cells and Cr 2 Nb, which distribute paralleled to the growth direction. The microstructure becomes more refined with the increasing withdrawal rate, accompany with the evolution of eutectic cells morphology. After heat treatment, Nbss phase connects and forms a continuous matrix, and the Cr 2 Nb phase becomes smaller and distributes more dispersedly. After heat treatment at 1450 °C for 10 h, the alloy achieves balance between the optimization of microstructure and alleviation of solute segregation.

Synergy of Cr concentration and withdraw rate on microstructure evolution of directionally solidified Nb-14Si-24Ti alloys

Abstract The synergy of Cr concentration and withdraw rate on the microstructure of directionally solidified and heat treated Nb–14Si–24Ti–xCr (xu200a=u200a0, 2, 8, 10, 14 at-%) alloys was investigated. The phases present in the directionally solidified alloys were sensitive to Cr concentration and withdraw rate. The addition of Cr changed the solidification path of ternary alloys. High Cr concentration stabilised the Nb5Si3 and facilitated the formation of Cr2Nb during the directional solidification process. However, the effect of Cr was suppressed by the increase in withdraw rate. The competition between the effect of Cr addition and withdraw rate determined the microstructure of alloys and resulted in a four-phase microstructure. During heat treatment, the Nb3Si phases in Nb–14Si–24Ti–0Cr alloys were stable, while in the Cr contained alloys, the Nb3Si decomposed into NbSS/Nb5Si3 eutectoid through the eutectoid reaction (Nb3Si→NbSS+Nb5Si3).

The Effects of the Withdrawal Rate and Heat Treatment on the Microstructure of Directionally Solidified Nb-14Si-24Ti Alloy

Abstract In this work, an Nb-14Si-24Ti alloy (at.%) was prepared by liquid-metal-cooled directional solidification (DS) and then heat treated (HT) at 1450°C for 100 h. The effects of the withdrawal rate and heat treatment on the microstructure were investigated. The results show that the DS samples consisted of NbSS dendrites, Nb3Si blocks and eutectic NbSS/Nb3Si. As the withdrawal rate increased, the microstructure became finer, the volume fraction of eutectic NbSS/Nb3Si increased and the morphology of eutectic cell transformed from petaloid to regularly arranged granular morphology. The microstructure of the HT samples consisted of NbSS and Nb3Si. The metastable phase of Nb3Si did not discompose through the eutectoid reaction. The dendritic NbSSconnected through diffraction and the secondary dendrite arm got coarser. The boundary of eutectic NbSS/Nb3Si became blurry and the black Ti-rich phase of the DS samples in this region disappeared. The Nb3Si connected to form the continuous matrix. The granular NbSS of eutectics spheroidized and the lamellar NbSS of eutectic interconnected and became more bending distributed uniformly in the Nb3Si matrix.

Microstructure Evolution in Nb-12Si-22Ti-14Cr-2Al2Hf Alloy Fabricated by Directional Solidification

Abstract In this work, the microstructure evolution of the Nb-12Si-22Ti-14Cr-2Al-2Hf alloy (at.%) fabricated by liquidmetal-cooled directional solidification (LMC) was investigated and the solidification behaviors were discussed. The results revealed that the growth rate was of crucial in controlling the microstructure of this alloy and the higher growth rate was beneficial to the formation of metastable phase Nb3Si. The microstructure of non-melted region consisted of NbSS dendrites, NbSS/Nb5Si3 eutectics and Ti-rich NbSS/Cr2Nb eutectics while the microstructure of quasi-steady-state growth region of this alloy consisted of NbSS dendrites, Nb3Si laths and Ti-rich NbSS/Cr2Nb eutectic. Nb3Si had the advantage of nucleation at medium/high growth rate. Therefore, the microstructure evolution occurred in this study was owing to the increase of growth rate.