Failure mode transition of NbSS phase from cleavage to dimple in Nb-Si based alloys prepared by spark plasma sintering through controlling of NbSS powder size and morphology and alloying

The influence of Nb powder sizes and morphologies (equiaxed and flaky particles) on fracture toughness and fracture modes of Nb-16Si binary alloy and one multi-component Nb-16Si-22Ti-2Al-2Hf-7Cr alloy with an Nb/Nb5Si3 microstructure fabricated by spark plasma sintering (SPS) was investigated. A reduced Nb powder size from 83.8 m to 4.9 m led to an Nb matrix with fine grain (9.9m in size) plus Nb5Si3 islands two-pshase microstructure, which changed the fracture mode of the Nb phase from cleavage (Fig. 1a) to a mixed mode of dimple (arrows A in Fig. 1b), tear (arrows B in Fig. 1b) and cleavage (arrows C in Fig. 1b), and then the fracture toughness of the Nb-16Si binary samples was significantly improved from 8.2 MPa·m1/2 (Fig. 1a) to 12.4 MPa·m1/2(Fig. 1b). A further improvement of the fracture toughness to 15.8 MPa·m1/2 was achieved in the multi-component Nb-16Si-22Ti-2Al-2Hf-7Cr alloy owing to full dimples (Fig. 1c) on the finer fractured Nb grains with a size of 11.2m through addition of the toughening elements of 22 at.% Ti and 2 at.% Hf. It is interesting that when the flaky Nb powders (123.7 m in diameter and 15.2m in thickness) were used to prepare the Nb-16Si binary sample, the Nb grains still adopted the flake morphology and two fracture modes were observed in one fractured Nb grain, i.e., the cleavage mode in radial plane and the dimple or tear mode in axial plane of the flaky Nb grain (Fig. 1d). These kinds of fracture modes in one flaky Nb grain also improved the fracture toughness of the bulk Nb-16Si binary sample to 11.8 MPa·m1/2 (Fig. 1d). Please click Additional Files below to see the full abstract

Effect of Hf additions on microstructure and mechanical properties of a Co-9Al-9W-2Ta alloy at room and high-temperatures

AbstractIn The microstructural evolution, room/high-temperature mechanical properties of a Co-9Al-9W-2Ta alloy with 2, 5, 6, 9 at.% Hf additions (referred as to 2Hf, 5Hf, 6Hf and 9Hf alloy hereafter, and content of W+Hf = 9 at.% for all alloys) prepared by arc-melting were investigated. It was found that the as-cast 2Hf∼6Hf alloys showed a microstructure composed of Co-base solid solution γ phase (γ-CoSS) and eutectic of γ+ intermetallic compound Co23Hf6, and the 9Hf alloy was composed of primary Co23Hf6 and (γ+ Co23Hf6) eutectic. While after 1170°C/8h solution and 800°C/100h aging, the cubic γ’ phase with a size of 200nm∼700nm homogeneously and coherently precipitates on the γ matrix for the 2Hf ∼ 6Hf alloys, no γ’ particles were found in the 9Hf alloy. The 2Hf alloy exhibits yield stress anomaly at temperatures above 600°C, and the temperature corresponding to the anomalies stress peak is at about 700°C. However, the other three alloys show no yield stress anomalies

Microstructure, tensile behavior and oxidation resistance of the two step heat treated Nb- Ti-Si based alloy

Although the temperatures of the hottest section of advanced turbine engines are approaching the limit of nickel-based superalloys, the desire for continued performance improvements for gas turbine engines continues [1]. Nb-Ti-Si based alloys have attracted much attention for their high temperature melting points, relatively low densities and excellent high temperature mechanical properties [2]. In this alloy system, the constitute phases are niobium solid solution (NbSS), silicides (Nb5Si3 and / or Nb3Si) and with / without Cr2Nb. Among these phases, NbSS ensures room temperature fracture toughness and tensile strength, silicides offers high temperature strength, and Cr2Nb provides the oxidation resistance [3].In this study, the Nb-Ti-Si based alloy was firstly prepared by arc-melting and then directional solidification at a rapid speed. Two-step heat treatments were carried out on the directionally solidified Nb-Ti-Si based alloy. The first-step heat treatment was performed at 1375, 1400, 1425 and 1450 °C for 10hours, respectively. According to the microstructure characteristic and their resulting tensile strength, the best condition for the first-step heat treatment is 1425 °C/10 h. Then the second step heat treatment was conducted at 1000, 1100, 1200 and 1300 °C for 50 hours, respectively. The precipitation of nano-scale Cr2Nb particles in the NbSS matrix substantially improved the tensile strength from 850 MPa to 950 MPa, and reduced the weight gain of the alloy from 218.27 to 144.41mg/cm2 by the second step heat treatment at 1000 °C. References [1] B. P. Bewlay, M. R. Jackson, P. R. Subramanian, J. C. Zhao, Metall. Mater. Trans., 34A (2003) 2043-2052 [2] L.N. Jia, J.F. Weng, Z. Li, Z. Hong, L.F. Su, H. Zhang. Mater. Sci. Eng., 623A (2015) 32-37 [3] L. F. Su, L. N. Jia, Y. B. Feng, H. R. Zhang, S. N. Yuan, H. Zhang, Mater. Sci. Eng., 560A(2013) 672-67

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Oxidation behaviours of Nb–22Ti–15Si–2Al–2Hf–2V–(2, 14)Cr alloys with Al and Y modified silicide coatings prepared by pack cementation

Al and Y modified silicide coatings on the Nb–15Si–22Ti–(2,14)Cr–2Al–2Hf–2V alloys (where the alloy with 2 at% Cr or 14 at% Cr is hereafter referred to as 2Cr and 14Cr alloy, respectively) was prepared by pack cementation. The microstructural evolution and the oxidation behaviours of the coating 2Cr and 14Cr samples at 1250 °C were studied. The 2Cr alloy consists of Nb solid solution (NbSS) and α-Nb5Si3 silicide, while the Laves C15–Cr2Nb phase arised in the 14Cr alloy. The coating structure of the coating 2Cr sample contained the outer (Nb, X)Si2+(Nb, X)5Si3 layer, the middle (Nb, X)5Si3 layer and the inner undeveloped intermetallic (Nb,Ti)3(Al,X) layer; the structure of the coating 14Cr sample consisted of the outer single (Nb, X)Si2 layer, the middle (Nb, X)5Si3 layer, the transition (Nb,Ti) (Cr,Al) layer and the inner (Cr, Al)2(Nb,Ti) layer. The coating 14Cr sample exhibited better oxidation resistance than the coating 2Cr sample. With an outer single (Nb, X)Si2 layer, a compact oxide scale consisting of SiO2 and TiO2 formed on the coating 14Cr sample, which can efficiently prevent the substrate from oxidising. For the coating 2Cr sample with an outer (Nb, X)Si2+(Nb, X)5Si3 layer, the oxide scale of the SiO2, TiO2, Nb2O5 and CrNbO4 mixture generated, and the scale spalled out from the surface of the sample, resulting in disastrous failure

Microstructural evolution and mechanical properties of Nb-Si-Cr ternary alloys with a tri-phase Nb/Nb5Si3/Cr2Nb microstructure fabricated by spark plasma sintering

In this work, pure Nb, Nb5Si3 and Laves Cr2Nb compound powders were used as raw materials to prepare Nb-Si-Cr ternary alloys by spark plasma sintering (SPS). A comprehensive estimation of the microstructure and properties, including room temperature fracture toughness, high temperature strength and oxidation resistance, of the Nb-Si-Cr ternary alloys as a function of the Nb/Nb5Si3/Cr2Nb phase volume fraction combinations was conducted. The results showed that Nb-Si-Cr ternary samples with the relative density larger than 98.42% were obtained by SPS processing, and the samples all consisted of Nb, Nb5Si3 and Cr2Nb phases that were distributed homogeneously. The fracture toughness KQ of the Nb/Nb5Si3/Cr2Nb microstructure, which was dominated by the Nb phase, naturally increased with the Nb fraction. As expected, the room-temperature Vickers hardness and the high-temperature strength of the bulk alloys increased monotonically with the increasing of the stiffening Nb5Si3 fraction. Interestingly, the binary Cr2Nb phase played a positive role in the high temperature strength and oxidation resistance. Finally, the fracture modes of the typical Nb/Nb5Si3/Cr2Nb microstructures under bending and compression conditions at room and high temperatures as well as the oxidation mechanism are described and discussed. Keywords: Nb-Si Alloy, Spark Plasma Sintering (SPS), Phase Fraction, Fracture Toughness, High-Temperature Strengt

High-throughput synthesis and oxidation behavior of a continuous composition spread (Nb–Ti)1-x-ySixCry alloy thin film

High-throughput method of three targets magnetron sputter co-deposition was adopted to prepare (Nb–Ti)1-x-ySixCry composition spread alloy film (CSAF). Early oxidation behavior of the as-deposited alloy thin film was investigated in atmospheric conditions at 1250 °C for 10 min. A (Nb–Ti)1-x-ySixCry combinatorial material library covering (9.34–32.31) Si-(10.43–20.27) Cr (at. %) is obtained and as-deposited film was composed of amorphous phase. The oxidized film was separated into three regions having different oxidation products. The composition range for establishment of a protective scale (SiO2 and CrNbO4) and a small amount of Nb2O5 was ascertained efficiently across the entire alloy thin film

High-temperature oxidation behaviour of novel Co-Al-W-Ta-B-(Mo, Hf, Nb) alloys with a coherent γ/γ'–dominant microstructure

In this work, 2 at% Mo, 2 at% Nb and 2 at% Hf were substituted for the same amount of W into a Co-9Al-9W-2Ta-0.02B alloy (hereafter referred as to 2Mo, 2Nb and 2Hf alloys, respectively, while the original alloy is denoted as 0Me alloy). The effect of the Mo, Hf and Nb additions on the isothermal oxidation resistance, oxide scale evolution and failure mechanism, of the Co-9Al-9W-2Ta-0.02B alloy when exposed at 800 °C and 900 °C for 100 h was investigated. It was found the Mo, Hf and Nb additions degraded the oxidation resistance of the Co-9Al-9W-2Ta-0.02B alloy, while the 2Mo alloy always displayed the poorest oxidation resistance, resulted from heavy spallation of the oxide scale. An oxide scale composed of an outer Co3O4+CoO layer, a middle complex oxide layer enriched with Al, W and Ta, and a γ/needle-like Co3W zone adhering to the γ/γ' substrate was gradually formed; moreover, a continuous or discontinuous Al2O3 layer and dispersive Al2O3 dots or slices were observed within the γ/needle-like Co3W zone, depending on the oxidation temperature and added elements (Mo, Hf and Nb). The formation of volatile MoO3 in the oxide scale of the 2Mo alloy enhance the exfoliation of the oxide products, resulting in severe spallation and poor oxidation resistance

Experimental studies and modeling for the transition from internal to external oxidation of three-phase Nb-Si-Cr alloys

A modified oxidation model, which assumes the cylindrical-like morphology of Cr2Nb phase, has been proposed to examine the conditions to form external scale of three-phase Nb-base alloys in this paper. The oxidation behavior of three-phase Nb-base alloys was tested at 1250 °C for 1 h and 7 h, and the experimental results were compared with the simulated results. The results show that the external CrNbO4 scale can be achieved by length-radius ratio and volume fraction increasing for Cr2Nb phase. The simulated results are consistent with the experimental results. Keywords: Intermetallics, Superalloys, Modeling studies, Oxidation, Nb-based alloy

A Quantitative Index to Evaluate the Commutation Failure Probability of LCC-HVDC with a Synchronous Condenser

Since thyristor cannot turn off automatically, line commutated converter based high voltage direct current (LCC-HVDC) will inevitably fail to commutate and therefore auxiliary controls or voltage control devices are needed to improve the commutation failure immunity of the LCC-HVDC system. The voltage control device, a synchronous condenser (SC), can effectively suppress the commutation failure of the LCC-HVDC system. However, there is a need for a proper evaluation index that can quantitatively assess the ability of the LCC-HVDC system to resist the occurrence of commutation failures. At present, the main quantitative evaluation indicators include the commutation failure immunity index and the commutation failure probability index. Although they can reflect the resistance of the LCC-HVDC system to commutation failures to a certain extent, they are all based on specific working conditions and cannot comprehensively evaluate the impact of SCs on suppressing the commutation failure of the LCC-HVDC system under certain fault ranges. In order to more comprehensively and quantitatively evaluate the influence of SCs on the commutation failure susceptibility of the LCC-HVDC system under certain fault ranges, this paper proposes the area ratio of commutation failure probability. The accuracy of this new index was verified through the PSCAD/EMTDC. Based on the CIGRE benchmark model, the effects of different synchronous condensers on LCC-HVDC commutation failure were analyzed. The results showed that the new index could effectively and more precisely evaluate the effect of SCs on commutation failures. Moreover, the proposed index could provide a theoretical basis for the capacity allocation of SCs in practical projects and it could also be utilized for evaluating the impact of other dynamic reactive power compensators on the commutation failure probability of the LCC-HVDC system under certain fault ranges