Abstract

The effects of Cr, Al and Ti on the microstructure and hardness of as cast and heat treated Nb–18Si–5Hf–5Cr (YG1), Nb–18Si–5Hf–5Al (YG2) and Nb–18Si–5Hf–24Ti (YG3) alloys were studied. The phases present in the as cast alloys were the Nb_ss, Nb₃Si, Nb₅Si₃ and HfO₂ in YG1-AC, the Nb_ss, Nb₅Si₃ and HfO₂ in YG2-AC and the Nb_ss, Nb₃Si and Nb₅Si₃ in YG3-AC. In the YG1 the formation of Nb₃Si was sensitive to cooling rate, in the YG3 the volume fraction of Nb₅Si₃ was low and in the YG3 and YG1 Ti and Hf rich areas were formed respectively in the Nb_ss and Nb₅Si₃. The phases present in the heat treated alloys were the Nb_ss, Nb₅Si₃ and HfO₂ in YG1-HT and YG2-HT and the Nb_ss, Nb₅Si₃ (including the hexagonal ¦ÃNb₅Si₃), Nb₃Si and HfO₂ in YG3-HT. Nb₂N and TiN formed near the surfaces of YG1-HT and YG3-HT. Compared to YG1, in the microstructure of YG2 (i) there was a significantly higher volume fraction of a thermally stable lamellar Nb_ss and Nb₅Si₃ microstructure and (ii) a lower volume fraction of Nb_ss. In YG1 the eutectoid transformation of Nb₃Si was enhanced and the solubility of Si in Nb_ss was reduced significantly. In the Nb_ss a reduction in the Hf solubility led respectively to increase and decrease in the Cr and Al solubility in YG1 and YG2. In YG3 the synergy of Hf and Ti stabilised the 5–3 silicide and the transformation Nb₃Si → Nb_ss + ¦ÁNb₅Si₃ (tI32) was replaced by an alternate eutectoid transformation in which the 5–3 silicide had the hexagonal (hP16) structure. In YG3-HT the consumption of Ti to ≤12 at% to form TiN near the surface led to the formation of a heavily contaminated fine lamellar structure via the eutectoid decomposition (Nb,Ti)₃Si → (Nb,Ti)_ss + (Nb,Ti)₅Si₃. The hardness of Nb₅Si₃ was reduced as the solubilities of Hf and Al in the silicide increased.

Abstract

The effects of Hf, Mo and W on the microstructure and hardness of as cast and heat treated (1500 °C/100 h) Nb–20Si–5Hf–5W (YG5), Nb-20Si–5Mo–3W (YG6) and Nb–20Si–5Hf–5Mo–3W (YG8) alloys were studied. The macrosegregation of Si was strong in the as cast alloy YG6 and decreased in the order of the synergy of alloying elements as follows (Mo + W), (Hf + W) and (Hf + Mo + W). All three alloys were contaminated by oxygen. The phases present in the as cast alloys YG5, YG6 and YG8 were the Nb_ss, Nb₅Si₃ and HfO₂, Nb_ss, Nb₃Si, and Nb₅Si₃ and Nb_ss, Nb₅Si₃ and HfO₂ respectively, with a lamellar Nb_ss + Nb₅Si₃ microstructure formed in all three alloys. In the as cast alloys YG5 and YG6 there were Nb_ss grains with no Si content. There was microsegregation of Hf and W in the Nb_ss in the as cast alloy YG8 and microsegregation of Hf in Nb₅Si₃ in the as cast and heat treated alloys YG5 and YG8. It is concluded that the concentrations of Si and refractory metal (RM) in the Nb_ss depend on the synergy of RMs with/without Hf. The Nb_ss with no Si and Nb₅Si₃ were present in all three heat treated alloys and HfO₂ was formed in the heat treated alloys YG5 and YG8 via the consumption of the Hf in the Nb_ss. The stability of the Nb_ss + Nb₅Si₃ lamellar microstructure decreased in the order of the heat treated alloys YG5, YG8 and YG6. In the presence of Hf the transformation of ¦ÂNb₅Si₃ to ¦ÁNb₅Si₃ was enhanced. It is concluded that the equilibrium microstructures of the alloys consist of the Nb_ss with no Si and ¦ÁNb₅Si₃ phases. Alloying with Hf caused reduction of the hardness of the alloys YG5 and YG8 after heat treatment.

Highlights

? Data on macrosegregation of Si. ? Data on contamination of alloys by oxygen and nitrogen. ? Data on microsegregation of refractory metals in Nb_ss and Nb₅Si₃. ? Formation of Si free Nb_ss. ? Data on alloy hardness.

Abstract

The effects of alloying on the microstructures, solidification path, phase stability and oxidation kinetics of Nb_ss/Nb₅Si₃ base in situ composites of the Nb–Ti–Si–Al–Cr–Mo–Hf–Sn system have been investigated in this study. All the studied alloys are classified as hyper-eutectic Nb silicide base in situ composites and have lower densities compared to nickel-based superalloys. The Nb₃Si silicide formed in the Hf-free alloys and transformed to Nb_ss and ¦ÁNb₅Si₃ during heat treatment at 1500 °C. This transformation was enhanced by the addition of Ti. The Nb_ss and Nb₅Si₃ were the equilibrium phases in the microstructures of the Hf-free alloys. In the presence of Ti, the ¦ÂNb₅Si₃ only partially transformed to ¦ÁNb₅Si₃, suggesting that Ti stabilises the ¦ÂNb₅Si₃ to lower temperatures (at least to 1300 °C). Furthermore, alloying with Hf stabilised the hexagonal ¦ÃNb₅Si₃ (Mn₅Si₃-type) silicide in the Hf-containing alloys. The addition of Sn promoted the formation of the Si-rich C14 Laves phase and stabilised it at 1300 °C. This is attributed to the Sn addition decreasing the solubility of Cr in the Nb_ss of the Nb–Ti–Si–Al–Cr–Mo–Hf–Sn system whilst increasing the Si solubility. The Si solubility in the C14 Laves phase was in the range ∼6.6 to 10.5 at%. The lattice parameter of the Nb_ss in each alloy increased after heat treatment signifying the redistribution of solutes between the Nb_ss and the intermetallic phases. The oxidation resistance of the alloys at 800 °C and 1200 °C increased significantly by alloying with Ti and Sn. Pest oxidation behaviour was exhibited by the Nb–18Si–5Al–5Cr–5Mo (as cast), Nb–24Ti–18Si–5Al–5Cr–5Mo (as cast), Nb–24Ti–18Si–5Al–5Cr–2Mo (heat treated) and Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf (heat treated) alloys at 800 °C. Pesting was eliminated in the alloy Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf–5Sn at 800 °C, indicating that the addition of Sn plays an important role in controlling the pest oxidation behaviour at intermediate temperatures. The oxidation behaviour of all the alloys at 800 °C and 1200 °C was controlled by the oxidation of the Nb_ss and was sensitive to the area fraction of Nb_ss in the alloy.

Abstract

The role of Fe and Sn additions in the microstructures of the as cast and heat treated alloys Nb–24Ti–18Si–5Cr–5Fe (NV3) and Nb–24Ti–18Si–5Cr–5Fe–5Sn (NV5) were studied. The microstructure of the as cast alloy NV3 consisted of the (Nb,Ti)_ss, (Nb,Ti)₅Si₃, (Nb,Ti)₃Si and metastable Nb 3-1 silicides, FeNb₄Si and a Ti rich oxide. The (Nb,Ti)₅Si₃ exhibited severe cracking everywhere in the ingot. The microstructure of the heat treated alloy NV3 consisted of the (Nb,Ti)_ss, (Nb,Ti)₅Si₃, FeNb₄Si and metastable Nb 3-1 silicides, with the latter at the higher volume fraction. The addition of Fe in the alloy NV3 retarded the Nb₃Si → Nb_ss + ¦ÁNb₅Si₃ eutectoid transformation during solidification, and enhanced the sensitivity of (Nb,Ti)₃Si formation on cooling rate. The phases present in the microstructure of the as cast alloy NV5 were the (Nb,Ti)_ss, (Nb,Ti)₃Sn, (Nb,Ti)₅Si₃, (Nb,Ti)₃Si and metastable 3-1 silicides, FeNb₄Si, Fe₇Nb₆ and a Ti rich oxide. The formation of the Fe₇Nb₆ intermetallic was sensitive to cooling rate. The microstructure of the heat treated alloy NV5 consisted of the (Nb,Ti)₃Sn, metastable Nb 3-1 silicide, FeNb₄Si, the Ti rich oxide and (Nb,Ti)₅Si₃. It is concluded that in Nb–24Ti–18Si-based alloys the addition of 5 at.% Fe enhances the stability of the (Nb,Ti)₃Si and that the addition of 5 at.% Fe and 5 at.% Sn destabilises the (Nb,Ti)_ss.

Abstract

The effects of Sn and Ti on the microstructure and hardness of the as cast and heat treated Nb–18Si–5Sn (NV9) and Nb–24Ti–18Si–5Sn (NV6) alloys were studied. In both alloys the phases present in the as cast and heat treated microstructures were Nb_ss, Nb₃Sn and Nb₅Si_3. In NV9, Sn suppressed the formation of Nb₃Si, partitioned in Nb_ss stronger than in Nb₅Si₃ and did not affect significantly the solubility of Si in the Nb_ss. In NV6, the solubility of Ti in (Nb,Ti)_ss increased in the presence of Sn, the concentration of Ti in Nb₅Si₃ was sensitive to cooling rate and the solubility of Sn in Nb₅Si₃ decreased as the concentration of Ti increased. The Ti controlled the partitioning of Si between (Nb,Ti)_ss and Nb₃Sn and was considered responsible for the macrosegregation of Si in the as cast ingot. The transformation of ¦Â to ¦Á Nb₅Si₃ was enhanced by the synergy of Sn and Ti. The addition of Ti did not destabilise the Nb₃Sn. Silicon increased the hardness of Nb₃Sn significantly, Sn did not affect the hardness of Nb₅Si₃ and Ti reduced the hardness of Nb₃Sn and Nb₅Si₃ significantly. The hardness of NV9 and NV6 decreased and increased, respectively, by heat treatment. The reduction of the hardness of NV6-AC compared to NV9-AC is attributed to the strong effect of Ti on the hardness of Nb₃Sn and Nb₅Si₃.

Abstract

Nanostructured 13 wt% Al₂O₃–8 wt% Y₂O₃–ZrO₂ (13AlYSZ) coatings were developed by atmospheric plasma spraying (APS). The phase structure and the morphology of the 13AlYSZ coatings were characterized using X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). It was found that the as-sprayed coatings mainly consisted of tetragonal zirconia, with the Al element solid solution in ZrO₂. Heat treatment at 1100 °C increased the average grain size of the ZrO₂ phase from 61 to 120 nm and decreased the porosity from 23.8 to 18%. The addition of the nano-Al₂O₃ can effectively inhibit the grain growth of the zirconia phase. The mechanism on inhibiting the grain growth of nanostructured 8 wt% Y₂O₃–ZrO₂ thermal barrier coatings has been discussed in detail.

Si-Zr-Y co-deposition coatings on an Nb-Ti-Si-Cr base ultrahigh temperature alloy were prepared by pack cementation processes at 1250 ¡æ for 8 h with different halide activators in the packs. The structure, constituent phases and formation process of the coatings were investigated. The results show that all the coatings are mainly composed of an (Nb, X)Si₂ (X represents Ti, Cr and Hf elements) outer layer, a (Ti, Nb)₅Si₄ transitional layer, and an Al, Cr and Y-rich (Nb, X)₅Si₃ inner layer located between the coating and the substrate. In the five kinds of (NaF, NH₄F, NH₄Cl, NaBr and NaCl) activators investigated, the coatings prepared respectively with NaF and NH₄F activators are thicker and denser, and furthermore, the contents of both Zr and Y elements are larger in the coating prepared with NaF as the activator. However, the coatings prepared respectively with NH₄Cl, NaBr and NaCl as the activators are thiner, and lots of holes and ZrO₂ and HfO₂ particles are observed in the surface layer of the coatings. The higher equilibrium partial pressures of gaseous halide silicides in the packs resulted in thicker and denser co-deposition coatings, while the equilibrium partial pressures of  gaseous halide Zr and Y in the packs have less effect on their contents in the coatings.

ʹÓò»Í¬ÖÖÀà´ß»¯¼Á(NaF, NH₄F, NH₄Cl, NaBrºÍNaCl)¾­1250 ¡æ±£ÎÂ8 hÀ©É¢¹²Éø, ÔÚÐÂÐÍNb-Ti-Si-Cr»ù³¬¸ßκϽð±íÃæÖƱ¸ÁËSi-Zr-Y¹²Éø²ã, ·ÖÎöÁ˸÷¹²Éø²ãµÄ½á¹¹¼°Ïà×é³É, ²¢¶ÔÆä×éÖ¯Ðγɹý³Ì½øÐÐÁËÌÖÂÛ. ½á¹û±íÃ÷: ²»Í¬ÖÖÀà´ß»¯¼ÁÖƱ¸µÄ¹²Éø²ãÓÉÍâÖÁÄÚ¾ùÓÉ(Nb, X)Si₂(X±íʾԪËØTi, CrºÍHf)Íâ²ã, (Ti, Nb)₅Si₄¹ý¶É²ã¼°¸»Al, CrºÍYµÄ (Nb, X)₅Si₃ÄÚ²ã×é³É. ÔÚ5ÖÖ´ß»¯¼ÁÖÐ, ²ÉÓÃNaFºÍNH₄FʱÖƱ¸µÄ¹²Éø²ã½Ïºñ¡¢×éÖ¯ÖÂÃÜ, ÇÒ²ÉÓÃNaFʱËùÖƱ¸µÄ¹²Éø²ãÖÐZrºÍYµÄº¬Á¿½Ï¸ß; ²ÉÓÃNH₄Cl, NaBrºÍNaClʱÖƱ¸µÄ¹²Éø²ã½Ï±¡ÇÒ±íÃæ·Ö²¼ÓдóÁ¿µÄ¿×¶´ÒÔ¼°ZrO₂ºÍHfO₂Ñõ»¯Îï. Éø°üÄÚSiµÄ±»¯ÎïƽºâÆøÏà·Öѹ½Ï¸ßʱÖƱ¸µÄ¹²Éø²ã½ÏºñÇÒ×éÖ¯ÖÂÃÜ; ¶øZrºÍYµÄ±»¯ÎïƽºâÆøÏà·Öѹ¶ÔÆäÔÚ¹²Éø²ãÖеĺ¬Á¿Ã»ÓÐÏÔÖøÓ°Ïì.

Abstract

The effect of Zr addition on the microstructures and mechanical properties of cast Nb–22Ti–16Si–xZr (x = 0–4) alloys was investigated. The results show that the Zr addition leads to coarseness of primary Nbss phase promotes formation and coarseness of Nbss and ¦Ã(Nb)₅Si₃ lamellar eutectic structure. In addition, the Zr addition increases the hardness of Nbss and (Nb)₃Si phases. The yield strength at room temperature of the alloys improves with increasing Zr content. The fracture strain reaches a peak value in 3 at.% Zr content, and then drops with the increase of Zr content. The high temperature strength decreases first and reaches a minimal value in 3 at.% Zr content, and then increases again. These variations are attributed to the microstructure and composition changes of the alloys with increasing Zr content.

Nb-Ti-Si base ultrahigh temperature alloys that possess higher melting points, relatively lower densities and attractive high temperature strength have received worldwide attention for their potential applications as next-generation turbine blade materials. In this work, integrally directional solidification of an Nb-Ti-Si base ultrahigh temperature alloy was conducted at different withdrawing rates (2.5, 5, 10, 20, 50 and 100 μm/s) with a constant melt temperature of 2000¡æ. Effect of solidifying rate on the integrally directionally solidified eutectic microstructure and solid/liquid interface morphology of this alloy has been investigated by XRD, SEM and EDS, and its directional solidification behavior has been discussed. The results show that the directionally solidified microstructure is mainly composed of petal-like Nb_ss/α(Nb,X)₅Si₃ eutectic colonies (Eutectic I) and coupled grown lamellar Nb_ss/γ(Nb,X)₅Si₃ eutectic (Eutectic II) which distributed in the intercellular area. The Eutectic I and Eutectic II are both aligned straight and uprightly along the growth direction. When the solidifying rate increases from 2.5 μm/s to 100 μm/s, the microstructure becomes finer and finer, and the petal-like eutectic colonies evolve from round morphology to tetragonal morphology. Either silicides or fine eutectics locate in the centers of round eutectic cells, while cross-like Nb_ss locates in the centers of tetragonal eutectic cells. Eutectic II exhibits a well-aligned lamellar structure on longitudinal-section. The solid/liquid interface of the alloy undergoes an evolution from cellular dendrite, dendrite and finally to cellular dendrite morphologies.

ÔÚÈÛÌåζÈΪ2000¡æµÄÌõ¼þÏÂ,·Ö±ðÒÔ2.5, 5, 10, 20, 50ºÍ100μm/sµÄ³éÀ­ËÙÂʶÔNb-Ti-Si»ù³¬¸ßκϽð½øÐÐÁËÓÐÛáÛöÕûÌ嶨ÏòÄý¹Ì.²ÉÓÃXRD, SEMºÍEDSµÈ·ÖÎö·½·¨,Ñо¿Á˳éÀ­ËÙÂʶԶ¨ÏòÄý¹Ì¹²¾§×éÖ¯¼°¹Ì/Òº½çÃæÐÎòµÄÓ°Ïì,²¢·ÖÎöÁ˸úϽðµÄÄý¹Ì¹ý³Ì. ½á¹û±íÃ÷:ºÏ½ð¶¨ÏòÄý¹Ì×éÖ¯Ö÷ÒªÓÉÑØ×ÅÊÔ°ôÖáÏòÅÅÁеĺá½ØÃæ³Ê»¨°ê×´µÄ¹²¾§°ûEutectic I(Nb_ss/α(Nb,X)₅Si₃)ÒÔ¼°·Ö²¼ÓÚ¹²¾§°ûÖÜΧµÄÑØÊÔ°ôÖáÏòñîºÏÉú³¤µÄ¹²¾§×éÖ¯EutecticII(Nb_ss/γ(Nb, X)₅Si₃×é³É.Ëæ×ÅÄý¹ÌËÙÂʵÄÔö´ó, ×é֯ϸ»¯,»¨°ê×´¹²¾§°ûÓÉÒԹ軯Îï»òϸС¹²¾§ÎªÖÐÐĵĽüËÆÔ²ÐÎÐÎòÖð½¥ÑݱäΪÒÔÊ®×ÖÐÎNb_ssΪÖÐÐÄ¡¢α (Nb,X)₅Si₃³ÊƬ״ÏòÍâ·øÉäÉú³¤µÄËıßÐÎÐÎò; EutecticIIÔò³ÊÑØ×ÝÏòñîºÏÉú³¤µÄ²ãƬ״ÐÎò. ¹Ì/Òº½çÃæÐÎò¾­ÀúÁËÓÉ°û֦״→Ê÷֦״→°û֦״µÄÑݱä¹ý³Ì.

Abstract

Nb-16Si-24Ti-6Cr-6Al-2Hf-xCe (x=0, 0.05, 0.1, 0.25, 0.5, 1 (%, atom fraction)) in situ composites were prepared by arc melting. The microstructure and the effect of rare earth element cerium on 1250 °C oxidation resistance of the composites were investigated with scanning electron microscopy (SEM) and X-ray energy disperse spectrum (EDS), as well as X-ray diffraction (XRD). The experimental results showed that the high temperature oxidation resistance of the alloy was improved by adding a proper amount of cerium (Ce). The effect of Ce was considered as the concurrent of the following three factors: first, the oxide of Ce formed in the interface reduced the internal oxidation rate: second, the lath shaped oxide containing Ce increased the cracking resistance and reduced the expansion of the oxide scale; and third, the decrease of the silicide volume fraction on account of Ce addition reduces the power of the sample resisting oxygen penetration.