Ultrahigh Temperature Alloy Research Articles

High temperature deformation behavior of an optimized Nb–Si based ultrahigh temperature alloy

The high temperature deformation behavior of an optimized Nb–Si based ultrahigh temperature alloy was investigated by isothermal compression. The constitutive equation as well as the deformation microstructure has been evaluated and discussed. The activation energy of deformation during the steady softening stage is 366–437kJ/mol. Dynamic recovery of Nbss, brittle fracture of large silicide blocks and refinement of small silicide particles have been recognized as the softening mechanisms of the Nb–Si based alloy under present deformation conditions.

Alloying effects on the microstructure and properties of Nb–Si based ultrahigh temperature alloys

Eight multi-component Nb–Si based ultrahigh temperature alloys were prepared by vacuum non-consumable arc melting. The effects of Hf, B and Cr alloying on the phase selection, phase stability, both non-equilibrium and equilibrium microstructure, room-temperature fracture toughness, hardness and oxidation resistance at 1250 °C of the alloys have been investigated and estimated systematically. The results show that the addition of B or Cr promotes the formation of hypereutectic structures. The alloying with both Hf and B suppresses the formation of β(Nb,X)5Si3 and promotes the formation of α(Nb,X)5Si3 and γ(Nb,X)5Si3, while the alloying with Cr has no effect on the crystal structures of 5-3 silicides. The room-temperature fracture toughness of the alloys is always degraded by the addition of Cr but almost not influenced by the combined additions of Hf and B. The hardness of 5-3 silicides exhibits a tendency of γ > α > β. The macrohardness of the alloys increases with Cr addition, and it obviously reduces in the presence of Hf after 1450 °C/50 h heat-treatment. The best oxidation-resistant performance has been obtained for the alloy with both B and Cr additions. However, in the presence of B and/or Cr, the oxidation resistance of the alloys has been degraded by further addition of Hf.

Precipitation and martensitic transformation of fcc-Ti in Nb–Ti–Si based ultrahigh temperature alloys

The morphologies, crystallographic characteristics and phase transitions of Ti precipitates developed in arc-melted and heat-treated Nb–Ti–Si based ultrahigh temperature alloys have been investigated by high resolution transmission electron microscopy (HRTEM). Rod-like Ti precipitates with fcc structure could only be found at interfaces in the arc-melted specimens, while ultrafine fcc-Ti precipitates formed inside Nbss in the heat-treated specimens. The fcc-Ti precipitates at interfaces partially transformed into hcp-Ti lamellas through a martensitic transformation during heat-treatment, and twins composed of different directional hcp-Ti lamellas formed in the fcc-Ti matrix with the orientation relationship (OR): [110]fcc−Ti//[112¯0]M//[1¯1¯20]T. The precipitation mechanisms of fcc-Ti grains in Nbss and hcp-Ti lamellas in fcc-Ti matrix have been analyzed.

Effect of Cr Addition on the Microstructure and Room-temperature Fracture Toughness of Nb-Si Based Ultra-high Temperature Alloys

Effect of Cr Addition on the Microstructure and Room-temperature Fracture Toughness of Nb-Si Based Ultra-high Temperature Alloys

Microstructure, mechanical properties and oxidation resistance of Nb silicide based ultrahigh temperature alloys with Hf addition

Four Nb silicide based ultrahigh temperature alloys with compositions of Nb–22Ti–16Si–3Cr–3Al–2B–xHf (x=0, 2, 4 and 8) (at%) were prepared by vacuum non-consumable arc melting. The effects of Hf content on the phase selection, microstructure, room-temperature fracture toughness, compressive performance and isothermal oxidation behavior at 1250°C of the alloys have been investigated. The results show that the formation of α(Nb,X)5Si3 is suppressed and instead the formation of γ(Nb,X)5Si3 is promoted by Hf addition. The Hf addition (especially 8at%) improves the room-temperature fracture toughness of the alloy. The hardness of γ(Nb,X)5Si3 increases with its Hf concentration. The compression strength of the alloy is the highest at 4at% Hf, and then obviously reduces at 8at% Hf due to embrittlement of the alloy (especially that of γ(Nb,X)5Si3). The oxidation resistance of the alloy has been firstly ameliorated by the lower Hf addition (2at%) and then degraded by the higher Hf addition (especially 8at%). The details have been discussed.

Research on Oxidation Resistant ZrSi2–NbSi2 Bilayer Coatings for an Nb–Ti–Si–Cr Based Ultrahigh Temperature Alloy

Oxidation-resistant ZrSi2–NbSi2 bilayer coatings were prepared on an Nb–Ti–Si–Cr based ultrahigh temperature alloy using a two-step (first sputtering Zr film, and then Si–Y co-deposition) process. The coatings prepared with different Zr film thicknesses possess a similar structure, consisting of a ZrSi2 outer layer, a (Nb,X)Si2 (X represents Ti, Cr and Hf) middle layer and a (Ti,Nb)5Si4 inner layer. The higher co-deposition temperature (Si–Y co-deposition at 1350 °C) would cause cracks at the interfaces between the constituent layers of the coatings. The coated specimens possess much lower mass gains than the base alloy after exposure of same static oxidation times at 1250 °C. After oxidation, a dense scale consisting of a mixture of SiO2, ZrSiO4, ZrO2, Al2O3, TiO2 and Cr2O3 formed, which could protect the specimen from oxidation at least for 100 h at 1250 °C in air.

Microstructural characteristics of Nb–Si based ultrahigh temperature alloys with B and Hf additions

Four Nb–Si based ultrahigh temperature alloys with compositions of Nb-22Ti-16Si-5Cr-3Al-mHf-nB ((m, n) = (0, 0), (0, 2), (4, 0) and (4, 2), respectively) (at.%) were prepared by vacuum non-consumable arc melting and then heat-treated at 1450 °C for 50 h. The effects of B and Hf additions on the phase selection, phase stability, microstructure and microhardness of these alloys under both as-cast and heat-treated conditions have been investigated. The results show that the microstructures of all the four alloys are composed of primary silicide blocks, Nbss dendrites together with one or two types of eutectic colonies around. However, the crystal structures of silicides, types of eutectic and amounts of constituent phases have obviously varied with B or Hf addition. Moreover, a low melting point three-phase eutectic is also observed in Hf-containing alloys. After 1450 °C/50 h heat-treatment, the microstructural uniformity of the alloys has been significantly ameliorated as well as their equilibrium phases have been basically obtained, and also some phase transformation reactions have occurred. The microhardness of the constituent phases present in the alloys is dependent on their types or crystal structures.

MICROSTRUCTURE AND OXIDATION BEHAVIOR OF ZrSi2-NbSi2 MULTILAYER COATINGS ON AN Nb-Ti-Si-Cr BASE ULTRAHIGH TEMPERATURE ALLOY

The rather poor oxidation resistance of Nb-Si base ultrahigh temperature alloys has seriously limited their practical applications at high temperatures. Niobium disilicide coatings, especially those modified by reactive elements (RE) such as Zr and Y, have been shown to possess good anti-oxidation properties at high temperatures due to the formation of a protective RE-containing SiO2 scale. Halide activated pack cementation (HAPC) is one of the most widely used techniques for preparing protective coatings on Nb-Si base ultrahigh temperature alloys, because compact coatings and metallurgical substrate/coating bonds can be obtained with using this technique. However, only a very limited amount of Zr and Y can be diffused into the coatings by a single co-deposition pack cementation process as a result of their large atomic radii and high melting points. To solve this problem, a method such as magnetron sputtering, which can be used for producing overlay coatings with different composition ratios of coating elements, seems to be feasible. In the present study, ZrSi2-NbSi2 multilayer coatings were prepared on an Nb-Ti-Si-Cr base ultrahigh temperature alloy by first magnetron sputtering 2 μm thick Zr-film, and then Si-Y *国家自然科学基金项目51371145, 51431003和U1453201,高等学校学科创新引智计划项目B080401和凝固技术国家重点实验室 自主研究课题项目116-QP-2014资助 收到初稿日期: 2014-09-09,收到修改稿日期: 2015-04-07 作者简介:李 轩,男, 1982年生,博士生 DOI: 10.11900/0412.1961.2014.00498 第693-700页 pp.693-700

Effects of Cr and Hf additions on the microstructure and properties of Nb silicide based ultrahigh temperature alloys

Four Nb silicide based ultrahigh temperature alloys with compositions of Nb–22Ti–16Si–3Al–2B–xHf–yCr ((x, y)=(0, 0), (0, 5), (4, 0) and (4, 5)) (at%) were prepared by vacuum non-consumable arc melting. The effects of Cr and Hf additions on the phase selection, microstructure, room-temperature fracture toughness and oxidation resistance at 1250°C of the alloys have been investigated. The results show that all the four alloys are comprised of primary silicides and Nbss dendrites together with one or two types of eutectic. The Cr addition does not change the crystal structures of 5-3 silicides but imposes a significant influence on the amount, size and morphology of the constituent phases. The Hf addition suppresses the formation of α(Nb,X)5Si3 but promotes the formation of γ(Nb,X)5Si3. The combined additions of Cr and Hf also cause the formation of Cr2(Nb,X) containing three-phase eutectic. The room-temperature fracture toughness of the alloys decreases with Cr addition while increases with Hf addition. The oxidation-resistant performance of the alloys has been ameliorated by Cr addition, while it has been significantly degraded by Hf addition in the absence of Cr. Furthermore, the formation mechanism of the scale has been illustrated.

Co-deposition of Si and B to form oxidation-resistant coatings on an Nb–Ti–Si based ultrahigh temperature alloy by pack cementation technique

In this work, we study the oxidation behavior of a B-modified silicide coating at 900–1350°C. The coating was prepared by pack cementation, which mainly consisted of a (Nb,X)Si2+(Nb,X)B2 outer layer, a (Nb,X)Si2 intermediate layer and an inner layer. At 900 and 1250°C, the evolution of the mass gain versus time showed parabolic-like kinetics; while at 1350°C, the evolution seemed to follow a linear-like kinetic, but could protect the alloy from oxidation for at least 60h. The good oxidation resistance of the B-modified silicide coating was attributed to the formation of a dense glass-like scale.

Microstructure of Nb–Ti–Cr–Si based ultrahigh temperature alloy processed by integrally directional solidification

The Nb–Ti–Cr–Si based ultrahigh temperature alloy was integrally directionally solidified with the using of special ceramic crucibles. The effects of different withdrawing rates on the microstructure and the solidification path in a Nb–Ti–Cr–Si based ultrahigh temperature alloy were investigated. The results showed that the microstructure was composed of primary niobium solid solution (Nbss) dendrites, Nbss/(Nb,X)5Si3 (here X represents Ti and Hf elements) eutectic colonies and fine Ti rich phase layer between Nbss dendrites and Nbss/(Nb,X)5Si3 eutectic colonies when the withdrawal rates vary from 2·5 to 10 μm s−1. However, the directionally solidified microstructures were composed of primary Nbss dendrites, Nbss/(Nb,X)5Si3 eutectic colonies and finer (Ti,Nb)ss/Nb3Si/Cr2Nb colonies colonies when the withdrawal rate was reached and exceed than 20 μm s−1. The primary Nbss and Nbss/(Nb,X)5Si3 eutectic aligned the growth direction when the withdrawal rates vary from 2·5 to 20 μm s−1; however, the discontinuous (Nb,X)5Si3 plates were present when the withdrawal rates vary from 50 to 100 μm s−1. The solidification path was studied according to the microstructure of solid/liquid interface.

Structure and Oxidation Behavior of Zr–Y Modified Silicide Coatings Prepared on an Nb–Ti–Si–Cr Based Ultrahigh Temperature Alloy

Zr–Y modified silicide coatings have been prepared on an Nb–Ti–Si–Cr based ultrahigh temperature alloy by a pack cementation process. The effects of amount of Zr powder in the pack mixture and co-deposition temperature on the coating structure were assessed. The coatings prepared at 1,250 °C with different amounts of Zr powders in the pack mixture have similar structures, mainly consisting of a thick (Nb,X)Si2 (X represents Ti, Cr and Hf elements) outer layer, a thin (Ti,Nb)5Si4 middle layer and a 1–2 µm thick (Nb,X)5Si3 inner layer. The increased amount of Zr powders in the pack mixtures led to a significant decrease in the coating thickness. The coatings prepared at 1,150 and 1,200 °C have similar structures to that prepared at 1,250 °C. However, when the co-deposition temperature increases to 1,300 and 1,350 °C, a (Ti,Nb)5Si3 outermost layer formed in addition to the three layers mentioned above. The Zr–Y modified silicide coating can protect the base alloy from oxidation at least for 100 h at 1,250 °C in air. The good oxidation resistance of the Zr–Y modified silicide coating is attributed to the formation of a dense scale consisting of SiO2 and TiO2.

Improvement in oxidation resistance of silicide coating on an Nb–Ti–Si based ultrahigh temperature alloy by second aluminizing treatment

Modified silicide coating was prepared on an Nb–Ti–Si based alloy by a two-step pack cementation process including first siliconizing and then aluminizing. The two-step coating is composed of an Al-rich outermost layer, a (Nb,X)Si2 outer layer and a (Ti,Nb)5Si4 inner layer. The coating possesses an enhanced oxidation resistance at 1250°C due to the presence of the Al-rich outermost layer. The mass gain of the coating after oxidation at 1250°C for 100h is 3.7mg/cm2, and a duplex scale consisting of an Al2O3 outer layer and a SiO2-matrix inner layer forms.

Hot corrosion behavior of silicide coating on an Nb–Ti–Si based ultrahigh temperature alloy

Hot corrosion behavior of an Nb–Ti–Si alloy and its silicide coating in Na2SO4–25wt.% NaCl molten salts at 900°C was investigated. For the bare alloy, a thick and porous corroded scale (Na-containing oxides) was formed after hot corrosion. The silicide coating showed a good oxidation resistance at 900°C, while the presence of the molten salts aggravated the damage process of the silicide coating by forming a thick corroded scale which mainly consisted of a Na2TiSi2O7 outer layer and a porous and cracked NaNbO3+Na2TiSi2O7 inner layer. The possible hot corrosion mechanism of the silicide coating has been explored.

Effects of B addition on the microstructure and properties of Nb silicide based ultrahigh temperature alloys

Four Nb silicide based ultrahigh temperature alloys with compositions of Nb–22Ti–16Si–5Cr–4Hf–3Al–xB (x = 0, 2, 5 and 10, respectively) (at%) were prepared by vacuum non-consumable arc melting. The effects of B content on the phase selection, microstructure, oxidation resistance at 1250 °C, room-temperature fracture toughness and microhardness of the alloys were investigated. The results showed that the microstructures of the four alloys were all comprised of primary silicide blocks, Nbss dendrites and eutectic colonies. However, the crystal structures or types of silicides and the amounts of constituent phases obviously varied with increase in B content in the alloys. The oxidation resistance of the alloys was significantly ameliorated by B addition. The room temperature fracture toughness of the alloys was improved with 2 at% B addition but degraded with 5 or 10 at% B addition. The microhardness of Nbss rose slightly with increase in B content in the alloys, while that of silicides was dependent on their crystal structures, types and concentrations of alloying elements.

Effect of withdrawal rates on microstructures and room temperature fracture toughness in a directionally solidified Nb–Ti–Cr–Si based alloy

Abstract Integrally directional solidification of an Nb–Ti–Cr–Si based ultrahigh temperature alloy is conducted with the use of special ceramic crucibles. The effect of withdrawal rates on microstructure and room temperature fracture toughness is investigated. The microstructure of arc-melted alloy is composed of primary niobium solid solution (Nbss), Nbss/(Nb,X) 5 Si 3 eutectic and fine Nbss/Cr 2 Nb eutectic colonies, and the directionally solidified microstructure is composed of primary Nbss, Nbss/(Nb,X) 5 Si 3 eutectic cells and fine Nbss/Nb 3 Si/Cr 2 Nb colonies (here X represents Ti and Hf elements). The room temperature fracture toughness decreased with increase in the withdrawing rates. The room temperature fracture toughness of the directional solidification specimen with withdrawing rates 10 and 20 μm/s are higher than the arc-melted counterparts. The effects of withdrawing rates on the microstructure and room temperature fracture toughness are discussed based on the microstructural and compositional characters.

Microstructure and growth kinetics of Ce and Y jointly modified silicide coatings for Nb–Ti–Si based ultrahigh temperature alloyMicrostructure and growth kinetics of Ce and Y jointly modified silicide coatings for Nb–Ti–Si based ultrahigh temperature alloyretain-->

In order to protect NbTiSi based ultrahigh temperature alloy from oxidation, pack cementation processes were utilized to prepare Ce and Y jointly modified silicide coatings. The Ce and Y jointly modified silicide coating has a double-layer structure: a relatively thick (Nb, X)Si2 (X represents Ti, Cr and Hf elements) outer layer and a thin (Ti, Nb)5Si4 transitional layer. The pack cementation experiments at 1150°C for 8h proved that the addition of certain amounts of CeO2 and Y2O3 powders in the packs distinctly influenced the coating thickness, the contents of Si, Ce and Y in the (Nb, X)Si2 outer layers, and the density of cavities in the coatings. In order to study the effects of Ce and Y joint modification in the silicide coatings, both only Ce and only Y modified silicide coatings were also prepared for comparison. The mechanisms of the beneficial effects of Ce and Y are discussed. A pack mixture containing 1.5CeO2–0.75Y2O3 (wt%) powders was employed to investigate the growth kinetics of the Ce and Y jointly modified silicide coating at 1050, 1150 and 1250°C. It has been found that the growth kinetics obeyed parabolic laws and the parabolic rate constants were 109.20μm2/h at 1050°C, 366.75μm2/h at 1150°C and 569.78μm2/h at 1250°C, and the activation energy for the growth of the Ce and Y jointly modified silicide coating was 197.53kJ/mol.

EUTECTIC MICROSTRUCTURE EVOLUTION OFDIRECTIONALLY SOLIDIFIED Nb-Ti-Si BASEULTRAHIGH TEMPERATURE ALLOY

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 100μm/s) with a constant melt temperature of 2000℃.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)_5Si_3 eutectic colonies(EutecticⅠ) and coupled grown lamellar Nb_(ss)/γ(Nb,X)_5Si_3 eutectic(EutecticⅡ) which distributed in the intercellular area.The EutecticⅠand EutecticⅡare both aligned straight and uprightly along the growth direction. When the solidifying rate increases from 2.5μm/s to 100μm/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.EutecticⅡexhibits 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.

FORMATION OF Ge-Y MODIFIED SILICIDE COATINGS ON Nb-Ti-Si BASE ULTRAHIGH TEMPERATURE ALLOY

Nb-Si base ultrahigh temperature alloy is a promising candidate for high tempera- ture applications due to its high melting point, high strength and relatively low density at elevated temperatures. However, the poor oxidation resistance of this alloy has still be a major barrier to its high temperature applications. Ge and Y modified silicide coatings on an Nb-Ti-Si based ultrahigh temperature alloy have been prepared successfully by co-depositing Si, Ge and Y in order to improve its high temperature oxidation resistance. The structure, constituent phases and formation process of the coatings have been revealed using XRD, SEM and EDS. The results showed that the coatings had obvious layered structure which was composed of a (Nb, X)(Si, Ge)2 (X represents Ti, Cr and Hf elements) outer layer and a (Ti, Nb)5(Si, Ge)4 inner layer. The concentration of Ge in the coating increased with increase in the distance away from the coating surface. Compared with the single Y modified silicide coating, the addition of Ge did not change the coating structrue, but reduced the absorption and deposition of Si and retarded the coating growth. The coatings prepared at 1250 for different time (0—8 h) had similar structures and the coating growth kinetics followed a parabolic law.

Effects of alloying and high-temperature heat treatment on the microstructure of Nb–Ti–Si based ultrahigh temperature alloys

The phase compositions, microstructure and especially phase interfaces in the as-cast and heat-treated Nb–Ti–Si based ultrahigh temperature alloys have been investigated. It is shown that β(Nb,X)5Si3 and γ(Nb,X)5Si3 are the primary phases in the Nb–22Ti–16Si–5Cr–5Al (S1) (at%) and Nb–20Ti–16Si–6Cr–4Al–5Hf–2B–0.06Y (S2) (at%) alloys, respectively. The Nb solid solution (Nbss) is the primary phase in Nb–22Ti–14Si–5Hf–3Al–1.5B–0.06Y (S3) (at%) alloy. An orientation relationship between Nbss and γ(Nb,X)5Si3 was determined to be (11¯0)Nb//(101¯0)γ and [111]Nb//[0001]γ in the as-cast S2 and S3 alloys. Some original β(Nb,X)5Si3 transformed into α(Nb,X)5Si3 because Al and Cr diffused from the β(Nb,X)5Si3 to Nbss during heat treatment at 1500°C for 50h in the S1 alloy. Meanwhile, Ti diffused from Nbss to β(Nb,X)5Si3, which induced αTi to generate near the interface between Nbss and Ti-rich β(Nb,X)5Si3. The orientation relationship between the newly-formed αTi and previous Nbss was (110)Nb//(11¯01¯)αTi and [001]Nb//(123¯1¯)αTi. Among the (Nb,X)5Si3 phases, the contents of Cr and Al in β(Nb,X)5Si3 are nearly the same as those in γ(Nb,X)5Si3 but obviously higher than those in the α(Nb,X)5Si3, whereas the content of Si in α(Nb,X)5Si3 is nearly the same as that in γ(Nb,X)5Si3 but higher than that in the β(Nb,X)5Si3.