Ultrahigh Temperature Alloy Research Articles

EFFECTS OF ACTIVATORS ON FORMATION OF Si-Zr-Y CO-DEPOSITION COATINGS ON Nb-Ti-Si-Cr BASE ULTRAHIGH TEMPERATURE ALLOY

Si-Zr-Y co-deposition coatings on an Nb-Ti-Si-Cr base ultrahigh temperature alloy were prepared by pack cementation processes at 1250 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

Effect of high temperature treatments on microstructure of Nb-Ti-Cr-Si based ultrahigh temperature alloy

To investigate the effects of homogenizing and aging treatments on the microstructure of an Nb-Ti-Cr-Si based ultrahigh temperature alloy, coupons were homogenized at 1 200-1 500 °C for 24 h, and then aged at 1 000 °C for 24 h. The results show that the heat-treated alloy is composed of Nb solid solution (Nbss), (Nb,X) 5Si 3 and Cr 2Nb phases. With the increase of heat-treatment temperature, previous Nbss dendrites transformed into equiaxed grains, and petal-like Nbss/(Nb,X) 5Si 3 eutectic colonies gradually changed into small (Nb,X) 5Si 3 particles distributed in Nbss matrix. A drastic change occurred in the morphology of the Laves phase after homogenizing treatment. Previously coarse Cr 2Nb blocks dissolved during homogenizing at temperature above 1 300 °C, and then much finer and crowded Cr 2Nb flakes precipitated in the Nbss matrix in cooling. Aging treatment at 1 000 °C for 24 h led to further precipitation of fine particles of Laves phase in Nbss matrix and made the difference in concentrations of Ti, Hf and Al in Nbss, (Nb,X) 5Si 3 and Cr 2Nb phases reduced.

Y and Al modified silicide coatings on an Nb–Ti–Si based ultrahigh temperature alloy prepared by pack cementation process

Y and Al modified silicide coatings were prepared on an Nb–Ti–Si based ultrahigh temperature alloy by co-depositing Si, Al and Y at 1150°C for up to 10h, respectively. The deposition of Al and Si occurred in a sequential manner during the pack cementation process. At the initial stage, the element deposited was primarily Al with very little Si and an Al3(Nb,X) (X represents Ti, Cr and Hf elements) layer formed preferentially. After a short period of holding time, Si started depositing and Si–Al co-deposition took place. However, this Si–Al co-deposition period was not long. When the holding time was longer than 1h at 1150°C, Si deposition dominated the coating growth process. The coating growth kinetics at 1150°C followed a parabolic law. The coating prepared at 1150°C for 10h had a multi-layer structure, with a thick (Nb,X)Si2 outer layer, a thin (Ti,Nb)5Si4 middle layer and an Al, Cr-rich inner layer. The coating could protect the Nb–Ti–Si based alloy from oxidation at 1250°C in air for at least 100h. The excellent oxidation resistance of the coating was attributed to the formation of a dense scale mainly consisted of TiO2, SiO2 and Al2O3.

Si-B Co-Deposition Coatings on an Nb Silicide Based Ultrahigh Temperature Alloy Prepared by Pack Cementation Technique

In order to improve the oxidation resistance of silicide coatings on an Nb silicide based alloy, Si-B co-deposition coatings were prepared at 1150°C for 8 h by using pack mixtures containing different amounts of B powders. The microstructural characteristics of the coatings were studied. The results showed that all coatings had a triple-layer structure. The middle layers of all coatings were composed of (Nb,X)Si2 (X represents Ti, Cr and Hf elements) and the inner layers of all coatings were composed of (Nb,X)5Si3, but the outer layers of the coatings had different phase constituents and morphologies when the amount of B powders in the pack mixture changed. When the amount of B powders in the pack mixture ranged from 0.5 wt% to 12 wt%, the outer layers were composed of (Nb,X)B2­, with a few CrB2 particles. When the amount of B powders in the pack mixture was in the range of 0.05~0.1 wt%, the outer layers were composed of (Nb,X)B2. However, when the amount of B powders in the pack mixture was in the range of 0.02~0.03 wt%, the outer layers were composed of (Nb,X)Si2, with needle-like (Nb,X)B2 particles dispersed in it.

Microstructure and microhardness of directionally solidified and heat-treated Nb-Ti-Si based ultrahigh temperature alloy

Directionally solidified (DS) specimens of Nb-Ti-Si based ultrahigh temperature alloy were heat-treated at (1 500 °C, 50 h) and (1 500 °C, 50 h) + (1 100 °C, 50 h), respectively. The results show that the microstructures become uniform, the long and big primary (Nb, X) 5Si 3 ( X represents Ti and Hf elements) plates in the DS specimens are broken into small ones, and the eutectic cells lose their lamellar morphology and their interfaces become blurry after heat-treatment. Meanwhile, the (Nb, X) 5Si 3 slices in the eutectic cells of the DS specimens coarsen obviously after heat-treatment. Homogenizing and aging treatments could effectively eliminate elemental microsegregation, and the segregation ratios of all elements in niobium solid solution (Nbss) in different regions tend to 1. After heat-treatment, the microhardness of retained eutectic cells increases evidently, and the maximum value reaches HV1 404.57 for the specimen directionally solidified with a withdrawing rate of 100 μm/s and then heat-treated at (1 500 °C, 50 h) + (1 100 °C, 50 h), which is 72.8 % higher than that under DS condition.

Morphology and phase constituents of mechanically alloyed Nb-Ti-Si based ultrahigh temperature alloy powders

Preparation of high-quality powder blends is the first step for hot-press sintering of Nb-Ti-Si based ultrahigh temperature alloys. In the present work, powder blends of Nb-22Ti-15Si-6Cr-4Hf-3Al-1.5B-0.06Y (at.%) were prepared by ball milling the elemental powder mixtures. The morphology and phase constituents of the powder blends were analyzed by XRD, SEM and EDS techniques. The mechanical alloying process, including diffusion, phase formation and powder deformation was revealed. The results show that supersaturated Nbss (Nb solid solution) formed during the milling process, the deformation of the powders became more serious and thus more crystal defects appeared in the powders with the prolonging of milling time at different rotating speeds. The powder size firstly increased and then decreased, and the size distribution of powders became more uniform with the increase in milling time. A small quantity of nanometer scaled spheroidal particles formed after ball milling for a certain time. It was found that the alloying process was faster upon a higher rotating speed ball milling, but with a lower powder yield.

Microstructure evolution and room temperature fracture toughness of an integrally directionally solidified Nb–Ti–Si based ultrahigh temperature alloy

Integrally directional solidification of an Nb–Ti–Si based ultrahigh temperature alloy was performed at 2000 °C with withdrawal rates varying from 2.5 to 100 μm s−1. The directionally solidified microstructure was composed of primary silicides and eutectic colonies that distributed evenly and aligned erectly along the growth direction. Both the average diameter and the lamellar spacing of eutectic cells decreased with increasing withdrawal rate. The room temperature fracture toughness was improved significantly by integrally directional solidification.

Microstructure of Si-Al-Y Co-deposition Coatings on an Nb-Ti-Si Based Ultrahigh Temperature Alloy

研究了包埋共渗温度和渗剂中Al含量对Nb-Ti-Si基超高温合金表面Si-Al-Y共渗层显微组织的影响. 采用组成为10Si-10Al-3Y-5NaF-72Al 2 O 3 (wt%)的渗剂, 分别在1050、1080和1150℃保温10h所制备的渗层具有相似的结构, 由(Nb,X)Si 2 (X代表Ti, Cr和Hf)外层、(Nb,X) 5 Si 3 中间层、(Cr,Al) 2 (Nb,X)和(Nb,X)Al 3 相构成的次内层及(Nb,X) 2 Al内层组成. 在1050℃/10h所制备渗层的外层和中间层的组成相随渗剂中Al含量增加而变化, 但次内层和内层的组成相不改变. 当渗剂中Al含量为15wt%时, 渗层外层仍为(Nb,X)Si 2 , 但中间层却由(Nb,X)Al 3 和(Nb,X) 5 Si 3 两相组成; 当渗剂中Al含量为20wt%时, 渗层外层转变为由(Nb,X)Si 2 和(Nb,X) 3 Si 5 Al 2 两相组成. 对1050℃/10h所制备的渗层(采用Al含量为15wt%的渗剂)进行1250℃/0.5h氧化, 氧化膜厚度约为10μm, 主要由Al 2 O 3 、TiO 2 和SiO 2 组成.

Effects of homogenizing and aging treatments on the microstructure and microhardness of an Nb-silicide based ultrahigh temperature alloy

Abstract To investigate the effects of homogenizing and aging treatments on the microstructure and microhardness of an Nb-silicide based ultrahigh temperature alloy, coupons were homogenized at 1 300–1 600°C for 50 h, and then aged at 1 100°C for 50 h. The results show that the phase constituents of the heat-treated alloys are still Nb solid solution and silicides, α-(Nb,X)5Si3 phase forms after heat-treatments at temperatures above 1 500°C. With the increase in heat-treatment temperature, the original big polygonal and plate-like silicides gradually break into small ones, and the amounts of retained typical lamellar and rod-like eutectic colonies reduce clearly. The variation of partitioning ratios of alloying elements between Nb solid solution and silicides causes the change in microhardness of the alloy. 1 500°C/50 h + 1 100°C/50 h is an optimum heat-treatment process.

Formation of Cr-modified silicide coatings on a Ti–Nb–Si based ultrahigh-temperature alloy by pack cementation process

Cr-modified silicide coatings were prepared on a Ti–Nb–Si based ultrahigh temperature alloy by Si–Cr co-deposition at 1250 °C, 1350 °C and 1400 °C for 5–20 h respectively. It was found that both coating structure and phase constituents changed significantly with increase in the co-deposition temperature and holding time. The outer layers in all coatings prepared at 1250 °C for 5–20 h consisted of (Ti,X) 5Si 3 (X represents Nb, Cr and Hf elements). (Ti,X) 5Si 4 was found as the only phase constituent in the intermediate layers in both coatings prepared at 1250 °C for 5 and 10 h, but the intermediate layers in the coatings prepared at 1250 °C for 15 and 20 h were mainly composed of (Ti,X) 5Si 3 phase that was derived from the decomposition of (Ti,X) 5Si 4 phase. In the coating prepared at 1350 °C for 5 h, single (Ti,X) 5Si 3 phase was found in its outmost layer, the same as that in the outer layers in the coatings prepared at 1250 °C; but in the coatings prepared at 1350 °C for 10–20 h, (Nb 1.95Cr 1.05)Cr 2Si 3 ternary phase was found in the outmost layers besides (Ti,X) 5Si 3 phase. In the coatings prepared at 1400 °C for 5–20 h, (Nb 1.95Cr 1.05)Cr 2Si 3 ternary phase was the single phase constituent in their outmost layers. The phase transformation (Ti,X) 5Si 4 → (Ti,X) 5Si 3 + Si occurred in the intermediate layers of the coatings prepared at 1350 and 1400 °C with prolonging co-deposition time, similar to the situation in the coatings prepared at 1250 °C for 15 and 20 h, but this transformation has been speeded up by increase in the co-deposition temperature. The transitional layers were mainly composed of (Ti,X) 5Si 3 phase in all coatings. The influence of co-deposition temperature on the diffusion ability of Cr atoms was greater than that of Si atoms in the Si–Cr co-deposition processes investigated. The growth of coatings obeyed inverse logarithmic laws at all three co-deposition temperatures. The Si–Cr co-deposition coating prepared at 1350 °C for 10 h showed a good oxidation resistance due to the formation of SiO 2 and Nb, Cr-doped TiO 2 scale after oxidation at 1250 °C for 10 h.

Oxidation Behavior of an Al-Modified Silicide Coating on an Nb-Silicide-Based Ultrahigh-Temperature Alloy

Abstract The oxidation behavior of an Al-modified silicide coating prepared through a siliconization and then an aluminization pack cementation process on an Nb-silicide-based ultrahigh-temperature alloy was investigated in air at 1,250°C. The coating consisted of a 50-μm-thick (Nb,X)Si2 (X represents Ti, Cr, and Hf elements) outer layer with (Nb,Ti)3Si5Al2 distributed discontinuously in its superficial part and a 90-μm-thick (Nb,Ti)Al3 inner layer containing Si-rich precipitates. The oxidation of this coating was controlled primarily by the preferential oxidation of Al with the formation of α-Al2O3 scale. The (Nb,Ti)Al3 inner layer could act as an Al reservoir for forming scale and maintaining the existence of (Nb,Ti)3Si5Al2 phase in the zone just beneath the scale. The parabolic oxidation rate constant of this coating was about 1.72 μm2/h, lower than that of the bare alloy by about 4 orders of magnitude.

Microstructure and High Temperature Oxidation Resistance of Si-Y<SUB>2</SUB>O<SUB>3</SUB> Co-deposition Coatings Prepared on Nb-silicide-based Ultrahigh Temperature Alloy by Pack Cementation Process

Microstructure and High Temperature Oxidation Resistance of Si-Y<SUB>2</SUB>O<SUB>3</SUB> Co-deposition Coatings Prepared on Nb-silicide-based Ultrahigh Temperature Alloy by Pack Cementation Process

Effects of B on the Microstructure and Oxidation Resistance of Nb-Ti-Si-based Ultrahigh-temperature Alloy

Nb-Ti-Si-based ultrahigh-temperature alloys concocted with boron ranging from 0 to 2 at% are prepared by arc-melting technology. The effects of adding boron on their as-melted microstructure and oxidation resistance are analyzed. The (Nb,Ti)ss, β-(Nb,Ti)5Si3 and γ-(Nb,Ti)5Si3 exist in Nb-22Ti-16Si-6Cr-3Al-4Hf alloy, while (Nb,Ti)ss, α-(Nb,Ti)5Si3 and γ-(Nb,Ti)5Si3 are present in Nb-22Ti-16Si-6Cr-3Al-4Hf-1B and Nb-22Ti-16Si-6Cr-3Al-4Hf-2B alloys. The oxidation of Nb-Ti-Si-based ultrahigh-temperature alloys is dominated by the diffusion of oxygen through (Nb,Ti)ss. Compared to boron-free alloys, the boron-containing alloys have significantly lower oxidation rate when oxidized at 1 200 °C for less than 50 h, but, for more than 50 h, their oxidation resistance deteriorates.

Structure and oxidation behavior of Si–Y co-deposition coatings on an Nb silicide based ultrahigh temperature alloy prepared by pack cementation technique

In order to improve the oxidation resistance of silicide coatings on Nb silicide based alloys, Y-modified silicide coatings were prepared by co-depositing Si and Y at 1050, 1150 and 1250 °C for 5–20 h, respectively. It has been found that the coatings prepared by co-depositing Si and Y at 1050 and 1150 °C for 5–20 h as well as at 1250 °C for 5 h were composed of a thick (Nb,X)Si 2 (X represents Ti, Cr and Hf elements) outer layer and a thin (Nb,X) 5Si 3 inner layer, while the coatings prepared by co-depositing Si and Y at 1250 °C for 10–20 h possessed a thin outer layer composed of (Ti,Nb) 5Si 3 and Ti-based solid solution, a thick (Nb,X)Si 2 intermediate layer and a thin (Nb,X) 5Si 3 inner layer. EDS analyses revealed that the content of Y in the (Nb,X)Si 2 layers of all the coatings was about 0.34–0.58 at.% while that in the outer layers of the coatings prepared by co-depositing Si and Y at 1250 °C for 10–20 h was about 1.39–1.88 at.%. The specimens treated by co-depositing Si and Y at 1250 °C for 10 h were selected for oxidation test. The oxidation behavior of the coating specimens at 1250 °C indicated that the Si–Y co-deposition coating had better oxidation resistance than the simple siliconized coating because the oxidation rate constant of the Si–Y co-deposition coating was lower than that of the simple siliconized coating by about 31%. The scale developing on the Si–Y co-deposition coating consisted of a thicker outer layer composed of SiO 2 and TiO 2 and a thinner SiO 2 inner layer.

INTEGRALLY DIRECTIONALLY SOLIDIFIED MICROSTRUCTURE OF AN NIOBIUM SILICIDE BASED ULTRAHIGH TEMPERATURE ALLOY

Integrally directional solidification of an advanced Nb silicide based ultrahigh temperature alloy was conducted with the use of special ceramic crucibles. The melt temperature was 2000°C and the withdrawing rate was respectively 5, 10, 15, 30, 50, 80 and 100 µm/s. The directionally solidified microstructure was composed of primary ( Nb ,X)5 Si 3 rods and ( Nb , Ti ) ss + ( Nb ,X)5 Si 3 eutectic colonies (here X represents Ti and Hf elements) that distributed evenly and aligned erectly along the growth direction. The quenched solid/liquid (S/L) interfaces had the morphology of a series of protruded parallel eutectic cells along the growth direction. ( Nb , Ti ) ss and ( Nb ,X)5 Si 3 slices grew in a coupled manner in each eutectic cell, forming a typical lamellar eutectic morphology. Both the average diameter of eutectic cells and lamellar spacing in them decreased with the increase of withdrawing rates. The solidification path was that primary ( Nb ,X)5 Si 3 rods formed firstly and then the retained liquid around solidified into ( Nb , Ti ) ss + ( Nb ,X)5 Si 3 eutectic cells.

Structure of Al-modified silicide coatings on an Nb-based ultrahigh temperature alloy prepared by pack cementation techniques

In order to prepare Al-modified silicide coatings on an Nb-based ultrahigh temperature alloy, both a two-stage pack cementation technique and a co-deposition pack cementation technique were employed. The two-stage process included siliconizing a specimen at 1150 °C for 4 h followed by aluminizing it at 800–1000 °C for 4 h. The coating prepared by pack siliconization was composed of a thick (Nb,X)Si 2 (X represents Ti, Cr and Hf elements) outer layer and a thin (Nb,X) 5Si 3 transitional layer; after the siliconized specimens were aluminized at or above 860 °C, a (Nb,Ti) 3Si 5Al 2 phase developed at the surface of the coating, and furthermore, when aluminizing was carried out at 860 °C, a new (Nb,Ti) 2Al layer formed in the coating between the (Nb,X) 5Si 3 layer and the substrate, but when aluminizing was performed at 900–1000 °C, the new layer formed was (Nb,Ti)Al 3. The co-deposition process was carried out by co-depositing Si and Al on specimens at 1000–1150 °C for 8 h under different pack compositions, and it was found that the structure of co-deposition coatings was more evidently affected by co-deposition temperature than pack composition. An Al-modified silicide coating with an outer layer composed of (Nb,Ti) 3Si 5Al 2, (Nb,X)Si 2 and (Nb,Ti)Al 3 was obtained by co-depositing Si and Al at 1050 °C.

Oxidation Behavior and Pack Siliconized Oxidation-Resistant Coatings of an Nb-Based Ultrahigh Temperature Alloy

The halide-activated pack cementation method was utilized to deposit silicide coatings on a multicomponent Nb-Ti-Si based alloy. The siliconized temperature was 1150 °C and the holding time was 10h. Both the specimens with siliconized coatings and without coatings were oxidized at 1250°C for 5, 10, 20, 50 and 100h respectively. The coating possessed a double layer structure with the composition of (Nb,X)Si2 (X represents Ti, Cr and Hf), and the outer layer was denser. The major structure in the outer layer was composed of columnar crystals perpendicular to the interface between the coating and the substrate, and that in the inner layer was mainly composed of equiaxed crystals. A transitional layer about 5μm thick was found between the coating and the substrate. After oxidation at 1250°C, the major constituents in the scale were SiO2 and TiO2 and the mole ratio of these two phases was about 2:1. The thickness of the (Nb,X)Si2 layer decreased and that of transitional layer increased as the oxidation time prolonged. The siliconized coating exhibited excellent oxidation-resistance at 1250°C within 50 hours.

Microstructure and Mechanical Properties of an Advanced Niobium Based Ultrahigh Temperature Alloy

The microstructure and mechanical properties including room temperature fracture toughness Kq, tensile strengthσb and elongationδ at 1250°C of the Nb based alloy directionally solidified in an electron beam floating zone melting (EBFZM) furnace have been evaluated. The microstructure is primarily composed of Nb solid solution (Nbss), α-(Nb)5Si3 and (Nb)3Si phases. After directional solidification with the moving rate of electron beam gun R being respectively 2.4, 4.8 and 7.2 mm/min, the primary Nbss dendrites, Nbss + (Nb)5Si3/(Nb)3Si eutectic colonies (lamellar or rod-like) and divorced Nb silicide plates align along the longitudinal axes of the specimens. When R = 2.4 mm/min, the best directional microstructure is obtained. Directional solidification has significantly improved theσb at 1250°C and Kq. The maximumσb occurs for the specimens with R = 2.4 mm/min and is about 85.0 MPa, meanwhile, the Kq is about 19.4 MPam1/2.

Development of an Oxidation Resistant Coating on Nb-Based Ultra-High Temperature Alloy

A coating with a duplex layer structure, outer β-NiAl and inner σ-Re-Cr-Ni layers, was formed on the third generation Nb-5Mo-15W-16Si- 5Hf-5C alloy by using successively Re-pack cementation, electroplating of Re-Ni film, and Cr/Al pack cementation. The duplex layer coating changed during high temperature oxidation to form a coating with a four layer structure: an outermost Ni2Al3, an outer Ni, an inner σ-Re-Cr-Ni, and an innermost χ-Re-Nb. The Re-pack cementation was carried in an alumina crucible where the specimen was buried in Re metal powder, in vacuum at 1573K to form a Re film, and then a Re-Ni film was electroplated onto the Re-pack treated alloy. The coated alloy formed a protective α-Al2O3 scale when oxidized at 1573K in air for 14.4ks. It was concluded that the σ-Re-Cr-Ni phase can act as a diffusion-barrier against both inward Al diffusion and outward diffusion of alloying elements from the alloy substrate to the β-NiAl.

Mechanical Testing of Ultra-high Temperature Alloys

Mechanical Testing of Ultra-high Temperature Alloys