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.
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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.
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.
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)5Si3 eutectic and fine Nbss/Cr2Nb eutectic colonies, and the directionally solidified microstructure is composed of primary Nbss, Nbss/(Nb,X)5Si3 eutectic cells and fine Nbss/Nb3Si/Cr2Nb 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 20μm/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.
•Si–B co-deposition coating was applied to Nb based alloy by pack cementation.•The Si–B co-deposition coating mainly consisted of (Nb,X)Si2 and (Nb,X)B2 phases.•The mass gains are very low after ...oxidation at 900 and 1250°C for 100h.•The good oxidation resistance is due to the formation of a dense glass-like scale.
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.
► Y and Al modified silicide coatings containing different Al contents were prepared. ► The microstructures of the coatings containing different Al contents were studied. ► The Al content in the ...coatings imposed a significant influence on their oxidation. ► The effect mechanism of Al on the oxidation of the coatings was investigated.
Several Y and Al modified silicide coatings containing different Al contents were prepared on an Nb–Ti–Si based alloy by pack cementation technique. The microstructure and oxidation resistance of these coatings were investigated. The existence of Al in the coatings was in the form of solid solutions in (Nb,X)Si2 outer layer and forming Al-rich inner layer. A suitable content of Al in the coatings improved their oxidation resistance at 1250°C for promoting the formation of SiO2 preferentially, while an excess of Al in the coatings imposed an adverse effect for destroying the integrality of the SiO2 scale.
Nb-Si based ultrahigh-temperature alloys with the composition of Nb-22Ti-15Si-5Cr-3Al (atomic percentage, at. %) were prepared by hot press sintering (HPS) at 1250, 1350, 1400, 1450 and 1500 °C. The ...effects of HPS temperatures on the microstructure, room temperature fracture toughness, hardness and isothermal oxidation behavior of the alloys were investigated. The results showed that the microstructures of the alloys prepared by HPS at different temperatures were composed of Nbss, βTiss and γ(Nb,X)
Si
phases. When the HPS temperature was 1450 °C, the microstructure was fine and nearly equiaxed. When the HPS temperature was lower than 1450 °C, the supersaturated Nbss with insufficient diffusion reaction still existed. When the HPS temperature exceeded 1450 °C, the microstructure coarsened obviously. Both the room temperature fracture toughness and Vickers hardness of the alloys prepared by HPS at 1450 °C were the highest. The alloy prepared by HPS at 1450 °C exhibited the lowest mass gain upon oxidation at 1250 °C for 20 h. The oxide film was mainly composed of Nb
O
, TiNb
O
, TiO
and a small amount of amorphous silicate. The formation mechanism of oxide film is concluded as follows: TiO
forms by the preferential reaction of βTiss and O in the alloy; after that, a stable oxide film composed of TiO
and Nb
O
forms; then, TiNb
O
is formed by the reaction of TiO
and Nb
O
.
Constitutive models for flow behaviors of an arc-melted Nb-15Si-22Ti-5Cr-3Al-2.5Hf alloy at temperatures of 1350 °C to 1500 °C and strain rates of 0.001 to 0.1 s
−1
have been successfully established ...during work hardening and dynamic softening stages, respectively, and relatively small average absolute relative errors of the predicted flow stresses are reached (7.7 pct for the work hardening stage and 5.7 pct for the dynamic softening stage). The hot processing map has also been established successfully for this Nb-Si-based ultrahigh temperature alloy. The favorable conditions for hot working of this alloy have been determined as 1350 °C to 1380 °C/0.001 to 0.003 s
−1
and 1380 °C to 1440 °C/0.001 to 0.01 s
−1
. The deformed microstructures under different conditions have been explored and the mechanisms for flow instability of this alloy have been revealed. Flow instability at relatively low temperatures and high strain rates (1350 °C and 1410 °C, 0.1 s
−1
) is mainly derived from the cracking of Nb
5
Si
3
and the debonding of Nbss/Nb
5
Si
3
interfaces, while flow instability at higher temperatures (1500 °C) should primarily result from the development of cracks and holes within the Nbss phase because of excessive strain accumulation.
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.
► A novel silicide coating modified simultaneously by Al and Y was prepared. ► The Al and Si deposition sequence and phase formation in the coating was revealed. ► The coating can protect the alloy from oxidation at 1250 °C for more than 100h.
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