Microstructure And Room Temperature Mechanical Properties Of Nb-Ti-Cr-Si Based Ultrahigh Temperature Alloy

Nb-silicide based ultrahigh temperature alloys have been studied as alternative materials to Ni-based superalloys because of their high melting points,low densities and favorable mechanical properties.However,a major barrier to the practical applications is their poor high temperature oxidation resistance.In order to achieve a balance among properties of good creep strength,excellent oxidation resistance and acceptable fracture resistance,the volume fraction and the morphology of each constituent phase must be controlled reasonably.It was found that addition of a higher content of Cr improved the high temperature oxidation resistance due to formation of the Laves phase,so a new kind of Nb–Ti–Cr–Si based alloys have attracted ever increasing attentions.Their phase constituents are Nb solid solution(Nbss),(Nb,X)5Si3(X represents Ti、Hf and Cr elements)and Cr2 Nb.Among these three phases,Nbss is introduced to improve the ambient temperature fracture resistance,while the silicide and Laves phases are introduced to improve high temperature creep strength and oxidation resistance.However,the Nb-Ti-Cr-Si based alloys possessed worse room temperature fracture toughness and higher temperature strength but better high temperature oxidation resistance than two phases Nb–Si based alloys.The aim of the present work is to reveal the effects of arc melting,the integrally directional solidification and homogenizing and aging treatments on the microstructure of Nb-Ti-Cr-Si based alloys and room temperature mechanical properties.The master alloy ingot was prepared by vacuum consumable arc-melting,and the chemical composition,constituent phases and microstructure of the alloy in different parts were analyzed.The integrally directional solidification of the alloy was conducted in a self-made high vacuum and ultrahigh temperature directional solidification furnace with the use of ceramic crucibles.The effects of melt temperatures on the directionally solidified microstructure were revealed.The microstructures in different zones of integrally directionally solidified(DS)specimens were analyzed.The effect of withdrawing rates on the directionally solidified microstructure,and S/L interface morphogology were studied.In order to eliminate or alleviate both the metastable phases and solute segregation in the alloys,high temperature homogenizing and aging treatments were conducted.At last,room temperature mechanical properties of the alloys were measured.The main conclusions of the present studies can be drawn as follows.The chemical compositions are distributed uniformly at different positions of the master alloy ingot except Cr whose concentration in the central of the ingot top is lower than that at other positions.The microstructure of the ingot is mainly composed of primary Nbss dendrites,Nbss/(Nb,X)5Si3 eutectic which exhibits typically lamellar or petal-like morphologies,and fine Nbss/Cr2 Nb eutectic colonies.However,some(Nb,X)3Si blocks are present at the top of the ingot alloy.The primary Nbss dendrites become coarser and directionally solidified character of Nbss/(Nb,X)5Si3 eutectic colonies disappears gradually from the edge to the centre of the ingot.The fine Nbss/Cr2 Nb eutectic colonies are distributed between primary Nbss and Nbss/(Nb,X)5Si3 eutectic colonies.With the withdrawal rate varying from 2.5 to 10μm/s,the DS microstructures of the alloy consist of primary Nbss dendrites,Nbss/(Nb,X)5Si3 eutectic colonies and α-Ti.However,the DS microstructures are composed of primary Nbss dendrites,Nbss/(Nb,X)5Si3 eutectic colonies and fine(Ti,Nb)ss/Nb3Si/Cr2 Nb eutectic colonies when the withdrawal rates varies from 20 to 100 μm/s.The(Ti,Nb)ss/Nb3Si/Cr2 Nb eutectic colonies appear due to the rapid withdrawal rate.The primary Nbss dendrites and Nbss/(Nb,X)5Si3 eutectic are aligned along the growth direction when the withdrawal rates varies from 2.5 to 20 μm/s.However,discontinuous(Nb,X)5Si3 plates are present when the withdrawal rates varies from 50 to 100 μm/s,and the fine(Ti,Nb)ss/Nb3Si/Cr2 Nb eutectic colonies are distributed between the primary Nbss dendtrites and Nbss/(Nb,X)5Si3 eutectic colonies.The Nb-Ti-Cr-Si based ultrahigh temperature alloy is composed of Nbss,(Nb,X)5Si3 and Cr2 Nb phases after various homogenizing and aging treatments.HfO2 is found in the alloy after heat treatment both at 1500°C for 24 h and 1500°C for 24 h then 1000°C for 24 h.With increase in heat-treatment temperature,previous Nbss dendrites in the arc-melted alloy transform into Nbss equaxied crystals.The previous Nbss/(Nb,X)5Si3 eutectic colonies break into small(Nb,X)5Si3 blocks in Nbss matrix,whereas the previous Nbss/Cr2 Nb eutectic colonies in arc-melted microstructure transform into needle-like Laves after homogenizing treatment at 1200°C for 24 h and become coarse Laves blocks after homogenizing at 1300°C for 24 h.The previous Laves Cr2 Nb particles dissolve into Nbss during homogenizing treatment at 1400°C for 24 h,and much finer and crowded Cr2 Nb platelets form during cooling.These observations suggest that the previous coarse Laves phase particles dissolve between 1300 and 1400°C,and the Ti and Cr concentrations decrease with increase in heat-treatment temperatures.Aging at 1000°C for 24 h after homogenizing treatments improves the precipitation of fine needle-like Cr2 Nb in Nbss matrix and Cr concentration in Nbss reduces.The variation of partitioning ratios of alloying elements among Nbss,(Nb,X)5Si3 and Cr2 Nb causes the change in microhardness of Nbss and(Nb,X)5Si3.The microhardness of Nbss and(Nb,X)5Si3 reaches the maximum value after 1300℃/24h+1000℃/24 h heat treatment.The microstructure of directional solidified specimen after heat treatment is composed of primary Nbss and Nbss/(Nb,X)5Si3 eutectic colonies,and the Cr2 Nb phase has been dissoloved,and fine fiber microstructure forms.Cr and Ti concentration in Nbss increases with increase in withdrawal rates.The microhardness of Nbss decreases firstly,then increases,and the microhardness of(Nb,X)5Si3 increases with withdrawal rates.The room temperature fracture toughness of the specimens has been improved by the integrally directional solidification with withdrawing rates of 10 and 20 μm/s.However,the room temperature fracture toughness of directionally solidified specimens with withdrawal rates of 50 and 100 μm/s is lower than that of the arc-melted alloy.The room temperature fracture toughness of the alloy has a trend to decrease with increase in the withdrawing rates.The maximum room temperature fracture toughness reaches to 17.4MPa.m1/2 when the withdrawal rate is 10 μm/s.The ambient fracture mechanism of the arc-melted alloy is quasicleavage mode,however,the fracture mechanism for the directionally solidified specimens is complicated,exhibiting intercrystalline cracking and transgranular cracking mode.The ambient tensile strength of arc-melted alloy reaches to 378.7MPa.In addition,the ambient fracture mechanism is characterized by brittle quasicleavage mode in both three point bending and uniaxial tensile fracture morphologies.