Mechanical Properties of DS NiAl/Cr(Mo) Alloys with Low Addition of Hf for High-temperature Applications

1.IntroductionAs a new type of structural materials based on the B2intermetallics,NiAl offers superior characteristics,suchas low density,high melting point and excellent oxida-tion resistance at high temperature[1].However,the poorfracture toughness at ambient temperature and low creepstrength at elevated temperature limit their applicationcurrently.Although the creep strength has been sig-nificantly improved by precipitation strengthening[2]orforming a particulate composite[3],and ductility…     

Behavior of Zr-rich phase in a cast NiAl-33.5Cr-0.5Zr alloy during heat treatment

There are many Zr particles in as-cast NiAl-33.5Cr-0.5Zr (at. pct) alloy, which usually exist at the edge of eutectic of beta -NiAl and cx-Cr. After air and furnace cooling solution treatments, far 1400 degreesC, 2 h and 1450 degreesC, 1 h, pure Zr phase remains in the furnace cooling (F.C.) state alloys and Ni2AlZr phase forms in the air cooling (A.C.) state alloys. During solution treatment at 1450 degreesC, bulk and "fish bone" shape Zr-rich phases form respectively in F.C. and A.C. state alloys. A 'river' shape Ni2AlZr phase forms after 1450 C for 1h F.C. and 850 degreesC for 12 h, F.C.. The alloy has less pure Zr and Ni2AlZr phase after 1400 degreesC with both air and furnace cooling followed by 850 C and 950 C for 12 h, F.C. aging treatments, respectively. Additionally, there is a ternary eutectic of NiAlZr and a phase enriched Zr and Cr forms at the edge of the eutectic of beta -NiAl and alpha -Cr in the alloy treated at 1400 degreesC, 2 h, F.C. and 950 degreesC, 12 h, F.C.     

Tensile Properties and Microstructure of DS NiAl—28Cr—5.8Mo—0.2Hf Alloy

A multiphase alloy NiAl-28Cr-5.8Mo-0.2Hf was directionally solidified in Ar atmosphere in an Al2O3-SiO2 mold by standard Bridgman method.The fracture toughness and tensile properties at 980℃ as well as tensile creep at 1050℃ were studied .It was found that the strength of the present alloy is higher that of many NiAl-based alloy and the stress exponent n for creep is about 6.69.Then the possible strengthening and creep mechanism are also discussed.     

Effect of Ho on the microstructure and compressive properties of NiAl-based eutectic alloy

The effect of various holmium content on the microstructure and compressive behavior of NiAl-28Cr-6Mo-0.15Hf eutectic alloy has been investigated by using combination of SEM, TEM and compression test. The results suggest that appropriate Ho addition improves the compressive ductility at room temperature and the high temperature strength at 1273 K of the alloy significantly. In addition, lamellar microstructure inside of eutectic cell and eutectic cell get refinement gradually, but the intercellular region become coarse, with increase of Ho content. Furthermore, Ni₂Al₃Ho phase, with hexagonal crystal structure, is observed in the alloy.     

Creep behaviour of carbon containing Fe3Al alloy

Abstract

Tensile creep of a Fe–16 wt-%Al–0·5 wt-%C alloy was investigated over a temperature range of 773 to 873 K and stress range of 80 to 200 MPa. Creep curves exhibited very limited primary and secondary creep regimes. An extended tertiary creep regime was observed for all the test conditions. Stress dependence of minimum creep rate can be represented by a power-law equation with stress exponents being in the range 4 to 5. The activation energy for creep was found to be ～340 kJ mol^?1. The observed stress exponent and activation energy for creep suggest that creep is controlled by dislocation climb. Creep fracture in Fe₃Al–C alloy is predominantly by transgranular ductile mode by nucleation, growth and coalescence of microvoids formed at FeAlC_0·5 particle/matrix interface by decohesion as well as fracture of elongated particles. Extended tertiary creep observed in the alloy was analysed in the light of the mechanisms proposed for nickel based superalloys.     

Static and cyclic creep behaviour of 2024/SiCp and its matrix alloy at high temperature

Abstract

The tensile creep and tensile-tensile cyclic creep behaviour of 2024/SiC_p composite and its matrix alloy have been investigated and analysed at high temperature. It was found that the creep threshold stress of the composite may not be caused by SiC_p alone: the matrix alloy also contributes to the threshold stress. The higher threshold stress of the composite compared with that of the matrix alloy can be explained in terms of load transfer in the composite and the value of threshold stress for the matrix alloy. A direct comparison between the composite and its matrix alloy indicates that only below a critical stress does the composite show a creep resistance higher than that of its matrix alloy. The two materials shown cyclic creep retardation in the tested stress range, cyclic creep showing a higher stress exponent and higher apparent activation energies in comparison with static creep. An analysis based on anelasticity is introduced to explain this result. The relationships between rupture lifetimes and applied stress, creep rate, and unloading amount show that the creep fracture mechanism is dominant in the present test condition.     

Precipitation behavior of Heusler phase (Ni2AlHf) in multiphase NiAl alloy

Precipitation behavior of Heusler phase (Ni₂AlHf) in a directionally solidified (DS) NiAl- 28Cr-5Mo-1Hf (at.%) alloy was examined using scanning electron microscope (SEM) and transmission electron microscope (TEM). In the as-cast alloy, the Ni₂AlHf phase generally appeared on the NiAl/Cr(Mo) interface, which degraded the NiAl/Cr(Mo) eutectic structure. In the heat-treated alloy, the density of the intercellular Ni₂AlHf phase was slightly reduced. In addition, the spherical Ni₂AlHf phase precipitated heterogeneously in the NiAl matrix, but the Ni₂AlHf phase did not precipitate in the lamellar Cr(Mo) phase. The precipitation behavior of the Ni₂AlHf phase could be explained in terms of the interfacial energy. A lattice model was also proposed to explain the NiAl↔Ni₂AlHf phase transformation.     

Contribution of coarsening of MX carbonitrides to creep strength degradation in high chromium ferritic steel

Abstract

The coarsening process of MX carbonitrides during creep and its effect on creep behaviour have been investigated for P92 steel (9Cr-0.5Mo-1.8W-VNb). At 1023 K after long term aging, creep rupture strength and tertiary stage creep rate rapidly decrease and increase, respectively. The stress exponent n of minimum creep rate decreases with increasing temperature, and the value of n at 1023 K was evaluated to be 5.7. MX carbonitrides coarsen during creep and the coarsening process corresponds to the early stage of Ostwald ripening. Creep deformation accelerates the coarsening of the MX carbonitrides. At 1023 K, the coherent strain between the MX carbonitrides and matrix reduces to a negligibly small value at times longer than 550 h, leading to a decrease in the resistance to dislocation motion. This contributes to the rapid increase of creep rate in the tertiary stage and the abrupt decrease of creep rupture strength in the long term.     

Microstructures and Creep Behavior of a Directionally Solidified NiAl-Fe(Nb) Multiphase Intermetallic Alloy

The microstrucure and creep behavior of a DS NiAl-Fe(Nb) multiphase intermetallic alloy have been investigated,This alloy exhibits dedritic structure ,in which dendritic arm is β-(Ni,Fe)(Fe,Al) phase surrounded by interdendritic region of γ′/γ phase ,The results of the creep test indicated that all of the creep curves have similar characteristic,which is a short primary creep stage and a dominant steady state crepp stage ,and the creep strain ranges from 18% to 52%,The apparent stress exponent and the apparent activation energy were analyzed and discussed,The mechanism of the creep deformation was also analyzed by the observation of TEM.     

High temperature creep behaviour of single crystal superalloy

Abstract

Creep behaviour of a newly developed single crystal nickel base superalloy DMSX-l (Ni-7.7Co-6.4Cr-8.4W-0.3Ti-S.S7 Al-7.6Ta-Nf-Y-Nb-Re, wt-%) with 〈001〉 orientation has been analysed and compared with the reported data ofSRR99. It was concluded that the shear stress based model developed for SRR99 is also applicable for the new alloy. Although the material constants for octahedral slip for the new alloy are not exactly the same as those of SRR99, the estimated values of initial creep rate and softening coefficients are nearly of the same order. Therefore it is concluded that in the absence of a material database for cube slip for this new alloy, those available for SRR99 could be used to predict the orientation dependence of its creep behaviour.     

定向凝固 NiAl/Cr(Mo,Hf)合金的微观组织及力学性能

研究了定向凝固NiAl-28Cr-5.85Mo-0.15Hf合金的微观组织与在293～1373K温度范围内的力学性能.结果表明:合金是由NiAl枝晶轴和枝晶间区(NiAl和Cr(Mo)相的共晶)组成的.经过长期固溶时效处理NiAl/Cr(Mo)合金析出少量弥散分布的Huesler相,其余Hf以固溶体方式存在.DSNiAl-28Cr-5.85Mo-0.15Hf合金具有明显的韧脆转变行为,韧脆转变温度依赖于应变速率.室温拉伸断口呈现明显的解理断裂,而韧脆转变温度在以上时,合金具有较大的变形量,断口上有许多韧窝,呈现明显的塑性断裂特征.     

Effect of Nb on the Microstructure and Mechanical Properties of Cast NiAl-Cr（Mo） Eutectic Alloy

The microstructure and mechanical behaviors of NiAl-28Cr-5Mo-1Nb eutectic alloy were investigated by using scanning electron microscopy, X-ray diffraction, transmission electron microscopy and compression tests, respectively. The alloy is mainly composed of three phases, which are the gray lamellar Cr（Mo） plate, black NiAI matrix and semicontinuously distributed Cr2Nb-type Laves phase. Through Nb addition, NiAl-Cr（Mo）/Nb alloy exhibits a reasonable balance of high temperature strength and room temperature compression ductility and its mechanical behaviors are superior to the NiAl-28Cr-6Mo eutectic alloy at all temperature. The elevated temperature compression deformation behavior of NiAl-Cr（Mo）/Nb alloy can be properly described by power-law equation.the National High Technology Committee of China （No. 863-715-005-0030） for financial supports.     

Tensile and creep properties of Al–7Si–0.3Mg alloy with Zr and Er addition

The combined and singular effects of Zr and Er addition on the microstructure, tensile and impression creep behaviour of the cast A356 alloy were investigated. The Zr and Er refined the α-Al dendrites and secondary dendrite arm spacing (SDAS) and modified the eutectic morphology. The simultaneous addition of Zr and Er exhibited the smallest α-Al dendrites and SDAS with a fibrous eutectic morphology, resulting in the highest yield (～118?MPa) and tensile (～190?MPa) strength, and the largest creep resistance. The stress exponent, and the activation energy, were within the range of 6.28–7.22 and between 121.6 and 148.2?kJ?mol^?1, respectively. The lattice self-diffusion climb-controlled creep was the dominant creep mechanism. The constitutive creep equation was developed for each alloy.     

BDT and Creep Behaviors of NiAl-25 at. pct Cr Alloy at Various Temperatures

1.IntroductionIntermetallic compound of NiAl with B2crystal struc-ture is regarded as a potential candidate of high tem-perature structural materials because it offers attractivechemical and physical properties,such as high meltingpoint,low density,good thermal conductivity,high resis-tance to oxidation and high stiffness[1,2].Unfortunately,this kind of intermetallic compound shows limited tough-ness at ambient temperatures and poor strength at hightemperatures.An efficient way to prepare int…     

Ni、Si元素配比对Cu-Cr-Zr合金组织与性能的影响

在Cu-Cr-Zr合金中添加Ni、Si元素,制备Cu-0.6Cr-0.15Zr、Cu-2.8Ni-0.7Si-0.6Cr-0.15Zr(w(Ni)/w(Si)=4∶1)、Cu-2.8Ni-0.9Si-0.6Cr-0.15Zr(w(Ni)/w(Si)4∶1)、Cu-2.8Ni-0.56Si-0.6Cr-0.15Zr(w(Ni)/w(Si)4∶1)共4种合金。研究了Ni、Si元素及其配比对合金组织及性能的影响。结果表明:Ni、Si元素细化了合金组织,增强了合金高温力学性能。合金时效初期先析出CrSi2化合物,时效后期析出相颗粒主要有CrSi2、Ni2Si、ZrCrSi2,形态为长条形、椭圆形及圆盘状。时效处理后,与Cu-0.6Cr-0.15Zr合金相比,加入Ni、Si元素后合金硬度从131HV上升到240HV以上;导电率从88%IACS左右降到40%IACS左右。Ni、Si元素配比对导电率的峰值影响有限,在4%IACS~9%IACS;对硬度峰值的影响在20HV~30HV之间。     

Effect of initial temper on the creep behavior of a Mg–Gd–Nd–Zr alloy

The effect of initial temper on the tensile creep behavior of a cast Mg–Gd–Nd–Zr alloy has been investigated. Specimens in unaged, underaged and peak-aged conditions exhibit a sigmoidal creep stage between the primary and steady-state creep stage, while the overaged specimens have no such creep stage. Transmission electron microscope observations revealed that sigmoidal creep stage was induced by the dynamic precipitation in the microstructure, and the rapid formation of β₁-phase and β-phase plates takes responsibility for the softening of material in this stage. Comparative evaluation of creep properties of the specimens showed that alloy in overaged condition had creep resistance superior to those in other conditions. Stress and temperature dependence of the steady-state creep rate were studied over a temperature range of 250–300 °C and stress range of 50–100 MPa, and a dislocation creep mechanism was proposed for the alloy.     

Precipitation characteristics of Cu–15Cr–0·15Zr in situ composite

Abstract

The precipitation characteristics of Cu–15Cr–0·15Zr in situ composite made by cold drawing have been studied using analytical TEM and hardness measurement. It has been observed that the in situcomposite reaches peak hardness at a lower aging temperature than the corresponding alloy which is not subjected to cold working prior to aging treatment. The Cr rich precipitates in the peak aged composite are much smaller than those in the peak aged alloy. Under peak aging conditions, precipitation free zones adjacent to the grain boundaries of the Cu matrix are also observed in both the alloy and the composite. The addition of Zr enhances the precipitation hardening effect in the alloy and the composite. The major mechanism of this enhancement may be through increasing the Cr rich precipitate nucleation rate and precipitate/matrix interfacial energy.     

Superplasticity and associated activation energy in Ti-3Al-8V-6Cr-4Mo-4Zr Alloy

The high temperature deformation characteristics of a commercial β -titanium alloy Ti-3Al-8V-6Cr-4Mo-4Zr have been studied in the temperature range 830–925∘C. The alloy exhibited superplasticity in a narrow temperature and strain rate range i.e. 850–865∘C and 5× 10^{− 5}–3× 10^{− 3} s^{− 1} respectively, with a maximum elongation of 634% at 855∘C. The superplastic behaviour in the alloy is considered to arise as a result of subgrain formation at the higher strain rates (region III) which enhances diffusional creep at lower strain rates (region II). The activation energy values for regions II and III were found to be close to the lower of the two activation energy values (129.2 KJ/mole) proposed to describe self diffusion in β -phase suggesting that the rate controlling mechanism during high temperature deformation of the alloy was that for lattice diffusion.     

Directional coarsening of γ′ phase in single crystal nickel based superalloys during tensile creep

Abstract

The γ′ precipitate rafting kinetics and morphological evolution for two model single crystal superalloys have been studied. The microstructure of the alloys after different stages of tensile creep at 1040°C and under a range of stresses are examined using TEM and SEM. The chemical compositions of both γ and γ′ phases are analysed by energy dispersive spectrometry. Results show that a meshlike γ′ raft structure is formed along the direction normal to the stress axis during primary creep. The applied stress causes a decrease in the coherent strain energy at the γ′/γ interfaces in the (001) crystal plane. The released energy is the driving force for the diffusion of elements, leading to the formation of the γ′ rafts. A longer time is required for the formation of γ′rafts in alloy 2 owing to its higher content of the refractory element W which obstructs the migration of the other elements in the diffusion field of the γ′ rafts.     

Low cycle fatigue behavior of Cu-Cr-Zr alloy with different cold deformation

The present study addressed the cyclic deformation behavior and fatigue properties of Cu-0.69Cr-0.07Zr alloy with different cold deformation (ε = 64%, 75%, and 84%) using low cycle fatigue test. Low cycle fatigue tests were conducted under fully-reversed conditions at different total strain amplitudes. The microstructure changes and fatigue fracture characteristics were analyzed by scanning electron microscope (SEM) and transmission electron microscope (TEM). The main findings suggest that the Bauschinger effect was significantly stronger with larger deformation at low total strain amplitude. And it was proved that the relationship between the total strain amplitude and the low cycle fatigue life of Cu-Cr-Zr alloy with different deformation can be expressed by the Manson–Coffin–Basquin formula. Further, the reason for the fatigue life was shorter and the cyclic softening rate decreased faster at high applied total strain amplitude was that the dislocation density decreased due to the rearrangement of the dislocations.