合金元素对Laves相增强Nb基合金的相组成

采用机械合金化与热压烧结工艺制备了添加合金元素V和Fe的Laves相增强的Nb基复合材料。研究了添加质量分数4%V和Fe的Nb/NbCr2-4.0V和Nb/NbCr2-4.0Fe配比成分的元素粉,经MA20h后在1250℃热压30min所获得的Nb/NbCr2合金的组织和性能。结果表明:在热压过程中原位合成出细小弥散分布的三元Laves相Nb(Cr,V)2和Nb(Cr,Fe)2,并且V和Fe原子只占据Laves相中的Cr原子位置。制备出的Laves相增强Nb基合金接近全致密,组织细小均匀,晶粒尺寸小于500nm。Nb/NbCr2-4.0V和Nb/NbCr2-4.0Fe合金的断裂韧性分别达到5.3和6.3MPa·m1/2,其中Nb/NbCr2-4.0Fe合金不仅抗压强度达到2256MPa,其屈服强度和塑性应变也分别达到2094MPa和6.03%。     

球磨时间对软第二相韧化Cr2Nb基复合材料组织与性能的影响

采用机械合金化 热压工艺制备了含Laves相的Cr2Nb基复合材料,研究了Cr、Nb元素粉经不同球磨时间后在1250℃× 30 min热压工艺下所获得Cr/Cr2Nb复合材料的组织和性能.结果表明,经20 h以上高能球磨的Cr-25Nb粉末在热压后可以制备出含高致密度的Laves相的Cr2Nb基复合材料.高能球磨40 h的Cr-25Nb试样的组织均匀细小,Laves相与软第二相间隔分布.其致密度高达99.2%.接近全致密.维氏硬度为8.93 GPa,抗压强度、屈服强度和塑性应变分别达到2153 MPa、2062 MPa和8.42%.与传统熔铸工艺相比,MA 热压方法制备的Laves相Cr2Nb基复合材料的强度及塑性都有显著提高,充分实现了细晶和软第二相综合增韧的效果.     

热压时间对Laves相Cr2Nb合金组织与性能的影响

采用机械合金化 热压工艺路线制备化学配比成分的单相Laves相Cr2Nb合金.研究了Cr、Nb元素粉经20h球磨后在1300℃不同时间热压所获得的Laves相Cr2Nb合金的组织性能.结果表明,采用机械合金化 热压工艺可以制备出高致密度的组织细小均匀的Laves相Cr2Nb合金,随着热压时间的延长,合金的致密度、晶粒尺寸和维氏硬度逐渐增大而断裂韧度逐渐减小.1300℃×30min的热压试样的平均晶粒尺寸达到亚微米级,致密度达到98.7%,断裂韧度达到5.07MPa·m1/2,与熔铸法制备的Laves相Cr2Nb相比,室温断裂韧性提高,实现了细晶组织对Laves相Cr2Nb合金的增韧效果.     

MA和软第二相对Cr/Cr2Nb复合材料组织与性能的影响

采用机械合金化(MA) 热压工艺来制备以Cr固溶体为软第二相的Cr/Cr2Nb复合材料.研究了不同配比成分的Cr,Nb元素粉经20 h球磨后,在1250 ℃,0.5 h热压工艺下所获得的Cr/Cr2Nb复合材料的组织和性能.结果表明,随着偏离Laves相化学配比的Nb含量减小,合金的致密度、强度、塑性应变均增加,而维氏硬度减小,不同配比成分的合金均具有良好的室温断裂韧性.Laves相Cr2Nb含量高达78%的Cr-25Nb合金的组织均匀,Cr固溶体与Laves相间隔分布,晶粒尺寸达到微/纳米级,屈服强度和抗压强度分别高达1949 MPa和2044 MPa,塑性应变达到了7.26%.与熔铸工艺制备的Cr/Cr2Nb复合材料相比,合金的强度及塑性都有明显提高,充分实现了细晶和软第二相综合增韧的效果.     

Zr(Cr,V)2 Laves相长周期堆垛结构的高分辨电子显微术观察

应用高分辨电子显微术对退火态Zr（Cr,V）2Laves相的显微结构进行了观察．结果表明退火态Zr（Cr,V）2Laves相在低倍显微观察下呈现层片状组织,并存在长周期堆垛结构;高分辨电镜和快速Fourier变换分析证明,退火态Zr（Cr,V）2长周期结构为C36（4H）结构,同时存在8H堆垛结构和少量其它过渡性多层堆垛结构、并就合金元素V对ZrCr2Laves相稳定性的影响进行了讨论．     

机械合金化-热压制备Nb/NbCr2复合材料的组织与性能

采用机械合金化与热压工艺制备以Nb固溶体为软第二相的Laves相Nb/NbCr2复合材料。研究不同成分的Nb、Cr元素粉经20 h球磨后在1 250℃,0.5 h热压工艺下所获得的Nb/NbCr2复合材料的组织和性能。结果表明：随着偏离Laves相化学配比的Nb含量的增大,材料的致密度、抗压强度、塑性应变均增加,而维氏硬度减小。Laves相含量为29%的Cr-77.5Nb合金的组织均匀,Nb固溶体与Laves相间隔分布,晶粒尺寸达到亚微米级,屈服强度为2 790 MPa,抗压强度为3 174 MPa,塑性应变达到5.44%,充分实现了细晶和软第二相综合增韧的效果。     

相组成对Cr-Nb合金高温氧化行为的影响

采用机械活金化 热压烧结法制备5种不同相组成的Cr-Nb合金,研究相组成对Cr-Nb合金在950～1200℃空气中氧化行为的影响。结果表明,Cr相能显著增加NbCr2合金950℃的抗氧化性能;而Nb相不利于合金高温抗氧化性的提高,甚至发生灾难性氧化。SEM和XRD分析显示,单相Cr-2.5Nb合金发生了Cr的外氧化,只形成单一的Cr2O3膜;而双相Cr-18.5Nb合金和单相Laves相NbCr2合金均发生Cr的外氧化和内氧化,形成两层结构的氧化膜。但随着氧化温度增加到1200℃,由于Cr2O3的挥发,导致Cr-Nb合金高温抗氧化性变差。因此,为满足实际高温应用要求,对富软第二相Cr或Nb的NbCr2基合金实施相应的表面防护是必须的。     

固溶处理对Inconel 625合金电弧增材组织的影响

采用GTAW电弧增材制造工艺进行了Inconel 625合金薄壁试样的制备,并研究了焊后固溶处理对Inconel625合金GTAW电弧增材制造组织和力学性能的影响.结果表明,增材试样的微观组织以柱状树枝晶为主,且晶粒生长方向呈现与焊接方向近似垂直的定向生长,同时发现,增材试样组织中主要存在Laves(Ni,Fe,Cr)_2(Nb,Ti,Mo)相和MC型碳化物两种析出相.经过680℃固溶处理,Laves相中Nb,Mo元素开始向基体中扩散,从而在Laves相附近析出针状δ相,随着固溶处理温度增加,Laves相溶解量增加,Laves相附近针状δ析出量明显增加,当固溶处理温度增加至1 080℃时,大量Laves相溶解,仅剩少量细小颗粒呈弥散状分布,大量Nb,Mo元素向基体中扩散,使得δ相消失.     

Microstructural Evolution in Multiphase NiAl-2.5Ta-7.5Cr Alloy during Annealing at Different Temperatures

研究了NiAl-2.5Ta-7.5Cr合金在不同的退火温度下的组织演变过程,结果表明该合金的铸态组织是由NiAl基底中包含Ta(CrNiAl)2的大晶粒Laves相和一些富含Cr的尺寸在400-500nm的小颗粒组成,其中大晶粒Laves相晶界处存在C14结构相。NiAl中Ta和Cr的浓度分别在~0.6at%和~2.5at%之间。将合金置于1000°C度下退火,有细小的棒状C15结构的Laves相在NiAl中开始弥散析出。而合金经过1200°C度退火2h后,这种颗粒的体积分数增加,同时NiAl基底中Ta的浓度减少到~0.2at%。当退火温度增加到1400°C,NiAl基底中的Laves析出相完全消失。因此,1000-1300°C温度范围内这种Laves相在NiAl基底中的析出,可归因于Ti元素在NiAl固溶后的过度饱和后发生扩散的缘故。     

Laves相金属间铬化物的制备研究进展

拓扑密排结构的金属间化合物是潜在的高温结构材料,Laves相金属间化合物是其中最大的一类。而Laves相金属间铬化物Cr2X(X为Ti、Nb、Ta、Zr、Hf等)近年来成为高温结构材料研究中的一个热点,因为这些化合物具有较高的熔点、较低的密度和比较好的抗氧化性。有多种方法可以制备Laves相金属间铬化物。对目前Laves相铬化物的各种制备工艺技术现状及研究进展进行了综述,重点介绍了国内外制备Laves相金属间铬化物的熔铸法、定向凝固、机械合金化(MA)、机械合金化 热固相反应、机械合金化 热压法及铸锭冶金等有效技术,并评述了这些制备工艺的优缺点,指出了研究中存在的问题和今后的发展方向。     

电弧熔炼Cr-40Ti-20Nb三元合金的组织与压缩性能

采用真空非自耗电弧熔炼的方法制备了Cr-40Ti-20Nb(原子分数)合金.利用光学显微镜(OM),X射线衍射(XRD),扫描电镜(SEM)分析了合金不同凝固位置处的相组成和组织形态.结果表明,合金的凝固组织主要是由Laves相Cr2(Nb,Ti)枝晶组织及丁i的固溶体组成.同时对合金的显微硬度和压缩性能进行了测试,显微硬度值在6 112～7 350MPa之间,比Laves相Cr2Nb的显微硬度低20％.另外室温压缩屈服强度可达1 334-1 524 MPa,与采用多种合金化元素的Cr-12Nb-4Re-2Al合金相当,这表明Ti元素的添加不仅可以改变Laves相Cr2Nhb合金的凝固组织,而且对其塑性也有很大程度的改善.     

锆合金中第二相的研究

将几种Nb、Fe含量不同的锆合金样品在1050℃保温0.5h后水冷,在560℃回火10h,用带能谱分析(EDS)的透射电子显微镜研究它们的第二相粒子.结果显示:随着合金样品中Nb元素含量的提高及Nb/Fe比的增加,第二相从Zircaly-4合金中密排六方结构的Zr(Fe,Cr)2向密排六方的Zr(Nb,Fe,Cr)2转...     

FeCrAl不锈钢包壳管挤压及退火工艺优化

利用光学显微镜、扫描电镜、电子背散射衍射和万能拉伸试验机等对FeCrAl合金包壳管挤压前后的微观组织、析出相及退火后的微观组织、析出相、再结晶及力学性能进行了研究。结果表明:Nb含量对FeCrAl合金中第二相的析出影响显著,高Nb含量下Laves相的析出温度和析出数量均大大提高;降低挤压温度有助于FeCrAl合金管坯中形成细小弥散第二相;随着退火温度升高,细小弥散相的析出数量呈现先增多后减少的趋势;在相同退火工艺下,800 ℃热挤压管坯的室温力学性能比950 ℃热挤压管坯的室温力学性能要更加优异。     

Effect of Grain Size on the Precipitation Behaviour in Super-Ferritic Stainless Steels During a Long-Term Ageing

A 27.6Cr-3.6Mo-2Ni alloy was solution treated and then aged for a long time to study the effect of grain size on precipitation behaviour by using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The experimental results demonstrated that the average grain size increased from 46.3 ± 6.2 to 101.8 ± 13.5 μm and the grain boundary length per unit area decreased from 3.3 × 10~4 to 1.7 × 10~4 m/m~2 with an increasing annealing temperature from 1100 to 1200 ℃. After ageing at 800 ℃, the σ-phase,χ-phase and Laves phase were observed. As the ageing time increased, the σ-phase notably increased, while the χ-phase and Laves phase gradually decreased before finally vanishing after ageing for 400 h. The σ-phase precipitation kinetics curves consisted of two parts, and the grain size had a significant effect on the first stage of the precipitation curves due to the abundance of nucleation sites in the specimens with finer grains. The Laves phase was transformed from Nb(C,N) particles by Nb diffusion. As the ageing time increased, the ferrite phase decreased due to the transformation of the ferrite phase to the σ-phase, and then C was expelled into the untransformed ferrite grains. Moreover, new Nb(C,N) particles were formed by Nb diffusion from the Laves phase, resulting in the absence of the Laves phase.     

Physical metallurgy and mechanical properties of transition-metal Laves phase alloys

This paper provides a comprehensive review of the recent research on the phase stability, point defects, and fracture toughness of AB₂ Laves phases, and on the alloy design of dual-phase alloys based on a soft Cr solid solution reinforced with hard XCr₂ second phases (where X=Nb, Ta and Zr). Anti-site defects were detected on both sides of the stoichiometric composition of NbCr₂, NbCo₂, and NbFe₂, while they were observed only on the Co-rich side of ZrCo₂. Only thermal vacancies were detected in the Laves phase alloys quenched from high temperatures. The room-temperature fracture toughness cannot be effectively improved by increasing thermal vacancy or reducing stacking fault energy through control of phase stability. Microstructures, mechanical properties, and oxidation resistance of dual-phase alloys based on Cr–NbCr₂, Cr–TaCr₂, and Cr–ZrCr₂ were studied as functions of heat treatment and test temperature at temperatures to 1200°C. Among the three alloy systems, Cr–TaCr₂ alloys possess the best combination of mechanical and metallurgical properties for structural use at elevated temperatures.     

Effect of Cr on Microstructure and Fracture Toughness of Nb-20Ti-16Si-xCr Alloys

The present paper reported the phase stability and microstructures evolution of Nb-20Ti-16Si-xCr (x = 0～20at.%) in-situ composites as function of Cr addition. It was suggested that with the addition of Cr less than 7%, the silicide phase in as-cast Nb-20Ti-16Si alloy was Nb3Si. The silicides changed to be α-Nb5Si3 by the addition of Cr above 7%. It was found that annealing of 1300 ℃ for 100 h could result in the decomposition of Nb3Si to NbSS and α-Nb5Si3 in Cr containing Nb-20Ti-16Si based alloys by means ...     

Effect of ball milling time on microstructure and properties of Laves phase NbCr2 alloys synthesized by hot pressing

Laves phase NbCr2 alloys with a composition of Nb-66.7Cr (molar fraction, %) were prepared by mechanical alloying and hot pressing. The microstructures and properties of the Laves phase NbCr2 alloys, prepared from elemental niobium and chromium powders under various ball milling time by hot pressing at 1 250 ℃ for 0.5 h, were investigated. The results indicate that if the ball milling time is longer than 40 h, the synthesizing reaction of Laves phase NbCr2 can be accomplished much sufficiently. Then the nearly full-dense Laves phase NbCr2 alloys can be prepared by hot pressing from ball milled powders with more than 40 h. The hot pressing sample with homogeneous and fine microstructure made from 40 h ball milled powders has the optimum microstructure and properties. It has a relative density of 98.1%, Vickers hardness of 11.4 GPa, compress strength of 1 981 MPa and fracture toughness of 4.82 MPa·m1/2. The effect of fine grain toughening is fully realized.     

Microstructure and properties of novel quinary multi-principal element alloys with refractory elements

Five equiatomic alloys(Ti Zr Hf VNb, Ti Zr Hf VTa, Ti Zr Nb Mo V, Ti Zr Hf Mo V and Zr Nb Mo Hf V) composed of five elements with high melting temperature, respectively were prepared by arc-melting to develop a novel high temperature alloy. The five alloys exhibit different dendritic and interdendritic morphologies. The Ti Zr Hf VNb, Ti Zr Hf VTa and Ti Zr Nb Mo V alloys formed disordered solid solution phases with body-centered cubic structure, and exhibited high compressive strength and good plasticity. The Ti Zr Hf Mo V and Zr Nb Mo Hf V alloys are composed with Laves phase(Hf Mo2) and disordered solid solution phases with body-centered cubic structure. The Ti Zr Hf Mo V and Zr Nb Mo Hf V alloys are harder and more brittle than the other three alloys due to the existence of hard and brittle Laves phases. At high temperatures, the strength decreases to below 300 MPa for the Ti Zr Hf VNb and Ti Zr Hf Mo V alloys. Solution strengthening is the primary strengthening mechanism of the Ti Zr Hf VNb, Ti Zr Hf VTa and Ti Zr Nb Mo V alloys, and brittle Laves phase is the main cause for the low ductility of the Ti Zr Hf Mo V and Zr Nb Mo Hf V alloys.