NbCr2/ Nb合金高温力学行为

在800～1200℃的变形温度,0.001～0.1 s~(-1)的应变速率条件下对通过机械合金化+热压工艺制备的成分为Nb-22.5Cr的NbCr_2/Nb合金进行了高温压缩试验,研究了合金的高温力学行为,并通过透射电子显微镜观察分析了合金的变形机制。结果表明:NbCr_2/Nb合金的峰值强度随着变形温度的升高,应变速率的降低而下降。基体Nb的变形机制主要为位错的滑移;而NbCr_2的变形机制是通过层错、孪晶、不全位错等方式进行。     

NbCr_2/Nb合金高温蠕变行为

采用机械合金化+热压制备了成分为Nb-22.5at.%Cr的细晶NbCr_2/Nb合金。通过Gleeble 3500型热模拟机上的恒应力压缩试验,研究了合金的高温蠕变行为,并采用透射电子显微镜观察了合金变形前后的组织。结果表明:NbCr_2/Nb合金的稳态蠕变速率随应力的增加和变形温度的升高而加快,1000℃和200 MPa条件下,NbCr_2/Nb合金的稳态蠕变速率为9.0×10-5s-1,1000℃下的应力指数为4.36,而200 MPa下的蠕变激活能为510.7 kJ·mol-1。蠕变变形过程中,Nb基体中位错的滑移、攀移和Laves相NbCr_2中的同步剪切是蠕变变形的主要方式;随着变形温度升高,Nb基体颗粒有形成亚晶的趋势,且两相颗粒界面处应力增大,Laves相NbCr_2颗粒中层错/孪晶密度增加。     

NbCr_2/Cr合金的高温蠕变行为

通过恒应力压缩试验,研究了成分为Cr-12Nb的细晶NbCr_2/Cr合金的高温蠕变行为。结果表明,NbCr_2/Cr合金的稳态蠕变速率随应力的增加和变形温度的升高而加快;Cr基体中位错的滑移和Laves相NbCr_2中的孪生是蠕变变形的基本方式;Cr基体的动态再结晶和NbCr_2相的同步剪切是合金蠕变抗力降低的主要原因。     

Laves相NbCr2/Nb两相合金高温流动应力行为及本构关系建立

采用Gleeble-3500型热模拟试验机对机械合金化+热压工艺制备的Laves相NbCr_2/Nb两相合金进行等温恒应变速率压缩实验,研究合金在800～1200℃,0.001～0.1s~(-1)条件下的流动应力行为,并分别基于双曲正弦函数型Arrhenius方程和逐步回归法建立合金的本构关系。结果表明,Laves相NbCr_2/Nb两相合金的韧脆转变温度在950～1000℃之间。当温度≤950℃时,合金尚未达到屈服就已发生断裂;当温度≥1000℃时,合金呈现出较好的塑性变形能力。合金流动应力随变形温度增加和应变速率降低而降低。合金在1050～1200℃、0.001 s~(-1)和1150～1200℃、0.01 s~(-1)条件下呈现流动稳态型特征;在1000℃、0.001 s~(-1),1000～1100℃、0.01 s~(-1)和1000～1200℃、0.1 s~(-1)条件下呈现流动软化型特征。基于双曲正弦函数型Arrhenius方程建立的峰值流动应力本构关系和应变补偿本构关系的平均绝对相对误差AARE分别为9.89%和13.859%;基于逐步回归法建立的全实验条件下的本构关系、流动应力稳态型曲线本构关系和流动应力软化型曲线本构关系的平均绝对相对误差AARE分别为8.63%、5.28%和6.83%。所建立的几种本构关系,可为Laves相NbCr_2/Nb两相合金的锻造工艺制定、锻造设备吨位选择以及锻造过程有限元数值模拟提供理论依据和基础数据。     

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.     

合金元素对新型Co-Al-W合金热腐蚀行为的影响

研究了由γ'-Co₃(Al,W)相沉淀强化的新型钴基Co-Al-W 高温合金在800℃、75% Na₂SO₄+25% NaCl熔盐中的热腐蚀动力学及合金元素Mo、Nb、Ta和Ti对合金热腐蚀行为的影响。研究发现,2Mo、2Nb、 2Ta和2Ti合金比9.8W合金具有更好的抗热腐蚀能力,Mo和Ti对提高合金耐热腐蚀能力的效果比Ta和Nb显著。加入合金元素的合金热腐蚀膜由三层组成,即主要由Co氧化物CoO和Co₃O₄组成的腐蚀膜外层,由合金元素、Al、 Co及W复杂氧化物组成的中间过渡层和由Al、Co氧化物组成的腐蚀膜内层。随着腐蚀时间的增加,中间过渡层厚度逐渐增加,热腐蚀膜内、外层厚度变化不大,但内层致密性逐渐增加。     

Al对AlxMo0.5NbTiVSi0.2高熵合金组织和性能影响

采用真空电弧熔炼工艺制备了不同Al含量的Al_xMo_0.5NbTiVSi_0.2(x=0.5,0.8,1.0,摩尔比)难熔高熵合金。研究了合金的相组成、微观组织、密度和力学性能。结果表明,Al_xMo_0.5NbTiVSi_0.2高熵合金的微观组织为典型的树枝晶结构,均由BCC固溶体相和M₅Si₃金属间化合物相组成。Al含量的增加并未使得合金的相组成发生改变。合金BCC基体相富集Al、Mo和V元素,M₅Si₃相富集Ti和Si元素,Nb元素在两相中分布较为均匀。随Al含量增加,合金的密度从6.18 g/cm³降至5.86 g/cm³,硬度提升了13.7%,压缩屈服强度增加约332 MPa,增幅达到37%,抗压强度从1 073 MPa提高到1 457 MPa,断裂应变从13.6%增加到14.4%。合金力学性能的提升主要是通过固溶强化、细晶强...     

细晶Cr-NbCr2合金的热稳定性

本文研究了通过机械合金化+热压工艺制备的细晶Cr-NbCr₂合金的热稳定性。结果表明：热暴露过程中,Cr基体的颗粒尺寸有一定程度的长大,而Laves相NbCr₂颗粒由于其高热稳定性无明显变化。随着热暴露的进行,Cr基体颗粒与Laves相NbCr₂颗粒间压应力增加,使得基体和Laves相中分别出现了位错和层错/孪晶结构。所有热暴露条件下的Cr-NbCr₂合金的强度均不低于热压态的。在800-1200℃热暴露50h后,Cr-NbCr₂合金仍保持了较高的屈服强度和良好的塑性。但随着热暴露时间的继续延长,此时颗粒的长大成为主导因素,使得Cr-NbCr₂合金的屈服强度和塑性明显下降。     

微量合金元素Ti、Zr对Mo金性能和显微组织的影响

采用粉末冶金法制备Mo-Zr、Mo-Ti合金,研究了Zr、Ti的添加方式及添加量对Mo的拉伸性能和显微组织的影响。结果表明,添加合金元素Zr、Ti大大提高了Mo的力学性能。合金元素Zr以纯Zr形式加入较以ZrH2形式为佳,其添加量在0.1%（质量分数,下同）时,合金性能最高。元素Zr仅有极少部分固溶到Mo基体中,大部分与合金中少量氧结合以ZrO2粒子相存在。合金元素Ti则以TiH2的方式添加为佳,添加量为0.8%时合金性能最高。元素Ti一部分固溶到Mo基体中,另一部分与Mo及合金中的少量氧结合以MoxTiyOz复合氧化物粒子相存在。     

微量合金元素Ti、Zr对Mo合金性能和显微组织的影响

采用粉末冶金法制备Mo-Zr、Mo-Ti合金,研究了Zr、Ti的添加方式及添加量对Mo的拉伸性能和显微组织的影响.结果表明,添加合金元素Zr、Ti大大提高了Mo的力学性能.合金元素Zr以纯Zr形式加入较以ZrH2形式为佳,其添加量在0.1%(质量分数,下同)时,合金性能最高.元素Zr仅有极少部分固溶到Mo基体中,大部分与合金中少量氧结合以ZrO2粒子相存在.合金元素Ti则以TiH2的方式添加为佳,添加量为0.8%时合金性能最高.元素Ti一部分固溶到Mo基体中,另一部分与Mo及合金中的少量氧结合以MoxTiO2复合氧化物粒子相存在.     

Si合金化对C15 NbCr2 Laves相稳定性和断裂韧性影响的第一性原理研究

基于第一性原理计算方法,通过对形成焓、结合能、原子自由体积和电子结构的计算,研究了Si合金化对C15 NbCr₂Laves相稳定性和断裂韧性的影响。位点占据能表示Si原子倾向于占据Cr位点。形成焓和结合能计算表明,随着Si含量的增加,Nb₈Cr_16-xSi_x(X= 0~ 5)相的形成能力和稳定性均得到提升且与Si含量保持线性相关性。原子自由体积计算表明,Nb₈Cr_16-xSi_x相的原子自由体积较NbCr₂基体相均得到增加,其中在Si含量为8.33 at%(Nb₈Cr₁₄Si₂)时,原子自由体积取得最大值,断裂韧性达到最优。电子结构计算表明,Si合金化使得DOS曲线右移,费米能级向赝能隙峰谷靠近,稳定了NbCr₂基体相,同时所有的成键峰变得下降和展宽,削弱了Nb-Cr原子的键合强度,使得剪切变形易于进行,从而提高韧性。     

Site preference of early transition metal elements in C15 NbCr2

The site preference of early 3d (Ti, V), 4d (Zr, Mo) and 5d (Hf, Ta, W) transition metal elements in C15 NbCr₂ Laves phase was studied using first-principles calculations. According to the present calculations, at T = 0 K, Zr, Hf and Ta consistently have a preference for the Nb sites in Nb-rich, Cr-rich and stoichiometric NbCr₂, while the site preference of Ti, V, Mo and W varies strongly with alloy composition. Using a statistical–mechanical Wagner–Schottky model based on the canonical ensemble, the finite temperature site occupancy behavior of those transition metal elements in NbCr₂ was further predicted. It was found that the site preference of Ti, V, Mo and W also depends strongly on temperature. The calculated results compare favorably with the experimental measurements using ALCHEMI and synchrotron X-ray diffraction techniques.     

Solidification of Bcc/T1/T2 three-phase microstructure in Mo–Nb–Si–B alloys

The three-phase, (Mo,Nb)_ss/(Mo,Nb)₅Si₃/(Mo,Nb)₅SiB₂, Bcc/T₁/T₂ microstructures that develop in Mo–Nb–Si–B alloys have been examined in arc cast and directionally solidified samples to identify the phase sequencing during solidification. A Mo-32.6Nb-19.5Si-4.7B (at.%) alloy was directionally solidified using an optical floating zone (OFZ) furnace in a flowing Ar gas atmosphere at a constant growth rate of 10 mm/h. The microstructure of the directionally solidified alloy is characterized by an elongated T₂ phase surrounded by bcc and T₁ phases with an interwoven morphology From the evaluation of the microstructures in arc cast ingots of several alloys at a constant 32.6 at%Nb composition, the path of the liquidus valleys with decreasing temperature has been determined to intersect at a common point that establishes the solidification reaction as a eutectic. The incorporation of the experimental results into a computational thermodynamic analysis provides insight on the partitioning of components within each phase during solidification.     

Laves phase fields in Cr–Zr–Nb and Cr–Zr–Hf alloy systems

The phase fields of the Laves phase in the Cr–Zr–Nb and Cr–Zr–Hf alloy systems were investigated at 1573 K. The Laves phase formed in the Cr–Zr–Nb alloy system had a broad off-stoichiometric range in the center of ZrCr₂–NbCr₂ pseudo-binary line, while the Laves phase formed in the Cr–Zr–Hf alloy system had a uniform and limited off-stoichiometric range along ZrCr₂–HfCr₂ pseudo-binary line. The results are discussed, in terms of geometric concept of atomic sizes of the constituent atoms.     

Effects of Si, W and W–Mo on isothermal oxidation behaviors of Nb/Nb5Si3 in situ composites at high temperature

The effects of Si, W and W–Mo on the isothermal oxidation behaviors of Nb/Nb₅Si₃ in situ composites in static air at 1000 and 1200 °C for 20–100 h were investigated on as-cast materials. The results show that the oxidation kinetics of each alloy was not changed whether at 1000 or 1200 °C, and the oxidation mechanism were not changed. The oxidation resistance of Nb/Nb₅Si₃ in situ composites was sensitive to Si content, and the oxidation rate of Nb-10Si alloy was more than twice as many that of Nb–20Si alloy. By alloying of W, the oxidation resistance of Nb–20Si–10W alloy was improved significantly, because the WO₃ scale can provide the adherence for the creaked Nb₂O₅ scale and reduce the diffusion of oxygen through the scale. Comparing to alloying with W, the poor oxidation resistance of Nb–20Si–10W–10Mo alloy was attributed to the evaporation of MoO₃ and highly porous scale.     

Structural, microstructural and magnetic properties of amorphous/nanocrystalline Ni63Fe13Mo4Nb20 powders prepared by mechanical alloying

This paper focuses on the magnetic, structural and microstructural studies of amorphous/nanocrystalline Ni₆₃Fe₁₃Mo₄Nb₂₀ powders prepared by mechanical alloying. The ball-milling of Ni, Fe, Mo and Nb powders leads to alloying the element powders, the nanocrystalline and an amorphization matrix with Mo element up to 120 h followed by the strain and thermal-induced nucleation of a single nanocrystalline Ni-based phase from the amorphous matrix at 190 h. The results showed that the saturation magnetization decreases as a result of the electronic interactions between magnetic and non-magnetic elements and finally increases by the partial crystallization of the amorphous matrix. The coercive force increases as the milling time increases and finally decreases due to sub-grains formation.     

Preparation, microstructure and properties of molybdenum alloys reinforced by in-situ Al2O3 particles

The conventional molybdenum alloys, lacking of hard particles enhancing wear property, have relative poor wear resistance though they are widely used in wear parts. To resolve the above question, Mo alloys reinforced by in-situ Al₂O₃ particles are developed using powder metallurgy method. The in-situ α-Al₂O₃ particles in molybdenum matrix are obtained by the decomposition of aluminum nitrate after liquid-solid incorporation of MoO₂ and Al(NO₃)₃ aqueous solution. The α-Al₂O₃ particles well bonded with molybdenum distribute evenly in matrix of Mo alloys, which refine grains of alloys and increase hardness of alloys. The absolute density of alloy increases firstly and then decreases with the increase of Al₂O₃ content, while the relative density rises continuously. The friction coefficient of alloy, fluctuating around 0.5, is slightly influenced by Al₂O₃. However, the wear resistance of alloy obviously affected by the Al₂O₃ particles rises remarkably with the increasing of Al₂O₃ content. The Al₂O₃ particles can efficiently resist micro-cutting to protect molybdenum matrix, and therefore enhances the wear resistance of Mo alloy.     

Comparative studies on densification behavior,microstructure, mechanical properties and oxidation resistance of Mo-12Si-10B and Mo3Si-free Mo-26Nb-12Si-10B alloys

Two bulk Mo-Si-B based alloys (Mo-12Si-10B and Mo-26Nb-12Si-10B (at.%), abbreviated as 0Nb and 26Nb alloy respectively) were fabricated by mechanical alloying and then hot pressing. Comparative studies were carried out on the densification behavior, microstructure, room-temperature fracture toughness, elevated temperature compression and oxidation resistance of these two alloys. The results showed that alloy 0Nb was composed of (Mo), Mo₃Si and Mo₅SiB₂, while alloy 26Nb was free of Mo₃Si and had higher (Mo) content and a little γNb₅Si₃. Compared to the alloy 0Nb, alloy 26Nb presented better compactibility, higher room-temperature fracture toughness (8.84 ± 0.17 vs. 6.77 ± 0.20 MPa·m^1/2) and elevated temperature compression strength (851.7 ± 11.7 vs. 644.2 ± 10.2 MPa) but worse oxidation resistance.     

Alloying of MoSi2 for improved mechanical properties

Effects of alloying on mechanical properties of solidification processed polycrystalline MoSi₂, ternary (Mo, 2.5 at.% Re)Si₂, Mo(2 at.% Al, Si)₂, (Mo, 1 at.% Nb)Si₂, and quaternary (Mo, 1 at.% Re)( 2 at.% Al, Si)₂ alloys were evaluated by microhardness testing at room temperature and compression testing at elevated temperatures. Alloying with 2.5 at.% Re resulted in an anomalously high solid solution hardening in MoSi₂, while alloying with 2 at.% Al lowered the brittle-to-ductile transition temperature, in compression, of polycrystalline MoSi₂ from 900 ^oC to ⩽25 ^oC. Similar to the effects observed in the quaternary (Mo, 1 at.% Re)(2 at.% Al, Si)₂ alloy, 1 at.% Nb containing samples exhibited concurrent enhanced ambient temperature ductility and higher elevated temperature strength compared to pure MoSi₂. The effects of alloying MoSi₂ with 2.5 at.% Re, 1 at.% Re+2 at.% Al, and 1 at.% Nb on hardness and yield strength are discussed.     

Al2O3陶瓷Ti+Nb/Mo金属化对其与Kovar合金钎焊连接的组织和力学性能的影响

采用磁控溅射在Al₂O₃陶瓷表面沉积了Ti+Nb/Mo金属层,实现了氧化铝陶瓷的金属化,并通过电镀镍提高了金属化效果.采用AgCu28钎料,实现了金属化Al₂O₃陶瓷与Kovar合金的可靠连接.通过扫描电子显微镜和能谱观察了钎缝的微观组织.结果表明,钎料与母材发生了明显的界面反应.Cu元素扩散进入Kovar合金,同时Ni元素扩散进入钎料的富铜区,从而促进AgCu/Kovar连接界面的形成;金属化层在Al₂O₃/AgCu钎料界面处,起到了关键作用,其中铌可以抑制脆性化合物形成,缓解残余应力.金属化层镀镍后,钎缝中AgCu共晶区明显,且钎缝较宽,对提高镀镍试样的钎焊接头强度有一定作用.