粉末冶金TiVNbTa难熔高熵合金的组织和力学性能

将机械合金化(MA)与放电等离子烧结(SPS)相结合制备了难熔TiVNbTa高熵合金,研究了这种合金的机械合金化过程、相组成和显微组织,以及烧结温度和O、N含量对其力学性能的影响。结果表明:机械合金化后高熵合金粉末为BCC结构,放电等离子烧结成的块体高熵合金由BCC基体和FCC析出相组成,其析出相为TiN+TiC+TiO的复合物。烧结温度为1100℃的高熵合金具有良好的综合力学性能,压缩屈服强度达到1506.3 MPa,塑性应变为33.2%。随着烧结温度的提高,合金发生了从准脆性到塑性再到脆性断裂的转变。O和N含量的提高对高熵合金强度的影响较小,但是使其塑性显著降低。     

球磨时间对镍基ODS合金拉伸性能的影响

研究了球磨时间对Y2O3氧化物弥散强化(ODS)镍基高温合金机械合金化和拉伸性能的影响.镍基高温合金采用机械合金化和热压烧结方法制备.镍基ODS高温合金粉末是在行星式球磨机上进行球磨.采用扫描电镜及X射线衍射分析了球磨时间对镍基ODS合金粉末形貌和物相的影响.研究结果表明,Y2O3氧化物弥散强化镍基高温合金机械合金化粉末尺寸随研磨时间的增加先增大后减小,8h粉末颗粒尺寸达到最大,之后粉末颗粒尺寸逐渐减小,28h后,镍基ODS合金粉末尺寸稳定且均匀.拉伸结果表明,采用研磨28h的合金粉末制备的镍基ODS合金具有最高的抗拉强度(1300MPa).     

化学法制备ODS钢的新工艺技术

氧化物弥散强化(Oxide Dispersion Strengthened,ODS)铁素体钢,由于其优异的高温力学性能和良好的抗辐照能力被认为是快堆与超临界水堆燃料包壳管候选材料之一。传统ODS钢的制备方法是采用机械合金化法向铁素体钢中添加高熔点弥散细小的氧化物Y2O3而具有优良的高温强度,但合金的塑性和冲击韧性较差,热加工中存在严重的组织和性能各向异性,给合金制备或薄壁管带来极大的困难。     

ODS钢中纳米氧化物析出长大机制的研究进展

纳米氧化物弥散强化(Oxide dispersion strengthened, ODS)钢得益于基体中弥散分布的超高数量密度的纳米氧化物粒子，具有优异的综合服役性能，被视为第四代裂变堆包壳以及未来聚变堆包层的优选结构材料。传统制备ODS钢最主要的方法是机械合金化(Mechanical alloying, MA)等先进粉末冶金技术，且对其制备样品中氧化物粒子性质的研究较为深入。由于机械合金化在工程应用上有一定局限性，近年来提出以液态金属(Liquid metal, LM)路线制备ODS钢。目前液态金属路线中的真空熔炼法是最常用也是相对比较成功的方法，但与机械合金化相比仍有一定的差距。本文主要总结了机械合金化制备的ODS钢中纳米氧化物的析出机制、长大行为以及其他元素的添加对其产生的影响等，同时对液态金属路线的工艺进程及真空熔炼法的研究现状进行概述，并对机械合金化未来所需解决的问题和液态金属路线的后续发展、工艺优化等进行了展望，为液态金属路线的后续研究工作提供参考。     

机械合金化应用的新领域

本文简述了机械合金过程和原理,着重介绍了机械合金化制备金属间化合物、非晶态合金、TiC的合成及非氧化物弥散强化的工艺、性能特点和机理。认为机械合金化技术有着广阔的前景。     

FeCr-ODS铁素体合金的氧化+粉锻工艺制备及其微观结构

提出一种氧化+粉锻(粉末锻造)新工艺并用其制备了FeCr-ODS铁素体合金。使用SEM、XPS、EPMA和TEM等手段对其表征,研究了粉末表面和内部氧化物的生成、演变以及合金中纳米氧化物弥散相的种类和分布特征。结果表明,在低温氧化过程中粉末表面生成了一层Fe的氧化膜,在随后的加热过程中粉末表面的O元素转移并与Y和Ti元素反应生成了Y-Ti-O纳米氧化物弥散相。通过纳米氧化物弥散相在粉末成型过程中的演变,阐明了粉锻对位错和纳米氧化物析出相形成的贡献。用这种工艺制备的ODS铁素体合金,大量细小的Y₂TiO₅析出相均匀地分布在基体中,晶界上只有少量大颗粒Y₂O₃。     

机械合金化氧化物弥散强化合金再结晶过程的研究

机械合金化氧化物弥散强化合金的极高再结晶温度无法用γ析出物熔解、弥散粒子粗化、或常规晶粒长在理论合理解释。这类合金再结晶前晶粒极其细小，在微米以下量级。分析表明，在这种情况下，超细晶粒的晶界交结线在强列钉轧作用，阻止再结晶成核启动。本工作基于这种考虑，建立模型，较完善地解释了机械合金化氧化物弥散强化合金的奇异再结晶现象。     

氧化物弥散强化高温合金及其在航空上的应用

氧化物弥散强化高温合金,简称ODS高温合金(Oxide Dispersion Strengthened Su-peralloys )是粉末高温合金的一大分枝,也是高温合金的一个重要发展方向。 高温合金的氧化物弥散强化是指在高温合金中加入一定量的热力学稳定的氧化物,使其弥散分布于基体中,形成不溶于基体的氧化物弥散相,从而使合金得以进一步强化的一种新的强化途径。     

氧化物弥散强化高温合金MGH956的基本焊接性研究

氧化物弥散强化(ODS)高温合金MGH956是机械合金化工艺制造的氧化物弥散强化高温合金,针对MGH956材料采用电子束、氩弧焊、钎焊及扩散处理等连接工艺方法进行了基本的焊接性研究.对接头组织及接头性能进行了测试分析,三种焊接方法室温接头力学性能相当,接头强度系数均达到95%左右,氩弧焊焊缝中的气孔较多,而电子束焊焊缝中气孔相对比较少、室温塑性比较明显.从接头高温强度而言,真空钎焊工艺有明显优势.     

机械合金化和粉末冶金法制备Fe-Mn-Si基形状记忆合金的研究进展

铁基形状记忆合金由于价格低廉、强度高、加工性能好、可焊接等优点引起广泛重视。机械合金化(MA)和粉末冶金(PM)作为制备材料的新工艺,可以用来制备性能优越的形状记忆合金。本文详述了机械合金化和粉末冶金工艺在制备Fe-Mn-Si基形状记忆合金过程中对合金相变、组织与性能的影响,以及此类合金在新领域的应用。最后提出了现阶段在研究MA/PM工艺制备Fe-Mn-Si基SMA中有关工艺参数、相变机制以及回复应力和低温应力松弛所存在的问题。     

Preliminary study on the fabrication of 14Cr-ODS FeCrAl alloy by powder forging

A simple powder forging process was presented herein to fabricate an Fe-14Cr-4.5Al-2W-0.4Ti-0.5Y2O3 ODS FeCrAI alloy.The forged alloy exhibits a high density that exceeds 97％of the theoretical density.The ODS alloy was investigated in terms of the residual porosity,morphology and phase structure of oxide nanoparticles,impact toughness and tensile properties.It was found that refined grains were obtained during powder forging.A residual porosity less than 1.1％has no impact on the precipitation of oxide nanoparticles.The average diameter of the oxide particles is 7.99 nm,with a number density of 2.75 x 1022 m-3.Almost all of the oxides are identified as orthorhombic YAlO3 particles.The refined grains and uniformly distributed oxide nanoparticles enable the alloy to show excellent mechanical strength and ductility below 700℃,and enable the ductile-to-brittle transition temperature to be close to room temperature.However,a slight decrease in strength at 1000℃and the Charpy upper shelf energy has been suggested to be due to the residual porosity.These results indicate that powder forging can be used as a promising technique for the fabrication of ODS alloys.     

ODS ferritic steel engineered with bimodal grain size for high strength and ductility

An attractive way to enhance the ductility of ODS ferritic steels is to develop an alloy with a bimodal grain size distribution, in which the micron-sized coarse grains provide high ductility. The nanometer-sized fine grains enhance the tensile strength. The microstructures were obtained by blending the gas-atomized powders and mechanical alloyed powders, followed by hot forging and annealing. The homogeneously distributed nanometer-sized oxide nanoparticles can also be detected. Mechanical properties tests revealed a great improvement in ductility in comparison with other ODS ferritic steels, and high strength over the whole range of test temperatures, owing to the fine grains and oxide nanoparticles. The combination of high ductility and high strength makes this ODS ferritic steel much promising in high-temperature application.     

Microstructure characterisation and tensile properties of 18Cr-4.5Al-0.3Zr- oxide dispersion strengthened steel

The 18Cr–4.5Al–0.3Zr–oxide dispersion strengthened (ODS) steel was fabricated by mechanical alloying (MA) and spark plasma sintering (SPS) technique. A microstructural characterisation was performed on an 18Cr–4.5Al–0.3Zr–ODS steel using high angle annual dark field (HAADF) and synchrotron small angle X-ray scattering (SAXS). HAADF and SAXS results showed that high-density nanoscale oxides are formed in 18Cr–4.5Al–0.3Zr–ODS steel. The oxides in the specimen can be roughly divided into two categories according to their compositions: (1) core/shell structure oxides with Al–O oxide cores and Y shells; (2) nm-scale trigonal-phase Y₄Zr₃O₁₂ oxides. In addition, tensile testing results revealed that the specimen exhibited better tensile strength and ductility as compared with another commercial ODS steels with similar composition.     

Enhancing the Mechanical Properties of Vanadium-Microalloyed Medium-Carbon Steel by Optimizing Post-Forging Cooling Conditions

The effects of different cooling conditions after forging on the microstructural characteristics and mechanical properties of a kind of high-content V-microalloyed medium-carbon steel 37MnSiVS were investigated. The effects were studied by using optical microscopy, transmission electron microscopy and tensile tests. Increasing direct cooling rate after forging is found to increase strength while slightly decrease ductility. A significant increase of strength could be obtained after forced air cooling and then short time isothermal holding at 873 K, while strength decreases gradually with further increasing holding time. The variations of microstructural characteristics especially V(C,N) precipitation strengthening effects with cooling conditions are mainly responsible for these variations of tensile properties. For the investigated high V-containing MA steel, a direct cooling strategy after finish forging is proposed, which includes accelerated cooling with forced air in ferrite range, following by short time isothermal holding or very slow cooling at around 873 K and then air cooling.     

Influences of residual stress and micro-deformation on microstructures and mechanical properties for Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy produced by laser powder bed fusion

Laser powder bed fusion(LPBF)yields unique advantages during the fabrication of titanium alloys.In the present work,Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy specimens with excellent mechanical performances were fabricated by LPBF.The as-built specimens displayed relatively high strength and ductility under modest volume energy densities(VEDs),whereas they manifested high strength with low ductility under high VEDs.To investigate the key reason of this phenomenon,the specimens were designed with two VEDs ranges of 60 J/mm3 and 85J/mm3.Special attention was paid to the influences of residual stress and micro-deformation on microstructures and mechanical properties for the first time.The results indicated that the residual stresses and relative density of the 60 J/mm3 range specimens were higher than that of the 85 J/mm3 range specimens.Dislocation multiplication and dislocation movement promoted by the residual stress were hindered by the initial α'phase grain boundary(prior-α'GB),leading to the formation of α'metastable structures.The mean tensile strength and elongation of the 60 J/mm3 range specimens were 1248.1 MPa and 12.3％,respectively,whereas the corresponding values for the 85 J/mm3 range specimens were 1405.3 MPa,5.0％,respectively.During deformation,the strength and ductility of the specimens were first improved by lamellar structures generated from prior-α'phases,and then effectively enhanced by the interaction between the{10-12}twins and dislocations.However,pores significantly reduced the ductility;hence,high VED specimens with large twins and numerous large pores increased the strength and reduce the ductility.     

含钛氧化物弥散强化钢的微观组织和力学性能

采用粉末冶金工艺制备了含Ti氧化物弥散强化钢。使用电子背散射衍射方式研究了这种钢的晶粒形貌,使用透射电镜和高分辨率透射电镜表征了钢中析出相的形貌及种类,使用以同步辐射装置作为光源的小角度X射线散射技术和X射线吸收精细结构技术研究了钢中纳米尺寸析出相的分布特征和氧化物弥散强化钢中Y元素的存在形式,并测量了钢的力学性能。结果表明,含Ti氧化物弥散强化钢的晶粒多呈等轴状、平均晶粒尺寸为1.24 μm。钢中富Y、Ti、O纳米尺寸析出相的分布密度为1.39×10²⁴/m³,平均直径为2.23 nm。向材料中添加Ti元素改变了材料中Y原子的存在形式,生成了具有烧绿石结构的Y₂Ti₂O₇相和少量的富Cr、Mn相。这种钢的室温抗拉强度达到1324 MPa,随着测试温度的提高抗拉强度逐渐降低,延伸率逐渐提高。     

Effects of processing parameters on tensile properties of selective laser melted 304 stainless steel

Selective laser melting (SLM) technology based on powder bed has been used to manufacture 304 stainless steel samples. The effects of slice thickness, overlap rate, building direction and hatch angle on tensile properties of SLMed 304 stainless steel samples are investigated. It is found that tensile properties of SLMed 304 stainless steel are independent of slice thickness and overlap rate, but increase slowly with increasing interval number of deposited layers. The hatch angle of 105° with the maximum interval number of deposited layers and vertical building direction are preferred to get excellent tensile properties. Importantly, all the SLMed samples feature much higher σ_0.2/UTS values of nearly 0.8. The tensile strengths and ductility of SLMed samples at proper parameters are higher than those of the wrought 304 stainless steel.     

Relation between microstructure and Charpy impact properties of an elemental and pre-alloyed 14Cr ODS ferritic steel powder after hot isostatic pressing

This article describes the microstructure and Charpy impact properties of an Fe–14Cr–2W–0.3Ti–0.3Y₂O₃ oxide dispersion strengthened (ODS)-reduced activation ferritic (RAF) steel, manufactured either from elemental powders or from an Fe–14Cr–2W–0.3Ti pre-alloyed powder. ODS RAF steels have been produced by mechanical alloying of powders with 0.3 wt% Y₂O₃ nanoparticles in either a planetary ball mill or an attritor ball mill, for 45 and 20 h, respectively, followed by hot isostatic pressing (HIPping) at 1,150 °C under a pressure of 200 MPa for 4 h and heat treatment at 850 °C for 1 h. It was found that the elemental ODS steel powder contains smaller particles with a higher specific surface area and a two times higher oxygen amount than the pre-alloyed ODS steel powder. After HIPping both materials exhibit a density higher than 99%. However, the pre-alloyed ODS steel exhibits a slightly better density than the elemental ODS steel, due to the reduced oxygen content in the former material. Charpy impact experiment revealed that the pre-alloyed ODS steel has a much larger ductile-to-brittle transition temperature (DBTT) (about 140 °C) than the elemental ODS steel (about 25 °C). However, no significant difference in the upper shelf energy (about 3.0 J) was measured. TEM and SEM–EBSD analyses revealed that the microstructure of the elemental ODS steel is composed of smaller grains with a larger fraction of high-angle grains (>15°) and a lower dislocation density than the pre-alloyed ODS steel, which explains the lower DBTT value obtained for the elemental ODS steel.     

Effect of carbon on mechanical properties of powder-processed Fe-0.35%P alloys

The present paper records the results of mechanical tests on iron-phosphorus powder alloys which were made using a hot powder forging technique. In this process mild steel encapsulated powders were hot forged into slabs, hot rolled and annealed to relieve the residual stresses. These alloys were characterized in terms of microstructure, porosity content/densification, hardness and tensile properties. Densification as high as 98.9% of theoretical density, has been realized. Microstructures of these alloys consist of single-phase ferrite only. Alloys containing 0.35 wt% P, such as Fe-0.35P-2Cu-2Ni-1Si-0.5Mo and Fe-0.35P-2Cu-2Ni-1Si-0.5Mo-0.15C show very high strength. It was observed in this present investigation that, the alloying additions, such as Si, Mo, Ni, and C to Fe-P based alloys caused increase in strength along with reduction in ductility. Cu reduces porosity of Fe-P alloys. Alloys developed in the present investigation were capable of hot working to very thin gauge of sheets and wires.     

Enhancement of the mechanical properties of EN52CrMoV4 spring steel by deep cryogenic treatment

The effect of the deep cryogenic treatment on the micro-structure and mechanical properties (tensile strength, toughness, residual stress and fatigue strength) of the medium carbon spring steel, which is subjected to different heat treatment steps, is investigated. Deep cryogenic treatment causes spring steel to keep compressive residual stress more efficiently due to an increase in the density of the crystalline defects, retardation in the stress relief after the phase transformations and nano-cluster carbide formations. If deep cryogenic treatment is applied before the tempering then the homogeneously distributed fine carbides form after the tempering and the grains remain relatively fine. The microstructure with homogeneously distributed fine carbides and fine grains cause spring steels to have simultaneously enhanced tensile strength, ductility and fatigue strength. If deep cryogenic treatment is applied after the conventional heat treatment (quenching+tempering), however, the coarse carbides form in the micro-structure and the improvement in the mechanical properties of the spring steel is limited.