基于三态组织的近 α 钛合金近 β 锻+固溶时效工艺参数匹配

基于正交试验结果,对近β锻+固溶时效工艺参数进行了显著性分析,并详细讨论了工艺参数对TA15钛合金显微组织的影响及合理的工艺参数,以获得性能优异的三态组织。结果表明：变形温度、固溶温度和固溶时间是3个最为重要的工艺参数,分别对等轴α_p相的体积分数和直径、片层α_s相的体积分数及片层α_s相的厚度影响最大。较合理的TA15钛合金处理工艺参数为970 ℃/0.1 s^-1/60%变形程度/水淬+930 ℃/1.5 h/空冷+550 ℃/5 h/空冷。     

基于m值高效法的TA15合金超塑性变形

对TA15合金在拉伸试验机上进行应变速率敏感因子(m值)高效超塑性变形试验,研究合金的超塑性性能和显微组织。结果表明:在780~950℃变形时,TA15合金呈现出良好的超塑性能;900℃变形时,该合金的超塑性能最好,m值达到0.62,最大伸长率为1287%;随着变形温度的升高,合金的超塑性能降低,950℃时伸长率仅为567%。显微组织分析表明:TA15合金在超塑性变形过程中,晶粒始终保持等轴状;由于变形温度升高,晶粒合并长大,950℃时发生?→?相转变,初生?相体积分数大幅度降低。与最大m值法相比较,m值高效超塑性变形不仅使TA15合金获得了良好超塑性能,变形效率也显著提高。     

Ti600合金在(α+β)相区的热变形行为和本构模型

对具有片层状初始组织的Ti600合金的热变形行为进行了研究。变形温度范围为800~960 ℃,应变速率范围为10^-3~1 s^-1。随后提出了应变硬化指数（n）来表征流动软化和加工硬化之间的竞争。并且通过分析流变曲线和观察显微组织研究了该合金的软化行为。结果表明,变形参数对Ti600合金的流变行为有显著影响。当变形超过峰值应变之后,n值逐渐降低,动态软化过程开始占主导地位。微观组织分析表明：热变形过程中,α相的弯曲、破碎、动态回复和动态再结晶行为是造成Ti600合金软化的主要原因。最后基于实验数据,建立了3种本构模型,分别是应变补偿Arrhenius模型、Hensel-Spittel模型和修正的Arrhenius模型,来表征Ti600合金的流变行为。将3种模型预测的流变应力与实验结果进行比较,并计算其相关系数值和平均相对误差值来评估模型的准确性。3种模型的相关系数值分别为0.965、0.989和0.997,平均相对误差值分别为12.86%,9.74%和3.26%。这些结果表明,这3种模型都可以描述Ti600合金的流变行为,而修正的Arrhenius模型具有最高的预测精度。     

TA15合金超塑性变形空洞演化及断裂行为

对TA15合金在SANS CMT4104型高温电子拉伸实验机上进行恒应变速率超塑性拉伸试验,研究了合金超塑性变形过程中空洞演化及断裂行为。结果表明:超塑性变形过程中,TA15合金空洞含量和大小受变形量、应变速率和应变速率敏感性指数m值的影响较大。随变形量增大,空洞分别沿拉伸轴方向和垂直于拉伸轴方向发生了聚合和连接,空洞长大由形核时的一般扩散机制向塑性变形机制转变。TA15合金超塑性拉伸试样断口呈针点状,断口上含有大量的韧窝状空洞,空位聚集-空洞连接是TA15合金超塑性断裂的主要机制。     

耐腐蚀性能和拉伸性能提升的 α 型钛合金设计

通过2个电子参数（结合次数Bo_t和d轨道能级Md_t）提出了新设计的α型钛（α-Ti）合金。新设计合金Ti-5Al-4Zr-3.6Sn、改性合金Ti-5Al-3Sn-1.9Zr和参考合金Ti-5Al-2.5Sn具有相同的Bo_t值（3.847）以及不同的Md_t值（2.430,2.426,2.422）。测试了3种α-Ti合金的极限抗拉伸强度（σ_UTS）、断裂应变（?_f）和热盐腐蚀性能。3种α-Ti合金均采用冷坩埚悬浮熔炼技术进行制备。结果表明,3种合金样品均具有均匀的微观结构。在3种α-Ti合金中测量到的α单相晶粒尺寸约为600 μm。Ti-5Al-4Zr-3.6Sn合金的σ_UTS和?_f值为801 MPa和16%,Ti-5Al-3Sn-1.9Zr合金的σ_UTS和?_f值为708 MPa和15%,Ti-5Al-2.5Sn合金的σ_UTS和?_f值为603 MPa和15%。热盐腐蚀测试进行28.8 ks后显示Ti-5Al-4Zr-3.6Sn、Ti-5Al-3Sn-1.9Zr和Ti-5Al-2.5Sn合金的失重率为2.61%、2.83%和3.10%。σ_UTS、?_f和耐热盐腐蚀结果表明,新设计合金Ti-5Al-4Zr-3.6Sn是一种有实际应用潜力的钛合金材料。     

TA7钛合金高温变形行为研究

为了分析TA7钛合金的热变形工艺参数,通过高温压缩试验对TA7钛合金的高温变形行为进行了研究。试验温度为1123~1273K,应变速率为0.001~1s_-1。此外,提出了一种修正并联本构模型用来分析应变速率、变形温度及应变对流动应力的影响。然后,基于动态模型,建立了TA7钛合金的热加工图,并通过微观组织分析对加工图的准确性进行了验证。结果表明,TA7钛合金合理的工艺参数为变形温度1223K,应变速率0.001s_-1,而其非稳态区域位于低温高应变速率区。     

Mn微合金化对Ti-Al-Mo-Zr-Fe-B合金组织演变和力学性能的影响

通过OM、EBSD和TEM等研究了Mn微合金化对新型近α Ti-Al-Mo-Zr-Fe-B合金微观组织和力学性能的影响。结果表明：添加0.5%（质量分数）的Mn元素可以将合金的铸态微观组织从3.28 μm细化到2.65 μm,使其抗拉伸强度从882 MPa提高到966 MPa,但延伸率从7.8%下降到5.1%。锻造后的2种合金的晶粒尺寸趋于一致,微观组织趋于等轴化且Mn微合金化后的组织更加均匀。锻造后,Ti-Al-Mo-Zr-Fe-B合金的抗拉伸强度和延伸率增加到966 MPa和16.4%,而含有0.5%（质量分数）Mn元素的合金具有更高的抗拉伸强度,达到了1079 MPa,同时延伸率达到了15.8%。结论表明,强度的提高可以归因于Mn元素的固溶强化效应,同时Mn微合金化处理使合金中的Al元素富集于α相,有利于提高合金的强度和塑性。     

热腐蚀对GH4169高温合金 δ 相析出和拉伸性能的影响

分别在650、750、850 ℃热处理条件下,研究了高温合金GH4169在75Na₂SO₄+25NaCl （质量分数）熔盐环境下的热腐蚀行为,之后进行组织表征和力学性能测试。结果表明：随着热处理温度的不断升高,高温合金的抗拉极限强度（UTS）和屈服强度（YS）都出现急剧退化,伸长率显著提高。但在750 ℃条件下,晶界处析出的针状δ相能提高高温合金的强度,导致合金出现沿晶脆性断裂现象,降低了合金的塑性。在650和750 ℃下,腐蚀机理符合II型热腐蚀,但在850 ℃条件下符合I型热腐蚀,2种不同类型腐蚀都促进了基体δ相的析出。     

TC4钛合金板材超塑性性能研究

采用最大m值法、恒应变速率法在850~910℃下测试TC4钛合金板材的超塑性性能,分析了工艺参数对TC4钛合金板材的流动应力、应变速率敏感性指数和微观组织演变的影响。结果表明:该合金的最佳超塑性变形温度在850℃左右,在该温度下的基于最大m值法、恒应变速率法拉伸的伸长率均达到了最大且分别为1031%和631%,而在850℃下最大m值法拉伸能获得材料的最佳超塑性;当变形温度为850~910℃时,最佳变形速率0.00031~0.001 s~(-1);随变形温度的升高、应变速率的降低,该合金的流动应力降低,最大为70 MPa;该合金在850℃、应变ε=0.1条件下的应变速率敏感性指数m值最大且为0.58,并随着变形温度、应变量的增加而降低:超塑性变形中其内部发生了明显的动态再结晶,温度越高,晶粒越粗大。     

Ti6Al4V钛合金冷轧板材的超塑性变形行为研究

在温度830～890℃和应变速率0.0005～0.005 s~(-1)下对Ti6Al4V钛合金冷轧板材进行超塑性拉伸实验。利用光学显微镜和扫描电镜观察变形后的微观组织和断口形貌。研究了该合金的超塑性变形行为和变形机理。结果表明:在应变速率为0.0005、0.005 s~(-1)时,随着变形温度的升高,伸长率先升高后降低;在应变速率为0.001 s~(-1)时,随着变形温度的升高,伸长率逐渐降低;在830℃和0.001 s~(-1)条件下伸长率达到最大值1259.0%;超塑性最优变形参数区间为温度830～850℃、应变速率0.0005～0.001 s~(-1)。合金的应变速率敏感性指数m值随温度升高先增加,850℃时达到最大值0.472,随后逐渐减小;超塑性变形下的平均激活能为259 k J/mol。超塑性变形过程发生了明显的动态再结晶,微观组织完全转变为等轴组织。超塑性变形的主要机制为晶界滑移。Ti6Al4V合金板材超塑性拉伸断裂属为于沿晶断裂。     

Microstructures And Mechanical Properties Of In-Situ Al2o3/Tial Composites By Exothermic Dispersion Method

In-situ Al2O3/TiAl composites were fabricated by pressure-assisted exothermic dispersion （PAXD） method from elemental powder mixtures of Ti, Al, TiO2, and Nb2O5. The microstructures and mechanical properties of the as-sintered composites are investigated. The results show that the as-sintered products consist of γ-TiAl, α2-Ti3Al, Al2O3, and NbAl3 phases. Microstructure analysis indicates that Al2O3 particles tend to disperse on the grain boundaries. Application of a moderate pressure of 35 MPa at 1200℃ yields Al2O3/TiAl composites with fine Al2O3 reinforcement and a discontinuous network linking by Al2O3 particles. The aluminide component has a fine submicron γ ＋α2 lamellar microstructure. With increasing Nb2O5 content, Al2O3 particles are dispersed uniformly in the matrix. The hardness of the composites increases gradually, and the bending strength and fracture toughness of the composites reach to the maximum value, respectively.     

Effect of boron and carbon on the fracture toughness of IN 718 superalloy at room temperature and 650 °C

The effect of B and C microadditions on the fracture toughness of IN 718 superalloy was investigated at room temperature (RT) and at 650 °C. At RT, the fracture toughness was observed to increase with increasing B and C concentrations. C had a relatively weak effect on the fracture toughness at 650 °C, but the influence of B was significant. At RT the highest fracture toughness value was obtained for the alloy with 29 ppm B and 225 ppm C at RT, and at 650 °C the alloy with 60 ppm B and 40 ppm C had the highest fracture toughness. An increase in the concentration of B to 100 ppm, however, resulted in a reduction in the fracture toughness at 650 °C. Fractographic observations showed that the formation and coalescence of microvoids was the predominant fracture mechanism at RT. In contrast, at 650 °C, the fracture surface exhibited intergranular cracking in the alloy with lower B concentrations and transgranular cracking coupled with fine dimples in the alloy with higher B concentrations. It is suggested that B impedes intergranular cracking by increasing the cohesion of grain boundaries and improving the grain boundary stabilization. The RT increase in the fracture toughness of the material caused by the addition of C is attributed to the formation of intergranular and intragranular carbides that increased the resistance to the plastic deformation.     

MODELING OF ＇BANDING＇ MICROSTRUCTURE FORMATION IN CENTRIFUGALLY SOLIDIFIED Ti-6Al-4V ALLOY

Numerical investigations of the ＇banding＇ microstructure formation during solidification of Ti-6Al-4 V alloy in the centrifugal casting are conducted using a multi-scale model, which combines the finite difference method （FDM） at the macroscale with a cellular automaton （CA） model at the microscale. The macro model is used to simulate the fluid flow and heat transfer throughout the casting. The micro model is used to predict the nucleation and growth of microstructures. With the proposed model, numerical simulations are performed to study the influences of the nucleation density, mould rotation speed, and casting size upon the ＇banding＇ microstructure formation. It is noted that changing the nucleation density has a minor effect on the microstructure formation. The rotation speed promotes the formation of ＇banding＇ microstructure, which is more noticeable for larger size castings. The ＇major mechanism responsible for this ＇banding＇ phenomenon is the spatial variation in cooling rates created by centrifugal force.     

Mechanical alloying of dispersion-hardened Ni3Al-B from elemental powder mixtures

Mechanical alloying and hot extrusion were studied as a means to dispersion harden an intermetallic compound based on Ni ₃ Al- B from elemental powder mixture. The oxide used for the dispersoids was partially stabilized zirconia. During mechanical alloying the microstructure evolved according to the characteristic stages found in other mechanical alloying systems. Completion of the alloying reaction required 16 h, beyond which loss of the crystalline property set in. Experimental observation of the grain refinement during mechanical alloying agreed with a prediction based on an existing model. Compared to V- cone mixing, the mechanical alloying produced a homogeneous distribution of fine dispersoids. The refined grain structure and dispersoids resulted in a high tensile yield strength over a wide range of temperatures.     

The direct observation and investigation of the oxidation of aluminum in the transmission electron microscope

The authors studied the oxidation of thin aluminum films free of oxide layers in situ prepared by evaporation directly in the electron microscope under ultra-high-vacuum conditions. The oxidation was realized at various temperatures (350–500°C) and at various oxygen pressures (1–10^–3 Pa). The formation and growth of the amorphous and crystalline products have been studied.     

Microstructural Analysis of TiAl x N y O z Coatings Fabricated by DC Reactive Sputtering

TiAl _x N _y O _z coatings were prepared by DC reactive sputtering on AISI D2 tool steel substrates, using a target of Ti-Al-O fabricated from a mixture of powders of Ti (22.60 wt.%), Al (24.77 wt.%), and O (52.63 wt.%). The coatings were deposited on substrates at room temperature in a reactive atmosphere of nitrogen and argon under a pressure of 8.5 × 10⁻³ mbar. X-ray diffraction, electron dispersive spectroscopy, Raman scattering, and nanoindentation techniques were employed to investigate the coatings. The results show that the increment in the nitrogen flow affects the structure and the mechanical properties of the coatings. The sample with the lowest nitrogen flow presented the highest hardness (10.5 GPa) and the Young’s modulus (179.5 GPa). The hardness of the coatings TiAl _x N _y O _z as a function of crystalline grain size shows a behavior consistent with the Hall–Petch relation.     

A new approach on the determination of ease of machining by EDM processes

Recently, the ease of machining a workpiece by electrical discharge machining (EDM) processes has been determined by λ·θ theory, which is the product of the thermal conductivity (λ) and melting point (θ) of the workpiece in relation to the machining time. This paper presents a fundamental study of the total energy of discharge pulses required to machine different workpiece materials, and a new theory, referred to as λ·θ·ρ theory, is proposed, where ρ is the electrical resistivity of the workpiece. Unlike the λ·θ theory, the λ·θ·ρ theory includes the electrical resistivity of the workpiece material, because it involves the electric current transfer to create the discharge pulse. Using discharge pulse count monitoring, it was also revealed that the machining time cannot be used as a parameter to measure the ease of machining, since it is affected by many complications such as adhesion, cavitations, and short-circuiting. Parameters that are independent of machining complications are introduced in order to measure the ease of machining; the total energy of discharge pulses, discharge pulse number, average discharge pulse energy, discharge pulse density, and tool electrode wear. The results show that the coefficient of correlation for each parameter from the λ·θ·ρ theory is much higher than the λ·θ theory. Thus, the λ·θ·ρ theory is better than the λ·θ theory in determining the ease of EDM processes.     

The influence of cryogenic deformation on Bi2223 superconducting tapes

The influence of cryogenic deformation on Bi2223 superconducting tapes has been investigated in this paper. Mechanical deformation was carried out on Ag-sheathed mono- and multi- filamentary tapes and silver alloy-sheathed multi- filamentary tapes at liquid nitrogen temperature (77 K) and room temperature, respectively. Results show that the critical current of the Bi2223 tapes is significantly enhanced by cryogenic deformation. SEM microstructure observations show that the cryogenic-tapes have higher core density, smoother silver/superconductor interface, and better grain alignment. Tensile tests indicate that the tapes pressed at liquid nitrogen temperature have better tensile behavior than those pressed at room temperature. This article is based on a presentation in “The 7th Korea-China Workshop on Advanced Materials” organized by the Korea-China Advanced Materials Cooperation Center and the China-Korea Advanced Materials Cooperation Center, held at Ramada Plaza Jeju Hotel, Jeju Island, Korea on August 24–27, 2003.     

Synthesis and characterization of multilayer CrN/ZrN coatings

In this study, multilayer CrN/ZrN coatings with various bi-layer periods (δ) were deposited by closed-field unbalanced magnetron sputtering and their compositions, crystalline structures and morphologies were characterized by auger electron spectroscopy (AES), X-ray diffraction (XRD) and transmission electron microscopy (TEM). Additionally, the mechanical properties of the coatings were characterized by nano-indentation and wear tests. The XRD results showed that the diffraction peak of multilayer CrN/ZrN coatings is observable between the CrN (111) and the ZrN (111). This peak has a strong tendency to increase the intensity and sharpen the width as the substrate rotation speed increases, indicating that the grain size of the multilayer CrN/ZrN coatings gradually increases. The hardness of films was dependent on the bi-layer period (δ), and the maximum hardness and elastic modulus were approximately 31.8 GPa and 321.5GPa, respectively. Moreover, the results of the wear tests showed that the multilayer CrN/ZrN coatings exhibited greatly improved wear resistance compared to the monolithic CrN coatings.     

Bioleaching of pyrite by A. ferrooxidans and L. ferriphilum

Pyrite oxidation rates were examined under various conditions in the presence of A. ferrooxidans and L. ferriphilum, in which different pulp concentration, inoculation amount, external addition of Fe^3＋ and initial pH value were performed. It is found that A. ferrooxidans and L. ferriphilum show similar behaviors in the bioleaching process. The increasing pulp concentration decreases the leaching rate of iron, and external addition of high concentration Fe^3＋ is also adverse to leaching pyrite. The increased inoculation amount and high initial pH value are beneficial to leaching pyrite, and these changed conditions bring more obvious effects on leaching pyrite by L. ferriphilum than by A. ferrooxidans. The results also show that adjusting the pH values in leaching process baffles leaching pyrite due to the formed jarosite. Jarosite formed in leaching process was observed using XRD, SEM and energy spectrum analysis, and a considerable amount of debris with a crystalline morphology is present on the surface of pyrite. The results imply that the indirect action is more important for bioleaching pyrite.