Ti40合金的超高温氧化行为研究

对Ti40合金进行1000~1500℃的超高温氧化实验。应用SEM、OM、XRD和EDS分析氧化表面以及氧化表面到基体的组织和元素的变化。结果表明,Ti40合金在1000~1500℃氧化过程中,氧化表层分别经历了V2O5的熔化蒸发、TiO_2晶粒的长大,以及表面氧化层由于TiO_2的生长应力而导致的开裂剥落的过程。当氧化层脱落后,原有的氧化次表层推至氧化表面,形成致密的、具有较好保护作用的TiO_2和Cr_2O_3混和氧化层。同时,在氧化层和基体界面依然稳定存在V、Cr元素的富集层。TiO_2、Cr_2O_3混和氧化层和V、Cr元素的富集层使得氧化表层脱落后合金依然具有良好的抗O元素扩散能力。     

Oxidation behavior of Ti3AlC2 at 1000-1400 °C in air

The isothermal oxidation behavior of bulk Ti₃AlC₂ has been investigated at 1000-1400 °C in air for exposure times up to 20 h by means of TGA, XRD, SEM and EDS. It has been demonstrated that Ti₃AlC₂ has excellent oxidation resistance. The oxidation of Ti₃AlC₂ generally followed a parabolic rate law with parabolic rate constants, k_p that increased from 4.1×10⁻¹¹ to 1.7×10⁻⁸ kg² m⁻⁴ s⁻¹ as the temperature increased from 1000 to 1400 °C. The scales formed at temperatures below 1300 °C were dense, adherent, resistant to cyclic oxidation and layered. The inner layer of these scales formed at temperatures below 1300 °C was continuous α-Al₂O₃. The outer layer changed from rutile TiO₂ at temperatures below 1200 °C to a mixture of Al₂TiO₅ and TiO₂ at 1300 °C. In the samples oxidized at 1400 °C, the scale consisted of a mixture of Al₂TiO₅ and, predominantly, α-Al₂O₃, while the adhesion of the scales to the substrates was less than that at the lower temperatures. Effect of carbon monoxide at scale/substrate was involved in the formation of the continuous Al₂O₃ layers.     

Si掺杂放电等离子合成Ti2AlC/Ti3AlC2材料及理论分析

以Ti粉、Al粉、活性炭和Si粉为原料,采用放电等离子工艺分别以摩尔比为2.0Ti/1.1Al/1.0C、2.0Ti/1.0Al/0.1Si/1.0C、2.0Ti/1.0Al/0.2Si/1.0C、2.0Ti/0.9Al/0.2Si/1.0C和2.0Ti/1.0Al/0.3Si/1.0C,在1 200 ℃合成了Ti2AlC/Ti3AlC2块体材料.通过合成试样的X射线衍射谱,确定了放电等离子合成试样的物相组成,并用扫描电镜结合能谱仪观察了合成试样的显微结构和微区成分.结果表明:以2.0Ti/1.1Al/1.0C为原料放电等离子合成了层状结构明显的Ti2AlC材料;掺Si后所有试样都由Ti2AlC、Ti3AlC2和Ti3SiC2 3种物相组成;当掺Si量逐渐增大,即Al与Si的量比减小时,试样中Ti3AlC2和Ti3SiC2的含量增加,而Ti2AlC的含量降低,同时颗粒得到细化.应用量子化学计算结果解释了掺Si后不利于Ti2AlC的生成,而有利于Ti3AlC2的生成机理,说明了掺Si后固溶体的产生过程.     

Microstructure and Residual Stress of Alumina Scale Formed on Ti2AlC at High Temperature in Air

Ti₂AlC ternary carbide is being explored for various high temperature applications due to its strength at high temperatures, excellent thermal-shock resistance, and high electrical conductivity. A potential advantage of Ti₂AlC over conventional Al₂O₃-forming materials is the near-identical coefficient of thermal expansion (CTE) of Ti₂AlC and α-Al₂O₃, which could result in superior spallation resistance and make Ti₂AlC a promising option for applications ranging from bondcoats for thermal barrier coatings to furnace heating elements. In this study, isothermal and cyclic oxidation were performed in air to examine the oxidation behavior of Ti₂AlC. Isothermal oxidation was performed at 1000, 1200 and 1400 °C for up to 25 h and cyclic oxidation consisted of 1,000 1-hour cycles at 1200 °C. Characteristics of the oxide scale developed in air, including mass change, residual stress in the α-Al₂O₃ scale, phase constituents and microstructure, were examined as functions of time and temperature by thermogravimetry, photostimulated luminescence, x-ray diffraction, scanning electron microscopy, and transmission electron microscopy via focused ion beam in situ lift-out. Above a continuous and adherent α-Al₂O₃ layer, a discontinuous-transient rutile-TiO₂ scale was identified in the oxide scale developed at 1000 and 1200 °C, while a discontinuous-transient Al₂TiO₅ scale was identified at 1400 °C. The continuous α-Al₂O₃scale thickened to more than 15 μm after 25 h of isothermal oxidation at 1400 °C, and after 1,000 1-hour cycles at 1200 °C, yet remained adherent and protective. The compressive residual stress determined by photoluminescence for the α-Al₂O₃ scale remained under 0.65 GPa for the specimens oxidized up to 1400°C for 25 hours. The small magnitude of the compressive residual stress may be responsible the high spallation-resistance of the protective α-Al₂O₃ scale developed on Ti₂AlC, despite the absence of reactive element additions.     

放电等离子烧结可加工Ti2AlC陶瓷的研究

以自蔓延高温合成（SHS）的Ti2AlC粉体为原料，利用放电等离子烧结技术（SPS）研究了Ti2AlC陶瓷的烧结制备。结果表明：烧结温度1250℃，压力20MPa，真空烧结，保温5min，可获得相对密度98．6％，维氏硬度为4．3GPa的致密烧结块体；烧结样品的维氏硬度随烧结温度升高而增大，但高于1250℃后随温度升高反而减小，SPS方法烧结Ti2AlC陶瓷的最佳温度为1250℃，当烧结温度≥1350℃时Ti2AlC分解；SEM分析表明，SPS技术烧结制备的Ti2AlC陶瓷片层尺寸随烧结温度的升高而增大。     

Effects of Chromia Coatings on the High-Temperature Behavior of F17Ti Stainless Steel in Air: Analytical Studies of the Effect of Rare-Earth-Element Oxides

The MOCVD deposition of neodymium oxide and/orchromium oxide provided beneficial effects both onisothermal- and cyclic high-temperature behavior ofcommercial F17Ti stainless steel. Fracture crosssections provided information about the morphology ofthe oxide scales formed on bare steel and coatedspecimens. The chromia scales developed small equiaxedgrains on the Nd₂O₃-coated samplesand columnar grains on the uncoated ones. Neo dymium segregatedwithin a surface layer composed ofMn_1.5Cr_1.5O₄ spineloxide. A complex phase (close to the structure ofCeTi₂₁O₃₈) was identified in thiszone. It could act as a source of neodymium ions, which couldsegregate to the grain boundaries of the chromia scale.Polished cross sections associated with X-ray mappingstudies confirmed the scale structure and the location of the rare-earth element in the outer part ofthe oxide layer.     

SHS/PHIP法制备Ti3AlC2可加工陶瓷的塑性特征

研究了用SHS/PHIP技术制备出的Ti3AlC2可加工陶瓷的塑性变形特征。应变速率为1×10^-3/s,从室温到1300℃的压缩实验结果表明,室温到800℃的压缩断裂方式为脆性断裂,但存在显微塑性。主裂纹的偏转与分岔、晶粒的分层与扭折是主要变形机制;1000℃到1300℃,位错运动带来了塑性流变的结果。800℃到1000℃被称为韧脆转变温度区间,在此温度区间以上的应力与应变曲线存在着“硬化区域”,并且随着温度的升高,“塑性区”要大于“硬化区”。     

Oxidation Behavior of a Ti3Al–Nb Alloy with Surface Thin Oxide Films

Thin films of aluminum, cerium, and yttrium oxides were applied onto the surfaces of Ti₃Al–11Nb samples using an electrodeposition technique. The oxidation behaviors of the Ti₃Al–Nb alloy, with and without these surface-applied films, were studied in air at 800–1000°C. The results showed that the oxidation rate of the alloy can be reduced by Ce oxide and Y oxide films, and the greatest improvement comes from oxidation of the Y oxide-coated specimens at 800°C. With increasing oxidation temperature, the difference between the Co-oxide and Y-oxide films becomes smaller. The results also indicated that the Ce-oxide and Y-oxide films can significantly improve the oxide scale-spallation resistance. On the other hand, Al-oxide films result in detrimental effects on the oxidation and scale-spallation resistance of the Ti₃Al–Nb alloy. Based on the experimental results, the effects of the different surface films on the oxidation mechanism are discussed.     

Ti3AlC2陶瓷在空气中的高温氧化行为

采用热压烧结法,在1450 ℃保温2 h制备Ti3 AlC2陶瓷,研究其在1000 ℃空气中的断续氧化行为.结果表明,Ti3AlC2在1000 ℃的空气中氧化30 h后,动力学曲线服从抛物线规律.氧化层厚度约为350 μm,由多层组成,从外到内依次为TiO2的富集层,TiO2和Al2O3的富集层,TiO2、Al2O3以及还有少量孔洞层,Al2O3富集层.整个氧化过程主要由Al3+和Ti4+的向外扩散和O2-的向内扩散组成.     

Ti3AlC2/Cu复合材料的制备及其性能研究

采用粉末冶金的方法在1000℃和30 MPa的热压条件下,烧结制备了以Ti3AlC2为增强相的Ti3AlC2/Cu复合材料,研究了增强相含量(10%～40%)对复合材料的显微结构、抗弯强度、硬度和电阻率的影响.结果表明:Ti3AlC2能够有效增强铜,当Ti3AlC2含量为30%时,增强效果最佳,复合材料的抗弯强度达1033 MPa,最大形变为2.5%,增强相含量继续增加时,复合材料的强度反而降低;随着增强相含量的增加,复合材料渐趋脆性断裂,同时复合材料的电阻率基本呈线性升高.     

Ti3AlC2-Al2O3-TiAl3 composite fabricated by reactive melt infiltration

Porous preforms were fabricated by cold-pressing process using powder mixture of TiC, TiO₂ and dextrin. After pyrolysis and sintering, Al melt was infiltrated into the porous preforms, leading to the formation of Ti₃AlC₂-Al₂O₃-TiAl₃ composite. Effects of cold-pressing pressure of preforms on microstructures and mechanical properties of the composites were studied. Synthesis mechanism and toughening mechanism of composite were also analyzed. The results shows that TiO₂ is reduced into Ti₂O₃ by carbon, the decomposition product of dextrin, which causes the spontaneous infiltration of Al melt into TiC/Ti₂O₃ preform. Then, Ti₃AlC₂-Al₂O₃-TiAl₃ composite is in-situ formed from the simultaneous reaction of Al melt with TiC and Ti₂O₃. With the increase of cold-pressing pressure from 10 MPa to 40 MPa, the pore size distribution of the preforms becomes increasingly uniform after pre-sintering, which results in the reduction of defects, and the decrease of property discrepancy of composites. Nano-laminated Ti₃AlC₂ grains and Al₂O₃ particles make the fracture toughness of TiAl₃ increase remarkably by various toughening mechanisms including stress-induced microcrack, crack deflection and crack bridging.     

Strengthening of Ti2AlC by substituting Ti with V

Ti₂AlC is strengthened by substituting Ti with V to form (Ti,V)₂AlC solid solutions. The Vickers hardness, flexural strength, shear strength and compressive strength are enhanced by 29%, 36% and 45% for (Ti_0.8,V_0.2)₂AlC solid solution, respectively. The strengthening mechanism is discussed.     

High-Temperature Oxidation of Ni Coated with La2O3 by Atomic-Layer Chemical-Vapor Deposition (ALCVD)

The effects of superficial (30–100 nm) La₂O₃ surface coatings on the oxidation kinetics of Ni from 700 to 1100°C in air and the oxide morphology of the NiO scales have been investigated. The parabolic rate constant is lower than for uncoated Ni by a factor of 5 to 10. The oxide morphology changes with the La₂O₃ coatings: The oxide scale consists of an outer fine-grain layer with an inner region of coarser, but still equiaxed, grains. SIMS shows that the majority of the La remains at the surface where a highly oxygen-defective spinel, La₂Ni₄O₇, was found by TEM. Two-stage oxidation followed by SIMS profiling reveals that the oxide growth occurs inside the scales.     

Ti2AlC陶瓷的电化学腐蚀性能研究

将原料粉体按TiC：Ti：Al：C=0．5：1．5：1：0．5的配比充分混合，在石墨模具中加压到30MPa，升温至1400℃保温2h后自然冷却到室温，得到高纯致密的Ti2AlC烧结体。用电化学方法测定了高纯Ti2AlC在不同腐蚀介质中的极化曲线。结果表明，Ti2AlC在HCl中的腐蚀电流密度最大，腐蚀严重；而在NaOH中自腐蚀电压最负，腐蚀趋势最大；除了在HCl中表现为活性溶解外，在其它的腐蚀介质中均出现钝化现象。     

Cyclic-Oxidation Behavior of Ti3SiC2-Base Material at 1100°C

The cyclic-oxidation behavior of Ti₃SiC₂-base material was studied at 1100°C in air. Scale spallation and weight loss were not observed in the present tests and the weight gain would just continue if the experiments were not interrupted. The present results demonstrated that the scale growth on Ti₃SiC₂-base material obeyed a parabolic rate law up to 20 cycles. It then changed to a linear rate with further increasing cycles. The scales formed on the Ti₃SiC₂-base material were composed of an inward-growing, fine-grain mixture of TiO₂+SiO₂ and an outward-growing, coarse-grain TiO₂. Theoretical calculations show that the mismatch in thermal expansion coefficients between the inner scale and Ti₃SiC₂-base matrix is small. The outer TiO₂ layer was under very low compressive stress, while the inner TiO₂+SiO₂ layer was under tensile stress during cooling. Scale spallation is, therefore, not expected and the scale formed on Ti₃SiC₂-base material is adherent and resistant to cyclic oxidation.     

Oxidation Behavior and Breakdown of an Aluminide Coating on DZ40M Alloy at High Temperatures

DZ40M alloy is a new Co-base superalloy, which is suitable for the blade material of gas turbines. In this paper, isothermal oxidation of an aluminide coating on this alloy was examined at 900–1100°C in air. It was observed that the weight gain at lower temperatures (900 and 1000°C) was greater than that at the higher temperature (1050°C), which was due to the formation of both -Al₂O₃ and -Al₂O₃ at 900 and 1000°C but only -Al₂O₃ at 1050 and 1100°C.     

Oxidation of Cr2AlC coatings in the temperature range of 1230 to 1410 °C

The isothermal oxidation behavior of Cr₂AlC coatings on alumina substrates was investigated in the temperature range of 1230 to 1410 °C. The structure, surface morphology, microstructure evolution and chemistry of the reaction products have been investigated. In the investigated temperature range, the Cr₂AlC films form a dense continuous oxide scale consisting of α-Al₂O₃ on Cr carbides. The oxidation rates determined by thermo gravimetric analysis (TGA) were parabolic, indicating that diffusion through the scale is the rate limiting mechanism. The activation energy for oxidation was determined to be 348 kJ mol^− 1 and the parabolic rate constant at 1230 °C was 7.1 × 10^− 10 kg² m^− 4 s^− 1. Hence, the oxidation behavior is comparable to NiAl in the temperature range and time intervals investigated. With increasing oxidation time voids form at the interface between oxide and Cr carbides and the amount of Cr₇C₃ increases at the expense of Cr₃C₂. Based on our thermodynamic calculations the oxygen partial pressure below the oxide scale increases as Al is depleted and Cr carbides oxidize, resulting in CO gas- and Cr₂O₃-formation. The formation of gas may together with the depletion of Al and Cr lead to the significant void formation observed in the Cr carbide interlayer. Observation of both Cr carbide precipitates and the formation of (Al,Cr)₂O₃ solid solution support this notion. For comparison bulk Cr₂AlC was oxidized. It is argued that the absence of pores in oxidized bulk Cr₂AlC is due to the considerably larger amount of Al available.     

High-Temperature Oxidation Behavior of the Co-Base Superalloy DZ40M in Air

The oxidation behavior of the Co-base superalloy DZ40M was studied in air at900–1100°C for times of up to 2000 hr. The results indicated thatthis alloy can grow a protective oxide scale at 900 and 1000°C duringisothermal oxidation, but not at 1100°C because of serious cracking andspalling of the oxide scales. Moreover, an internal-precipitate zone formedin the subsurface region of the alloy at all temperatures and times. Theprecipitates were rich in Cr in the vicinity of the alloy–scaleinterface and rich in Al deep in the alloy. The internal-precipitatemorphology changed from a granular to needlelike shape with increasingoxidation temperature.     

Effect of SI Content on the Oxidation Resistance of Ti3Al1-x Si x C2 (x⩽ 0.25) Solid Solutions at 1000–1400°C in Air

The oxidation behavior of Ti₃Al_1-x Si _x C₂ (x ⩽ 0.25) solid solutions was investigated in flowing air at 1000–1400°C for up to 20 hrs. Similar to Ti₃AlC₂, Ti₃Al_1-x Si _x C₂ (x⩽ 0.15) solid solutions display excellent oxidation resistance at all temperatures because of the formation of the continuous α-Al₂O₃ protective layers. However, Al₂(SiO₄)O formed during oxidation of Ti₃Al_1-x Si _x C₂ (x=0.2 and 0.25) solid solutions at and above 1100°C, which is believed to be responsible for the deterioration of oxidation resistance of Ti₃Al_0.75Si_0.25C₂ at 1300°C. Additionally, Ti₅Si₃ was also found in the oxidized samples. This implies that Ti₅Si₃ precipitated from Ti₃Al_1-x Si _x C₂ solid solutions during oxidation. But it has been proven that Ti₅Si₃ has little effect on the oxidation resistance of the material, which is attributed to an interstitial carbon effect.     

TiAl3对燃烧合成Ti3AlC的影响

以单质粉末Ti，Al和碳黑为原料，研究了添加金属间化合物TiAl3对燃烧合成Ti3AlC的影响。实验结果表明，仅以单质粉末Ti，Al和碳黑为原料，按Ti3AlC化学计量比配料，燃烧产物主要物相是Ti2AlC和TiC，无Ti3AlC。但在保持原料配比不变的情况下，在反应物原料中添加金属间化合物TiAl3（0～23．5％，质量分数）后，可得到Ti3AlC相物质，其含量随TiAl3的增加而显著增多，成为燃烧产物的主要物相之一。从动力学和热力学角度探讨了TiAl3对燃烧合成Ti3AlC的影响机理。