Effect of Ternary Elements on the Oxidation Behavior of Aluminized TiAl Alloys

The effects of ternary elements added to TiAl on the coating layer formed by the pack-aluminizing process was studied with respect to oxidation resistance and mechanical properties. All the TiAl specimens, with various amounts of Nb, Cr, Fe, and V, were pack aluminized under identical conditions using a high-activity process. Among the ternary alloying elements tested, Nb showed the best property of the TiAl₃ coating layer formed on the surface and, consequently, the best oxidation resistance. The TiAl₃ coating layer becomes thicker and has a finer grain size as the content of Nb or Cr is increased. Microhardness tests revealed that the addition of Nb or Cr improved the toughness of the coating layer and thus improved the cracking resistance. Cyclic oxidation tests showed that the TiAl₃ coating layer formed on the TiAl alloy has better oxidation resistance with increasing Nb content. The ductility and oxidation resistance of the TiAl₃ coating layers improved with Nb addition, which contributes to the grain refinement of TiAl₃. The Nb present in the TiAl₃ coating layer inhibits grain growth by the solute-drag effect and retards inward diffusion of Al to the TiAl matrix by forming (Nb, Ti)Al₃ precipitates during high-temperature oxidation.     

镧对TiAl3金属间化合物显微组织及力学性能的影响

采用机械合金化及退火工艺制备镧改性的TiAl3金属间化合物.通过X射线衍射、粒度分析、透射电镜、硬度、延伸率及断裂韧性研究镧对TiAl3显微组织以及室温力学性能的影响.结果表明:添加镧可使DO22型TiAl3的晶体结构向有序立方Ll2型转化,断裂韧性最大提高143.75%,可有效改善其室温脆性.镧的加入使TiAl3的硬度有所损失,试样的室温维氏硬度为3450-3500 MPa.     

Sulfidation properties of TiAl–2 at.% X (X=V,Fe, Co,Cu, Nb,Mo, Ag and W) alloys at 1173 K and 1.3 Pa sulfur pressure in an H2S–H2 gas mixture

TiAl–2 at. % X (X=V, Fe, Co, Cu, Nb, Mo, Ag and W) alloys were sulfidized at 1173 K for 86.4 ks at a 1.3 Pa sulfur pressure in an H₂–H₂S gas mixture. The structure, phases, and compositions of the external sulfide scale and alloy surface layer were measured using EPMA and X-RD. The TiAl–2Ag and –2Cu alloys sulfidized faster than TiAl, and the alloy surface layer was thicker than that of TiAl. Sulfidation amounts of the TiAl–2X (X=V, Co, Fe, Mo, W and Nb) alloys were almost the same as that of TiAl, while the thickness of the alloy surface layer decreased in the order: V>Co>Fe>Mo>[Cr (by Narita T, Izumu T, Yatagai M, Yoshioka T. Intermetallics, 2000;8:371)]>W>Nb. The sulfide scale was composed of multi-layer structures: an outermost (rich in Ti-sulfides), an outer (rich in Al₂S₃), an inner (a mixture of Ti-sulfides and Al₂S₃), and an innermost (rich in Ti-sulfides) layer. The alloy surface layer also had a multi-layer structure, and was classified into four groups: group 1 for TiAl–2V and –2Co alloys as well as TiAl binary alloy where the surface layer consists of alloy substrate/TiAl₂/TiAl₃/sulfide scale, group 2 for TiAl–2Nb, –2Mo, and –2W (and also– 2Cr) alloys with alloy substrate/TiAl₂/TiAl₃/(Nb, Mo, W or Cr)–Al alloy/sulfide scale, group 3 for TiAl–2Cu and –2Ag alloys with alloy substrate/TiAl₂/Ti (Al, Ag or Cu)₃ with an L1₂ structure/TiAl₃/sulfide scale, and group 4 for TiAl–2Fe alloy with alloy substrate/TiAl₂/Ti(Al,Fe)₃ with an L1₂ structure/TiAl₃/FeAl₃/sulfide scale. Diffusion paths for these four groups were shown in a tentative Ti–Al–X ternary phase diagram.     

Strengthening behavior of beta phase in lamellar microstructure of TiAl alloys

β phase can be introduced to TiAl alloys by the additions of β stabilizing elements such as Cr, Nb, W, and Mo. The β phase has a body-centered cubic lattice structure and is softer than the α₂ and γ phases in TiAl alloys at elevated temperatures, and hence is thought to have a detrimental effect on creep strength. However, fine β precipitates can be formed at lamellar interfaces by proper heat treatment conditions and the β interfacial precipitate improves the creep resistance of fully lamellar TiAl alloys, since the phase interface of γ/β retards the motion of dislocations during creep. This paper reviews recent research on high-temperature strengthening behavior of the β phase in fully lamellar TiAl alloys.     

复相Al3Ti—8Mn基合金的显微组织

在Ａｌ－Ｔｉ－Ｎｂ－８Ｍｎ四元系邻近Ｌ１２型Ａｌ３Ｔｉ－８Ｍｎ成份区合金中,随Ｎｂ含量变化,其显微组织发生明显变化。通过对不同合金的微观组织的考察,进一步确认了在Ａｌ６７Ｍｎ８Ｔｉ２５基金中加Ｎｂ并相应调整Ａｌ和Ｔｉ含量可形成ＤＯ２２型Ａｌ３（Ｔｉ,Ｎｂ）,Ｌ１０型ＴｉＡｌ和Ｇａ２Ｈｆ型Ａｌ２Ｔｉ３种类型的第二相。本文列举了Ａｌ６７Ｍｎ８ｔＩ２３Ｎｂ２,Ａｌ６５Ｍｎ８Ｔｉ２６－５Ｎｂ０．５,Ａｌ５     

晶粒细化对Cr-25Nb合金高温氧化行为的影响

采用电弧熔铸和机械合金化+热压烧结技术制备晶粒尺寸相差较大的Cr-25Nb合金,研究其在950及1200 ℃空气中的氧化行为。结果表明,熔铸态及机械合金化Cr-25Nb合金氧化后均没有发生Cr的单一外氧化,而形成了以Cr2O3为外层、NbCrO4为内层的双层氧化膜结构;机械合金化Cr-25Nb合金在950及1200 ℃的氧化速度均小于熔铸态合金,特别是在1200 ℃氧化100 h后,熔铸态Cr-25Nb合金的氧化增重是机械合金化合金的2倍多。这主要是因为晶粒细化促进了氧化膜内应力的释放,提高了氧化膜与基体的粘附性     

溅射TiAlCr涂层对Ti-24Al-14Nb-3V抗氧化性能的影响

研究了溅射Ｔｉ５０Ａｌ１０Ｃｒ及Ｔｉ５０Ａｌ２０Ｃｒ涂层对Ｔｉ２４Ａｌ１４Ｎｂ３Ｖ抗高温氧化性能的影响。结果表明,在８００℃下,Ｔｉ５０Ａｌ１０Ｃｒ及Ｔｉ５０Ａｌ２０Ｃｒ涂层表面由于可形成粘附性良好的Ａｌ２Ｏ３膜,大大改善了Ｔｉ２４Ａｌ１４Ｎｂ３Ｖ的抗高温氧化性能。９００℃时在Ｔｉ５０Ａｌ１０Ｃｒ涂层表面长出大量ＴｉＯ２,导致其氧化增重较大;而在Ｔｉ５０Ａｌ２０Ｃｒ涂层表面生成了连续Ａｌ２Ｏ３,其抗氧化性能良好。然而,由于ＴｉＡｌＣｒ涂层与基体热膨胀系数不同,在循环氧化过程中涂层出现一些裂纹,导致其抗循环氧化性能有所下降。     

Isothermal oxidation behavior of powder metallurgy beta gamma TiAl–2Nb–2Mo alloy

A new TiAl–2Nb–2Mo beta gamma alloy was synthesized by powder metallurgy process. HIP’ed and vacuum heat treated specimens were isothermally oxidized at 800 °C and 900 °C in air up to 500 h. The TiAl–2Nb–2Mo alloy oxidized parabolically up to 500 h at both 800 °C and 900 °C. The oxides consisted of outer TiO₂ layer, intermediate Al₂O₃ layer, and inner TiO₂ rich mixed layer and the oxidation mechanisms of the alloy were identical at both temperatures. During oxidation, the degradation of lamellar colonies formed a diffusion zone just below the oxide/substrate interface consisting of γ-TiAl matrix and dispersed beta phases which contained high concentration of Nb and Mo. The oxidation rate of the TiAl–2Nb–2Mo alloy is more sensitive to temperature than those of the Ti–48Al–2Nb–2Cr and Ti–48Al–2Nb–2Cr–W alloys.     

Oxidation of Powder Metallurgy (PM) Ti–48%Al–2%Cr–2%Nb–(0–1%)W Alloys Between 800 and 1000°C in Air

Three powder metallurgy (PM) TiAl alloys with a fully lamellar structure were oxidized isothermally and cyclically between 800 and 1000°C in air in order to find the effect of W on the oxidation behavior of Ti–48Al–2Cr–2Nb alloys. The alloys oxidized parabolically during isothermal oxidation. Tungsten improved the isothermal and cyclic oxidation resistance. The oxide scales consisted primarily of an outer TiO₂ layer, an intermediate Al₂O₃-rich layer, and an inner TiO₂-rich layer. The alloying elements of Cr, Nb, and W tended to segregate in the lower part of the scale owing to their thermodynamic nobility. In the vicinity of the scale/matrix interface, TiN and Ti₂AlN coexisted.     

Plasma Alloying of TiAl with Niobium and Its Wear Resistance

将双层辉光离子渗铌技术应用于TiAl合金从而改善其耐磨性能。对TiAl渗铌合金层组织,室温和600℃高温摩擦学性能进行了研究。结果表明,TiAl经离子渗铌后表面形成厚度约12μm的铌合金层,渗层组织致密均匀,其组成物主要包括AlNb2、AlNb3、Ti2AlNb和Nb。铌合金层在600℃的耐磨性明显提高,并且室温耐磨和减摩性能均优于TiAl基材。     

STUDIES OF Nb–TiAl ALLOY BY POSITRON ANNIHILATION TECHNIQUES

利用正电子湮没寿命谱(PALS)和符合Doppler展宽(CDB)技术研究了Ti51Al49合金中Nb的掺杂效应.结果表明:低含量掺杂时,Nb原子主要偏聚在合金晶界处,提高了晶界位置的自由电子密度,有利于改善合金的室温韧性;而较高含量的Nb掺杂时,由于形成了新的晶体结构,合金基体及晶界处的自由电子密度减少,导致合金的脆性增加.     

用正电子湮没技术研究Zr和Nb在TiAl合金中的行为

测量了ＴｉＡｌ,Ｔｉ５０Ａｌ４８Ｚｒ２和Ｔｉ５０Ａｌ４８Ｎｂ２的正电子寿命谱,并利用正电子寿命参数分别计算了合金基体和缺陷态的自由电子密度。ＴｉＡｌ合金基体的自由电子密度比金属Ｔｉ和金属Ａｌ基体的低,当Ｔｉ和Ａｌ组成ＴｉＡｌ合金时,Ｔｉ原子和Ａｌ原子的部分价电子被局域化,ＴｉＡｌ合金中金属健和共键共存。ＴｉＡｌ合金晶界缺陷的开空间较大,晶界缺陷处的自由电子密度较低,金属键结合力较弱,材料易发生沿晶     

合金化韧化MoSi_2的研究

本文用XRD、SEM-EDS、EPMA、维氏硬度仪及压痕法研究了Nb、Al、Cr合金化对MoSi2组织结构、室温硬度及断裂韧性的影响.结果表明:2at%Al、4at%Al和2at%Cr三种合金均得到了单一四方Cllb相;6at%Nb合金因Nb含量高,导致部分四方C11b相转变为六方C40相,合金出现灰色Cllb基体相与亮白色层片状C40第二相双相组织,部分C40相中存在几乎平行排列的宽度贯穿裂纹.Nb、Al、Cr合金化都降低MoSi2的维氏硬度,提高断裂韧性,其中2at%Nb、4at%Al合金的断裂韧性最高,达到了5.90MPa·m1/2,提高了69%.     

不锈钢表面粉末包埋渗铝过程及渗铝层表征

采用固体粉末包埋法对00Cr17Ni14Mo2和1Cr18 Ni9Ti 不锈钢进行渗铝，形成富铝表层.阐述了不锈钢表面渗铝过程，并且对不锈钢渗铝层的成分、结构和形貌进行了表征.分析了合金中Ni元素对渗铝过程的影响.结果表明：渗铝层呈多层结构，渗层与基体及层间结合良好，界限明显、齐整.渗层组织主要由FeAl相组成，并含一定量的Ni3Al相.      

马氏体不锈钢堆焊层组织和耐冲蚀性能

采用2Cr13和经过V、Ti、Nb、N微合金化改性处理的1Cr13马氏体不锈钢进行埋弧堆焊试验,分析微合金元素和热处理工艺对马氏体不锈钢堆焊层显微组织和耐冲蚀性能的影响。研究结果表明:微合金元素V、Ti、Nb与C、N在奥氏体形核初期形成大量颗粒细小、弥散分布的稳定化合物,使堆焊层金属的晶粒得到细化;奥氏体晶粒稳定后,这些化合物又起到析出强化的作用;回火后,部分碳化合物在马氏体板条间析出,起到了二次强化的作用,同时降低马氏体中的碳含量,改善基体韧性。较好的基体强韧性配合最终提高了材料的耐冲蚀性。     

用正电子湮没技术研究Cr和Nb对TiAl合金中缺陷和d-d电子相互作用的影响

测量Al,Si,Ti,Cr,Nb等纯元素以及Ti50Al50,Ti50l48Cr2,Ti50Al48Nb2合金的符合正电子湮没辐射多普勒展宽谱和寿命谱,获得金属及合金中d电子和缺陷的信息.结果表明,二元TiAl合金的电子密度和3d电子的信号较低,晶界缺陷的开空间较大.在TiAl合金中加入Cr或Nb,合金中的d-d电子作用增强,基体和晶界处的电子密度均增加.Ti50Al48Cr2合金的多普勒展宽谱的d电子信号高于Ti50Al48Nb2合金.讨论了Cr和Nb对TiAl合金中缺陷和d-d电子相互作用的影响.     

Y, Nb对快速凝固TiAl合金组织和性能的影响

研究了元素Y和Nb对用熔体快淬法制备的TiAl基快速凝固合金组织及性能的影响。发现添加Y的快速凝固TiAl合金主要为等轴晶,主要组成相为α2和少量的γ相。随着Y含量的增加,γ相的含量增加,快速凝固TiAl合金的组织逐渐细化。不同Nb含量的快速凝固TiAl合金的组织为块状结构和层片状结构,主要由γ和α2两相组成,层片间距显著细化,为15～17nm。快速凝固TiAl合金的硬度比其铸态合金显著提高。     

Al、Si及Y多元合金化对Cr-20Nb合金高温氧化行为的影响

针对软第二相Cr稍微降低Laves相NbCr2合金的1200℃抗氧化性,采用Al、Si及Y多元合金化来提高Cr-20Nb合金的高温抗氧化性能。结果表明,多元合金化的Cr-20Nb合金1100℃及1200℃抗氧化性均好于加入单一合金化的及纯Cr-20Nb合金,并随着Si合金元素含量增加,Cr-20Nb合金的氧化增重变小,抗氧化性变好;SEM结果表明,添加合金元素后,氧化膜与基体的粘附性得到了明显提高。     

Investigation on the oxidation behaviour of gamma titanium aluminides coated with thermal barrier coatings

In the present study, the applicability of thermal barrier coatings (TBCs) on γ‐TiAl alloys was investigated. Two alloys with the chemical compositions of Ti‐45Al‐8Nb‐0.2B‐0.15C and Ti‐45Al‐1Cr‐6Nb‐0.4W‐0.2B‐0.5C‐0.2Si were used. Before TBC deposition, the specimens were pre‐oxidised in laboratory air or low partial pressure oxygen atmosphere. Yttria partially stabilised zirconia top coats were then deposited using electron‐beam physical vapour deposition (EB‐PVD). The oxidation behaviour of the γ‐TiAl specimens with TBC was studied by cyclic oxidation testing in air at 850 and 900 °C. Post‐oxidation analysis of the coating systems was performed using scanning electron microscopy with energy‐dispersive X‐ray spectroscopy (EDS). No spallation of the TBC was observed for pre‐oxidised specimens of both alloys when exposed to air at 850 °C for 1100 cycles of 1 h dwell time at high temperature. SEM micrographs of the thermally grown oxide scale revealed outer mixed TiO₂/Al₂O₃ protrusions with a columnar structure. The protrusions contained small particles of zirconia and a low amount of about 0.5 at% zirconium was measured by EDS analysis throughout this outer oxide mixture. The TBCs exhibited excellent adherence on the oxide scale. Intercolumnar gaps and pores in the root area of the TBC were filled with titania and alumina. Below the outer columnar oxide scale, a broad porous zone of predominant titania was observed. The transition region between the oxide scale and substrate consisted of a discontinuous nitride layer intermixed with alumina particles and intermetallic phases rich in niobium formed at the nitride layer/substrate interface. When thermally cycled at 900 °C, the oxide scales on the alloy Ti‐45Al‐8Nb‐0.2B‐0.15C pre‐oxidised in low partial pressure oxygen spalled off after 540 cycles. For the sample with TBC, spallation was observed after 810 cycles. Failure occurred in the thermally grown oxide near the oxide/nitride layer interface. Microstructural examinations revealed again oxide scales with columnar structure beneath the zirconia top coat and good adherence of the TBC on the thermally grown oxides formed at 900 °C.     

等离子渗Mo提高TiAl基合金抗高温氧化性能的研究

研究了等离子渗Mo处理对Ti-46.5Al-1.0V-2.5Cr(原子分数)合金抗高温氧化性能的影响。结果表明:等离子渗Mo处理能显著提高TiAl基合金的抗高温氧化能力,这主要是因为TiAl基合金经等离子渗Mo处理后在表面形成含Ti-Al-Mo的致密合金层,该致密合金层具有良好的抗高温氧化性能。