Effect of boron addition on tensile ductility in lamellar TiAl alloys

Various cast or wrought fully lamellar TiAl-based alloys with and without boron addition have been assessed. It has been found that titanium boride precipitates are the predominant factor influencing the room temperature tensile ductility. Large sized titanium boride precipitates often observed in high-alloyed TiAl alloys (such as Ti–44Al–8Nb–1B) cause premature failure in as-cast samples through promoting crack propagation via debonding between boride-matrix interfaces or cracking through boride precipitates themselves, giving rise to a typical tensile ductility of 0.3%. Refinement in titanium boride precipitates, via hot working or fast cooling during casting, will significantly improve the tensile ductility. In low-alloyed alloys (such as Ti–48Al–2Cr–2Nb–1B) the effect of boride precipitates is not as significant as it is in the high-alloyed alloys mainly because of their small sizes.     

Microstructure of diffusion-brazing repaired IN738LC superalloy with uneven surface defect gap width

Slots with uneven width were cut by femtosecond laser in small plates of IN738LC superalloy to imitate service cracks. The ‘cracks’ were repaired by diffusion brazing using BNi-1a or a mixed filler alloy at 1100°C. The joint region was composed of isothermal solidification zone (ISZ), diffusion affected zone (DAZ) and precipitate zone (PZ). The compositions were different between the upper and lower ISZ due to the variation of gap width. The sample, repaired with two kinds of filler metals, had similar DAZ microstructure. PZ of mixed filler alloy bonded sample had a similar microstructure with that of BNi-1a, but less borides. The maximum gap sizes of complete isothermal solidification were almost the same for different filler alloys, followed a square root relationship with time. However, PZ of BNi-1a bonded was larger, resulting from a more base metal dissolution. The relationship between the PZ, ISZ and crack width is discussed.     

Infrared joining strength and interfacial microstructures of Ti–48Al–2Nb–2Cr intermetallics using Ti–15Cu–15Ni foil

Infrared joining of Ti–48Al–2Nb–2Cr using Ti–15Cu–15Ni (wt%) foil as brazing filler metal was investigated at the temperature range of 1100∼1200°C for 30∼60 s in a flowing argon environment. The compressive tests show three types of fracture morphologies in which type I fails at the joint interface, but types II and III are fractured in the base-metal with the crack direction parallel to and perpendicular to the loading axis, respectively. Most of joined specimens were fractured through the base metal indicating that the infrared joined interface has relatively good joint strength. The compressive strength of type I specimen is about 319–322 MPa. Experimental results show that the shorter the real holding time or the higher the joining temperature, the larger the strength variation will be. The observed interfacial microstructures of Ti–48Al–2Nb–2Cr joint interfaces indicate that seven characteristic zones can be distinguished in the joint interfaces and each characteristic structure corresponds to one or more stable phases at T_w temperature. The observed microstructures and their evolutions of each zone are explained in detail in this study. The major difference between joint interfaces of Ti–48Al–2Nb–2Cr and Ti₅₀Al₅₀ alloys takes place on the base-metal interface zone and the columnar two-phase zone. The existence of Nb and Cr atoms in Ti–48Al–2Nb–2Cr alloy also has some influences on the microstructural evolution of the columnar two-phase zone and the continuous α₂-layer.     

化学成分对GH4169合金组织与力学性能的影响

针对不同Nb、Al、Ti含量的GH4169合金,对其进行固溶时效热处理,研究微观组织与拉伸性能的变化规律。研究表明,一定的Al/Ti和(Al+Ti)/Nb值下,随Nb含量增加,δ相含量增加,并且具有较好的高温稳定性,高温固溶可有效阻止晶粒的长大,主要通过稳定的δ相来保证合金的细晶强化。当Al/Ti与(Al+Ti)/Nb比值较低时,时效后γ″、γ'含量有所降低,γ″相形貌由唇状变为圆盘或芝麻粒状,其中Nb元素主要形成δ相。力学性能表明,Al/Ti和(Al+Ti)/Nb值一定时,拉伸强度主要受晶粒尺寸影响,而δ相含量影响较小。但当Al/Ti与(Al+Ti)/Nb比值较低时,由于强化相γ″和γ'数量与形貌的变化,拉伸力学性能显著降低,其微观硬度也随拉伸强度的降低而降低。     

Grain refinement in coarse-grained heat-affected zone of Al–Ti–Mg complex deoxidised steel

The particles and microstructure characteristics of coarse-grained heat-affected zone (CGHAZ) in Al–Ti–Mg killed steels with different Al contents were investigated. The results show that inclusion in high Al steel consists of Al–Mg oxide surrounded with a layer of MnS. However, inclusion in low Al steel is Al–Ti–Mg oxide covered with a layer of MnS, effectively promoting the formation of acicular ferrite. The precipitates of both steels are (Ti, Nb)N, the finer and more dispersed inclusions and precipitates in low Al steel can effectively inhibit austenite grain growth by grain boundary pinning during the thermal cycle. The fine-grained microstructure is obtained in CGHAZ of the low Al steel due to the pinning effect of finer particles and the high density of acicular ferrite.     

Effects of heat shock at 800 °C on mechanical properties of γ-TiAl alloys

Effect of heat shock on the mechanical properties and the microstructure of TiAl alloys (Ti–47Al–2Cr–2Nb and Ti–45.3Al–2Cr–2Nb–0.1W–0.15B) with near fully lamellar structure were investigated. After heat shock process from room temperature to 800 °C for 500 cycles, the microstructure demonstrated that lamellar microstructure has been destructed by the presentation of some γ and α₂ block phases in lamellar structure, resulting in the ductility of as-polished Ti–47Al–2Cr–2Nb alloy decreased by about 70% and the ultimate tensile strength (UTS) reduced by about 25%, and the ductility of as-unpolished Ti–47Al–2Cr–2Nb alloy decreased by more than 70% and the ultimate tensile strength (UTS) was reduced by about 35%. The ductility of Ti–45.3Al–2Cr–2Nb–0.1W–0.15B alloy decreased by about 60% and the ultimate tensile strength (UTS) reduced by about 18% after heat shock, which was resulted from the appearance of small α₂ block phase at interfaces of lamellar colonies and microcracks at the interfaces of ribbon boride and lamellar structure.     

Grain refinement of DHP copper by particle inoculation

Abstract

The viability of grain refining phosphorus deoxidised, high residual phosphorus Cu (DHP-Cu) by particle inoculation through the formation of in situ Zr, Ti, Nb, La, Ce, Gd and Nd oxide particles, additions of WC and NbB₂ particles as well as additions of Ti+B, Zr+B, Al+B and Ca+B with the purpose of forming in situ boride particles have been investigated under TP-1 casting conditions. Nominal additions of P in the range of 300–690?wt?ppm and of Ag of 150?wt?ppm have also been explored. In comparison, it appears that it is more difficult to grain refine DHP-Cu by particle inoculation than it is by the combined addition of P+Ag solute elements to a residual content of 396?ppm P and 143?ppm Ag (and having very low contents of Ti, Zr, Al, Ca and B microalloying elements) for the conditions studied.     

Microstructural characterization in 7 at% Al-containing NiTi-based alloys

The microstructural evolution of Ni–42Ti–7Al and Ni–41Ti–7Al alloys as a function of solution and aging heat treatment was investigated using transmission electron microscopy（TEM）, electron probe, and X-ray diffraction（XRD）. The results reveal that the volume fraction of Ti2 Ni phase as well as its composition does not change significantly after as-solution treated at 1200 °C and aged at 850 °C. At the early stage of the aging treatment at 850 °C for 1 h, the cuboidal β＇ precipitate keeps coherency with the matrix; further aging, β＇ precipitate coarsens, and the semicoherency between the β/β＇ two phases are observed.The shape of coarsened β＇ precipitates changes to the globule, and the interface dislocations are introduced accompanied by the occurrence of semicoherent precipitates. Under the same heat treatment, compared to the Ni–42Ti–7Al alloy, the lattice misfits of the Ni–41Ti–7Al alloy between the β and β＇ two phases are larger, so the β＇ precipitates in Ni–41Ti–7Al alloy are coarsened severely and easily lose coherency with the matrix. The thermal stability of Ni–41Ti–7Al alloy is much worse when aging at 850 °C.     

Crack propagation behavior in TiAl–Nb single and Bi-PST crystals

The fracture toughness of directional solidified Ti–(45,47)Al–3Nb, Ti–(45,47)Al–3Nb–0.2Si–0.1C, Ti–(45,47)Al–3Nb–0.3Si–0.2C type I alloys and their contribution to crack growth resistance of TiAl–Nb alloys were studied using PST (polysynthetically twinned) crystals produced by directional solidification in FZ (floating zone) furnace. Lamellar orientations in the individual colonies are described using two angles defined with respect to the notch orientation: an in-plane kink angle and a through-thickness twist angle. Therefore, lamellar misorientation across an individual colony boundary is quantified as differences in these angles across the boundary. Crack growth resistance in colony boundary was identified by three-point bend test and crack advance was monitored by interrupted in situ test. From three-point bend test, it was found that the colony boundary could offer significant resistance to crack growth under large twist angle difference. Fracture toughness of type I specimens (in which crack propagates against lamellae boundaries) of the alloys decreased slightly with increasing Si and C contents and increased rapidly with decreasing Al content. The toughness for type I specimens was controlled by α₂–α₂ spacing in which the delamination-type separation occurred. Compared to 47Al alloys, α₂–α₂ spacing in 45Al alloys increased by decreasing Al content, therefore, fracture toughness increased rapidly. These results are discussed and the ability to improve toughness by changing Al content, Si and C addition in TiAl–Nb alloys produced by directional solidification is suggested.     

Numerical study of convection-induced peritectic macro-segregation effect at the directional counter-gravity solidification of Ti–46Al–8Nb alloy

Unidirectional counter-gravity (upward) melt solidification of Ti–46Al–8Nb (at%) intermetallic alloy was performed in the three-zone resistive tube electro-furnace with power-down thermal profile operation. The laboratory refinement of cylindrical ingots growth technique was developed in course of terrestrial preparation experiments in the facility specially designed for a sounding rocket flight. Despite Ti–46Al–8Nb is nominally slightly pro-peritectic composition, an axial elongated channel-like area with peritectically-transformed microstructure was observed in solidified ingots, where Al content locally exceeds 47 at%. For revealing the reasons of this microstructural inhomogeneity formation, the numerical modeling was applied. The real-time-scale 2D temperature field mapping, macro-scale study of melt hydrodynamics, heat–mass transfer, segregation effects, mushy zone evolution and solidification dynamics of TiAl–Nb melt/solid system have been performed accordingly. It was found that appearance of peritectic axial “spindle” in the solid is induced by the joint action of two factors in the melt: (i) rejection of Al solute ahead of the concave growth interface (dendrite tips front); (ii) development of weak laminar thermo-gravitational convective flow that picks-up, pulls upward and stratifies an Al-enriched stream along the axis of melt column. The driving force of convection is a radial thermal gradient that depends on the furnace operational thermal conditions. To prevent the segregation, it was shown numerically that single regular convective cell could be broken into several cells by the appropriate variation of power-down cooling rate. The resulting uniform as-solidified microstructure of alloy obtained confirms the modeling findings.     

The effect of annealing on microstructure and tensile properties of Ti–22Al–25Nb electron beam weld joint

This study investigated the microstructure evolution and tensile properties of Ti–22Al–25Nb EBW joints. The fusion zone of the as-welded joint exhibited a fully B₂ microstructure. Widmanstätten O particles precipitated out of B₂ matrix after annealing and their size increased within a temperature range from 750 °C to 900 °C. In the heat affected zone, there was a transition of microstructure moving away from the fusion zone towards the base material. Strength and elongation of the as-welded sample were significantly improved after annealing, which was attributed to the strengthening effect of O precipitates and the slip transmission between O and B₂ phases. Samples tensile tested at 650 °C all failed within the fusion zone and exhibited intergranular failure instead of transgranular failure at room temperature. The room temperature strength and hardness of the joints decreased with annealing temperature due to the coarsening of O precipitates. At 650 °C, failure occurred by intergranular fracture in the fusion zone and the joint strength of all annealed samples was similar due to similar B₂ grain boundary strength.     

Oxidation behavior of Ti–46Al–8Nb alloy with boron and carbon addition under thermal cycling conditions

Oxidation behavior of Ti–46Al–8Nb (in at.%) alloy with boron and carbon addition under thermal cycling conditions was investigated. Oxidation of Ti–46Al–8Nb, Ti–46Al–8Nb–1B and Ti–46Al–8Nb–1B-0.25C (in at.%) alloys was carried out at 1073 K in laboratory air for 42 cycles (1 cycle, 24 h), 1008 h in total. The mass loss rates of Ti–46Al–8Nb and Ti–46Al–8Nb–1B measured during the oxidation were similar. The best oxidation resistance was found for Ti–46Al–8Nb–1B-0.25C alloy with the smallest mass change. XRD and SEM-EDS investigations showed that in all cases, the oxide scales compositions were substantially similar. The scale consisted of an outer layer built of Al₂O₃ with the presence of some amounts of TiO₂, an intermediate layer of the scale consisting of TiO₂, an inner layer composed of oxides and nitrides. Additionally, niobium rich particles at the scale/substrate interface were present. The oxidation mechanism of Ti–46Al–8Nb was studied via two-stage isothermal oxidation (24 h in ¹⁶O₂ followed by 24 h in ¹⁸O₂) at 1073 K combined with secondary neutral mass spectroscopy (SNMS). These results indicate that the oxidation mechanism depends on a mixed diffusion process, consisting of outward transport of cations and simultaneous oxygen inward transport.     

Effect of Cu on the evolution of precipitation in an Fe–Cr–Ni–Al–Ti maraging steel

The evolution of precipitates in an Fe–Cr–Ni–Al–Ti stainless maraging steel alloyed with Cu was investigated during aging at 525 °C. Atom probe tomography was used to reveal the development of precipitates and to determine their chemical composition. Two types of precipitates were observed to form during the aging process. Based on their chemical composition these are assumed to be NiAl B2 and Ni₃(Ti,Al) (η-phase). The two phases of precipitates were found to develop independently of each other and the addition of Cu was found to accelerate precipitation. However, the effect of Cu on the nucleation of these phases is different: on the one hand, in the case of NiAl, Cu is incorporated and thus reduces the activation energy by reducing the lattice misfit; on the other hand, Cu acts as a nucleation site for the precipitation of Ni₃(Ti,Al) by forming independent Cu clusters.     

Microstructure of Vacuum-Brazed Joints of Super-Ni/NiCr Laminated Composite Using Nickel-Based Amorphous Filler Metal

Vacuum brazing of super-Ni/NiCr laminated composite and Cr18-Ni8 stainless steel was carried out using Ni-Cr-Si-B amorphous filler metal at 1060, 1080, and 1100 °C, respectively. Microstructure and phase constitution were investigated by means of optical and scanning electron microscopy, energy-dispersive spectroscopy, x-ray diffraction, and micro-hardness tester. When brazed at 1060-1080 °C, the brazed region can be divided into two distinct zones: isothermally solidified zone (ISZ) consisting of γ-Ni solid solution and athermally solidified zone (ASZ) consisting of Cr-rich borides. Micro-hardness of the Cr-rich borides formed in the ASZ was as high as 809 HV_50 g. ASZ decreased with increase of the brazing temperature. Isothermal solidification occurred sufficiently at 1100 °C and an excellent joint composed of γ-Ni solid solution formed. The segregation of boron from ISZ to residual liquid phase is the reason of Cr-rich borides formed in ASZ. The formation of secondary precipitates in diffusion-affected zone is mainly controlled by diffusion of B.     

Zr对高强度钢热影响区第二相粒子及韧性影响

研究了0.012 4%锆对低合金高强度钢焊接热影响区粗晶区第二相粒子和冲击韧性的影响.结果表明,模拟20 kJ/cm焊接线能量下无锆钢焊接热影响区粗晶区中第二相粒子为Al-Ti复合氧化物和（Ti,Nb） N析出物.而含锆钢则是Zr-Al-Ti复合氧化物及（Al,Ti,Nb） N和（Ti,Nb） N析出物.同时,定量数据分析表明含锆钢中氧化物和氮化物粒子密度更高且尺寸更加细小.这些高密度的细小的第二相粒子在焊接过程中能有效钉扎晶界移动,抑制奥氏体晶粒粗化,在焊接热影响区粗晶区中得到尺寸相对细小均匀的原奥氏体晶粒,使得含锆钢焊接热影响区粗晶区呈现韧性断裂和极好的低温冲击韧性.     

Combined ab initio and experimental study of A2 + L21 coherent equilibria in the Fe–Al–X (X = Ti,Nb, V) systems

A combined theoretical and experimental study of ordering and phase separation in α-Fe alloys in the Fe–Al–X (X = Ti, Nb, V) systems is presented. The theoretical part is dedicated to the assessment of the BCC-based phase equilibrium diagram in the iron-rich zone of the ternary systems via a truncated cluster expansion, through the combination of Full-Potential-Linear augmented Plane Wave (FP-LAPW) electronic structure calculations and of Cluster Variation Method (CVM) thermodynamic calculations in the irregular tetrahedron approximation. The stability and the solid solubility of Al in the Fe₂X Laves phases are also included in the discussion of the ternary BCC ground state diagrams. The approach was employed in order to explore particular vertical sections of the ternary systems where a coherent two-phase microstructure can be generated with an optimal combination of volume fraction of Fe₂AlX (L2₁) Heusler type precipitates and Al content in the α-Fe (A2) matrix. The results indicate that in the Fe–Al–Ti and Fe–Al–V systems there are two kinds of phase separations of the BCC phase, (A2+ L2₁) and (B2 (FeAl structure) + L2₁). A tie-line separates both two-phase fields that shrinks and moves toward the Fe-X binary system while its direction remains almost parallel as the temperature increases. Selected experiments were performed on three alloys of the Fe–Al–Ti system belonging to vertical sections that contain this tie-line. The microstructure and composition of the matrix and precipitate phases were characterized by transmission electron microscopy (TEM) and Energy Dispersive Spectroscopy (EDS), the theoretical predictions were borne out.     

Oxidation behavior of titanium aluminides of high niobium content

The oxidation behaviour of two titanium aluminides of composition Ti–24Al–20Nb and Ti–24Al–27Nb were studied in oxygen in the range 1125–1325 K. The parabolic rate constants of the alloys in the present investigation were higher than that of titanium aluminides of lower Nb content [Ti–24Al–11Nb and Ti–24Al–15Nb (Roy TK, Balasubramaniam R, Ghosh A. Metall Mater Trans A 1996;27A:3993)]. The major constituent of the oxides was TiO₂ while Nb₂O₅ and Al₂O₃ were the minor constituents. The formation of Nb₂O₅ as a separate phase is possibly related to the lower oxidation resistance of these alloys compared to titanium aluminides of lower Nb content.     

激光立体成形Rene88DT高温合金γ′相的高温粗化行为研究

采用激光立体成形技术(LSF)制备Rene88DT高温合金,对其在760～840℃温度区间进行高温短时(4～16h)时效处理,采用微观测试分析方法对高温短时时效处理后γ′相形态、尺寸变化及粗化动力学行为进行了研究。结果表明:激光立体成形Rene88DT高温合金在高温短时时效条件下,γ′相分布均匀,形态基本为球形,时效温度对γ′相的影响比时效时间更为显著;γ′相的粗化规律符合Lifshitz-Slyozov-Wagner(LSW)理论,γ′粗化激活能Q=211.65kJ/mol,γ′相的粗化行为主要由Ti和Al在基体中的扩散所控制。     

Formation of Exclusive Al2O3 Scale on Nb and Nb-Rich Alloys by Two-Step Oxygen–Aluminum Diffusion Process

External Al₂O₃ scale formation behavior by a diffusion aluminizing process on Nb and Nb–X (X?=?Mo, Re, and Ta) alloys with different oxygen solubilities was investigated. The oxygen content in Nb and Nb–X alloys was controlled by oxygen diffusion treatment at 1,100?°C using Nb/NbO mixture. Nb–aluminide, NbAl₃ and Nb₂Al, layers were developed on the low-oxygen Nb substrate by an aluminum diffusion treatment using Al/Al₂O₃/NH₄Cl mixture at 1,100?°C; whereas an adhesive Al₂O₃ scale developed on the Nb substrate with higher oxygen content. The alloy oxygen content decreased by an addition of Ta, Mo and Re, in this order, and the decrease in alloy oxygen content resulted in the transition of Al₂O₃ formation from external scale to internal precipitates. This transition was not observed when low Al activity powder mixture, Ni50Al/Al₂O₃/NH₄Cl, was used for Al diffusion process, and only external Al₂O₃ scale was developed on all samples. In the present study, the effects of oxygen solubility limit, oxygen content, and the activity of Al in diffusion process on the formation of Al₂O₃ scale will be discussed.     

Properties of passive film formed on CP titanium,Ti–6Al–4V and Ti–13.4Al–29Nb alloys in simulated human body conditions

The passive film behavior of CP Titanium, Ti–6Al–4V and Ti-based intermetallic Ti–13.4Al–29Nb was investigated as a function of immersion time in simulated body condition (Hank's solution), utilizing potentiodynamic polarization and electrochemical impedance spectroscopy techniques. All the alloys were spontaneously passivated on immersion in the electrolyte. Potentiodynamic polarization experiments conducted after 1 and 168 h of immersion in Hank's solution indicated similar passive current densities. The susceptibility of Ti–13.4Al–29Nb alloy for passive film breakdown was ascribed to the greater Al content in the alloy. Electrochemical impedance spectroscopy studies indicated that the resistance of the passive film increased with duration of immersion for CP Titanium and Ti–13.4Al–29Nb. Therefore, the order of corrosion resistance under simulated human body conditions is Ti–Nb>Ti>Ti–V.