固溶处理对铸造625合金显微组织和拉伸性能的影响

研究1050～1250℃ 固溶处理对铸造625合金显微组织和拉伸性能的影响.采用SEM、EDS、EPMA和DTA研究合金的显微组织及凝固特征.结果表明,合金的凝固顺序为L→L+γ→L+γ+MC→L+γ+MC+γ/Laves→ γ+MC+γ/Laves.经1225和1250℃固溶处理后,组织中Laves相发生初熔.经不同...     

MAR-M247合金高温断裂机制的原位研究

利用原位高温拉伸台在扫描电镜中研究了镍基铸造高温合金MAR-M247在室温、400 ℃与760 ℃拉伸过程中的动态组织演变和断裂机制。原位测试结果表明,在室温到760 ℃范围内,MAR-M247合金的屈服强度与抗拉强度随温度的升高略有下降,拉伸塑性略有提高。室温原位拉伸过程中,并没有出现滑移带;400 ℃与760 ℃的原位拉伸,只在样品断口附近存在少量的滑移带。随拉伸温度的提高,合金的断裂机制并无明显变化,均表现为韧性穿晶断裂。合金的微裂纹主要来源于变形过程中碳化物的破裂,晶内与晶界都存在因碳化物破裂而形成的微裂纹。     

Effects of annealing on microstructure and mechanical properties of γ-TiAl alloy fabricated via laser melting deposition

The microstructure evolution and mechanical properties of the as-deposited γ-TiAl-based alloy specimen fabricated via laser melting deposition and as-annealed specimens at different temperatures were investigated. The results show that the microstructure of as-deposited specimen is composed of fine α₂(Ti₃Al)+γ lamellae. With the increase of annealing temperature, the bulk γ_m (TiAl) phase gradually changes from single γ phase to γ phase + acicular α₂ phase, finally small γ phase + lamellar α₂₊γ phase. Compared with the mechanical properties of as-deposited γ-TiAl alloy (tensile strength 469 MPa, elongation 1.1%), after annealing at 1260 °C for 30 min followed by furnace cooling (FC), the room-temperature tensile strength of the specimen is 543.4 MPa and the elongation is 3.7%, which are obviously improved.     

Discontinuous Precipitation of γ′ Phase in Ni-Co-Al Alloys

Decomposition of nickel-base alloys by precipitation of the γ′ phase occurs either continuously (homogeneously) or discontinuously. Under certain conditions of solute content and temperature, discontinuous precipitation is observed. Ni-Al-Co alloys have been characterized by coupling atom probe tomography with transmission electron microscopy studies. The primary focus was to investigate the discontinuous precipitation of γ and γ′ phases. When subjected to fast quenching after solution treatment, the γ′ precipitates exhibit a near-spherical shape and monomodal size distribution with an average size of less than 5 nm. After early stage annealing at 600°C for 10 min, discontinuous precipitation nucleated near the grain boundaries while some regions of homogeneous γ′ precipitates were observed. Discontinuous γ + γ′ product was completely transformed throughout the grain after 600°C/1 h. On long-term annealing (600°C/256 h), coarser γ + γ′ lamellae products replaced the fine discontinuous products that exhibited after 1 h annealing at 600°C. Equilibrium compositions of the γ and γ′ phases were achieved in this coarsening stage. The γ′ phase has an Al content of 25 at.%, which is consistent with the as-quenched condition where the (Ni + Co)₃Al stoichiometry is maintained after 600°C/256 h annealing.     

Upper nose temper embrittlement of a Ni-Cr steel

Temper embrittlement of a Ni-Cr steel was investigated both isothermally and with temperature changes. Embrittlement was most rapid in two temperature ranges: 490° to 550°C and just below the Ae₁, near 675°C. Embrittlement in the lower range was accompanied by rapid grain boundary attack by ethereal picric acid and fracture along austenite grain boundaries. Embrittlement in the upper range was accompanied by slow attack by ethereal picric acid and fracture mostly along ferrite grain boundaries. No increase in ferrite grain size was observed, but carbide particles grew during treatments in the upper temperature range. Embrittlement during slow cooling from 675°C appeared to be associated with the lower range.     

Effect of WC addition and cooling rate on microstructure,magnetic and mechanical properties of Ti(C0.6,N0.4)-WC-Mo-Ni cermets

Ti(C_0.6,N_0.4)-8Mo-xWC-25Ni (x = 0, 3, 6 and 9 wt%) cermets were synthesized under different cooling rates by vacuum sintering. The influence of WC addition and cooling rate on microstructure, magnetic and mechanical properties of the as-prepared Ti(C,N)-based cermets was investigated by using X-ray diffraction (XRD), scanning electron microscopy (SEM), vibrating sample magnetometer (VSM) and physical property measurement system (PPMS). The results revealed that the grain size of the Ti(C,N)-based cermets became finer with WC addition. Furthermore, room-temperature saturation magnetization (M_s), remanence (M_r) and Curie temperature (T_c) of the Ti(C,N)-based cermets initially decreased with increasing WC content, followed by a gradual increase. Cermets bacame paramagnetic at x = 6 under the cooling rate of 2 °C/min, x = 6 and 9 under the cooling rate 35 °C/min, respectively. The decrease in magnetic properties could be ascribed to the enhanced solid solubility of alloy elements in Ni-based binder phase. Moreover, the hardness and transverse rupture strength (TRS) of the Ti(C,N)-based cermets initially increased and followed by a gradual decrease, whereas the fracture toughness initially decreased followed by an increase with increasing WC content. At the same value of x, the Ti(C,N)-based cermets exhibited better magnetic and mechanical properties at the cooling rate of 35 °C/min than that at the cooling rate of 2 °C/min, which could be attributed to the grain refinement strengthening and solid-solution strengthening of the binder phase.     

Tempered martensite embrittlement in a 32NiCrMoV125 steel

This study focused on tempered martensite embrittlement in a 32NiCrMoV125 steel through examination of the effects of austenite grain size and tempering temperature on the mechanical properties and fracture morphology of this material. Two different austenite grain sizes were obtained by austenitizing at 870 and 950 °C. After quenching, the specimens were tempered in the temperature range of 200–650 °C. The results obtained in this research indicate that by increasing the tempering temperature, the strength and hardness decrease, but ductility increases. However, impact testing indicated that tempered martensite embrittlement occurred when samples were tempered in the range of 250–400 °C. Fractography revealed intergranular and quasi-cleavage fracture. In summary, increasing the austenite grain size decreased strength, but increased impact toughness, except for samples tempered between 200 and 350 °C.     

Tensile properties and fatigue crack propagation of solution-annealed and aged superaustenitic stainless steel

The effect of aging in air at 650°C for 100~1000 h on the tensile properties of superaustenitic stainless steel in the range RT-750°C and the fatigue crack growth behaviour at RT and 650°C was studied. Yield strength and ultimate tensile strength were almost the same between the as-received and the aged specimen. The fracture strain, however, decreased significantly from aging, and the fracture surface of the aged specimen at RT test was intergranular. The fatigue crack growth rate at RT is enhanced by aging at the high stress intensity factor range. This is due to the occurrence of an intergranular fracture in the aged specimen. At 650°C the fatigue crack growth behaviour of both the as-received and the aged specimen was almost same with no intergranular fracture.     

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.     

Simple Heat Treatment for Production of Hot-Dip Galvanized Dual Phase Steel Using Si-Al Steels

This work presents relevant metallurgical considerations to produce galvanized dual phase steels from low cost aluminum-silicon steels which are produced by continuous strip processing. Two steels with different contents of Si and Al were austenized in the two-phase field ferrite + austenite (α + γ) in a fast manner to obtain dual phase steels, suitable for hot-dip galvanizing process, under typical parameters of continuous annealing processing line. Tensile dual phase properties were obtained from specimens cooled from temperature below Ar₃, held during 3 min, intermediate cooling at temperature above Ar₁ and quenching in Zn bath at 465 °C. The results have shown typical microstructure and tensile properties of galvanized dual phase steels. Finally, the synergistic effect of aluminum, silicon, and residual chromium on martensite start temperature (M _s), critical cooling rate (C _R), volume fraction of martensite, and tensile properties has been studied.     

Microstructural evolution and mechanical properties of forged β-solidified γ-TiAl alloy by different heat treatments

The microstructural evolution and tensile properties of a forged Ti?42Al?5Mn alloy subjected to different heat treatments were studied. The results showed that, when the forged alloy was aged at 800 °C for 24 h, the interlamellar spacing (λ) and γ grain size at colony boundaries are generally coarsened. Whereas, when the alloy was first annealed at 1300 °C and then aged at 800 °C for 24 h, this coarsening of related microstructures appears less pronounced. The suggested annealing temperatures for the forged Ti?42Al?5Mn alloy are in the range of 1250?1300 °C. It was found that, on the condition of the same annealing system, both the strength and ductility were improved as the aging temperature changed from 1000 to 800 °C. The secondary precipitated β_o (β_o,sec) at colony boundaries could be responsible for improving the strength, and the γ phase at colony boundaries with the grain size about 6 μm might be one of the main reasons for the better ductility.     

热加工对电子束焊接TC11/Ti2AlNb双合金接头组织和性能的影响（英文）

研究热加工对电子束焊接TC11/Ti2Al Nb双合金接头显微组织的影响,对焊接件热暴露前后的室温拉伸性能进行测试。结果表明:电子束焊接TC11/Ti2Al Nb双合金熔合区主要由β相组成;经过变形和热处理后,熔合区主要由β、α2和α相组成,同时原始铸态的晶界在变形过程中破碎。在拉伸试验中,熔合区是薄弱区域;在不同的变形条件下,试样(热暴露前后)在此区域发生断裂。热处理后试样的最大室温拉伸强度达到1190 MPa;锻后水冷试样具有较好的塑性,其伸长率达到4.4%。相比较而言,经过(500°C,100 h)的热暴露后,试样的室温拉伸强度略有上升,但塑性变化较小。拉伸断口SEM观察显示,在不同变形条件下穿晶断裂为主要的断裂机制。     

Superplastic behavior of a powder metallurgy TiAl alloy with a metastable microstructure

Superplasticity in a powder metallurgy (P/M) TiAl alloy (Ti–47Al–2Cr–1Nb–1Ta) with a metastable B2 phase coexisted with a fine-grained γ+α₂ duplex structure has been studied. Alloy samples were tested at temperatures ranging from 650 to 1100°C, and at strain rates ranging from 10⁻⁶ to 10⁻⁴ s⁻¹. An elongation of over 300% was obtained at a strain rate of 2×10⁻⁵ s⁻¹ and at a temperature of 800°C, which is close to the ductile-to-brittle transition temperature of the alloy. This is in contrast to the prior observations of superplastic behavior of TiAl alloys in which a typical temperature of 1000°C is usually required for achieving superplasticity. It is suggested that the occurrence of low-temperature (800°C) superplasticity in the present alloy is primarily due to the presence of a metastable B2 phase in addition to a fine-grained (α₂+γ) duplex microstructure. The metastable B2 phase continues to decompose into fine-grained α₂ and γ phases, which promotes grain boundary sliding during superplastic deformation. The retained fine B2 grains accommodate the sliding strains to reduce the propensity of cavitation at grain triple junctions and thus delay the cavitation and fracture process.     

Hot cracking in cast alloy 718

Hot cracking susceptibility of the Fe–Ni-based precipitation hardening cast superalloy Alloy 718 was studied by Varestraint weldability testing. The effect of two hot isostatic pressing (HIP) treatments commonly employed in the aerospace industry was investigated in reference to the as cast condition. It was found that the heat affected zone (HAZ) liquation cracking susceptibility increased for samples with pre-weld HIP treatments. The as cast condition disclosed the best response for liquation cracking followed by HIP-1120 (1120°C/4h (HIP)?+?1050°C/1h and furnace cooling to 650°C/1h in vacuum?+?950°C/1h) and HIP-1190 (1190°C/4h (HIP)?+?870°C/10h and furnace cooling to 650°C/1h in vacuum?+?950°C/1h). The amount of the secondary precipitates and base metal grain size was found to be important parameters influencing the cracking susceptibility. Regarding solidification cracking susceptibility, the three conditions appear to behave similarly.     

Precipitates and hydrogen permeation in HSLA steel produced by TMCP

Abstract

The precipitates and hydrogen permeation behaviour were investigated in high strength low alloy steel produced by thermomechanical controlled processing with air/water cooling after finishing rolling, and the water cooled specimens were further tempered at various temperatures. Two types of precipitates have been found in the specimens. One is TiN with the size ranging from 50 to 300 nm, and the other one is fine NbC. The cooling and tempering treatment conditions affect the precipitation behaviour of NbC particles in α-Fe, leading to the difference in hydrogen permeation. The apparent hydrogen diffusivity in the air cooled specimen is lower than that in the specimen quenched and subsequently tempered at 300°C when the charging current density is 10 mA cm^?2. Increasing the tempering temperature to 500°C leads to the decrease of apparent hydrogen diffusivity; but the value is still higher than that in the air cooled specimen. However, the apparent hydrogen diffusivity slightly increases with further increasing tempering temperature from 500 to 650°C.     

Computation of synthetic surface heat transfer coefficient of 7B50 ultra-high-strength aluminum alloy during spray quenching

According to inverse heat transfer theory, the evolutions of synthetic surface heat transfer coefficient (SSHTC) of the quenching surface of 7B50 alloy during water-spray quenching were simulated by the ProCAST software based on accurate cooling curves measured by the modified Jominy specimen and temperature-dependent thermo-physical properties of 7B50 alloy calculated using the JMatPro software. Results show that the average cooling rate at 6 mm from the quenching surface and 420–230 °C (quench sensitive temperature range) is 45.78 °C/s. The peak-value of the SSHTC is 69 kW/(m²·K) obtained at spray quenching for 0.4 s and the corresponding temperature of the quenching surface is 160 °C. In the initial stage of spray quenching, the phenomenon called “temperature plateau” appears on the cooling curve of the quenching surface. The temperature range of this plateau is 160–170 °C with the duration about 3 s. During the temperature plateau, heat transfer mechanism of the quenching surface transforms from nucleate boiling regime to single-phase convective regime.     

Grain growth behaviour and high strain rate tensile properties of gas tungsten arc welds in iridium alloy DOP–26

Abstract

The high strain rate tensile ductilities of gas tungsten arc welds in an Ir–0.3 wt-%W alloy containing 60 wt-ppm Th (designated DOP–26) have been determined at test temperatures of 900–1200°C. Within this temperature range, the welded specimens of DOP–26 exhibited tensile ductilities of 9–15%, independent of the test temperature. These values are comparable to those of unwelded DOP–26 tensile specimens tested at temperatures below 1000°C, but significantly lower than (approximately half) those of unwelded DOP–26 tested above 1000°C. Elongation measurements at points along the gauge length of tensile tested specimens indicated that ductility was fairly uniform across the base metal and weld regions. At a tensile test temperature of 900°C, fracture occurred in the base metal with a mixed intergranular–transgranular failure mode. At 980°C and above, fracture occurred along the grain boundaries in the centreline of the weld. Scanning electron microscopy of fracture surfaces revealed the presence of numerous secondary phase particles along grain boundaries in the weld region. These particles were rich in thorium and were identified as an Ir–Th eutectic phase (melting point ～2080°C) that formed as the weld pool cooled. These particles, and the larger grain size of the fusion zone compared with the base metal, contributed to the lower tensile ductilities of the welded specimens compared with unwelded specimens. Because high strain rate tensile ductility in this alloy is strongly dependent on grain size, the grain growth behaviour of welded specimens of the alloy was also studied. In as welded specimens, the average grain diameters (measured through the thickness of the specimens in a plane perpendicular to the welding direction) in the base metal, weld centreline, and fusion zone were ～21, 41, and 72 µm respectively. For annealing times up to 1065 h at 1400°C and up to 100 h at 1500°C, grain sizes in the weld centreline and in the fusion zone did not change significantly. For these same anneals the base metal grain size increased gradually to 45 and 58 µm for 1400 and 1500°C annealing respectively. The base metal grain sizes were comparable to previous data from unwelded specimens of this alloy. However, excessive grain growth for an annealing time of 250 h at 1500°C was observed and as yet is unexplained.     

Thermal Stability Study of Ultrafine Grained 304L Stainless Steel Produced by Martensitic Process

An ultrafine grain 304L stainless steel with average grain size of about 650 nm was produced by martensitic process. 10 mm as-received sheets were 80% cold rolled in the temperature of ?15 °C and then annealed at 700 °C for 300 min to obtain ultrafine grained microstructure. The results showed that the ultrafine grained 304L steel has yield strength of 720 MPa, tensile strength of about 920 MPa, and total elongation of 47% which is about twice that of coarse grain structure. The effect of annealing temperature (750-900 °C) on the grain growth kinetics was modeled by isothermal kinetics equation which resulted in the grain growth exponent (n) and activation energy for grain growth of 4.8 and 455 KJ/mol, respectively. This activation energy was also compared with those for other austenitic steels to better understanding of the nature of grain growth and atoms mobility during annealing. It was found that activation energy for grain growth is about twice higher than self-diffusion activation energy of austenite that is related to the Zener pinning effects of the second phase particles.     

Improved High-Temperature Microstructural Stability and Creep Property of Novel Co-Base Single-Crystal Alloys Containing Ta and Ti

The influence of Ta and Ti additions on microstructural stability and creep behavior in novel Co-Al-W base single-crystal alloys has been investigated. Compared to the ternary alloy, the γ′ solvus temperature and γ′ volume fraction were raised by individual additions of Ta and Ti, and increased further in the quinary alloy containing both alloying additions. In contrast to ternary and quaternary alloys, an improved microstructural stability with the stable γ–γ′ two-phase microstructure and more than 60% γ′ volume fraction existed in the quinary alloy after prolonged aging treatment at 1050°C for 1000 h. The creep behavior at 900°C revealed lower creep rates and longer rupture lives in the quaternary alloys compared to the ternary alloy, whereas the quinary alloy exhibited even better creep resistance. When the creep temperature was elevated to about 1000°C, the creep resistance of the quinary alloy exceeded the previously reported Co-Al-W-base alloys and first-generation Ni-base single-crystal superalloys. The improved creep resistance at approximately 1000°C was considered to be associated with high γ′ volume fraction, γ′ directional coarsening, and dislocation substructure, which included γ–γ′ interfacial dislocation networks and the sheared γ′ precipitates containing stacking faults and anti-phase boundaries.     

Abnormal grain growth and grain boundary faceting in a model Ni-base superalloy

Normal or abnormal grain growth in a model Ni-base superalloy is observed to depend on the grain boundary structure when heat-treated in a solid solution temperature range above the solvus temperature (1150°C) of the γ′ phase. When heat-treated at 1200°C abnormal grain growth occurs and most of the grain boundaries are observed to be faceted by optical microscopy, transmission electron microscopy, and scanning electron microscopy at the intergranular fracture surface. Some of the grain boundary facet planes are expected to be singular corresponding to the cusps in the polar plot of the boundary energy against the inclination angle, and it is proposed that if these boundary segments move by a boundary step mechanism, the abnormal grain growth can occur. When heat-treated at 1300°C normal grain growth occurs, the grain boundaries are defaceted, and hence atomically rough. Normal growth is expected if the migration rate of the rough grain boundaries increases linearly with the driving force arising from the grain size difference. The correlation between the grain boundary structural transition and the growth behavior thus appears to be general in pure metals and solid solution alloys.