Hot working characteristics of powder metallurgy Nimonic AP-1 superalloy

Abstract

A simple instability condition based on the Ziegler's continuum principles as applied to large plastic flow, is extended for delineating the regions of unstable metal flow during hot deformation of Nimonic AP-1 superalloy. The optimum hot working conditions for this material are suggested.     

Microstructures and mechanical properties of hot isostatically pressed powder metallurgy Alloy APK-1

The influence of Hot Isostatic Pressing (HIP) and heat treatment parameters on the microstructure and mechanical properties of powder metallurgy alloy APK-1 (a low carbon modification of Astroloy) has been investigated. Poor mechanical properties result if carbide networks are present along the prior particle boundaries (PPBs). These networks which form during powder consolidation can be avoided by manipulating HIP parameters. Heat treatments which produce a large volume fraction of fine γ’ make this alloy susceptible to environment-assisted tensile embrittlement around 760 °C.     

The substitution of nickel for cobalt in hot isostatically pressed powder metallurgy UDIMET 700 alloys

Nickel was substituted in various proportions for cobalt in a series of five hot-isostatically-pressed powder metallurgy alloys based on the UDIMET 700 composition. These alloys were given 5-step heat treatments appropriate for use in turbine engine disks. The resultant microstructures displayed three distinct sizes of γ′ particles in a γ matrix. The higher cobalt-content alloys contained larger amounts of the finest γ′ particles, and had the lowest γ-γ′ lattice mismatch. While all alloys had approximately the same tensile properties at 25 and 650°C, the rupture lives at 650 and 760°C peaked in the alloys with cobalt contents between 12.7 and 4.3 pct. Minimum creep rates increased as cobalt contents were lowered, suggesting their correlation with the γ′ particle size distribution and the γ-γ′ mismatch. It was also found that, on overaging at temperatures higher than suitable for turbine disk use, the high cobalt-content alloys were prone to sigma phase formation.     

Processing map for hot working of powder

The constitutive flow behavior of a metal matrix composite (MMC) with 2124 aluminum containing 20 vol pct silicon carbide particulates under hot-working conditions in the temperature range of 300 °C to 550 °C and strain-rate range of 0.001 to 1 s^-1 has been studied using hot compression testing. Processing maps depicting the variation of the efficiency of power dissipation given by [2m/(m + 1)] (wherem is the strain-rate sensitivity of flow stress) with temperature and strain rate have been established for the MMC as well as for the matrix material. The maps have been interpreted on the basis of the Dynamic Materials Model (DMM). [3] The MMC exhibited a domain of superplasticity in the temperature range of 450 °C to 550 °C and at strain rates less than 0.1 s^-1. At 500 °C and 1 s^-1 strain rate, the MMC undergoes dynamic recrystallization (DRX), resulting in a reconstitution of microstructure. In comparison with the map for the matrix material, the DRX domain occurred at a strain rate higher by three orders of magnitude. At temperatures lower than 400 °C, the MMC exhibited dynamic recovery, while at 550 °C and 1 s^-1, cracking occurred at the prior particle boundaries (representing surfaces of the initial powder particles). The optimum temperature and strain-rate combination for billet conditioning of the MMC is 500 °C and 1 s^-1, while secondary metalworking may be done in the super- plasticity domain. The MMC undergoes microstructural instability at temperatures lower than 400 °C and strain rates higher than 0.1 s^-1.     

Powder metallurgy T15 tool steel: Part I. Characterization of powder and hot isostatically pressed material

The microstructure and constitution of T15 tool steel processed from gas-atomized powder have been characterized. From the atomized powder, four particle size ranges (≤840, 250 to 840, 44 to 100, and ≤44 Μm) were consolidated to full density by hot isostatic pressing (“hipping”) at 1130 ‡C or 1195 ‡C. Both atomized powder and consolidated material were examined by means of optical and electron microscopy, X-ray diffraction, chemical analysis, and micro-hardness. A segregated structure exists in the gas-atomized powder, independent of particle size; MC and M₂C carbides are present, primarily at cell boundaries. The matrix of the powders is a mix of martensite and retained austenite. Weight fraction and overall composition of the carbides are insensitive to particle size, but the proportion of MC carbides increases with decreasing particle size. After consolidation, MC, M₆C, and M₂₃C₆ carbides are present in a ferrite matrix. The carbide size distribution is skewed to larger carbide sizes at the higher consolidation temperature, independent of the prior particle size fraction, but there is no significant change in carbide volume fraction. For a given consolidation temperature, the size distribution of the MC and M₆C carbides is broader for the coarser particle size fractions.     

Identification of flow instabilities during hot working of powder metallurgy superalloy IN 100

Abstract

A simple instability condition based on the Ziegler's continuum principles as applied to large plastic flow, is extended for delineating the regions of unstable metal flow during hot deformation of powder metallurgy materials. This criterion has been applied to the existing flow stress data of powder metallurgy superalloy IN 100 and compared with the reported microstructural observations. The optimum hot working conditions for the PM superalloy IN 100 are suggested.     

Microporosity in hot isostatically pressed Ti-6Al-4V powder compacts

The existence of micropores of less than 100 nm size is reported in hot isostatically pressed Ti-6Al-4V compacts that were expected to be fully dense. The micropores are in a film-like arrangement which traverses individual alpha grains. Several possible origins of the micropores are discussed. Of these, the dissolution of oxide film on prior powder particles appears most probable. Formerly Graduate Student at Case Western Reserve University, Cleveland, OH 44106.     

Processing map for hot working of alpha-zirconium

The hot deformation characteristics of alpha-zirconium in the temperature range of 650 °C to 850 °C and in the strain-rate range of 10_-3 to 10² s^-1 are studied with the help of a power dissipation map developed on the basis of the Dynamic Materials Model.[7,8,9] The processing map describes the variation of the efficiency of power dissipation (η =2m/m + 1) calculated on the basis of the strain-rate sensitivity parameter (m), which partitions power dissipation between thermal and microstructural means. The processing map reveals a domain of dynamic recrystallization in the range of 730 °C to 850 °C and 10⁻² to 1⁻¹ with its peak efficiency of 40 pct at 800 °C and 0.1 s^-1 which may be considered as optimum hot-working parameters. The characteristics of dynamic recrystallization are similar to those of static recrystallization regarding the sigmoidal variation of grain size (or hardness) with temperature, although the dynamic recrystallization temperature is much higher. When deformed at 650 °C and 10^-3 s^-1 texture-induced dynamic recovery occurred, while at strain rates higher than 1 s^-1, alpha-zirconium exhibits microstructural instabilities in the form of localized shear bands which are to be avoided in processing. Formerly Divisional Head, Physical Metallurgy Division, Bhabha Atomic Research Centre.     

Processing and microstructure of powder metallurgy Al-Fe-Ni alloys

Prealloyed rapidly solidified Al-Fe-Ni alloy powder with dispersoid volume fractions of 0.19, 0.25, and 0.32 FeNiAl₉ was produced by air atomization. The powder was degassed, canned, and consolidated to full density by vacuum hot pressing and extrusion or by direct extrusion. Microstructures in the alloy powder and consolidated material were characterized by means of optical, scanning (SEM), and transmission electron microscopy (TEM) and constituent phases identified by X-ray diffraction. The coarsening kinetics of the FeNiAl₉ dispersoid were monitored by differential scanning calorimetry (DSC) and by quantitative metallography. Atomized powders exhibited two scales of microstructures: optically featureless regions and regions with a coarse dispersoid morphology. Within the featureless regions, there are three morphologies, namely, a fine uniform precipitate microstructure, a cellular microstructure, and an eutectic microstructure. The only dispersoid observed in the atomized powders and consolidated material was FeNiAl₉. The two scales of microstructure were retained after consolidation, and after hot extrusion, the typical microstructure consisted of a recovered matrix structure with a grain size of 0.2 to 0.3 μm and equiaxed intermetallics of average diameter 0.1 μm. The microstructure was resistant to coarsening up to approximately 370 °C. Coarsening kinetics in this alloy system were consistent with a grain boundary diffusion model (activation energy 146 kJ/mol) and were not appreciably affected by dispersoid volume fraction.     

Microstructural control in hot working of IN-718 superalloy using processing map

The hot-working characteristics of IN-718 are studied in the temperature range 900 °C to 1200 °C and strain rate range 0.001 to 100 s⁻¹ using hot compression tests. Processing maps for hot working are developed on the basis of the strain-rate sensitivity variations with temperature and strain rate and interpreted using a dynamic materials model. The map exhibits two domains of dynamic recrystallization (DRX): one occurring at 950 °C and 0.001 s⁻¹ with an efficiency of power dissipation of 37 pct and the other at 1200 °C and 0.1 s⁻¹ with an efficiency of 40 pct. Dynamic recrystallization in the former domain is nucleated by the δ(Ni₃Nb) precipitates and results in fine-grained microstructure. In the high-temperature DRX domain, carbides dissolve in the matrix and make interstitial carbon atoms available for increasing the rate of dislocation generation for DRX nucleation. It is recommended that IN-718 may be hot-forged initially at 1200 °C and 0.1 s⁻¹ and finish-forged at 950 °C and 0.001 s⁻¹ so that fine-grained structure may be achieved. The available forging practice validates these results from processing maps. At temperatures lower than 1000 °C and strain rates higher than 1 s⁻¹ the material exhibits adiabatic shear bands. Also, at temperatures higher than 1150°C and strain rates more than 1s⁻¹, IN-718 exhibits intercrystalline cracking. Both these regimes may be avoided in hotworking IN-718.     

316LN不锈钢管道热加工过程的加工图及可加工性

建立了316LN超低碳控氮不锈钢管道的热加工图,并确立了最适宜热加工的工艺范围。利用Gleeble-3500热模拟实验机进行热压缩实验,用以模拟316LN超低碳控氮不锈钢的热加工过程。实验的温度范围是1 173~1 473 K,实验的应变速率范围是0.001~1 s-1。利用热压缩模拟实验得到的真应力-真应变数据,分别绘制出了材料能量耗散效率图和材料失稳图,并将二者叠加绘制出了316LN超低碳控氮不锈钢的加工图。从绘制出的图形可以看出,当温度为1 375~1 450 K、应变速率为0.01~0.1 s-1时,该材料的能量耗散效率达到最大值41%,此时发生了明显的动态再结晶。因此,该区域被确定为316LN超低碳控氮不锈钢热加工的最佳工作范围。     

粉末高温合金FGH97疲劳裂纹扩展行为

测定不同晶粒尺寸、γ'相以及不同Hf含量的粉末高温合金FGH97在650℃高温条件下的疲劳裂纹扩展速率,并将其与FGH95和FGH96两代粉末合金的疲劳裂纹扩展速率进行对比.用定量分析的方法对FGH97合金在疲劳断裂各个阶段的行为特征进行分析.较大晶粒尺寸的FGH97合金具有较低的裂纹扩展速率,合理的二次和三次γ'相匹配析出,可以获得较高的疲劳寿命;Hf元素的添加使合金的整体疲劳寿命增大;FGH97合金与FGH95和FGH96相比,具有较高的疲劳裂纹萌生抗力,更低的高温疲劳裂纹扩展速率.     

C元素对FGH96粉末高温合金显微组织和力学性能的影响

对C质量分数分别为0.05%和0.09%的FGH96合金进行了显微组织和力学性能的分析。结果发现:C元素含量较高时,除了碳化物数量明显增多外,还会在晶界处形成较大尺寸的块状MC碳化物;但是,C元素含量并未对原始颗粒边界(prior particle boundary,PPB)、晶粒度及γ′相产生明显的影响。C元素含量较高会提高FGH96合金在650℃时的抗拉强度和屈服强度,但会降低其塑性。在低周疲劳试验中,C元素含量较高,形成的大块MC型碳化物分布于表面或亚表面,将会作为裂纹起源从而显著降低合金的低周疲劳性能。     

涡轮盘用高性能粉末高温合金的优化设计探讨

从成分和组织两个方面分析近年来国内外涡轮盘用粉末高温合金的优化设计过程,总结新的合金化途径对合金力学性能的影响规律,探讨合适的热处理工艺对获取最佳显微组织的重要性,并为国内开展新型高性能粉末高温合金优化设计提出建议.     

The influence of microstructure and environment on the crack growth behavior of powder metallurgy nickel superalloy RR1000

A study of crack growth in vacuum and air at 725 °C (T/T _m=0.6) highlights the relative importance of creep and environmental crack-tip damage mechanisms in Powder Metallurgy (P/M) disc alloy RR1000. Both of these mechanisms are associated with a transition to intergranular fracture during fatigue crack growth at 0.25 Hz. Crack growth under sustained loads reveals the precise nature of these mechanisms in RR1000. The severity of creep and environmental mechanisms is controlled by the grain-boundary microstructure and the crack-tip stress. Near-tip cavitation leads to fracture in vacuum. Sigma-phase precipitation causes an increase in crack growth rate through increased crack-tip cavity nucleation. Rapid near-tip stress relaxation induced by γ′ coarsening has a beneficial effect on the severity of this type of damage. In air, increases in crack growth rates are associated with near-tip intergranular oxidation. It is proposed that the extent of this damage and subsequent growth rates are increased by sigma-phase precipitation through enhanced oxidation due to chromium depletion and subsequent decreased passivation. Again, a beneficial effect of rapid near-tip stress relaxation due to selective γ′ coarsening is apparent and environmental damage is reduced under these conditions.     

新型镍基粉末高温合金相变温度的测定

分别采用热力学计算(thermo-calc,TC),差热分析(differential thermal analysis,DTA)和金相法(metallography observation)测定一种新型镍基粉末高温合金(CSU-A)的铸态母合金及其热挤压态合金的相变温度,分析和对比升温测试和降温测试对差热分析结果的影响。结果表明,热力学计算可准确预测合金的固、液相线温度;铸态合金的γ′相先后发生2次析出/固溶,且γ′相的完全固溶温度高于挤压态合金的γ′相完全固溶的温度;DTA降温曲线的相变温度低于升温曲线的相变温度,凝固过冷度(TL)和γ′相析出过冷度(T1γ′,T2γ′)分别为16,35和43℃。最终确定挤压态CSU-A合金的γ′相完全固溶温度为1 145±5℃,铸态合金的γ′相完全固溶温度为1 196℃,固、液相线温度分别为1 25 9和1 356℃。     

镍基粉末高温合金枝晶间亚稳碳化物

对急冷凝固镍基高温合金松散粉末热等静压成型合金中亚稳碳化物及其相间反应进行了研究.随着热处理温度的升高,粉末中合金元素的分布逐渐均匀化,但枝晶间MC能在较高的温度下保持稳定,使Ti和Zr在该处仍有较高含量.原始粉末中枝晶间主要分布着块状和花状的MC型碳化物,在预热处理过程中粉末枝晶间块状碳化物分解,发生M₂₃C₆和M₆C的析出反应,而花状碳化物的成分及形貌则保持相对稳定.成型合金残余枝晶间分布的碳化物主要由块状M₆C和MC及花状MC组成,形变再结晶可以促进枝晶间碳化物的溶解.     

FGH91粉末高温合金与K418B铸造叶环热等静压扩散连接研究

采用热等静压工艺进行了FGH91粉末高温合金和K418B铸造叶环扩散连接试验,研究了FGH91-K418B双合金的界面成分扩散和连接接头的组织和力学性能。结果表明,在连接界面足够清洁的条件下,选择1190 ℃+170 MPa的热等静压工艺,可以实现FGH91和K418B两种合金良好的冶金连接。进一步观察和分析发现,扩散连接接头致密完整,无夹杂物和连续的第二相析出物,扩散区宽度80～120 μm。FGH91-K418B双合金的拉伸性能、持久性能和显微组织具有良好的一致性,试样断裂均未发生在界面结合处。     

热挤压对FGH96镍基粉末高温合金微观组织的影响

研究了热挤压温度、挤压比、挤压速度、挤压前预处理对FGH96镍基粉末高温合金微观组织的影响规律,确定了获得晶粒尺寸小于10 μm的超塑性细晶组织的热挤压方法.研究结果表明,热等静压后FGH96合金发生了再结晶,实现了粉末的完全致密化成形,但晶粒大小极不均匀,且存在明显的原始颗粒边界(PPB)缺陷.采用热挤压前预处理工艺...