Effect of γ′ precipitation during hot isostatic pressing on the mechanical property of a nickel-based superalloy

The effect of the precipitation of γ′ phase during hot isostatic pressing (HIPing) on the mechanical property of a nickel-based superalloy, GTD-111, was evaluated by conducting tensile and creep-rupture tests at 871 °C. In the 4-h two-step HIP process, the coupons were isostatically compressed (at 120 MPa) and heated to 1230 °C, well above the dissolution temperature of γ′ precipitates into the γ matrix, for the first 2 h, and cooled down to a temperature to induce the precipitation of γ′ phase and held for the last 2 h at 120 MPa or at ambient pressure. The precipitates were controlled in size by varying the temperature for the last half of the process. According to the result of the tensile test, the mechanical properties of the alloy were varied upon the microstructural evolution, and improved more than 40%, compared to those of the untreated ones. The precipitation of γ′ phase under high pressure further improved in the properties, suggesting that the precipitation of γ′ phase at high pressure provides an advantage for the rigidity of the structure. Based on these findings, a 6-h three-step HIP process was tried, and proved to be an effective substitute for the normal heat treatment, especially in terms of creep properties. This feature was mostly attributed to the homogenized microstructure of HIPed ones, as evidenced by the X-ray diffraction patterns.     

Consolidation of nanoscale iron powders

The consolidation behavior of two types of nanoscale iron powders-vacuum condensed (nanograins in nanoparticles) and ball-milled (nanograins in microparticles), was studied. The consolidation of two microscale powders, atomized and ground, was also characterized for comparison. Consolidation techniques investigated were cold closed die-compaction, cold isostatic pressing (CIPing), and after CIPing, sintering or hot isostatic pressing (HIPing). The mechanical properties, density, and microstructure of the resulting compacts were found to depend on the original powder type and its consolidation history. Significant differences were found between the microscale and nanoscale powders. An additional reason, besides the dissimilarity in grain size, for the differences observed relates to the fact that the nanograin powders contained significant amounts of oxygen, which ultimately resulted in a distinctly two-phase bulk microstructure. The vacuum condensed powder achieved satisfactory green strength on CIPing, and high hardness (440 Hv) on low temperature sintering. While unnecessary for complete consolidation, HIPing at 500 °C was found to be beneficial and compacts of this powder thus treated were found to have a hardness of 520 Hv and high compressive yield strength (1800 MPa). For ball-milled powders, HIPing was found to be essential for achieving effective consolidation: ball-milled material, which remained friable after CIPing and sintering at 580 °C, achieved exceptionally high hardness (820 Hv) when HIPed at 580 °C and 175 MPa. The ductility was greatly improved when HIPed at temperatures between 700 °C and 850 °C, while preserving its relatively high strength. The behavior of these nanoscale powders can be understood by invoking the usual densification, particle bonding, and grain growth mechanisms. Optimization of these processes may result in unique mechanical properties of ball milled powders.     

Ionic Conductivity and Mechanical Properties of Post-HIPed Cubic Zirconia/Alumina Composites

8 mol.% yttria-doped cubic zirconia (8Y-CSZ)/AI₂O₃ composites containing 0-30 vol.% Al₂O₃ particles were fabricated by sintering, followed by hot isostatic pressing (post-HIPing). All composites were densified to at least 99·5% of the theoretical density by post-HIPing. The bending strength of composites sintered at 1500°C in air was independent of A1₂O₃ content, but a significant improvement in the bending strength was achieved by the post-HIPing technique. The bending strength and the fracture toughness of the HIPed composites increased with increasing A1₂O₃ content. Ionic conductivity of the composites was evaluated and the total, lattice, and grain boundary conductivities slightly decreased with increasing A1₂O₃ content. The HIPed composites containing up to 20 vol.% A1₂0₃ appear to be suitable candidate materials as electrolyte for solid oxygen fuel cell.     

Hot Isostatic Pressing Modifications of Pore Size Distribution in Plasma Sprayed Coatings

Plasma sprayed coatings contain relatively large amount of pores. This is primarily due to the nature of deposition by the liquid droplets upon impact. This paper reports the modifications made in the pore size distribution of plasma sprayed yttria stabilized zirconia (YSZ) and Ti-6Al-4V/hydroxyapatite (HA) composite coatings following hot isostatic pressing (HIP). The pore size distribution was measured by a mercury intrusion porosimeter (MIP). The results indicated that the YSZ coatings which were HIPed for 1 hour and 3 hours in the temperature range 1000^° to 1200^°C and ∼185 MPa showed a small decrease in the average porosity (∼2.5%) for the 1 hour samples. However, the hardness increased ∼39%, and there was a corresponding increase in the coating density. This was due to reduction of the average pore size in the HIPed coatings. Thus, in the YSZ coatings, the pores responded to HIP by a general breakdown of large pores to smaller ones and effectively forming many 'new' interparticle contacts. Whilst the overall porosity was reduced marginally, the increase in physical property like hardness was significant because of the increase in interparticle and inter-lamellae contacts following HIP treatment. In the Ti-6A1-4V/HA composite coatings, the reduction of pores is most significant amongst the small pores. The porosity of the as sprayed 20 wt% HA composite was ∼19%. This value was reduced to 17% for the sample HIPed at 1,000%C for 1 hour. Although the reduction was relatively minor, the interesting aspect was the drastic reduction of small pores less than 0.3 μm. The average pore diameter was observed to increase from 0.1676 μm in the as sprayed coating to 0.787 μm in the sample HIPed at 1,000^°C, as a result of the elimination of the micro-pores. Physical properties such as microhardness, Young's modulus and density increased substantially. This is believed to be aided mainly by the plastic deformation of the ductile Ti-6A1-4V phase during HIP. Thus modification of the pore size distribution or even average pore size can elicit substantial improvement in the properties in two different material coating, albeit the difference in the manner the modification occur.     

Hot isostatic pressing of Y-TZP powder compacts

A fundamental study of hot isostatic pressing (HIPing) was performed on Y-TZP powder compacts, and the effect of the various HI Ping process variables (time, temperature and pressure) was examined on the densification behaviour. The results were analysed using Ashby's HIPing model, which was found to be applicable for this system. The densification was found to be governed by the grain-boundary diffusion of cations. The use of grain-boundary transport characteristics determined in this study enabled HIP maps to predict the densification behaviour of HIPing within an accuracy of a factor of three for all stages of densification, except for the results at low HIPing pressure and the region of near full density.     

In-situ shelling via selective laser melting: Modelling and microstructural characterisation

This study focuses on the use of selective laser melting (SLM) to produce tooling (a shell) that is filled with powder and subsequently consolidated via hot isostatic pressing (HIPing) so that the tooling, rather than being removed, becomes part of the sample. The microstructures of the HIPed samples were studied using scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD) to assess the bond between the shell and the consolidated powders. The columnar grains in the SLM-built tooling bonded with the powder although the grains in the tooling were found to be much coarser than those in the HIPed powder, leading to preferential failure in the tooling. However, failure occurred in a fairly ductile mode and reasonable tensile strengths and ductility were obtained. Finite element models were developed to define the required initial shape of the shell in order to obtain the correct geometry after HIPing. It was found that the final shapes predicted are consistent with the observations on HIPed samples.     

Consolidation behavior of L12 phase (Al + 12.5 at.% Cu)3Zr powder with nanocrystalline structure during CIP and subsequent sintering

The mechanically alloyed (Al + 12.5 at.% Cu)₃Zr powders were consolidated by cold isostatic pressing (CIP) and subsequent sintering. Effects of CIP pressure and sintering temperature on the stability of metastable L1₂ phase and nanocrystalline structure were investigated. Before sintering, the powders were CIPed at 138, 207, 276, and 414 MPa. The relative densities of the CIP compacts were not greatly affected by the CIP pressure. However, the L1₂ phase of the specimen CIPed at pressures greater than 276 MPa was partially transformed into D0₂₃. The optimum consolidation conditions for maintaining L1₂ phase and nanocrystalline microstructure were determined to be CIP at 207 MPa and sintering at 800 °C for 1 h for which the grain size was 34.2 nm and the relative density was 93.8%. Full density specimens could be prepared by sintering above 900 °C, however, these specimens consisted of L1₂ and D0₂₃ phases. The grain sizes of all the specimens were confirmed by TEM and XRD, and were found to be less than 40 nm. This is one of the smallest grain sizes ever reported in trialuminide intermetallic compounds.     

MgO·1.5Al2O3透明陶瓷的无压/热等静压烧结制备

镁铝尖晶石透明陶瓷是典型的结构功能一体化材料, 具有优异的光学和机械性能。实验合成了颗粒细小、均匀的单相MgO·1.5Al₂O₃陶瓷粉末, 并且利用XRD全谱拟合软件Fullprof和尖晶石位置分配程序SIDR两步法确定其晶体结构为(Mg_0.46Al_0.54)^IV[Mg_0.26Al_1.64□_0.09]^VIO₄。再通过真空无压烧结结合热等静压烧结制备出了高性能的透明陶瓷, 热等静压18 MPa下1850℃烧结4 h所得样品的致密度达到99.75%, 厚度为2 mm的烧结样品可见光透过率达到65%, 红外波段透过率达到80%以上, 维氏硬度为(12.75±0.12) GPa, 杨氏模量为277 GPa。     

Sintering behaviour and microstructure development of T42 powder metallurgy high speed steel under different processing conditions

High speed steel powders (T42 grade) have been uniaxially cold-pressed and subsequently densified through different sintering routes including: supersolidus liquid phase sintering (SLPS) under vacuum and different nitrogen pressures (0.2, 0.9, and 8 bar) and through solid state sintering (SSS) by hot isostatic pressing (HIP). HIP temperatures as low as 850 °C led to near full densification of the material (>98% theoretical density) with average size of M₆C and MC carbides lower than 1 μm and grain size ≈3 μm. Pressureless sintering under different nitrogen pressures (up to 0.39 wt.%N absorption) led to a significant reduction of the optimum sintering temperature (OST) and a pronounced increase in the sintering window (SW) as compared to vacuum sintering. Pressureless sintering under 8 bar N₂ led to a further reduction in OST together with the precipitation of massive eutectic structures. Therefore, the SW was judged to be negligible. The response of the as-sintered materials to the heat treatment is basically determined by the amount of C available in the matrix prior to quenching and the grain size. The highest hardness achievable for the sintering conditions evaluated ranges 700–1100 HV2 after austenitizing at 1100 °C, oil quenching and multitempering at 500–550 °C.     

Fabrication of dense thin sheets of γ-TiAl by hot isostatic pressing of tape-cast monotapes

A method is described for the fabrication of dense thin sheets of γ titanium aluminide (γ-TiAl) by a powder metallurgy route involving hot isostatic pressing (HIP) of tape-cast monotapes. Gamma-TiAl powder (particle size <90 μm) was incorporated into a concentrated slurry by mixing with an organic binder in a solvent and the system was tape-cast to form sheets with a thickness of 400–600 μm. After insertion of the tape-cast sheet into a HIP can and binder removal in situ by thermal decomposition, HIP at 1100 °C under a pressure of 130 MPa produced dense sheets with a thickness of 250–400 μm. The free, dense sheets with a fine-grain microstructure were obtained by dissolution and oxidation of the HIP can. The carbon content of the fabricated sheets was 0.035 wt.%. Facile adaptation of the process to the production of γ-TiAl thin sheets with complex shapes is expected.     

AlN-BN复相陶瓷的热等静压制备与性能研究

以氮化铝(AlN)和氮化硼(BN)为原料, 无烧结助剂、热等静压烧结制备了AlN-BN复相陶瓷, 研究了热等静压温度和压强对两种不同原料配比(摩尔比)烧结试样的微观结构和性能的影响。结果表明: 增加BN的添加量对复相陶瓷的烧结致密化影响较小, 但逐渐降低硬度和热导率、增大体积电阻率。相同原料配比下, 复相陶瓷的密度越高, 其热导率、体积电阻率、硬度越高。热导率和体积电阻率的实测值与两相复合模型方程较为符合。当n_AlN:n_BN=75:25时, 在温度为1600℃、压强为90 MPa、保温3 h的热等静压工艺下可以制备出相对密度达98.03%、热导率为77.29 W/(m·K)、体积电阻率为1.35×10¹⁵ Ω·cm的复相陶瓷。     

Densification of sintered lead zirconate titanate by hot isostatic pressing

The effects of hot isostatic pressing (HIPing) on sintered lead zirconate titanate are presented. Densities up to 98% were obtained by HIPing for 1 h at 1300°C with argon gas pressures of either 20.7 or 138 MPa. The microstructural changes observed after HIPing, and the rapid initial kinetics for densification and pore shrinkage, indicate that prssure-enhanced grain rearrangement and solution-precipitation processes are primarily responsible for densification. The persistance of large voids after HIPing suggests that it may be impossible to completely eliminate gross processing-related defects in lead zirconate titanate by HIPing.     

细晶MgO·1.44Al2O3透明陶瓷的制备及其性能研究

晶粒细化是提高镁铝尖晶石透明陶瓷机械性能的有效途径之一。本研究采用单相MgO·1.44Al₂O₃陶瓷粉体, 首先通过放电等离子烧结进行成型和预致密化, 然后无压烧结达到烧结末期, 最终在180 MPa下1500 ℃热等静压烧结5 h, 制备出细晶MgO·1.44Al₂O₃透明陶瓷。无压烧结的结果表明: 缩窄气孔尺寸分布、降低平均气孔尺寸有助于显著促进陶瓷的致密化, 得到平均晶粒尺寸为1.4 μm、致密度为96.7%的闭气孔烧结体。透明陶瓷的平均晶粒尺寸为1.9 μm, 维氏硬度为(13.94±0.20) GPa, 杨氏模量为289 GPa。同时, 样品具有良好的光学透过率, 厚度为2 mm的样品在可见光和红外波段的最大直线透过率分别为70%和80%。     

Effect of HIPing on conductivity and impedance measurements of DyBi5Fe2Ti3O18 ceramics

X-ray diffraction, a.c. impedance and conductivity (a.c. and d.c.) have been used to characterize DyBi₅Fe₂Ti₃O₁₈. Samples were prepared by solid state double sintering method. A few samples were also subjected to hot isostatic pressing (HIP) at 800°C for 2 h at 100 MPa pressure. The data on XRD, impedance and conductivity of two sets of samples are compared to understand study of effect of HIPing on the properties of DyBi₅Fe₂Ti₃O₁₈     

Microstructural characterization of P/M Ni3Al consolidated by HIP

Rapidly solidified powder of Ni₃Al doped with boron was produced by inert gas atomization and consolidated by hot isostatic pressing (HIP). Morphology and microstructure of the powder were studied. From the particle morphology, it could be deduced that the solidification time was similar at least to the time necessary for complete fragmentation of the liquid. The powder showed a two-phase microstructure that was finer the smaller the particle size. The presence of dendrites of NiAl (β) phase was consistent with the diagram proposed by Schramm and not with the traditional diagram of Singleton et al. The microstructure of the material consolidated at 1100°C and 1200°C was studied. A monophasic structure was observed after HIP, and no relevant microstructural differences were seen between the two temperatures used.     

多晶铈掺杂硅酸镥闪烁陶瓷的制备和发光性能

本工作对铈离子掺杂多晶硅酸镥(LSO:Ce)闪烁材料的制备方法进行了系统研究。将LSO:Ce前驱体溶胶喷雾干燥后得到了球形LSO:Ce前驱粉体, 该前驱粉体在1000℃和1100℃的温度下煅烧后分别得到了不同晶型的的单相LSO : Ce球形粉体。显微结构观察显示: 粉体颗粒的平均直径约为2 µm, 是由几十纳米大小的LSO:Ce纳米晶粒堆积而成。A型球形LSO:Ce粉体经1200℃/80MPa的放电等离子体烧结(SPS)后获得了平均晶粒尺寸为1.3 µm, 相对密度高达99.7%的LSO:Ce闪烁陶瓷。由A型球形LSO:Ce粉体压制的素坯在1650℃的空气气氛下烧结4 h后可获得相对密度达98.6%, 平均晶粒尺寸为1.6 μm的LSO:Ce陶瓷。该陶瓷经1650℃/150 MPa的热等静压(HIP)处理1 h后, 获得了相对密度为99.9%的半透明LSO:Ce闪烁陶瓷, 其平均晶粒尺寸为1.7 μm, 晶界干净。该LSO:Ce陶瓷的光产额可达28600 photons/MeV, 发光衰减时间为25 ns。     

热处理对选区激光熔化17-4PH不锈钢力学性能的影响

对选区激光熔化成形的17-4PH不锈钢分别进行真空热处理、热等静压高压淬火处理和组合热处理(热等静压固溶后快淬和马弗炉时效后水冷),在1040℃固溶处理2 h和在480℃时效4 h,观察其显微组织并研究了热处理对其力学性能的影响。结果表明,17-4PH不锈钢由回火马氏体和淬火马氏体组成,热处理后沉淀相弥散分布于晶粒内部,其颗粒尺寸为100~150 nm。真空热处理使合金内部孔隙的尺寸减小到3~7 μm,而热等静压使内部孔隙几乎完全闭合,使钢的密度基本上达到理论值。真空热处理+水淬使沉积态17-4PH不锈钢的抗拉强度和硬度都显著提高(分别提高到1300 MPa和448.5HV);热等静压在提高沉积态17-4PH不锈钢抗拉强度的同时使其延伸率显著提高到22.4%。断口分析结果表明,沉积态和热等静压样品的断口形貌为典型的韧性断裂,热等静压样品的韧窝更深、尺寸更大。真空热处理和组合热处理样品的断口形貌具有部分脆性断裂的特征且出现裂纹,与沉积态相比塑性略有降低。     

Formation of metal–TiN/TiC nanocomposite powders by mechanical alloying and their consolidation

Fe–TiN, Ni–TiN, and SUS316–TiC nanocomposite powders were prepared by ball-milling Fe–Ti, Ni–Ti, and SUS316–TiC powder mixtures in a nitrogen or argon gas atmosphere. Fe–63vol.% TiN and Ni–44–64vol.% TiN milled powders were dynamically compacted by use of a propellant gun to produce bulk materials of nanocrystalline structure and having grain sizes between about 5 and 400 nm. SUS316–2.8–5.6vol.% TiC milled powders were consolidated by hot isostatic pressing (HIP) to produce bulk materials having grain sizes between about 100 and 400 nm. The possibility of using fine-dispersed TiN/TiC particles to pin grain boundaries and thereby maintain ultra-fine grained structures of grain sizes down to the nanocrystalline scale has been discussed.     

Densification of ultrafine SiC powders

Recent results on the densification behaviour of ultrafine SiC powders (below 20 nm) are presented and compared with results on the densification of ultrafine silicon-based ceramic powders given in the literature. A study of different powder processing routes and their influence on the pore-size distribution is given. Pressureless sintered green bodies having pore sizes of about 20 nm show extreme coarsening without significant densification. The results indicate a significant influence of green density on shrinkage. Encapsulated hot isostatic pressing (HIPing) led to a reduction of pore size and to considerable density increase at temperatures below 1600 °C. But even then full density without extensive grain growth was difficult to achieve. The applied method to determine grain sizes (X-ray diffraction measurements, XRD, using the Scherrer formula, scanning electron microscopy, SEM, and transmission electron microscopy, TEM) gave similar results for TEM and SEM but lower values for XRD. A possible explanation is presented. Density and grain growth both during pressureless sintering and HIPing showed significant differences between samples with and without sintering additives (B and C). Whether or not the use of sintering agents is favourable in reaching high densities and fine grain sizes, is discussed. HIP densification was modelled assuming diffusion to be the dominant mechanism. Grain growth according to a t ^1/4 dependence and an activation energy of 6.8 eV was introduced into the model. Results on the properties (hardness, also at elevated temperatures, fracture toughness, bending and compression tests, thermal conductivity) of the hot isostatically pressed samples, are presented.     

Strength and toughness of beryllium-niobium intermetallic compounds

Bend and compression strengths, fracture toughness, and high-temperature microhardness of Be---Nb intermetallic compounds were measured at temperatures up to 1200 °C. Be₁₂Nb and Be₁₇Nb₂ materials exhibited brittle behavior at temperatures below 1100 °C in bending and below 800 °C in compression. Hot isostatically pressed (HIP) Be₁₂Nb had the highest low-temperature strengths (250 MPa in bending and 2750 MPa in compression) resulting from its greater fracture toughness (K_IC = 4 MPa m^1/2) compared with the other Be---Nb materials, vacuum hot pressed (BHP) Be₁₂Nb, and HIP Be₁₇Nb₂, which had . Measured strengths for the HIP Be₁₂Nb were more than twice that measured for the VHP Be₁₂Nb or for HIP Be₁₇Nb₂. The HIP Be₁₂Nb also exhibited good high-temperature mechanical properties, having a bend strength of 250 MPa at 1200 °C, compared with less than 100 MPa for the VHP Be₁₂Nb. However, intergranular embrittlement was observed at intermediate temperatures, reducing the HIP Be₁₂Nb bend strength and fracture toughness below those measured for the other materials. HIP Be₁₇Nb₂ exhibited poor low-temperature properties, but high-temperature bend strengths of 740 MPa at 1100 °C and 400 MPa at 1200 °C were measured. Strength in compression was similar for all materials above 800 °C, decreasing sharply to about 600 MPa at 1000 °C and to 200 MPa at 1200 °C. Microhardness and indentation creep tests also revealed similar high-temperature behavior among the materials. Power-law creep exponents ranging from 4.1 to 6.6 and activation energies of 220–290 kJ mol⁻¹ were measured for the beryllides, with the HIP Be₁₂Nb having the highest activation energy for creep.