Hot Isostatic Pressing Process Research Articles

Hot isostatic pressing of a near α-Ti alloy: Temperature optimization, microstructural evolution and mechanical performance evaluation

Hot isostatic pressing (HIP) shows distinct advantages in the manufacture of difficult-to-process materials into components with geometric complexity and high mechanical performance. In this work, a near-α titanium alloy Ti–6Al–2Zr–1Mo–1V (TA15) designed for service temperatures of ~500 °C in the aerospace industry was processed by HIP at three representative temperatures. A systematic study on the phase constitutions, microstructural evolution, and tensile properties measured at 20, 500, 600, and 700 °C was carried out for the assessment of HIP TA15 specimens. The relationships among HIP temperature, crystallographic characteristic, and mechanical performance of HIP TA15 were elucidated in detail. Spark plasma sintering (SPS) was selected to compare with the HIP technique regarding the fabrication of TA15 alloy. The tensile strength and ductility of the HIP specimens at 500 °C were 14.8% and 52.8% higher than those of the SPS counterparts. Furthermore, the high-temperature tensile properties of HIP TA15 specimens were comparable to those of the forged IMI 685 and even better than those of sintered Ti60 parts. These findings can serve as a guideline for the HIP process of TA15 alloy and the HIP parameter optimization for other near-α Ti alloys applied in harsh environments.

Measurement of residual strain in tantalum-clad tungsten after hot isostatic pressing

Tantalum-clad tungsten targets are a popular choice for spallation neutron production, due to the combination of high neutron yield and corrosion resistance. Such targets typically use the Hot Isostatic Press (HIP) process to bond the cladding to the core; this produces a strong bond but also introduces large residual stresses in the target and cladding. This is of particular interest at the ISIS neutron source, because cladding breaches are currently believed to limit the lifetime of ISIS TS2 targets. Two different and complementary methods were used to measure the residual strain in a tantalum-clad tungsten strip manufactured using the same HIP process as ISIS targets. The strip was produced with deliberately asymmetric cladding, causing it to deflect in proportion to the residual stress. FEA simulations were used to back-calculate the stress from the measured deflection. The strip was then placed on the ISIS instrument ENGIN-X, which allowed detailed through-thickness strain profiles to be measured via neutron diffraction. The results of both methods confirm the presence of large residual strains, and agree reasonably well with FEA simulations of the cladding process.

The Effects of Post-processing in Additively Manufactured 316L Stainless Steels

This paper provides a systematic examination of microstructure and corrosion behavior of additively manufactured 316L stainless steel after post-processing by surface finishing, isothermal heat treating, or hot isostatic pressing. The effects of isothermal heat treatments from 500 °C to 1300 °C and hot isostatic press processing from 1000 °C to 1200 °C were correlated to the evolution of microstructure, pore morphology and volume fraction, microhardness measurements, and corrosion behavior to reveal the different post-processing temperature regimes and their corresponding characteristics. In particular, this study found that the AM 316L microstructures and properties are stable up to nearly 800 °C. Higher temperatures eliminate the fine solidification structure, causing a drop in microhardness and corrosion resistance. Corrosion was primarily driven by the porosity content of the exposed surface and near-surface regions, which is unaffected by isothermal or HIP post-processing, and the corrosion was not significantly affected by recrystallization, which begins around 1050 °C and is nearly complete by 1200 °C. The amount of porosity was not markedly affected by isothermal post-processing, but could be significantly reduced by HIP processing.

Numerical simulation of residual stresses in hot isostatic pressed SiC/GH4738 composites

In this paper, the residual stresses in hot isostatic pressed SiC/GH4738 composites were investigated using numerical simulation. First, the finite element model of the SiC/GH4738 composites was built using ABAQUS. Then, the hot isostatic press process was simulated, where the paths of temperature and pressure were considered. The residual stress distributions were obtained, and the changes in interfacial residual stresses with angle were analyzed. Finally, the effect of process parameters on residual stresses was analyzed. The simulation results show that maximum temperature, holding time, and fiber volume content have little influence on the distribution of interface residual stress but have great influence on the magnitude of interfacial residual stress. The results will help to influence the design and preparation of SiC/GH4738 composites.

Investigating the hot isostatic pressing of an additively manufactured continuous carbon fiber reinforced PEEK composite

Abstract Additive manufacturing has recently begun to explore its role in the composites industry, utilizing the capability of complex fiber placement and part geometry to further reduce the weight of composite structures. Additively manufactured (AM) thermoplastic composites suffer weak interlayer bonding and high void contents. This study investigates the use of hot isostatic pressing (HIP) to post-process AM continuous carbon fiber reinforced polyetheretherketone (PEEK) composites to improve their flexural, interlaminar shear, tensile, and compressive properties. Isostatic pressure (200 psi) and elevated temperatures were used in combination to compress internal voids, promote PEEK crystallization, and enhance inter-filament polymer diffusion.Three different HIP temperatures of 200 °C, 250 °C, and 300 °C, between the glass transition and heat deflection temperatures of the composites, were considered. The post-processing was found to improve properties, with flexural strength and interlaminar shear strength improvements of up to 46% and 30%, respectively, and tensile strength and modulus reaching values as high as 1312 MPa and 92 GPa. The accompanying changes in void content and polymer structure for various HIP processing parameters were investigated to account for the changes in mechanical properties and failure mechanisms. Specifically, increasing the treatment temperature was found to monotonically reduce the void content. The polymer degree of crystallinity (DOC), however, was found to change appreciably only for the highest treatment temperature (300 °C), which negatively impacted the flexural and interlaminar composite properties. Treatment at 300 °C is understood to embrittle the polymer matrix, thereby increasing the stress concentration factors of the internal porosity. HIP demonstrates a fast and robust method to post-process AM composite parts, resulting in a significant improvement in their mechanical performance whereas a high crystallinity with void presence could degrade the performance of AM composites. The outcome of this study can be utilized to improve both the additive manufacturing process and post-HIP treatment to enhance the performance of AM composites.

Hot isostatic pressing of MRI compatible Zr-1Mo components manufactured by laser powder bed fusion

To avoid the “susceptibility artifacts” in magnetic resonance imaging (MRI) and realize patient-specific medical devices with complicated shapes, a novel MRI compatible Zr-1Mo alloy with low porosity (c.a. 0.4%) were manufactured by laser powder bed fusion process(L-PBF) using gas-atomized Zr-1Mo(wt%) alloy powder. However, as-built Zr-1Mo(wt%) components using optimized process parameters show relatively high mechanical performance(elongation of 6.4%, UTS of 1175 MPa), but contain 0.4 vol% defects which show a spherical morphology and distribute in the builds randomly. These defects definitely deteriorate the mechanical performance(especially ductility, fatigue behavior) of Zr-1Mo components manufactured by L-PBF. In order to eliminate defects in as-built Zr-1Mo components, hot isostatic pressing(HIP), which has been known as an effective process to decrease the porosity of additive manufactured products, was executed. Due to the elevated temperature and high pressure during HIP treatment, the randomly distributed defects were eliminated after the HIP process. Moreover, the α-martensite structure in the as-built alloy was transformed into a basketweave α + β structure at low HIP temperature(1023 K and 1073 K) or lamellar α + β structure at relatively high HIP temperature(1173 K and 1273 K). In the meantime, the elongation of HIP-treated Zr-1Mo components significantly increased, but the strength shows the opposite trend. From the viewpoint of mechanical property, Zr-1Mo components HIP-treated at 1023 K possessed elongation of 19.9%, and UTS of 628 MPa should be optimized in this study. More importantly, porosity regrowth behavior was investigated to evaluate subsequent heat treatment feasibility for further improvement of mechanical performance. Due to intact raw material power as well as vacuum/low pressure inside the original defect caused by evaporation of metals, porosity regrowth can not be confirmed in HIP-treated Zr-1Mo components after subsequent heat treatment(holding temperature higher than the β transus of Zr-1Mo alloy). Therefore, further improvement of mechanical performance by subsequent heat treatment for HIP-treated Zr-1Mo components is very promising and should conduct further research in the future

A comparison of field assisted hot pressing and hot isostatic pressing for gas atomized Ti–22Al–26Nb(at.%) and Ti–22Al–26Nb–5B(at.%) powders

Ti–22Al–26Nb(at.%) and Ti–22Al–26Nb–5B(at.%) compacts were processed from gas atomized prealloyed powders by field assisted hot pressing (FAHP) for between 600 and 1200 s at temperatures between 725 and 1070 °C and the resulting microstructures were compared. The average powder diameters ranged between 50 and 100 μm, where both the B-containing powders and resulting FAHP compacts contained elongated B-rich needles. The microstructure in the FAHP compacts depended on the processing temperature. For processing temperatures less than or equal to 960 °C, porosity was evident on the FAHP compact, where greater porosity was observed with decreasing temperature. Processing temperatures greater than or equal to 980 °C resulted in minimal porosity and three-phase microstructures containing a mixture of the orthorhombic, α2, and body-centered-cubic (BCC) phases, where longer processing times led to microstructural coarsening. The microstructures of the FAHP compacts were compared to those from compacts consolidated at 1027 °C for 4 h using hot isostatic pressing (HIP). For both HIP and FAHP processing, the B-containing compacts exhibited finer prior-BCC grain sizes. Compositional analysis of the B-rich phase within the Ti–22Al–26Nb HIP compact suggested that it could be the B27 orthorhombic structure with a B2TiNb stoichiometry. Overall, FAHP was shown to be a viable powder metallurgy processing technique for intermetallic Ti2AlNb based intermetallic alloys.

Microstructure and properties of TiAl processed via an electron beam powder bed fusion capsule technology

Titanium aluminide (TiAl) alloys are considered for high-temperature applications at a progressive rate and processing issues become increasingly important. Especially β-solidifying TiAl alloys are subjected to extensive research activities. In our research, we assessed a novel manufacturing technique combining electron beam powder bed fusion (PBF-EB/M) for manufacturing near net shape capsules and hot isostatic pressing (HIP) for consolidating sintered powder particles inside the capsule. This novel processing route is intended to overcome the disadvantages from PBF-EB/M process (heterogeneous aluminum evaporation during melting) and HIP process (formation of shell structures at particle boundaries). The formation of shell structures at previous particle boundaries (PPB) is a phenomenon typically related to powder metallurgy (PM). In this study a β-solidifying TiAl alloy, termed TNM, with a nominal composition of Ti-43.5Al–4Nb–1Mo-0.1B (in atomic percent, at.%) is used as raw material. Additively manufactured capsules containing sintered powder particles are hot isostatic pressed to the final part dimension. Microstructures from both, powder particles as well as consolidated material, are presented in detail regarding aluminum (Al) evaporation and the formation of shell structures. The mechanical properties of produced specimen are compared to results from conventional manufacturing techniques. In summary, our results reveal the capability of the novel PBF-EB/M capsule technology approach.

Effect of HIP treatment on the microstructure and mechanical properties of a Ni-based superalloy fabricated by selective laser melted method

Selective laser melting (SLM) is one of the most emerging additive manufacturing (AM) method through the melting of metal powder to manufacture the full density parts, which will lead to an orientation microstructure and anisotropy mechanical properties. Thus, corresponding post-processing treatments, such as solution treatment (ST) and hot isostatic pressing (HIP), were subjected to the specimens before tests. The effect of the HIP on the microstructure and mechanical properties of SLM Inconel 625 was investigated through uniaxial tension tests at room and elevated temperature. The fracture mechanism was systematically characterized and analyzed using scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS). It was found that the microstructure after the HIP process is composed of equiaxed austenite, and the annealing twins are reduced. The tensile test results reveal that the HIP has a significant impact on the plastic behavior of the alloy at high-temperature, and is the main reason for the greatly improved elongation, compared to the ST process.

The size dependence of microstructure and hardness on the MA powders for the MA-HIP processed Cu-Y2O3 dispersion-strengthened alloys

Abstract Dispersion-strengthened (DS) copper alloys with Y2O3 particles are known as high strength and high thermal conductivity materials having potential for heat sink materials of fusion devices. In the previous study on mechanical alloying (MA) and hot isostatic pressing (HIP) processing of DS-Cu alloys, the MA powders showed various morphology and size distribution which influenced the properties of the MA-HIP samples. In this study, the MA alloyed powders with 3 wt% Y2O3 particles were classified by their size and characterized. The particles ranged from fine particles of under 46 μm to coarse particles of 212 μm over, and a wide particle size distribution was confirmed. The peeled Cu-O fragments from coarser MA powders are the main source of finer grain with 46 μm. The finer grains had oxide-like behavior as microstructural and mechanical properties. On the other hand, the coarser powder having over 212 μm had metal-like behavior as microstructural and mechanical properties. We thought that the classification of the MA powder before HIP process was one of the suitable process to optimize the microstructure and several properties of ODS-Cu alloy.

Effect of HIP at 800 and 900 °C on microstructure and properties of extruded Be-Ti composites

• Various beryllides are formed during HIP of the extruded Be-Ti composite. • TiBe 12 has a fine grained microstructure with a mean grain size of 190 nm. • Beryllides have a very high microhardness up to 1680 HV. • In the extruded composite, beryllium and titanium phases interact mainly in the temperature range of 670–740 °C. Hot isostatic pressing (HIP) has been proposed for manufacturing large hexagonal TiBe 12 blocks for neutron multiplication in the new reference design of the DEMO blanket. This paper investigates the effect of HIP at 800 and 900 °C on the microstructure and the properties of extruded Be-Ti composites with the main aim of optimizing HIP parameters. Be-Ti composites produced by powder extrusion at 650 °C consist exclusively of Be and Ti phases. During HIP, they interact forming titanium beryllides with a volume fraction of 70–79%. These beryllides have a very high microhardness of 1130–1680 HV. X-ray diffraction showed that the main beryllide phase is TiBe 12 . After HIP at 900 °C, it has a very fine-grained microstructure with a mean grain size of 190 nm. Auger electron spectroscopy revealed that the TiBe 2 beryllide forms a thin layer surrounding the remaining Ti phase. Ti 2 Be 17 can be found at the prior titanium phase locations in the form of small particles. The titanium and beryllium phase do not dissolve completely during the used HIP process. In addition, the beryllium phase exhibited a higher porosity after HIP. This results in densities as low as 94.4% and 95.9% of the theoretical density of TiBe 12 after heat treatment at 800 °C and 900 °C, respectively. Differential scanning calorimetry showed that beryllides are mainly synthesized in the temperature range of 670–740 °C, which can be monitored by the observation of a heat release. To accomplish diffusion processes in the Be-Ti composite, HIP should be carried out at temperatures exceeding 900 °C. Heating rates should be less than 10 K/min to avoid excessive overheating.

Effect of HIP process and subsequent heat treatment on microstructure and mechanical properties of direct metal laser sintered AlSi10Mg alloy

PurposeThe purpose of this paper is to evaluate the effect of post process combinations, e.g. hot isostatic pressing (HIP) only, HIP + T6 heat treatments, and T6 only, with different aging time, on surface properties, microstructure and mechanical properties of stress-relieved AlSi10Mg parts produced by direct laser metal sintering.Design/methodology/approachHIP process and HIP + T6 heat treatments were applied to as stress-relieved direct laser metal sintered (DMLS) AlSi10Mg parts. Aging times of 4 and 12 h are selected to examine the optimum duration. To analyze the advantages of HIP process, a T6 heat treatment with 4 h of aging was also applied. Densities, open porosities and roughness values of as stress-relieved, HIPed, HIP + T6, and T6-only samples were measured. The samples were characterized by OM and SEM together with EDX analysis. An image analysis study was made to evaluate the inner pore structure, thereby to understand the mechanical behavior.FindingsHIP process does not cause a significant change in surface porosity; yet it has a positive influence on inner porosity. HIP process results in a microstructure of the aluminum matrix surrounded by a network of micron and nano size Si particles. Additional heat treatment results in larger particles and precipitation. After HIPing, ductility increases but strength decreases. Samples aged 4 h present improved yield and tensile strength but decreased elongation, yet samples aged for 12 h reach a combination of optimum strength and ductility. The lower level of tensile strength and ductility in T6-only condition indicates that HIP process plays a crucial role in elimination of the porosity thus improves the effectiveness of subsequent heat treatment.Originality/valueThe study investigates the effect of post-process conditions and optimizes the aging time of the T6 heat treatment after HIP process in order to obtain improved mechanical properties. The stress-relieved state was chosen as the reference to prevent distortion during HIPing or heat treatment.

Compressive fatigue properties of additive-manufactured Ti-6Al-4V cellular material with different porosities

Selective laser melting (SLM) is a novel additive manufacturing (AM) technique for producing cellular metallic materials with designed pore structures. SLM Ti-6Al-4V cellular alloys have been extensively studied for biomedical and other applications. The objective of this study was to investigate the effects of porosity (33 vol%, 50 vol%, and 84 vol%) on the compressive fatigue performance and fracture mechanism of an SLM Ti-6Al-4V cellular material with a new cuboctahedron COH-Z unit cell. Furthermore, the influences of hot isostatic pressing (HIP) at 1000 °C/150 MPa on the fatigue properties and fracture were also examined in this study.The results showed that the fatigue strain slowly accumulated when the stress cycle was below the 70% fatigue life. After the 70% fatigue life, the ratcheting strain rate (dε/dN) gradually increased. A shear band along a direction inclined at ~45° to the stress axis could be definitely identified at about 95% fatigue life, as demonstrated by the digital image correlation technique. With further increases in the stress cycle, the shear band was intensified and finally led to rapid fatigue failure. The HIP process did not alter the fracture behaviors of the SLM cellular alloy, although the fatigue life was much improved by several times. On the other hand, the fatigue endurance ratio of the SLM cellular alloy with 33 vol% was 0.5. Raising the porosity from 33 vol% to 84 vol% reduced the endurance ratio from 0.5 to 0.15 due mainly to the high notch sensitivity of α′-martensite in the SLM cellular alloy. The HIP treatment changed the microstructure from α′-martensite to lamellar α+β phases and thus significantly improved the fatigue endurance performance. With the combination of the COH-Z unit cell and HIP treatment, the endurance ratios at 106 cycles of the Ti-6Al-4V cellular alloy with porosities of 33 vol% to 84 vol% were as high as 0.5. This superior endurance ratio is advantageous to the long-term application of SLM Ti-6Al-4V cellular alloy.

Iodosodalite synthesis with hot isostatic pressing of precursors produced from aqueous and hydrothermal processes

Iodosodalite powders were synthesized using aqueous (70 °C) and hydrothermal (180 °C) methods and then hot isostatically pressed (HIP), without or with (10 or 20 mass%) sodium borosilicate glass binders, at 900 °C and 175 MPa for 3 h to convert into glass-bonded iodosodalite waste forms. After HIPing, comparisons were made of the structures, compositions, morphologies, porosities, and chemical durabilities of the iodosodalite samples produced through aqueous and hydrothermal methods. X-ray diffraction patterns of HIPed samples showed that iodosodalite remained as the dominant phase. However, the iodosodalite phase concentrations decreased after HIPing, indicating iodosodalite decomposition (or amorphization) during the HIP process. Scanning electron microscopy and elemental mapping analyses on cross sections of HIPed samples showed homogeneous elemental distributions for aqueous-grown iodosodalite, whereas hydrothermally grown iodosodalite samples had greater heterogeneity and porosity. The densities of HIPed samples with more glass binder were generally higher than samples HIPed without binder. The leach tests on samples containing 20 mass% glass binder showed that the iodine dissolution rate of HIPed hydrothermally grown iodosodalite was about 4 × higher than iodosodalite grown with the aqueous method. This study provides alternative techniques for immobilizing iodine streams in iodosodalite waste forms.

Effect of Post Processing Heat Treatment Routes on Microstructure and Mechanical Property Evolution of Haynes 282 Ni-Based Superalloy Fabricated with Selective Laser Melting (SLM)

Selective laser melting (SLM) is one of the most widely used additive manufacturing technologies. Fabricating nickel-based superalloys with SLM has garnered significant interest from the industry and the research community alike due to the excellent high temperature properties and thermal stability exhibited by the alloys. Haynes-282 alloy, a γ′-phase strengthened Ni-based superalloy, has shown good high temperature mechanical properties comparable to alloys like R-41, Waspaloy, and 263 alloy but with better fabricability. A study and comparison of the effect of different heat-treatment routes on microstructure and mechanical property evolution of Haynes-282 fabricated with SLM is lacking in the literature. Hence, in this manuscript, a thorough investigation of microstructure and mechanical properties after a three-step heat treatment and hot isostatic pressing (HIP) has been conducted. In-situ heat-treatment experiments were conducted in a transmission electron microscopy (TEM) to study γ′ precipitate evolution. γ′ precipitation was found to start at 950 °C during in-situ heat-treatment. Insights from the in-situ heat-treatment were used to decide the aging heat-treatment for the alloy. The three-step heat-treatment was found to increase yield strength (YS) and ultimate tensile strength (UTS). HIP process enabled γ′ precipitation and recrystallization of grains of the as-printed samples in one single step.

Hot isostatic pressing in metal additive manufacturing: X-ray tomography reveals details of pore closure

Hot isostatic pressing (HIP) of additively manufactured metals is a widely adopted and effective method to improve the density and microstructure homogeneity within geometrically-complex metal structures fabricated with laser powder bed fusion (LPBF). The role of pores in the fatigue performance of additively manufactured metal parts is increasingly being recognized as a critical factor and HIP post-processing is now heralded as a method to eliminate pores, especially for high-criticality applications such as in the aerospace industry. Despite the widely reported positive influence on fatigue performance and high efficiency of pore closure, examples have been reported in which pores have not been entirely closed or have subsequently re-opened upon heat treatment. A variety of porosity distributions and types of pores may be present in parts produced by LBPF and the effectiveness of pore closure may differ depending on these pore characteristics. In this work, X-ray tomography was employed to provide insights into pore closure efficiency by HIP for an intentional and artificially-induced cavity as well as for a range of typical process-induced pores (lack of fusion, keyhole, contour pores, etc.) in coupon samples of Ti6Al4V. The same samples were imaged non-destructively before and after HIP and aligned carefully for side-by-side viewing. High pore closure efficiency is demonstrated for all types of cavities and pores investigated, but near-surface pores of all types are shown to be problematic to varying degrees, in some cases perforating the superficial surface and creating new external notches. Subsequent heat treatments (annealing after HIP) in some cases resulted in internal pore reopening for previously closed internal pores as well as a new “blistering” effect observed for some near-surface pores, which the authors believe is reported for the first time. Implications of these results for quality control and HIP processing of LPBF parts are discussed. Finally, the utility of using HIP to consolidate intentionally-unmelted powder in order to improve production rates of powder bed fusion has great potential and is preliminarily demonstrated.

Structural designation and mechanical properties of TiNi/Ti2Ni laminated composites

TiNi/Ti2Ni laminated composites were fabricated using hot isostatic pressing (HIP) furnace together with structural designation by the control of Ti and Ni foil thicknesses. During HIP processing, Ti reacts with Ni, leading to the formation of softer TiNi and harder Ti2Ni. Through adjusting the thickness ratio of Ti and Ni foils, the ratio of TiNi and Ti2Ni fraction can be controlled. Hard-soft-hard, soft-hard-soft and hard-intermediate-soft laminated composites were designed, where hard layer was made by 50 μm Ti and 20 μm Ni, soft layer was made by 40 μm Ti and 20 μm Ni, and the intermediate structure between hard and soft ones was made by 45 μm Ti and 20 μm Ni. The mechanical properties were characterized using uniaxial tension, uniaxial compression, single edge-notched beam and split-Hopkinson pressure bar. The results show that all three laminated composites have compression strength of 1690 ∼ 1853 MPa and fracture toughness of 27.2 ∼ 37.9 MPa• m1/2 due to the same constituents of TiNi and Ti2Ni. However, the soft-hard-soft structure shows the best response under high strain rate because it was fractured into larger pieces while the other two structures were fractured into smaller pieces together with smaller fracture strain.

Hot Isostatically Pressed Zirconolite Wasteforms for Actinide Immobilisation

In order to demonstrate the deployment of Hot Isostatic Pressing (HIP) for the immobilisation of Pu stocks and residues, a series of active and inactive zirconolite formulations have been processed and characterised. In this instance, Ce, U, and Th have been applied as chemical surrogates for Pu4+. A range of formulations targeting isovalent Zr4+ site substitution (i.e. to simulate CaZr1-xPuxTi2O7) have been processed by HIP and characterised by powder X-ray diffraction, and scanning electron microscopy, in order to determine surrogate partitioning between the host zirconolite phase, and accessory phases that may have formed during the HIP process.

A study of reaction processes in SiC fiber reinforced Ni-Fe matrix composite during HIP

ABSTRACT Binary Ni-Fe metal matrix composite containing continuous W-core SiC fibers was fabricated by hot isostatic pressing (HIP) method to study the interface reaction processes. Interfacial reaction products were analyzed via field emission scanning electron microscope, electron probe microanalysis and micro-zone X-ray diffractometer. The results indicate that the reactions are so dramatical that even W core can decompose during the HIP process. The whole reaction processes of W-core SiC fiber can be divided into four stages. In stage 1, SiC starts to react with metal matrix to form Ni silicides with dissolved Fe element, graphite and Kirkendall void zone. In stage 2, SiC decomposes further and W core is exposed, then W core participates in reactions. In stage 3, SiC reacts completely, W core further reacts. In stage 4, W core decomposes completely and transform to be steady WC particles, which means the whole processes terminate. A model to clarify the mechanism of the reaction processes is proposed.

Influences of a Hot-Working Process on the Microstructural Evolution and Creep Performance of a Spray-Formed Nickel-Based Superalloy

A new third generation nickel-based powder metallurgy (PM) superalloy, designated as FGH100L, was prepared by spray forming. The effects of hot isostatic pressing (HIP) and isothermal forging (IF) processes on the creep performance, microstructure, fracture, and creep deformation mechanism of the alloy were studied. The microstructure and fracture were characterized by optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The coupled HIP and IF process improved the creep performance of the alloy under the creep condition of 705 °C/897 MPa. As for both the HIPed and IFed alloys, the creep process was dominated by the accumulation of dislocations and stacking faults, cutting through γ′ precipitates. The microstructural evolution was the main factor affecting the creep performance, which mainly manifested as coarsening, splitting, and morphology change of γ′ precipitates. Both the creep fractures of the HIPed and IFed alloys indicated intergranular fracture characteristics. In the former, wedge-shaped cracks usually initiated at the trigeminal intersection of the grain boundaries, while in the latter, cavity cracks generate more easily around the serrated curved grain boundary and carbides.