Prior Particle Boundaries Research Articles

Influences of PPB, PPB affect zone, grain boundary and phase boundary on crack propagation path for a P/M superalloy FGH4096

The fatigue crack growth (FCG) tests for hot isostatic pressed (HIPed), isothermal forged (IFed) and heat treated (HTed) FGH4096 superalloys were conducted in air at 650–750 °C with and without dwell time to investigate the effect of microstructural characteristics on FCG paths. Optical microscope (OM) and scanning electron microscope (SEM) with energy dispersive spectrometer (EDS) were used to observe and analyze the microstructures. After FCG tests, fracture surfaces and central sectioned surfaces were observed to investigate the crack propagation paths. FCG rate curves were also plotted to assist the analysis of FCG behaviors. The results indicated that prior particle boundaries (PPBs) were prevalent in HIPed specimens, but they transformed to PPB affect zones (PAZs) in IFed specimens by isothermal forging. After the standard heat treatment, PAZs were eliminated, and grain boundaries which were decorated with continuously distributed carbides became prevalent in HTed specimens. Grain boundaries in HTed specimens made the greatest contribution to FCG, and the highest FCG rate was attributed to the predominant intergranular failures. PPBs also had a pronounced influence on FCG, and the dominant failures of interparticle resulted in the high FCG rate in HIPed specimens. However, PAZs made the least contribution to FCG mainly owing to their particular microstructures. Thus, the prevalent failures of PAZs resulted in the lowest FCG rate in IFed specimens. Phase boundaries of large γ′ precipitates in PPBs or PAZs also had accelerating effect on FCG, while their contributions were relatively small.

The role of yttrium and titanium during the development of ODS ferritic steels obtained through the STARS route: TEM and XAS study

Oxide Dispersion Strengthened Ferritic Steels (ODS FS) are candidate materials for structural components in future fusion reactors. Their high strength and creep resistance at elevated temperatures and their good resistance to neutron radiation damage is obtained through extremely fine microstructures containing a high density of nanometric precipitates, generally yttrium and titanium oxides.This work shows transmission electron microscopy (TEM) and extended X-ray absorption fine structure (EXAFS) characterization of Fe-14Cr-2W-0.3Ti-0.24Y ODS FS obtained by the STARS route (Surface Treatment of gas Atomized powder followed by Reactive Synthesis), an alternative method to obtain ODS alloys that avoids the mechanical alloying to introduce Y2O3 powder particles. In this route, FS powders already containing Ti and Y, precursors of the nanometric oxides, are obtained by gas atomization. Then, a metastable Cr- and Fe-rich oxide layer is formed on the surface of the powder particles. During consolidation by HIP at elevated temperatures, and post-HIP heat treatments above the HIP temperature, this oxide layer at Prior Particle Boundaries (PPBs) dissociates, the oxygen diffuses, and Y-Ti-O nano-oxides precipitate in the ferritic matrix.TEM characterization combined with XAFS and XANES analyses have proven to be suitable tools to follow the evolution of the nature of the different oxides present in the material during the whole processing route and select appropriate HIP and post-HIP parameters to promote profuse and fine Y-Ti-O nanometric precipitates.

Fatigue behavior of freestanding AA2024 and AA7075 cold spray deposits

This paper examines the fatigue properties of freestanding AA7075 and AA2024 Cold Spray (CS) deposits. In this study, high pressure cold spray was used to make deposits large enough to machine mechanical test specimens free from the substrate. To quantify the mechanical performance, monotonic tensile and stress-controlled fatigue experiments were carried out on the freestanding CS and wrought AA7075 and AA2024 specimens. The experimental fatigue results revealed a reduction in the stress-life behavior of the CS deposits compared to the corresponding wrought material. However, when normalized by their ultimate tensile strengths, the CS specimens compared very well with the wrought materials, especially for the CS AA7075. Post-mortem analysis of the CS specimens revealed that fatigue cracks typically initiated from layer interface defects and subsequent crack propagation was influenced by the CS prior-particle boundaries. The results from this study indicate that refining the layer interface bonding may increase the fatigue resistance of freestanding aluminum alloy CS deposits.

Effects of Temperature and Pressure of Hot Isostatic Pressing on the Grain Structure of Powder Metallurgy Superalloy.

The microstructure with homogeneously distributed grains and less prior particle boundary (PPB) precipitates is always desired for powder metallurgy superalloys after hot isostatic pressing (HIPping). In this work, we studied the effects of HIPping parameters, temperature and pressure on the grain structure in PM superalloy FGH96, by means of scanning electron microscope (SEM), electron backscatter diffraction (EBSD), transmission electron microscope (TEM) and Time-of-flight secondary ion spectrometry (ToF-SIMS). It was found that temperature and pressure played different roles in controlling PPB precipitation and grain structure during HIPping, the tendency of grain coarsening under high temperature could be inhibited by increasing HIPping pressure which facilitates the recrystallization. In general, relatively high temperature and pressure of HIPping were preferred to obtain an as-HIPped superalloy FGH96 with diminished PPB precipitation and homogeneously refined grains.

Effect of Powder Characteristics and Oxygen Content on Modifications to the Microstructural Topology During Hot Isostatic Pressing of an Austenitic Steel

The effect of powder size distribution and oxygen content on the extent of multiple twinning and spatial distribution of oxide inclusions in hot isostatic pressed (HIPed) 316L steels was investigated using powders with different characteristics. Modifications to, and differences in their microstructural topology, were tracked quantitatively by evaluating the metrics related to twin related domains (TRDs) on specimens produced by interrupting the HIPing process at various points in time. Results revealed that powder size distribution has a strong effect on the extent of multiple twinning in the fully HIPed microstructure, with specimens produced using narrow distribution showing better statistics (i.e., homogeneously recrystallized) than the onesproduced using broad size distribution. The oxide inclusion density in fully HIPed microstructures increased with the amount of oxygen content in the powders while prior particle boundaries (PPBs) were only observed in the specimens that were HIPed using broad powder distribution. More importantly, results clearly revealed that the spatial distribution of the inclusions was strongly affected by the homogeneity of recrystallization. Implications of the results are further discussed in a broader context, emphasizing the importance of utilizing the occurrenceof solid state phase transformations during HIPing for controlling the microstructure evolution.

The connections between powder variability and coating microstructures for cold spray deposition of austenitic stainless steel

This article explores the variability in austenitic stainless steel feed-stock powders and its effect upon low-pressure cold spray deposition. Four, commercial, austenitic stainless steel powders were deposited using cold spray deposition with helium gas at a temperature of 230°C and a pressure of 1.7MPa. Electron microscopy and X-ray diffraction were used to characterize the microstructure of both the powders and the resultant coatings. The particle size for the powders ranged from 17 to 40μm, while the crystallites ranged from 2 to 6μm in size. Only the 304L stainless steel powder was purely austenite in phase; while the three 316L powders had ferrite contents ranging from 23 to 50%. The deposition efficiency of the four powders varied considerably from 12 to 42%, but this variation did not correlate with the ferrite content alone. The spatial location of the ferrite in the cold sprayed material was highly dependent upon the powder type used: ferrite primarily on the outside of austenite particles, mixtures of wholly ferrite and wholly austenite particles, and particles with both ferrite and austenite internally. After cold spray deposition, all of the coatings exhibited a composite microstructure containing highly deformed prior particle centers with small (300nm) crystallites near the prior particle boundaries.

Influence of Powder Surface Contamination in the Ni-Based Superalloy Alloy718 Fabricated by Selective Laser Melting and Hot Isostatic Pressing

The aim of this study was to gain a deep understanding of the microstructure-mechanical relationship between solid-state sintering and full-melting processes. The IN718 superalloy was fabricated by hot isostatic pressing (HIP) and selective laser melting (SLM). Continuous precipitates were clearly localized along the prior particle boundary (PPB) in the HIP materials, while SLM materials showed a microstructure free of PPB. The mechanical properties of specimens that underwent SLM + solution treatment and aging were comparable to those of conventional wrought specimens both at room temperature and 650 °C. However, a drop was observed in the ductility of HIP material at 650 °C. The brittle particles along the PPB were found to affect the HIP materials’ creep life and ductility during solid-state sintering.

Studies on induction hardening of powder-metallurgy-processed Fe–Cr/Mo alloys

Induction hardening of dense Fe–Cr/Mo alloys processed via the powder-metallurgy route was studied. The Fe−3Cr−0.5Mo, Fe−1.5Cr−0.2Mo, and Fe−0.85Mo pre-alloyed powders were mixed with 0.4wt%, 0.6wt%, and 0.8wt% C and compacted at 500, 600, and 700 MPa, respectively. The compacts were sintered at 1473 K for 1 h and then cooled at 6 K/min. Ferrite with pearlite was mostly observed in the sintered alloys with 0.4wt% C, whereas a carbide network was also present in the alloys with 0.8wt% C. Graphite at prior particle boundaries led to deterioration of the mechanical properties of alloys with 0.8wt% C, whereas no significant induction hardening was achieved in alloys with 0.4wt% C. Among the investigated samples, alloys with 0.6wt% C exhibited the highest strength and ductility and were found to be suitable for induction hardening. The hardening was carried out at a frequency of 2.0 kHz for 2–3 s. A case depth of 2.5 mm was achieved while maintaining the bulk (interior) hardness of approximately HV 230. A martensitic structure was observed on the outer periphery of the samples. The hardness varied from HV 600 to HV 375 from the sample surface to the interior of the case hardened region. The best combination of properties and hardening depth was achieved in case of the Fe−1.5Cr−0.2Mo alloy with 0.6wt% C.

Influence of Sintering Temperature and Cooling Rate on Microstructure and Mechanical Properties of Pre-alloyed Fe–Cr–Mo Powder Metallurgy Steel

Influence of carbon content, sintering temperature and cooling rate on microstructure and mechanical properties of a Fe–Cr–Mo alloy has been investigated. Graphite powder with varying wt% (0.4 and 0.8) was mixed with Fe–Cr–Mo prealloyed powder. The green preform compacts of 32 mm diameter were prepared by pressing in a cylindrical die at 650 MPa. The compacts were sintered for 1 h in a flowing gas mixture of 90% N2 +10% H2. Two different sintering temperatures (1120 and 1200 °C) were used. The sintered compacts were cooled at 3.5 °C/min or 6.0 °C/min. It was found that the higher sintering temperature of 1200 °C resulted in improved mechanical properties. Ferrite with pearlite was mostly observed in alloy with 0.4 wt% C while in 0.8 wt% C alloy, a carbide network was also observed. Graphite particles were observed at the prior particle boundaries in some of the alloys. The Fe–Cr–Mo–0.4C alloy processed at 1200 °C with 6.0 °C/min exhibited best strength and ductility. The observed properties were correlated with the microstructure.

Hot isostatic pressing of in-situ TiB/Ti-6Al-4V composites with novel reinforcement architecture, enhanced hardness and elevated tribological properties

In this study, titanium borides reinforced Ti-6Al-4V composites have been successfully prepared by hot isostatic pressing (HIPing). The microstructure of the as-fabricated samples was investigated using X-ray diffraction technique, secondary electron microscopy and electron backscatter diffraction and the mechanical properties evaluated through micro-hardness and wear resistance measurements together with nano-indentation. It was found that during HIPing the additive particles TiB2 have transformed into TiB needles which tend to decorate at prior particle boundaries of the consolidated powder particles to form a network structure. Under the same HIPing condition, the needles became increasingly coarser and agglomerated with increased addition of TiB2. The micro-hardness of the synthesized materials increased with increased volume fraction of TiB. Nano-indentation measurement demonstrates that the TiB network structure shows much higher nanohardness than the surrounding matrix regions. The friction coefficient of the synthesized composites decreased continuously with increased volume fraction of TiB, indicating improved wear resistance. High resolution transmission electron microscopy analysis on wear debris revealed the formation of a series of oxides suggesting that chemical reaction between alloy elements and oxygen in air may have happened. It is thus believed that the wearing of the current samples is a result of both friction and chemical reaction.

Development of Oxide Dispersion Strengthened (ODS) Ferritic Steel Through Powder Forging

Oxide dispersion strengthened (ODS) ferritic steels are candidates for cladding tubes in fast breeder nuclear reactors. In this study, an 18%Cr ODS ferritic steel was prepared through powder forging route. Elemental powders with a nominal composition of Fe-18Cr-2 W-0.2Ti (composition in wt.%) with 0 and 0.35% yttria were prepared by mechanical alloying in a Simoloyer attritor under argon atmosphere. The alloyed powders were heated in a mild steel can to 1473 K under flowing hydrogen atmosphere. The can was then hot forged. Steps of sealing, degassing and evacuation are eliminated by using powder forging. Heating ODS powder in hydrogen atmosphere ensures good bonding between alloy powders. A dense ODS alloy with an attractive combination of strength and ductility was obtained after re-forging. On testing at 973 K, a loss in ductility was observed in yttria-containing alloy. The strength and ductility increased with increase in strain rate at 973 K. Reasons for this are discussed. The ODS alloy exhibited a recrystallized microstructure which is difficult to achieve by extrusion. No prior particle boundaries were observed after forging. The forged compacts exhibited isotropic mechanical properties. It is suggested that powder forging may offer several advantages over the traditional extrusion/HIP routes for fabrication of ODS alloys.

Role of Microstructure Heterogeneity on Fatigue Crack Propagation of Low-Alloyed PM Steels in the As-Sintered Condition

Due to their lower production costs, powder metallurgy (PM) steels are increasingly being considered for replacing wrought counterparts. Nevertheless, the presence of a non-negligible volume fraction of porosity in typical PM steels makes their use difficult, especially in applications where cyclic loading is involved. On the other hand, PM offers the possibility of obtaining steel microstructures that cannot be found in wrought. Indeed, by adequately using alloying strategies based on admixing, pre-alloying, diffusion bonding or combinations of those, it is possible to tailor the final microstructure to obtain a distribution of phases that could possibly increase the fatigue resistance of PM steel components. Therefore, a detailed study of the effect of different microstructural phases on fatigue crack propagation in PM steels was performed using admixed nickel PM steels (FN0208) as well as pre-alloyed PM steels (FL5208). Specimens were pressed and sintered to a density of 7.3 g/cm3 in order to specifically investigate the effect of matrix microstructure on fatigue properties. Fatigue crack growth rates were measured at four different R-ratios, 0.1, 0.3, 0.5 and 0.7 for both PM steels. The negative effect of increasing the R-ratio on fatigue properties was observed for both alloys. The crack propagation path was characterized using quantitative image analysis of fracture surfaces. Measurements of roughness profile and volume fractions of each phase along the crack path were made to determine the preferred crack path. Weak Ni-rich ferritic rings in the FN0208 series (heterogeneous microstructure) caused a larger crack deflection compared to the more homogeneous microstructure of the FL5208 series. It was determined that, contrary to results reported in literature, crack propagation does not pass through retained austenite areas even though fatigue cracks propagated predominantly along prior particle boundaries, i.e., intergranular fracture.

Microstructure, Tensile Properties, and Hot-Working Characteristics of a Hot Isostatic-Pressed Powder Metallurgy Superalloy

A series of microstructure observation, tensile, and hot compression tests were conducted to investigate the variation of microstructure, tensile properties, and hot-working characteristics of a powder metallurgy (PM) superalloy with hot isostatic pressing (HIPing) temperature, to establish a basis for the parameter selection for PM superalloy preparation. The results show that the dendritic structure from the powder was not completely removed until the HIPing temperature is above the γ′ solvus; γ/γ′ eutectic formed when the powder particles were HIPed at 1533 K (1260 °C) or above. Prior particle boundaries (PPBs) were observed in alloys HIPed at 1513 K (1240 °C) and below; the PPB decoration is serious in alloys HIPed at 1483 K and 1513 K (1210 °C and 1240 °C), owing to melting and aggregation of the boride phase at the particle boundaries during HIPing; the PPBs were eliminated when the HIPing was done at 1533 K (1260 °C) or above. Tensile fracture mode of the alloy changes from inter-particle and transgranular mixed fracture to transgranular fracture with increasing HIPing temperature, which is in accordance with the change in precipitate distribution at the PPBs. The hot workability of alloy is poor for all combinations of HIPing/deformation conditions except for HIPing at sub-solvus temperature and deformation at low strain rates.

Effect of the Prior Particle Boundary on the Microstructure and Mechanical Properties of Hot-Isostatic-Pressed IN718 Alloy

Effect of the Prior Particle Boundary on the Microstructure and Mechanical Properties of Hot-Isostatic-Pressed IN718 Alloy

Microstructure and mechanical properties of cold sprayed 6061 Al in As-sprayed and heat treated condition

Microstructural and mechanical properties of cold sprayed 6061 aluminum deposits on 6061-T6 aluminum alloy substrates are investigated under various heat treatment conditions, i.e. as-deposited, stress relieved and T6. The local mechanical property variation in the as-deposited material are explored using nanoindentation technique, and correlated with microstructural characterization conducted via electron back-scattered diffraction. It is found that the prior particle boundaries have ~0.4GPa higher hardness than particle interiors, which is attributed to grain refinement in these regions promoted by local dynamic recrystallization. Also, the bulk-scale mechanical properties of the deposits are evaluated by microtensile testing in various post-heat treatment conditions and compared to those of conventionally processed 6061-T6 aluminum. The as-deposited material showed markedly higher ultimate strength (~460MPa) and lower ductility (~3%) compared to conventionally processed material and this is attributed to significant cold working during the cold spray deposition process and associated grain boundary strengthening and dislocation strengthening mechanisms. Heat treated specimens showed a slight improvement in both ultimate strength and ductility compared to the as-deposited condition. These improvements are attributed to an improvement in metallurgical bonding at prior particle boundaries and a modest increase in the density of strengthening precipitates. Fractography of the specimens revealed that the heat treatment also changes the fracture characteristics of the cold sprayed 6061 aluminum deposit. The residual stress profiles and bond strength of the deposits are also studied using x-ray diffraction, tensile pull-off and three lug shear testing, respectively.

Preparation of hot-isostatic-pressed powder metallurgy superalloy Inconel 718 free of prior particle boundaries

A new hot-isostatic-pressing (HIPing) scheme, which consists of a short time holding above the solidus and a long time annealing below the incipient temperature of Laves phase, has been proposed for Inconel 718 powder to avoid the formation of prior particle boundaries (PPBs). The powder metallurgy Inconel 718 compacted with the above HIPing scheme is free of PPBs, features transgranular ductile fracture mode during tensile tests, and possesses a stress rupture life that is much longer than that of the wrought alloy.

Microstructure and mechanical properties of the PM precipitation-hardening alloy N07626: effect of HIP temperature and solution annealing

The microstructure and mechanical properties of the precipitation-hardening powder metallurgy Ni-base alloys UNS N07626 are investigated. The material was compacted by hot isostatic pressing and solution treated adopting distinct conditions to reveal the effect of processing temperature on material properties. The microstructure is analysed by scanning electron microscopy, optical microscopy and electron backscatter diffraction and energy-dispersive X-ray spectroscopy. The prior particle boundaries (PPBs) are related to the presence of thermally stable nanometric Nb-carbides and aluminium oxides. An increase of processing temperature conditions is associated to a slight decrease of mechanical strength and to an appreciable improvement of toughness, as suggested by Instrumented V-Notch Charpy experiments. This behaviour is attributed to the reduction of the density of PPBs precipitates acting as void-nucleating sites in a micro-ductility mechanism. The precipitates’ density is related to a limited grain growth and the resulting separation of PPBs from grain boundaries.

ODS ferritic steels produced by an alternative route (STARS): microstructural characterisation after atomisation, HIPping and heat treatments

The conventional PM ODS Ferritic Steel (FS) processing route includes gas atomisation of steel powder and its mechanical alloying (MA) with Y2O3 powder particles to dissolve yttrium and form, during consolidation, a dispersion of oxide nanoparticles (Y–Ti–O) in a nanostructured matrix. This work presents an alternative route to produce ODS steels avoiding MA: STARS (Surface Treatment of gas Atomized powder followed by Reactive Synthesis). STARS FS powders with composition Fe–14Cr–2W–0.3Ti–0.23Y, already containing the nanoparticles precursors, were gas-atomized. Oxygen, Y and Ti contents were tailored to the required values to form Y–Ti–O nanoparticles during processing. Powders were HIPped at 900, 1220 and 1300°C. Specimens HIPped at 900 and 1220°C were heat treated (HT) at temperatures ranging from 1200 to 1320°C. The microstructural evolution with HIP and HT temperatures, including characterisation of nanoparticles and feasibility of achieving complete dissolution of prior particle boundaries (PPBs) were assessed.

An Investigation of Direct Powder Forging of Nickel Superalloy FGH96

Powder metallurgy (PM) nickel-based superalloy has been widely used in high temperature applications and is most commonly manufactured using hot isostatic pressing (HIP). However, HIP is an expensive process and takes a long time at high temperature which leads to the formation of networks of prior particle boundaries (PPBs). In this study, a recently developed processing method - direct powder forging (DPF) was employed to produce a PM nickel-based superalloy component, using a single acting hydraulic press under normal atmosphere. EBSD study has been conducted for its microstructure, grains size distribution, and grain boundary misorientation; and mechanical testing has been carried out for its hardness and tensile properties at room temperature and 650°C. It has been found that the DPFed material has reached full density in the whole component. Networks of PPBs have been broken in the direct powder forged FGH96 alloy. Compared with HIPed FGH96 alloy, the DPFed material has a substantially higher recystallisation nucleation degree and more recrystallised sub-grains. After heat treatment, the tensile properties of the direct powder forged FGH96 alloy match or surpass those of the material produced by HIP plus isothermal forging.

Preparation of Mg/Nano‐HA Composites by Spark Plasma Sintering Method and Evaluation of Different Milling Time Effects on Their Microhardness, Corrosion Resistance, and Biocompatibility

Herein, Mg/nano‐HA composites (10 wt% nano‐HA) are prepared by spark plasma sintering (SPS) technique. Indeed, hydroxyapatite (HA) is used as reinforcement material in order to maintain biocompatibility and bioactivity. The influence of high energy ball milling time are considered by different methods. The study finds that an appropriate time (5 h) during high energy ball milling process effectively can eliminate the prior particle boundary (PPB). Furthermore, the Mg/nano‐HA composites milled for 5 h has appropriate microhardness, stable corrosion resistant, and excellent biocompatibility, which can act as a potential candidate for orthopedic implants.