Intergranular Precipitation Research Articles

THE INFLUENCE OF Nb, Ta AND Ti MODIFICATION ON HOT-WORK TOOL-STEEL GRAIN GROWTH DURING AUSTENITIZATION

In order to explore the influence of niobium, tantalum and titanium modification on grain growth in high-thermal-conductive hot-work tool steel during austenitisation at high temperatures, three types of modified steels (sample HTCS-130 + 0.06 w/% Nb, sample HTCS-130 + 0.03 w/% Ta, sample HTCS-130 + 0.006 w/% Ti) based on reference (sample HTCS-130 – 0) were prepared. The effect of different austenitisation temperatures (1030, 1060, 1080 and 1100) °C on hardness after quenching and grain size were investigated. The results show that there is a positive effect on the mechanical properties and a decreased grain-growth effect in the modified steel samples. The precipitation behaviour of the carbides was also investigated with electron microscopy. The Mo-W carbides were relatively weak at retaining grain size, but their pinning effect was increased with the incorporation of other carbide-forming elements like Nb and Ta. MC-type carbides on the grain boundary were effective at grain-boundary pinning. Nb even further increased the resistance by forming NbC carbides. The addition of Ti, on the other hand, proved to be ineffective due to the intergranular precipitation of the formed carbonitrides.

Effect of particle size on grain growth of Nd-Fe-B powders produced by gas atomization

Gas atomized Nd-Fe-B powders of several compositions were separated in different size fractions by sieving. These fractions were annealed between 1100 °C and 1150 °C for 24 and 96 h. The oxygen content of the powders was measured before and after annealing for the different size fractions. The oxygen concentration of the powders depends strongly on the particle size and increases significantly during annealing, particularly in the case of small particle sizes. The effect of particle size on the microstructural changes was analyzed in detail, particularly on grain growth, using high resolution scanning electron microscopy and transmission electron microscopy. Electron back scattering diffraction was used to measure grain size. When the particle size rises, the degree of sintering decreases and the higher solid/vapor surface area reduces the mobility of grain boundaries. Oxidation also reduces grain growth rate and its effect is more evident for particles sizes below 45–63 μm and high Nd concentrations. Nb addition leads to the formation of intra- and intergranular precipitates. The size of these Nb-Fe-containing precipitates increases with the particle size for equivalent annealing conditions. At 1150 °C, Nb loses its effect as an inhibitor of grain growth in the particle size fractions larger than 45–63 μm. • Grain size of gas atomized powders increases with particle size from 2 to 10 μm. • Intragranular Nb nanoprecipitation in gas atomized powders was confirmed. • Oxygen (0.1–0.23 wt.%) delays grain growth in small particles (<45–63 μm). • Porosity reduced the mobility of grain boundaries in large particles (>20–63 μm). • Nb loses its effect as inhibitor of grain growth in large particles (>45–63 μm).

Effect of regression and re-aging treatment on tensile yield strength anisotropy of 7050 aluminum alloy

In this study, the evolution rule of yeild strength and microstructure along the axial, hoop and radial directions of 7050 aluminum alloy ring forging during Regression and re-aging (RRA) heat treatment was studied by using the experimental technology of tensile property test and transmission electron microscope (TEM) and Electron backscatter diffraction (EBSD) observation. During the RRA process, the evolution rule of intragranular and intergranular precipitates along axial, hoop and radial directions of 7050 aluminum alloy ring forging was described by establishing a model. After the pre-aging stage, the yield strength of samples along the axial, hoop and radial directions of forgings shows obvious anisotropy, and the anisotropy decreases gradually after regression and re-aging treatment. The results show that the cause of the larger yield strength anisotropy is the inhomogeneous size and distribution of precipitates after the pre-aging stage; After the regression stage, the cause of the reduction of the yield strength anisotropy is due to the homogeneous distribution of intragranular precipitates; After re-aging treatment, the reasons of further reduction of the yield strength anisotropy are because of more homogeneous distribution of intragranular precipitates and the discontinuous of intergranular precipitates.

Effect of retrogression treatment with different heating rates on microstructure, strength and corrosion behaviors of 7050 alloy

In order to achieve the combination of mechanical and corrosion properties for 7050 alloy, a novel retrogression and re-aging regime were proposed in this study. During the retrogression, the pre-aged alloys were heated from room temperature to 190 °C with different retrogression heating rates, and then were cooled quickly for following re-aging treatment. The size of precipitates and the width of precipitation free zones decreased with the increasing retrogression heating rate, the mechanical properties of 7050 alloy were improved, while the corrosion resistance was reduced. When the retrogression heating rate was controlled at 3 °C/min, a good combination of strength and corrosion resistance of 7050 alloys could be obtained. The excellent properties were attributed to the homogeneous distribution of intragranular precipitates (η’) and the coarsening of intergranular precipitates (η) after the novel retrogression and re-aging regime.

Weldability and mechanical properties of dissimilar laser welded aluminum alloys thin sheets produced by conventional rolling and Additive Manufacturing

The production of parts involving the combination of different materials has gained a lot of interest in recent years as a strategy for achieving weight reduction. Particular attention has been paid to the joining process of mechanical components obtained with different production processes. This paper aims at studying the feasibility of laser welding to join rolled 6082-T6 thin sheet to A357 sheet produced by Laser-based Powder Bed Fusion (LPBF). The role of welding parameters, LPBF scan strategy and post heat treatment on the weld bead formation has been studied by means of microstructural and mechanical characterization. The results showed that a higher welding speed reduces the porosity in the weld bead obtaining values of about 5% in terms of percentage of fused area, while the post heat treatment of the LPBF sheet has the greatest influence on the final properties of the joint compared to the scanning strategy used. In particular, a complete T6 heat treatment of the additive sheets leads to an ultimate tensile strength of about 200 MPa closes to the reference condition 6082−6082. Hardness measurements showed that a drop in the values occurred at the center of the bead regardless of the treatment conditions while Digital Image Correlation (DIC) strain analysis confirmed that higher deformation occurred in the Fused Zone and in the Heat Affected Zone. SEM-EDS analysis revealed the presence of intergranular precipitates which contain Fe, Si, Mg and Mn in the fused zone.

Failure Analysis of a C-276 Alloy Pipe in a Controlled Decomposition Reactor.

Failure analysis was carried out on a ruptured C-276 pipe heated externally at 1050 °C, which had been used for a few months in a controlled decomposition reactor (CDR) system. To catch the decomposed perfluorinated compounds (PFCs, e.g., CF4, SF6, NF3, C3F8 and C4F8) present in the exhaust gas, the C-276 reactor was periodically purged with water mist, which caused a temperature gradient from the external to the inner surface of the pipe. The precipitation of large amounts of intermetallic compounds along the grain boundaries were found to be corroded preferentially. The internal surface of the used pipe was covered with many fine cracks. The corrosion and cracking of grain boundary precipitates accounted for the short service life of the C-276 pipe. Compositional measurements by electron probe micro-analyzer (EPMA) and phase identification by electron backscatter diffraction (EBSD) confirmed the presence of δ and μ phases in the ruptured pipe. The coarse intergranular precipitates were the δ phase (Mo7Ni7), which were enriched in Mo and Cr. Moreover, the fine precipitates dispersed intergranularly and intragranularly were the μ phase (Mo6Ni7), which were abundant in Mo and W. The numerous precipitates present in the matrix and along the grain boundaries were responsible for an obvious loss in the strength and ductility of the used C-276 pipe.

Effect of grain-boundary θ-Al2Cu precipitates on tensile and compressive creep properties of cast Al–Cu–Mn–Zr alloys

Tensile and compressive creep tests were performed at 300 °C on high-temperature Al–Cu–Mn–Zr (ACMZ) alloys with 6 wt% Cu (6Cu) and 9 wt% Cu (9Cu) to evaluate the effect on creep properties of micron-size θ-Al2Cu intergranular precipitates. For compressive creep, the increased volume fraction of θ-precipitates at grain boundaries (from ∼0.7% in 6Cu to ∼ 6% in 9Cu) does not affect deformation rates across the investigated stress range of 15–110 MPa, consistent with creep being controlled by submicron θ′-Al2Cu precipitates within grains, whose size and fractions are the same in both alloys. In contrast, for tensile creep, 9Cu creeps faster than 6Cu at stresses above 20 MPa, and this difference increases with the stress level. This discrepancy between tensile and compressive creep behavior is explained by cavitation during tensile creep, which is favored by higher volume fraction and larger size of intergranular θ precipitates in 9Cu. Conversely, larger precipitates impede cavity linkage resulting in improved creep ductility of 9Cu as compared to 6Cu at 300 °C.

Comprehensive analysis of nitriding behavior and synergistic strengthening mechanism on quenched M50 steel

M50 secondary hardening steel is widely used as high-end bearing steel for engine shafts in the aerospace industry. In this study, plasma nitriding at different temperatures varying from 410 to 570 °C was performed on quenched M50 steel to investigate the response of the microstructure and mechanical properties to nitriding temperature and advance the scientific understanding of the mechanism involved. The nitrided samples were fully characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, electron microprobe analysis, Vickers hardness tester, and Pin-on-disc tribometer. The results revealed that the nitrided layer's thickness, microstructure, hardness, and wear resistance depended on the treatment temperature. When the nitriding treatment performs at secondary-hardening temperature, the samples were endowed with excellent wear resistance and gentle hardness profile, i.e., hardness profile with a gradually reducing hardness profile towards the core, attributed to the synergistic strengthening. Moreover, the thermodynamic calculation was involved to clarify the formation mechanism of coarse intergranular precipitation, which can be connected to form a wave shape line at higher temperatures.

Cavitation-resistant intergranular precipitates enhance creep performance of θ′-strengthened Al-Cu based alloys

Tensile and compressive creep properties of a quaternary Al-Cu-Mn-Zr (ACMZ) alloy and its commercial counterpart (Al-Cu-Mn-Zr with Ni, Co and Sb additions, RR350) are investigated at 300°C. At low stresses up to 30 MPa where diffusional creep dominates, creep resistance is the same in tension and compression and RR350 deforms more slowly than ACMZ, consistent with RR350 alloy's larger linear fraction of intergranular precipitates (Al7Cu2(NiFe) and Al9FeNi for RR350 vs. θ-Al2Cu for ACMZ) and a reduced fraction of precipitate-free zones near grain boundaries. At stresses between 30 and 80 MPa, dislocation creep with a stress exponent n ∼ 3 becomes rate-limiting in compression, which is expected to be controlled by θ′ precipitates within the grain bulk. By contrast, in tension, enhanced creep rate and higher apparent stress exponents are measured, consistent with cavitation at intergranular precipitates becoming increasingly dominant as the stress increases. In the dislocation creep regime, RR350 alloy is again more creep resistant than ACMZ alloy, which is related to three mechanisms (i) a reduced fraction of softer precipitate-free zones, (ii) more effective load transfer to intergranular precipitates, and (iii) reduced cavitation. A model for cavitation is applied to calculate tensile creep rates from compressive creep rates and the model successfully predicts the improved tensile creep resistance of the RR350 alloy. The present investigation underscores the importance of intergranular grain boundary precipitates, in addition to strengthening θ′ precipitates, in enhancing the creep resistance of Al-Cu alloys.

The Effect of Heat Treatment on Fracture Behavior in ENiCrFe-7 Weld Overlay Cladding Materials

There were more intergranular precipitates and intragranular NbC in the interdendrites for the heat treatment one after 615 °C/48h than that of as-weld, the intergranular precipitates increased resulted in the emergence of cracks in the grain boundary 615 °C/48h sample, which only worked in local region during the failure of the materials. The lattice distortion in the interdendrites reduced resulted from NbC precipitates increased by the heat treatment. As a result, the fractography was classified as brittle and transgranular fracture and the interdendrites played a major role in the failure of as-weld Alloy ENiCrFe-7, and the fractography was ductility fracture for the heat treatment one after 615 °C/48h.

Effect of Precipitates and Cold Working on the Reheat Cracking of STS 347 Austenitic Stainless Steel

The aim of this study was to investigate the effect of precipitates and cold working on the reheat cracking of STS 347 austenitic stainless steel. Three different heat treatments to form NbC and M23C6 precipitates were applied to the STS 347 austenitic stainless steel and 10% pre-strain was additionally introduced to simulate cold working. The NbC and M23C6 precipitates were examined by SEM and XRD analysis, and creep rupture test was conducted under various temperature and stress conditions. The specimen having intergranular NbC precipitate prior to cold working had enhanced intergranular creep resistance and the tendency to reheat cracking decreased at temperature conditions between 650 oC and 750 oC, compared to the specimens processed by solution treatment or M23C6 formation treatment. This means that for STS 347 that has not been subjected to stabilization treatment, the cold working strain limits required by ASME’s boiler and pressure vessel code should be lowered, or if stress relief treatment is impractical, the stabilization heat treatment should be performed prior to cold working. Although the precipitation of M23C6 is not the major cause of reheat cracking, it should still be considered as a factor that increase the sensitivity of reheat cracking because M23C6 forms mostly at the grain boundary and grows rapidly, adversely affects creep ductility.

Hydride embrittlement resistance of Zircaloy-4 and Zr-Nb alloy cladding tubes and its implications on spent fuel management

• Mechanical tests of hydrided Zircaloy-4 and Zr-Nb alloy with Ring Compression Test. • EBSD analyses on texture of reactor-grade hydrided Zircaloy cladding tubes. • Abrupt ductile to brittle transition at 560 and 490 wppm for Zr-4 and Zr-Nb alloy. • Larger grain size of Zr-4 reduces interlink of inter-granular hydrides. • Reduced hydride interlink increases hydride embrittlement resistance. In the spent fuel storage phase, nuclear fuel cladding is subjected to increased embrittlement owing to a large amount of hydride precipitation. This study compares the differences in the hydrogen-induced cladding embrittlement of cold work stress-relief annealed (CWSR or SRA) Zircaloy–4 and Zr-Nb alloy cladding with ring compression test at the temperature of the spent fuel pool, which is approximately 40 °C. Experiments demonstrate that an abrupt ductile to brittle (DTB) transition occurs at the critical hydrogen content of 560 and 490 wppm for Zircaloy-4 and the tested Zr-Nb cladding tubes, respectively. Even beyond the critical hydrogen content, sufficiently high cladding ductility with the offset strain >10% is maintained up to ∼ 90 MWd/kgU for both cladding materials on the rod-average basis. Extensive EBSD analyses coupled to thermodynamic modeling demonstrate that this is primarily due to the slightly larger grain diameter of Zircaloy-4 tube, which reduces the number of available sites for inter-granular hydride precipitation. Reduced inter-granular hydride precipitation prevents the extent of hydride interlink, thereby improving the hydride embrittlement resistance. Nevertheless, the tested Zr-Nb alloy cladding presents an extened discharge burnup limit for abrupt DTB transition owing the reduced in-core cladding oxidation rate. The presented understanding of microstructural effect on hydride interlink and resulting embrittlement may provide a basis for understanding the general hydride embrittlement phenomena of Zircaloy cladding which include, but not limited to, wet storage, dry storage, and post-accident ductility of high burnup Zircaloy cladding.

Precipitation behavior of M23C6 in high nitrogen austenitic heat-resistant steel

The precipitation behavior of M23C6 carbides in LF25 high nitrogen austenitic heat-resistant steel was investigated from 700 °C to 850 °C. The results demonstrated that the precipitates in LF25 steel included Nb (C, N) and M23C6 during the aging treatment. M23C6 carbides with four morphologies formed during aging followed a sequence of intergranular precipitates, parallel plate-like precipitates, cellular precipitates, and intragranular precipitates with increasing aging temperature. The four types of morphologies of M23C6 retained a cube-on-cube crystallographic relation with the matrix. Firstly, the M23C6 precipitated as discontinuous particles along the grain boundaries due to the faster diffusion rate of the elements. With the aging temperature increasing, intergranular M23C6 grew and connected to form a film-like morphology. The partial dislocations slipped out from the non-coherent twin boundaries due to the stresses resulting from quenching and elastic modulus together with atomic volume difference between the M23C6 and austenite matrix. M23C6 nucleated at the slipped dislocations and grew along the (111) or (110) planes to form a parallel plate-like morphology. Cellular precipitates were precipitated from supersaturated austenite matrix and then formed an alternating morphology of lamellae M23C6 and austenite during the discontinuous precipitation reaction. Due to high resistance to intracrystalline growth, the intragranular precipitates were short rods or granules with small sizes.

The effect of exposure to elevated temperatures on the microstructure and hardness of Mg–Ca–Zn alloy

Abstract The die cast Mg-5 wt.% Ca-6 wt.% Zn ternary alloy was exposed to 160 °C for different times of up to 40 days. Microstructural analysis and microhardness and hardness measurements of the samples after different exposure times enabled the thermal stability of the cast alloy to be monitored. The as-cast structure is composed mainly of α-Mg solid solution, elliptical grain boundary particles of CaMg2, and an intergranular eutectics of Mg and Ca2Mg6Zn3. The grain size of α-Mg and the structure of the alloy did not change during the treatment, but the amount of the intergranular phases increased slightly. Exposure to 160 °C resulted in a decrease in micro-hardness of the α-Mg grains, but no change in the overall hardness of the samples was observed. Microstructural analyses indicated diffusion of the solute elements from the α-Mg grains to the grain boundaries, resulting in a decrease in hardness of the α-Mg which is, however, compensated by an increase in hardness related to intergranular precipitates. Using a simple diffusion model, the diffusion coefficient of Ca atoms in Mg at 160 °C was estimated to be 2.1 · 10 –17 m2 s– 1, which is greater than that of Zn by two orders of magnitude.

Early stages of discontinuous precipitation reaction in an advanced Cr-Fe-Ni alloy isothermally aged at 800 °C

The initiation mechanism of the discontinuous precipitation (DP) reaction driven by migrating grain boundaries (GBs) in Alloy 33 (Cr-Fe-Ni-N) isothermally aged at 800 °C has been studied using analytical electron microscopy, which includes scanning/transmission electron microscopy (STEM/TEM), X-ray energy-dispersive spectroscopy (XEDS), and electron diffraction. Precipitation products including both the types of precipitated phases (FCC M23C6 and M6N with diamond-cubic structure) and the GB morphology generated in the early stages of the aging process have been investigated in detail to correlate the experimental findings of the present work with the DP initiation mechanisms reported in the literature. STEM-XEDS elemental maps and electron diffraction data confirmed the FCC M23C6 was the first precipitated phase at the GBs, generating a Cr-depleted zone along the boundary and around the intergranular precipitates. The most remarkable characteristic in the initiation of DP reaction observed in this alloy system refers to the consistent evidence that the GB migration occurred to a significant extent while the M23C6 precipitates remain at the original GB position. In all observations, GB migration occurred against the capillarity force of the concave-forward boundary curvature, thereby suggesting a strong chemical force acting on the GB. Micro and nano-scale analytical STEM-XEDS results corroborated the existence of a compositional gradient of Cr across all migrating GB, confirming that a chemical driving force is responsible for the displacement of the GB. It is concluded that in the overall precipitation phenomenon the necessary solute partitioning is operated by interface diffusion mechanism through the moving GB acting as reaction front. Extensive STEM-XEDS analyses of the precipitation products observed in this investigation have confirmed that diffusion-induced grain boundary migration (DIGM) plays a major role as precursor to the initiation of DP reaction in Alloy 33.

Effect of the nanostructuring by high-pressure torsion process on the secondary phase precipitation in UNS S32750 Superduplex stainless steel

In this work, the precipitation and the morphology of secondary phases after severe plastic deformation (SPD) processing followed by an isothermal treatment was investigated. High-pressure torsion (HPT) was the SPD process carried out on superduplex 2507 (UNS S32750) stainless steel material under P = 6 GPa at room temperature. At this high strain levels (ε up to 170) samples have shown grain size decrease and strained microstructure with high dislocation density and nanostructure features. After a short isothermal treatment at 830 °C, the sigma phase and chromium nitrides were revealed as the main secondary phases identified by scanning and transmission electron microscopy and element analysis by energy dispersive spectroscopy. Scanning precession electron diffraction and automated crystal orientation mapping have been carried out in order to confirm the precipitation of the secondary phases. In fact, the results provide evidence that the precipitation of chromium nitrides seems to be the preferred nucleation site for sigma phase at higher deformation strain, in addition to the intergranular precipitation of sigma. Both the sigma phases nucleated integranularly and besides chromium nitrides are randomly orientated.

Synergy of Intragranular and Intergranular Precipitation Behaviors in the Al-Mg-Xsi-Cu-Zn Alloys with High Corrosion Resistance

Synergy of Intragranular and Intergranular Precipitation Behaviors in the Al-Mg-Xsi-Cu-Zn Alloys with High Corrosion Resistance

Suppressing grain boundary embrittlement via Mo-driven interphase precipitation mechanism in martensitic stainless steel

The addition of small amount of molybdenum to martensitic stainless steels has been found to be beneficial for the combination of strength and ductility. This paper focuses on its underlying mechanism. It was found that molybdenum could motivate the formation of reversed austenite supersaturated with carbon/nitrogen interstitial atoms, thus leading to interphase precipitation phenomenon in the following tempering and aging process. This mechanism has been demonstrated to be effective in reducing intergranular precipitates, leading to superior strength-ductility synergy of Mo-containing steel. The functional mechanism of molybdenum has been revealed based on thermodynamics and kinetics simulations. The simulation results indicated that Mo significantly enhanced the stability of Fe–C/Fe–N system with face centered cubic (fcc) lattice structure. This work opens a new insight into the mechanism of Mo in martensitic stainless steels, thus provides a new route to develop advanced steels with good resistance to grain boundary embrittlement. A universal mechanism to suppress grain boundary embrittlement of martensitic stainless steel was introduced.

Intergranular precipitation-enhanced wetting and phase transformation in an Al0.4CoCrFeNi high-entropy alloy exposed to lead-bismuth eutectic

After exposure to oxygen-poor (10−13–10−14 wt%) liquid lead-bismuth eutectic (LBE) at 500 °C for 500 h, LBE penetrates more than one order of magnitude deeper in an FCC Al0.4CoCrFeNi high-entropy alloy (HEA) decorated with a network of BCC (Ni, Al)-rich intergranular (IG) precipitates than in a single-phase, FCC Al0.3CoCrFeNi HEA without the IG precipitate network. This deterioration of corrosion resistance is attributed to the energetic nature of the BCC/FCC interphase boundaries (IBs) and resultant IB wetting. The LBE ingress film selectively leaches nickel located at those low-indexed crystalline planes, resulting in phase transformation from FCC to BCC structure.

Effect of Sintering Temperature on Microstructure and Mechanical Properties of AZ91 Magnesium Alloy via Spark Plasma Sintering

Gas‐atomized AZ91 powder with superhigh compressive strength is sintered by spark plasma sintering (SPS) at the temperatures of 350, 400, and 450 ºC. The grain size, Mg17Al12 precipitate, and geometrically necessary dislocations (GNDs) density are investigated by transmission electron microscopy, X‐ray diffractometer, scanning electron microscopy, and electron backscattered diffraction. Microstructure characterization reveals that with the increase in sintering temperature, the grain size increases, while the Mg17Al12 precipitate content and GND density decrease. The eutectic intergranular Mg17Al12 precipitate partially transforms to lamella intragranular Mg17Al12 phase during SPS process. The compressive yield strength (CYS) decreases with the increases in sintering temperature, whereas the ultimate compressive strength (UCS) and strain show a contrary tendency. The theoretical strengthening mechanism of sintered AZ91 alloy is calculated, which indicates that the grain refinement, dislocation, and Orowan strengthening contribute 59.3–66.3, 28.8–34.5, and 4.9–6.2% to the CYS, respectively. It is found that the dislocation strengthening plays an important role in SPS AZ91 alloy. Compared with pure Mg, the sintered AZ91 alloy shows a higher σ 0 and a lower k. The enhanced UCS and fracture strain at higher sintering temperature are attributed to the lower content of MgO and Mg17Al12 phases and the coarser grain size.