Precipitation Of Laves Phase Research Articles

Effect of phosphorus content and grain size on the long-term phase stability of Ni-base superalloys

The effect of grain size on the phase stability of two nominally identical high refractory content powder-processed Ni-base superalloy with varying levels of P additions (0.013 wt% for alloy P1 and 0.026 wt% for alloy P3) was studied. Solutioning of the alloys at either sub-solvus or super-solvus temperatures were used to vary the grain size of the starting microstructure prior to long-term thermal exposures at 800 °C for up to 1000 h. EBSD analyses revealed that the sub-solvus solutioned samples had an average grain size of 10 μm and possessed a high angle grain boundary length density that was approximately 2.5 times greater than that found in the super-solvus solutioned samples with an average grain size of 16 μm. Differences in both the initial grain boundary character distribution and P content present in these alloys resulted in varying behavior with respect to the grain boundary precipitation of Laves and sigma phase during aging. Since P additions to Ni-base superalloys are known to promote the formation of Laves phases and segregate predominately to grain boundaries, sub-solvus heat treated samples exhibited a lower susceptibility of Laves phase precipitation compared with super-solvus heat treated samples for both P1 and P3 alloys. Sub-solvus heat treated P1 samples were found to be resistant to the formation of Laves phase after 1000 h exposure while the coarser grained super-solvus heat treated P3 samples exhibited extensive formation of intertwined Sigma-Laves along the grain boundaries just after 100 h exposure. The improvement in phase stability could be attributed to the reduced concentration of P along the grain boundary as increasing the length density of random high-angle grain boundary resulted in a more dilute spatial distribution of segregated P atoms.

Effect of long-term exposure at 650 °C on microstructural and creep characteristics of T92/Super304H dissimilar welds

This study reports the effect of 650 °C exposure on microstructural evolution and creep behavior of post-weld heat-treated dissimilar welds between the alloys T92 and Super304H. The dissimilar welds were exposed for durations of 24 h, 100 h, 250 h, 500 h, and 1000 h to investigate the thermal stability of the microstructure. Creep tests were carried out at 650 °C and 120 MPa on the post-weld heat-treated and on 1000 h exposed dissimilar metal welds. The isothermal exposure degraded the microstructure of T92-HAZ, resulting in type IV failure at an accelerated rate under creep conditions. Further, accelerated recovery of martensitic laths and extensive precipitation of Laves phase on M23C6 carbides sitting at the grain boundaries of the fine-grained heat-affected zone (FGHAZ) during creep deformation were observed. The isothermal exposure on Super304H-HAZ showed no significant microstructural changes except precipitation of thick M23C6 carbides at the grain boundaries. These prominent microstructural changes in the HAZ of T92 lead to the accelerated void formation in the FGHAZ and are responsible for consequent premature failure.

High-temperature Laves precipitation and its effects on recrystallisation behaviour and Lüders deformation in super ferritic stainless steels

The interaction of microstructural evolution, precipitation and mechanical properties of cold rolled 27Cr–4Mo–2Ni steels stabilised by Nb/Ti during annealing are investigated. Prior deformations provide an acceleration effect on bulk sigma precipitation and increase the starting precipitation temperature to ~1000 °C, which can induce a brittle fracture mode for annealed specimens. The precipitation temperature of Fe2Nb-type Laves phase is observed to be as high as 1050 °C, and these nanoscale Laves phases are intertwined with Nb2C particles, which are located at sub-grain boundaries. The abnormal relationship between the grain size and the annealing temperature, which is also related to the formation of Nb-containing particles, is determined. The transformation from Nb2C to Laves phase below 1050 °C release C atoms into the matrix around the Nb2C particles. This further leads to Lüders deformation during tensile test because of Cottrell atmosphere formation. The γ-fibre texture development during the annealing process is explained based on orientated nucleation and orientation growth theory. Super ferritic stainless steels exhibit a good combination of strength and ductility via Laves phase precipitation control during annealing at approximately 1050 °C for 5–60 min; these properties are advantageous for their application with strong γ-fibre texture.

Laves phase precipitation behavior and high-temperature strength of W-containing ferritic stainless steels

Small-size Laves phase precipitates have the advantage of providing superior high-temperature properties to ferritic stainless steels. However, the coarsening of precipitates becomes inevitable during long-term service at elevated temperatures, which causes degradation of high-temperature strength and thermal fatigue resistance. In this study, Laves phase precipitation behavior and high-temperature strength is elucidated in W-containing ferritic stainless steels in the temperature range of 800–1100°C. Laves phase preferentially precipitated at the grain boundaries and were uniformly dispersed within the grains after aging. The intragranular Laves phase almost completely vanished and only intergranular precipitates were present after long-term aging, especially at 900–1000°C. The W-containing steels exhibited higher tensile strength than W-free steel, which resulted from the formation of Laves phase at higher temperatures and solid-solution strengthening effect. Nevertheless, the size of Laves phase was increased with the increase of W content, which led to the decrease of precipitation strengthening effect after long-term aging. This change resulted in the apparent reduction of tensile strength with increased aging time, although the W-containing steels continued to exhibit higher tensile strength compared to W-free steel.

Creep strength breakdown and microstructure in a 9%Cr steel with high B and low N contents

A 9%Cr–3%Co steel with high B and low N contents exhibits creep strength breakdown at a temperature of 923 K after 3000 h. Specific feature of this steel is a distinct difference between short-term and long-term creep regimes for transient, steady state and tertiary creep stages. This behavior is unusual for high Cr steels and attributed to low density of M23C6 carbides precipitated on lath boundaries during tempering. Precipitation of Laves phase along these boundaries during transient creep followed by its coarsening affects significantly the creep mechanisms and results in the well-defined difference in mechanical behavior between the short- and long-term creep regimes. In contrast, the strain-induced formation of a small amount of Z-phase particles scarcely changes the creep behavior.

Laves Phase in a 12% Cr Martensitic/Ferritic Steel: Evolution and Characterization of Nanoparticles at 650 °C.

A 12% Cr martensitic/ferritic with tailor-made microstructure was characterized by HRTEM after tempering (780 °C/2 h) and isothermal aging at 650 °C during 2 months and 1 year. The investigation was focused on the precipitation behavior of Laves phase nanoparticles and its contribution to creep response at 650 °C. Although, dense precipitation of Laves phase occurs during the heat treatment (196 nm), addition of 3.8% tungsten (W) promotes a high growth rate of this particle at 650 °C. The excessive addition of W induces a high driving force for the growth of this particle reaching a mean size of 300 nm at 2 months and 400 nm at 1 year. However, some small Laves phase nanoparticles inside the grain were detected at 1 year of isothermal aging, suggesting a low coarsening rate maybe due to low diffusion of W in the matrix. Results of creep test at 650 °C suggest most of the precipitation hardening is granted by fine dispersion of Ta-rich carbonitrides (Ta-MX), i.e., Laves phase is too coarse to contribute to the pinning force of the subgrain structure in the designed alloy.

Effects of Yttrium on Microstructure Stability and Tensile Properties of China Low Activation Martensitic Steel

This study investigated the microstructural stability and mechanical properties exhibited by China low activation martensitic (CLAM) steels with different yttrium (Y) contents over 3000 h of aging at 550 °C. Scanning electron microscopy, electron backscatter diffraction analysis, and transmission electron microscopy were employed to investigate the microstructural evolution of the steels. Results indicated that grain boundary migration was slow and the Laves phase precipitation was delayed in Y-containing steels. Grain boundaries at different angles in 0Y and 6Y CLAM steels were significantly affected, and those in 36Y and 71Y alloys exhibited negligible changes during the long-term thermal aging. Moreover, Y contents had appreciable effects on the strength and toughness of the aged steels. The stable microstructure of Y-containing CLAM alloys is responsible for improved strength and impact toughness during aging.

The evolution behavior of second phases during long-term creep rupture process for modified 9Cr-1.5Mo-1Co steel welded joint

In this paper, evolution of Laves phase in weld metal (WM) during long-term creep exposure was investigated for modified 9Cr-1.5Mo-1Co steel welded joint (WJ) by microstructure characterization and theoretical calculation based on a series of creep tests at 620 °C. It was found that some rod-like Mo-rich Laves phase precipitated adjacent to M23C6 carbides on grain boundary and other isolated polygonal Laves phase particles formed in grains after long-term creep exposure. Number and size of Laves phase increased simultaneously at early stage of precipitation. The volume fraction nearly approached to equilibrium fraction while coarsening was still ongoing after 2500 h. Johnson-Mehl-Avrami and Ostwald ripening model indicated that precipitation of Laves phase was controlled by grain boundary diffusion with a decreasing rate. EDS results showed Laves phase in WM had the highest Mo content in the whole WJ. The pinning effects on grain boundaries by Laves phase became weaker during creep exposure due to its coarsening. More high angle boundaries and sub-structure in WM from EBSD results provided more potential sites for Laves phase nucleating. With Laves phase particles growth on grain boundaries in WM, solid solution elements like Mo in matrix were severely depleted, resulting in loss of solute strengthening effect and precipitation strengthening effect, thus leading to the rupture in WM.

Effects of Isothermal Aging on Microstructure and Mechanical Property of Low-Carbon RAFM Steel

In order to investigate the microstructure and mechanical property evolution of low-carbon reduced activation ferritic/martensitic (RAFM) steel during isothermal aging, the normalized and tempered specimens were aged at 600 °C for 500, 1000, and 3000 h, respectively. The microstructural evolution with aging time was analyzed, including the precipitation and growth of M23C6 and MX-type carbides as well as the formation of Laves phase. The results indicate that the coarsening of M23C6 is more obvious than that of MX with increase in aging time. During the long-term thermal exposure, the Fe2W Laves phase precipitates adjacent to M23C6 along the prior austenite grain boundaries and packet boundaries. Lower carbon content can delay the precipitation of Laves phase compared to the steel containing higher carbon. In addition, the Laves phase precipitated along boundaries can provide the precipitation strengthening, slightly increasing the tensile strength of low-carbon RAFM steel after aging for 3000 h.

Effects of Si on Phase Stability and Precipitation Behavior of C14 Laves Phase (Fe,Cr)2(Nb,Mo) in High Cr αFe-base Alloys

The growth mechanism of C14 Laves phase in the bcc αFe matrix was determined as the ledge mechanism on the (110)α//(0001)C14 habit plane in Fe-20Cr-0.5Nb-1Mo (at%) alloys annealed at 1073 K for 24 hours, using conventional and scanning transmission electron microscopy. Terrace planes are the basal plane of hcp-based C14 structure. Precipitation particles tend to grow in plate shape depending on the anisotropic difference of lattice mismatch. The addition of Si with Mo remarkably enhances C14 Laves phase precipitation. The area fraction of Laves phase increases from 5.9% to 12.1% by the 2Si addition on Fe-20Cr-0.5Nb-2Mo alloys. Contrary to this, the addition of Si is not effective to increase Laves phase precipitation. It is indicated that Si improves the phase stability of C14 Laves phase while the partitioning of Mo into C14 Laves phase would be promoted due to the attractive interaction between Mo and Si.

Understanding Laves phase precipitation induced embrittlement of modified 9Cr–1Mo steel

Precipitation of Laves phase in steels is often identified as the primary cause for the degradation of mechanical properties. Analytical electron microscopy and thermodynamic modelling have been used in this study to understand the Lave phase precipitation induced damage initiation process in modified 9Cr–1Mo steel, exposed to high temperature for 10,000 h. The results have suggested that in addition to void nucleation at the interface and de-cohesion, the limited plasticity of the Laves phase is also a predominant mechanism for crack initiation in this material. The thermodynamic propensity of Laves phase precipitation around the carbides and the underlying role of local chemistry have also been studied. Binary Laves phases have been found to be stabilized by ternary additions.

Microstructural evolution, precipitation and mechanical properties of hot rolled 27Cr-4Mo-2Ni ferritic steel during 800 °C aging

A 27Cr-4Mo-2Ni super ferritic stainless steel was hot rolled and aged at 800 °C for times ranging from 10 min to 4 h to study the microstructure, precipitation and their effects on mechanical properties by using SEM, EBSD and TEM techniques. Experimental results demonstrate that a great deal of substructure, especially shear bands is obviously observed within elongated ferrite grains after hot rolling. After aging at 800 °C, Laves, chi and sigma phases are identified, and precipitation of these phases is accelerated evidently by the substructure. Laves phase particles are observed formed at dislocations and distributed inside grains with sub-micron scale. Precipitates of chi and sigma phase are found at grain boundaries and along shear bands, as well as around the TiN particles. Recovery during 800 °C aging is impeded by the precipitation of Laves phase at dislocations and chi phase at shear bands. Tensile ductility and micro hardness are significantly affected by the precipitation of intermetallics. The breakage pattern after tensile test transformed from ductile to brittle fracture when the aging time increases from 10 min to 4 h.

Laves Phase Precipitation Behavior and the Orientation Relationship with BCC in Cr–Mo–Nb System

Laves Phase Precipitation Behavior and the Orientation Relationship with BCC in Cr–Mo–Nb System

HRTEM observation of silicides and Laves phase precipitates in Nb-Ti-Si based alloys

The morphologies, crystallographic characteristics and formation processes of silicides and Laves phase precipitates in Nb-Ti-Si based alloys have been systematically investigated by high-resolution transmission electron microscopy (HRTEM) observation. Silicide precipitate of δNb11Si4 usually exhibits homogeneous distribution with acicular and tetrapod morphologies, possessing uniform orientation relationships (ORs) with Nbss. The occurrence of these morphologies might be caused by the symmetry decrease during the phase transformation of Nbss ⟶ δNb11Si4 + Nbss1. In some cases, the δNb11Si4 precipitates also adopt heterogeneous distribution with plate morphology and identical ORs as tetrapod precipitates with Nbss. The Laves phase precipitate, Cr2Nb, possesses C15 structure and contains high densities of stacking faults (SFs). It always forms along defects or Nbss/γNb5Si3 interfaces. The coupling precipitation behaviors and precipitation reactions of δNb11Si4, Cr2Nb and γNb5Si3 in the Nb-Ti-Si based alloys have been discussed.

Effect of Heat Treatment on the Microstructure and Mechanical Properties of the Modified 718 Alloy

M718 alloy with an extra high Mo content of 7.50 wt% which reduced Nb addition and increased Al and Ti additions within the composition specifications of 718 alloy has been designed to increase the service temperature of 718 alloy. And the effect of the heat treatment on the microstructure and mechanical properties of M718 alloy has been investigated in this study. The results showed that Laves phase precipitated on the grain boundaries of M718 alloy instead of δ-Ni3Nb phase in 718 alloy, and γ′′ and γ′ phases precipitated in the matrix of M718 alloy as that in 718 alloy. Increasing the solution temperature from 960 to 1050 °C noticeably reduced the intergranular precipitation of Laves phase. The precipitation of Laves phase was appropriate at 1020 °C for improving the grain boundary cohesion. Increasing the two-stage aging temperatures markedly increased the sizes of γ′′ and γ′ phases. As a result, the strength of M718 alloy increased.

Effect of Compositional Changes of Laves Phase Precipitate on Grain Boundary Embrittlement in Long-Term Annealed 9 Pct Cr Ferritic Steel

In 9 to 12 pct chromium steels, the high-temperature mechanical properties are known to be strongly dependent on the formation and coarsening of Laves phase precipitates at boundaries. During high-temperature deformation, the Laves phase precipitate coarsening to over a critical size has been considered to trigger cavity formation at the precipitate-matrix interfaces. This coarsening, accompanied by the diffusion of W, Mo, and Cr, should change the mechanical properties and chemical composition of both Laves phase precipitates and the matrix. In this study, we aimed to clarify the effects of compositional changes of Laves phase precipitates on cavity formation during coarsening. The values of the Fe/Cr and W/Mo ratios in Laves phase precipitates were shown to induce different levels of strain energy in the vicinity of the Laves phase precipitate, consequently promoting the formation of cavities. Therefore, the compositional change of Laves phase precipitates was found to play a critical role in the grain boundary embrittlement of high Cr steel at elevated temperature.

High-temperature low-cycle fatigue behavior of a 9Cr-ODS steel: Part 2 - hold time influence, microstructural evolution and damage characteristics

Creep-fatigue (CF) interaction in a tempered martensitic Fe-9%Cr-based oxide dispersion strengthened (ODS) steel was studied at 650 °C by introducing hold-time of up to 30 min at peak tensile strain of 0.7%. The symmetrical loops under pure fatigue/continuous cycling (PF/CC) became asymmetrical due to stress relaxation during hold-time. Moreover, this also resulted in a reduction of cyclic life. For the investigated hold-time durations, the increase in tensile hold (TH) period had a negligible effect on peak stresses, but led to a further reduction in lifetime. Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were used to visualize microstructural evolution under both PF/CC and TH waveforms. In general, PF/CC resulted in: (1) rearrangement and/or annihilation of dislocations, (2) partial elimination of the original sub-grain structures, (3) grain growth, (4) M23C6 carbides coarsening and (5) Cr-W enriched Laves phase precipitation. Nevertheless, upon introducing tensile hold-time, no substantial additional microstructural changes were identified. To uncover reasons for specimens premature failure under TH waveforms, detailed investigations on their surfaces, cross-sections and fracture surfaces were carried out. These investigations led to two important conclusions. Firstly, due to comparatively longer high-temperature exposure, the extent of oxidation increased upon introducing TH which expedited damage progression. Secondly, TH induced intergranular damage in the form of creep cavities does not only provide additional crack initiation sites but also their growth/coalescence under tensile stresses act as a bridging link for accelerated crack propagation. These two findings are associated with a reduction of cyclic life due to introduction of hold-time. Hence, the effect of hold-times is primarily due to (1) oxidation-fatigue interaction and (2) creep-fatigue interaction.

Laves phase precipitation in Ti-Zr-Fe-Cr alloys with high strength and large plasticity

In this work, a novel serial of Ti-33Zr-xFe-yCr (x = 3, 5, 7 wt% and y = 2, 4 wt%) alloys was designed first with an expectation to obtain Laves phase in microstructure and then these alloys were produced using cold crucible levitation melting. Microstructure and phase analyses suggest the precipitation of a C15 type Laves phase with a dominating β phase in all the Ti-33Zr-xFe-yCr alloys except in Ti-33Zr-3Fe-2Cr alloy which has a single β phase microstructure. The volume fraction of the Laves phase increases upon increasing the Fe and Cr concentrations in Ti-Zr-Fe-Cr alloys. Mechanical properties evaluation reveals the highest ultimate compressive strength (~2 GPa) and superior plastic strain (33.6%) in Ti-33Zr-3Fe-4Cr alloy. Moreover, all the studied alloys exhibit extraordinarily high yield strength above 1 GPa and hardness exceeding 8 GPa. Embrittlement in Laves phase alloys, which is a common problem, is not encountered in this work as all the studied alloys display plastic strain including maximum elongation of 40%. The fracture behaviour of each studied alloy is also discussed in terms of the influence of Laves phase on a fracture, shear banding pattern and plastic deformation.

Creep behavior and microstructural evolution of a 9%Cr steel with high B and low N contents

The creep behavior of a 3%Co modified P92-type steel with high content of boron and low content of nitrogen was studied. The precipitation of Laves phase at lath boundaries provides a rapid decrease in the strain rate during transient creep. The finely dispersed (V,Nb)(C,N) carbonitrides uniformly distributed throughout and M23C6 carbides located at various boundaries/subboundaries result in extended steady-state creep. Gradual coarsening and an increase in the volume fraction of M23C6 carbides as well as coarsening of Laves phase particles is accompanied by an increase in the lath size during the steady-state creep. The coarsening of lath structure that is assisted by the dissolution of Laves phase particles at lath boundaries during steady-state creep leads to the onset of tertiary creep with a highly accelerated rate. The well-developed subgrain structure is observed in the ruptured samples, whereas the distance between high-angle boundaries does not change during the creep.

A novel ultra-high strength maraging steel with balanced ductility and creep resistance achieved by nanoscale β-NiAl and Laves phase precipitates

A novel ultra-high strength precipitation hardened martensitic steel with balanced ductility and creep resistance has been developed. It utilises a unique combination of nanometre scale intermetallic precipitates of Laves phases and β-NiAl to achieve such properties. The mechanical properties of this steel were assessed by tensile and creep testing. With different heat treatments, this steel showed a remarkable combination of mechanical properties: yield strength of >1800 MPa, ultimate tensile strength of ∼2000 MPa, tensile ductility up to ∼8% at room temperature and creep rupture life >2000 h under 700 MPa stress at 500 °C. The microstructures at different length scales were characterised using scanning/transmission electron microscopy and atom probe tomography. The austenisation and ageing temperatures were found be the key factors determining the microstructural development and resulting mechanical properties. Large primary Laves phase precipitates formed at lower austenisation temperatures resulted in reduced creep strength; whilst the small difference (20 °C) in ageing temperatures had significant impact on the spatial distribution characteristics of β-NiAl precipitates. Lower ageing temperature produced much smaller but more uniformly distributed β-NiAl precipitates which contributed to the higher observed yield strength. It is clear from this study that whilst this novel alloy system showed great potentials, careful design of heat treatment is still required to achieve balanced mechanical properties to meet the service requirements in aerospace propulsion systems.