Deformation Mechanism Of Alloy Research Articles

Effect of Al and Nb addition on the precipitation hardening and deformation mechanism of CoCrNi medium entropy alloy

Single-phase medium-entropy alloys (MEAs) with face-centered cubic (fcc) structure exhibit high elongation and toughness but suffer from insufficient strength. In this work, two novel (CrCoNi)Al4 and (CrCoNi)Al4Nb2 MEAs were designed with the aim of introducing precipitation hardening effect and thus improve strength of the alloys. It was revealed that addition of Al alone, only slightly increased the strength, but the elongation decreased severely. Conversely, the combination of Al and Nb can significantly increase strength and maintain acceptable elongation. The Al addition promotes the formation of a disperse distribution of BCC/B2 composite precipitates, however, the large size and incoherent nature of these composite precipitates produce minor hardening response. Except for the BCC/B2 composite precipitates, a high density of the nanoscale γ′ phase with L12 structure and stoichiometry of (Ni, Co, Cr)3(Al, Nb) is produced in the Al4Nb2 MEA, which significantly increases the strength of the alloy. After the addition of Al and Nb, the deformation mechanism of the alloys is changed from nanotwins to stacking fault (SF) dominated plasticity. Such phenomenon mainly relates to the increasing critical twinning stress affected by the channel width of the matrix. This study provides new insight for the composition design of precipitation-hardening MEAs.

Creep Damage of a Re/Ru-Containing Single Crystal Nickel-Based Superalloy at Elevated Temperature

By means of creep properties measurement, microstructure observation and contrast analysis of dislocation configuration, the creep behavior of a 4.5%Re/3.0%Ru-containing single crystal nickel-based superalloy at elevated temperature is investigated. Results show that the creep life of the alloy at 1040°C/160MPa is measured to be 725h to exhibit a better creep resistance at high temperature. In the primary stage of creep at high temperature, the γ phase in alloy has transformed into the N-type rafted structure along the direction vertical to the stress axis, the deformation mechanism of alloy during steady state creep is dislocations slipping in γ matrix and climbing over the rafted γ phase. In the latter period of creep, the deformation mechanism of alloy is dislocations slipping in γ matrix and shearing into the rafted γ phase. Wherein the dislocations shearing into the γ phase may cross-slip from {111} to {100} planes for forming the K-W locks to restrain the slipping and cross-slipping on {111} plane, which is thought to be the main reason of the alloy having a better creep resistance. As the creep goes on, the alternate slipping of dislocations results in the twisted of the rafted γ phase to promote the initiation and propagation of the cracks along the interfaces of γ/γ phase up to creep fracture, which is thought to be the damage and fracture mechanism of alloy during creep at high temperature.

Deformation Features of a High Mo Nickel-Based Single Crystal Superalloy during Creep at High Temperature

The deformation and damage features of a high Mo single crystal Ni-based superalloy during creep at high temperature are investigated by means of measuring creep properties and observing microstructure. Results show that, compared to 4%Mo single crystal nickel-based superalloy, the 6%Mo superalloy displays a better creep resistance, and the creep life of 6%Mo single crystal superalloy at 1040°C/137MPa is measured to be 556 h. In the ranges of applied temperatures and stresses, the creep activation energy of the alloy is measured to be 484.7kJ/mol. Wherein, the deformation mechanisms of the 6%Mo superalloy during steady state creep are dislocations slipping in ϒ matrix and climbing over the rafted ϒ' phase. In the later stage of creep, the deformation mechanism of alloy is dislocations shearing into the rafted ϒ' phase, the alternate activation of dislocations slipping results in the twisted of the rafted ϒ'/ϒ phases, as the creep goes on, to promote the initiation and propagation of cracks along the interface of the twisted ϒ/ϒ' phase perpendicular to the stress axis, up to creep fracture, which is thought to be the damage and fracture features of the alloy during creep at high temperature.

Microstructure evolution and creep behaviors of a directionally solidified nickel-base alloy under long-life service condition

By means of creep properties measurement, microstructure observation and lattice parameters measurement, the microstructure evolution and creep behaviors of a directionally solidified nickel-based alloy under long-life service condition are investigated. The results show that the creep life of the alloy at 980°C/90MPa is measured to be 9714h. During creep, the various morphologies of γ′ and γ phases display in the different regions of sample. Wherein, the γ′ phase in the region near fracture is firstly transformed into the rafted structure, while the γ′ phase in the stress-free region exhibits the bunch-like structure. The size of the rafted γ′ phase in thickness increases from 0.4µm to 1.8µm as the time of creep prolongs to 9714h, the coarsening regularity of the rafted γ′ phase in thickness obeys the parabolic law. Moreover, the parameters and misfits of γ′/γ phases in the alloy increase with the creep time. When the creeping time is less than 3000h, the deformation mechanisms of alloy are dislocations slipping in the matrix channels and climbing over the rafted γ′ phase. In the later stage of creep, the deformation mechanisms of alloy are dislocations slipping in the matrix channels and shearing into the rafted γ′ phase, the alternate activation of the main/secondary slipping dislocations promotes the coarsening and twisting of the rafted γ′/γ′ phases. Wherein, the coarsening of the rafted γ′ phase is thought to be a main reason of the alloy having a better creep resistance and longer life.

Influence of Temperature on Stacking Fault Energy and Creep Mechanism of a Single Crystal Nickel-based Superalloy

The influence of temperatures on the stacking fault energies and deformation mechanism of a Re-containing single crystal nickel-based superalloy during creep at elevated temperatures was investigated by means of calculating the stacking fault energy of alloy, measuring creep properties and performing contrast analysis of dislocation configuration. The results show that the alloy at 760 °C possesses lower stacking fault energy, and the stacking fault of alloy increases with increasing temperature. The deformation mechanism of alloy during creep at 760 °C is γ′ phase sheared by <110> super-dislocations, which may be decomposed to form the configuration of Shockley partials plus super-lattice intrinsic stacking fault, while the deformation mechanism of alloy during creep at 1070 °C is the screw or edge super-dislocations shearing into the rafted γ′ phase. But during creep at 760 and 980 °C, some super-dislocations shearing into γ′ phase may cross-slip from the {111} to {100} planes to form the K–W locks with non-plane core structure, which may restrain the dislocations slipping to enhance the creep resistance of alloy at high temperature. The interaction between the Re and other elements may decrease the diffusion rate of atoms to improve the microstructure stability, which is thought to be the main reason why the K–W locks are to be kept in the Re-containing superalloy during creep at 980 °C.

Creep properties and deformation mechanism of the containing 4.5Re/3.0Ru single crystal nickel-based superalloy at high temperatures

By means of creep properties measurement and microstructure observation, an investigation has been made into the creep behaviors of a Re/Ru-containing single crystal nickel-based superalloy at high temperatures. Results show that after fully heat treatment, the microstructure of the Re/Ru-containing single crystal nickel-based superalloy consists of the cuboidal γ′ phase about 0.4μm in size coherent embedded in the γ matrix. The creep life of the superalloy at 137MPa/1100°C is measured to be 321h. And the deformation mechanism of alloy during steady state creep are dislocations slipping in γ matrix and climbing over the rafted γ′ phase. In the latter stage of creep at 1085°C, the dislocations shearing into γ′ phase may cross-slip form {111} to {100} planes to form the K–W locks, which may restrain the slipping and cross-slipping of dislocations to improve the creep resistance of alloy. While the size of the rafted γ′ phase in thickness diminshes as the creep temperrature rises, which may enhance the climbing rate of dislocations to decrease the creep lifetimes of alloy.

Creep Behavior of a 4.5% Re Nickel-Base Single Crystal Superalloy at High Temperature

By means of heat treatment and creep property measurement, an investigation has made into the creep behaviors of a containing 4.5% Re nickel-base single crystal superalloy at high temperature. Results show that the elements W, Mo and Re are enriched in the dendrite arm regions, the elements Al, Ta, Cr and Co are enriched in the inter-dendrite region, and the segregation extent of the elements may be obviously reduced by means of heat treatment at high temperature. In the temperature ranges of 1070--1100 °C, the 4.5% Re single crystal nickel-based superallloy displays a better creep resistance and longer creep life. The deformation mechanism of the alloy during steady state creep is dislocations slipping in the γ matrix and climbing over the rafted γ′ phase. In the later stage of creep, the deformation mechanism of alloy is dislocations slipping in the γ matrix, and shearing into the rafted γ′ phase, which may promote the initiation and propagation of the micro-cracks at the interfaces of γ/γ′ phases up to the occurrence of creep fracture.

Influence of solution temperature on microstructure and creep property of a directional solidified nickel-based superalloy at intermediate temperatures

By means of solution treatment at different temperatures, creep properties measurements and microstructure observations, an investigation has been made into the influence of solution temperature on the elements segregation and creep behaviors of a directionally solidified nickel-based superalloy at intermediate temperatures. Results show that after solution treated at 1230°C, the bigger composition segregation and un-homogeneous microstructure exist still in the dendrite/inter-dendrite regions of the alloy. The fine γ′ precipitates are distributed in the dendrite arm regions, while the coarse ones are distributed in the inter-dendrite regions. After solution treated at 1260°C, the segregation extent of the elements reduces obvious and the fine γ′ precipitates distribute homogeneously in the dendrite/inter-dendrite regions. Moreover, the particle-like carbides are precipitated along boundaries, which may restrain boundary slipping. The deformation mechanism of alloy during creep at intermediate temperature is dislocations slipping in matrix and shearing γ′ phase; the super-dislocations shearing into γ′ phase may cross-slip from {111} plane to (100) plane to form the configuration of K–W locking, which can hinder dislocation slipping on {111} plane. In the latter stage of creep, the cracks may be initiated and propagated along the boundaries with different angles relative to the stress axis. The fact that the boundaries at about 45° angles relative to the stress axis bear the maximum shearing stress increases the probability of the cracks initiating and propagating along the boundaries.

Microstructure and creep behaviors of a high Nb-TiAl intermetallic compound based alloy

By means of heat treatment, creep properties measurement and microstructure observation, an investigation has been made into the influence of heat treatment on microstructure and creep properties of the high Nb-TiAl alloy. Results show that the microstructure of as-cast high Nb-TiAl alloy consists of lamellar γ/α2 phases with various orientations. The irregular serrated boundaries with single γ phase are located in between the lamellar γ⧸α2 phases with various orientations. After solution and aging treatment, the microstructure of alloy consists of uniform and regular lamellar γ⧸α2 phases, and the regular boundaries are located in between the lamellar γ⧸α2 phases. Under the applied stress of 200MPa at 800°C, the creep lifetime of the as-cast high Nb-TiAl alloy is measured to be 147h, the one of the alloy after heat treatment is measured to be 297h. In the ranges of the applied temperatures and stresses, the creep activation energy of alloy after heart treatment is measured to be Q=432kJ/mol. The deformation mechanism of alloy during creep is dislocations slipping in the lamellar γ⧸α2 phases, the creep dislocations may be decomposed to form the configuration of the partials plus stacking faults. The deformation of as-cast alloy during creep occurs mainly in the irregular serrated boundary regions with single γ phase. For the heat treated alloy, the primary/secondary slipping systems of dislocations are alternately activated during creep, which results in the bigger plastic deformation of alloy to contort the lamellar α2/γ phases. In the latter stage of creep, the cracks are firstly initiated along the boundaries parallel to the lamellar γ⧸α2 phases, and propagated along the boundaries vertical or at about 45° angles relative to the stress axis up to the occurrence of creep fracture.

Creep behaviors and role of dislocation network in a powder metallurgy Ni-based superalloy during medium-temperature

By means of creep propertiesmeasurement, microstructure observation and contrast analysis of dislocation configuration, the creep behaviors and role of dislocation networks in FGH95 powder metallurgy Ni-based superalloy during creep have been investigated. The results show that the microstructure of alloy consists of the fine γ′ phase coherently embedded in the γ matrix, and a few coarser γ′ particles are distributed in the boundary regions. In the ranges of the applied temperatures and stresses, the alloy displays a better creep resistance and longer lifetime. The deformation mechanisms of alloy during creep are the dislocations slipping in the matrix and shearing into the γ′ phase, and the dislocations shearing into the γ′ phase may be decomposed to form the configuration of the partials plus the stacking fault. During creep, two groups of dislocations on different slip planes may knit to form the quadrangular dislocation networks; while two groups of moving dislocations on the same slip plane may encounter and react to form the hexagonal dislocation networks. The strength of boundaries is responsible for the creep resistance of alloy, and the one is related to the deforming behaviors of coarse γ΄ particles distributed in the boundaries. Thereinto, the dislocation networks distributed in the interfaces of coarse γ′/γ phases may release the lattice-misfit stress and relax the stress concentration to delay the dislocation shearing into the γ′ phase, which is beneficial to keep the boundary strength and enhance the creep resistance of the alloy.

Mechanism of plastic deformation of a Ni-based superalloy for VHTR applications

Alloy 617 is considered a leading structural material for next generation nuclear power plants due to its good corrosion resistance and exceptional strength at high-temperatures. In the present work, a complementary study of deformation mechanisms in Alloy 617 was performed via tensile testing at temperatures up to 1000°C. Dynamic strain aging characterized by serrated flow leading to temperature independent yield strength was observed at intermediate temperatures. At temperatures higher than 700°C, the material’s strength reduced dramatically as a result of the predominance of dislocation creep and dynamic recrystallization. The changes in deformation mechanisms were established by direct observations of Electron Backscatter Diffraction mappings and by the analysis of texture development based on the pole figure mappings. Recrystallized grains were found to preferentially grow in particle-rich areas and on high-angle grain boundaries. Finally, fracture was initiated from particle cracks at temperatures up to 700°C and by triple junction cracks from 800 to 1000°C.

Influence of Phosphorus and Boron on Creep Behavior and Fracture Mechanism of GH4169 Superalloy

By means of creep properties measurement and microstructure observation, the influence of the traces P, B on creep behaviors and fracture mechanism of GH4169 alloy was investigated. Results showed that during creep the twinning was thought to be the main deformation mechanism of GH4169 alloy, however the deformation mechanisms of alloy containing P, B were the twinning and slipping of dislocations activated within the grains. And the fact that the slipping of dislocations activated within the grain can delay the stress concentration in the local regions to restrain the initiation of the cracks to improve the creep resistance of the alloy. Compared to GH4169 alloy, the fracture of GH4169G alloy displayed the non-smooth surface. The adding traces P, B were segregated in the region near the boundary, which may promote the particle-like δ phase precipitated along the boundary to restrain the boundaries slipping and the cracks propagation. This was thought to be the main reason for enhancing the strength of the boundary and prolonging the creep life.

Creep properties and effect factors of hot continuous rolled Ti–6Al–4V alloy

Abstract By means of heat treatment, measurement of creep properties and contract analysis of dislocations configuration, creep behaviors and effect factors of hot continuous rolled (HCR) Ti–6Al–4V alloy are investigated. Results show that, the microstructure of HCR alloy after solution treatment at temperature lower than β phase transus point consists of the super-saturation equiaxial α phase and “basket weave” structure. The quantity of “basket weave” structure increases with solution temperature, and the fully “basket weave” structure may be obtained after solution treatment at 1000 °C. Compared to the alloy solution treated at 940 °C, the alloy treated at 1000 °C has lower strain rate and longer creep lifetime under the conditions of applied stress of 575 MPa at 420 °C. In the range of the applied stress and temperature, the creep activation energy of the alloy is calculated to be Qa = 249.8 kJ/mol. Deformation mechanism of HCR alloy during creep is double orientations slips of dislocations activated within α phase with HCP structure, while deformation feature of the alloy solution treated at 940 °C is the wave-like 〈a + c〉 dislocations activated on the pyramidal planes in the α phase. Solution treated at 1000 °C, deformation mechanism of the alloy is that the multiple slips of (1/2)〈1 1 1〉 dislocations are activated within β phase with BCC structure, in which V-rich β phase with high volume fraction may enhance creep resistance, which is thought to be the main reason of the alloy possessing the low strain rate and longer creep lifetime.

Deformation behaviour of a newer tungsten heavy alloy

The present study attempts to investigate room temperature and high temperature flow behaviour of a tungsten heavy alloy (W-7.10Ni–1.65Fe–0.5Co–0.25Mo alloy). The alloy was deformed under compression at room temperature and elevated temperatures (400–700 °C) at different strain rates (1–0.0001 s −1) to observe plasticity under compression loading at these temperatures and strain rates. The alloy showed higher plasticity and positive strain rate sensitivity at room temperature. Samples hardness after 70% deformation at room temperature increased from 3.20 ± 0.14 to 5.08 ± 0.03 GPa. Barreling was observed in room temperature compression tested samples. Microstructure of the alloy after heavy compressive deformation at room temperature showed that severe deformation of W grains took place along a direction at 45° to the direction of applied stress. The alloy showed varying (positive and negative) strain rate sensitivity at elevated temperatures. Samples hardness after 70% deformation at elevated temperatures increased from 3.20 ± 0.14 to 4.60 ± 0.23 GPa. At 600 and 700 °C, the specimens failed by shear along the direction which is at an angle 45° to the direction of applied stress. Microstructural evidences indicate that the failure at these elevated temperatures seems to be triggered by the excessive void generation in the matrix and their coalescence under the influence of the applied stress. Room temperature deformation mechanism of tungsten heavy alloy was also studied by carrying out tensile testing at room temperature in strain rates ranging from 0.1 to 0.0001 s −1. The values of strain rate sensitivity, and apparent activation volume with respect to different level of strain suggest that the deformation mechanism of the alloy is similar to that of bcc metals such as tungsten and molybdenum, i.e. Peierls mechanism controls the dislocation motion.

Deformation mechanism of compression of 2024 Al alloy in semi-solid state

The deformation mechanism of alloy in the semi-solid state has been investigated, with the compression of 2024 Al alloy in the semi-solid state as an example. Experimental results showed that the compressive deformation of 2024 Al alloy in the semi-solid state was mainly caused by the liquid flowing under the hydrostatic compressive stress, the grain boundary sliding under the shear stress and the plastic deformation of grain under the normal compressive stress. Among them, the grain boundary sliding was mainly accommodated by the nucleation and growth of cavity under the hydrostatic tensile stress.