Effect of Aging Treatment on the Formation of α Precipitates in β-Type Ti–6Mo–6V–5Cr–3Sn–2.5Zr Alloys

The microstructural evolution of a novel β-type Ti–6Mo–6V–5Cr–3Sn–2.5Zr (wt%) alloy subjected to different aging treatments was investigated. The normalized intensity of the α precipitates reached a peak value at 450 °C. A nanoscale orthorhombic phase was observed to coexist with α precipitates in the β matrix, which followed the Burgers orientation relation of 〈\(1120\)〉_α//〈111〉_β and {0001}_α//{110}_β. Fine α precipitates were formed with metastable O and β′ phases, and the β phase was spinodally decomposed to β and β′ phases. The maximum hardness value of the specimen was obtained after aging at 450 °C. Compositional partitioning of Mo, V, and Cr elements occurred with the depletion of fine acicular α precipitates upon aging 450 °C.     

Improvement of strength and ductility of Al-Cu-Li alloy through cryogenic rolling followed by aging

To develop an improved approach in achieving an excellent combination of high strength and ductility, the solutionized Al-Cu-Li plates were subjected to rolling at cryogenic and room temperatures, respectively, to a reduction of 83%, followed by aging treatment at 160 °C. The results indicate that Al-Cu-Li alloys through cryogenic rolling followed by aging treatment possess better mechanical properties. Rolling at cryogenic temperature produces a high density of dislocations because of the suppression of dynamic recovery, which in turn promotes the precipitation of T₁ (Al₂CuLi) precipitates during aging. Such high density of T₁ precipitates enable effective dislocation pinning, leading to an increase in strength and ductility. In contrast, room temperature rolled alloys after aging treatment exhibit lower strength and ductility due to low density of T₁ precipitates in the grain interior and high density of T₁ precipitates around subgrain boundaries.     

热处理工艺对砂型铸造Al−2Li−2Cu−0.5Mg−0.2Sc−0.2Zr合金显微组织和力学性能的影响

研究不同热处理工艺对砂型铸造Al?2Li?2Cu?0.5Mg?0.2Sc?0.2Zr合金显微组织和力学性能的影响.设计三级固溶处理方案((460℃,32 h)+(520℃,24 h)+(530/540/550℃,4/12/24/32 h))和不同温度(125,175,225℃)时效处理方案用于比较.采用光学显微镜(OM...     

Non-isothermal aging behavior of in-situ AA2024−Al3NiCu composite

The effect of non-isothermal aging treatment on microstructure and mechanical properties of in-situ AA2024?Al₃NiCu composite fabricated by the stir casting process was examined. The Al₃NiCu intermetallic was created by adding 3 wt.% nickel powder during stir casting and homogenization treatment at 500 °C for 24 h after casting. The microstructural results obtained using optical and scanning electron microscope indicate that, after non-isothermal aging treatment, the S-Al₂CuMg precipitates become finer, forming a poor zone of this precipitate in the area between the dendrites. Also, adding nickel during stir casting reduces the precipitation rate and the contribution of S-Al₂CuMg precipitates in strengthening composite during non-isothermal aging. The maximum hardness, ultimate tensile strength, and toughness achieved in the 3 wt.% nickel-containing sample after non-isothermal aging at 250 °C are (121.30±4.21) HV, (221.67±8.31) MPa, and (1.67±0.08) MJ/m³, respectively. The maximum hardness and ultimate tensile strength of AA2024?Al₃NiCu composite are decreased by 6% and 4%, respectively, compared to those of nickel-free AA2024 aluminum alloy.     

Microstructural characterization in 7 at% Al-containing NiTi-based alloys

The microstructural evolution of Ni–42Ti–7Al and Ni–41Ti–7Al alloys as a function of solution and aging heat treatment was investigated using transmission electron microscopy（TEM）, electron probe, and X-ray diffraction（XRD）. The results reveal that the volume fraction of Ti2 Ni phase as well as its composition does not change significantly after as-solution treated at 1200 °C and aged at 850 °C. At the early stage of the aging treatment at 850 °C for 1 h, the cuboidal β＇ precipitate keeps coherency with the matrix; further aging, β＇ precipitate coarsens, and the semicoherency between the β/β＇ two phases are observed.The shape of coarsened β＇ precipitates changes to the globule, and the interface dislocations are introduced accompanied by the occurrence of semicoherent precipitates. Under the same heat treatment, compared to the Ni–42Ti–7Al alloy, the lattice misfits of the Ni–41Ti–7Al alloy between the β and β＇ two phases are larger, so the β＇ precipitates in Ni–41Ti–7Al alloy are coarsened severely and easily lose coherency with the matrix. The thermal stability of Ni–41Ti–7Al alloy is much worse when aging at 850 °C.     

Dual phases strengthening behavior of Mg–10Gd–1Er–1Zn–0.6Zr alloy

The microstructure evolution and strengthening mechanisms of Mg–10Gd–1Er–1Zn–0.6Zr (wt.%) alloy were focused in the view of the size parameters and volume fraction (f_p) of dual phases (long period stacking ordered (LPSO) structures and β′ precipitates). Results show that two types of LPSO phases with different morphologies formed, and the morphology and size of both LPSO phases varied with the solution conditions. However, the volume fraction decreased monotonously with increasing solution temperature, which in turn raised the volume fraction of β′ phase during aging. The alloy exhibited an ultimate tensile strength of 352 MPa, a yield strength of 271 MPa, and an elongation of 3.5% after solution treatment at 500 °C for 12 h and aging at 200 °C for 114 h. In contrast to the LPSO phase, the β′ phase seems to play a more important role in enhancing the yield strength, and consequently, a decreased f_LPSO/f_β′ ratio results in an increased yield strength.     

Precipitation of metastable phases and its effect on electrical resistivity of Al-0.96Mg2Si alloy during aging

The precipitation behavior and its influence on the electrical resistivity of the Al-0.96Mg₂Si alloy during aging were investigated with in-situ resistivity measurement and transmission electron microscopy (TEM). The precipitates of the peak aged alloy include both β″ and β′, but the amount ratio of β″ to β′ varies with the aging temperature and time increasing. The precipitates during aging at 175 °C are dominated by needle-like β″ phases (including pre-β″ phase), the size of which increases with the time prolonging, but does not increase substantially after further aging. The evolution of electrical conductivity is directly related to such microstructural evolution. However, the hardness of the alloy stays at the peak value for a long term. When the alloy is aged at 195 °C, the ratio of β″ to β′ becomes the main factor to influence relative resistivity (Δρ) value. The higher the temperature is, the smaller the ratio is, and the faster the Δρ value decreases. Moreover, the hardness peak drops with the decrease of the ratio. With the size and distribution parameters measured from TEM images, a semi-quantitative relationship between precipitates and the electrical resistivity was established.     

Heat Treatment Development for a Rapidly Solidified Heat Resistant Cast Al-Si Alloy

Existing heat treatment standards do not properly define tempers for thin-walled castings that solidified with high solidification rates. Recently emerged casting processes such as vacuum high pressure die casting should not require long solution treatment times due to the fine microstructures arising from rapid solidification rates. The heat treatment studies involving rapidly solidified samples with secondary dendrite arm spacing between 10 and 35 μm were conducted for solution times between 30 min and 9 h and temperatures of 510 and 525 °C and for various aging parameters. The metallurgical analysis revealed that an increase in microstructure refinement could enable a reduction of solution time up to 88%. Solution treatment resulted in the dissolution of Al₂Cu and Al₅Mg₈Si₆Cu₂, while Fe- and TiZrV-based phases remained partially in the microstructure. The highest strength of approximately 351 ± 9.7 and 309 ± 3.4 MPa for the UTS and YS, respectively, was achieved for a 2-step solution treatment at 510 and 525 °C in the T6 peak aging conditions, i.e., 150 °C for 100 h. The T6 temper did not yield dimensionally stable microstructure since exceeding 250 °C during in-service operation could result in phase transformation corresponding to the over-aging reaction. The microstructure refinement had a statistically stronger effect on the alloy strength than the increase in solutionizing time. Additionally, thermal analysis and dilatometer results were presented to assess the dissolution of phases during solution treatment, aging kinetics as well as dimensional stability.     

The effect of aging on the B2-R transformation behaviors in Ti-51at%Ni alloy

Aging is an effective way to control the transformation behaviors of Ti–Ni shape memory alloys. In the present study, the transformation behaviors of Ti-51at%Ni polycrystals after aging at various temperatures (250 °C–500 °C) for different time were investigated by DSC measurement. It is found that B2-R transformation temperature (R_S) of the aged samples is uniquely determined by the aging temperature for aging above 400 °C, being almost independent on aging time. Higher aging temperature causes a lowering of R_S temperature. At lower aging temperatures (e.g., 250 °C, 300 °C), R_S gradually increases with aging time and finally saturates. The saturated R_S value is also a unique function of the aging temperature. These facts can be explained by the phase equilibrium between Ti–Ni matrix (B2) and Ti₃Ni₄ precipitates. The Ti₃Ni₄ precipitates formed at different aging temperatures alter the Ni concentration in B2 matrix and consequently cause an aging temperature dependence of the R-phase transformation of B2 matrix. By considering the kinetics of precipitation reaction, the “time-independence” of R_S for high temperature aging and the “time-dependence” of R_S for low temperature aging can also be reasonably understood. Besides, the R–B19′ transformation temperature (M_S) gradually increases with aging time as well, which can also be understood by considering the growth of precipitates during aging.     

Effect of T6-treatments on microstructure and mechanical properties of forged Al-4.4Cu-0.7Mg-0.6Si alloy

Transmission electron microscopy (TEM), scanning electron microscopy (SEM), hardness tests and tensile tests were performed to investigate the effect of aging on microstructure and mechanical properties of forged Al-4.4Cu-0.7Mg-0.6Si alloy. The results show that the alloy exhibits splendid mechanical properties with an ultimate tensile strength of 504 MPa and an elongation of 10.1% after aging at 170 °C for 16 h. With tensile testing temperature increasing to 150 °C, the strength of the alloy declines slightly to 483 MPa. Then, the strength drops quickly when temperature reaches over 200 °C. The high strength of the alloy in peak-aged condition is caused by a considerable amount of θ′ and AlMgSiCu (Q) precipitates. The relatively stable mechanical properties tested below 150 °C are mainly ascribed to the stability of θ′ precipitates. The growth of θ′ and Q precipitates and the generation of θ phase lead to a rapid drop of the strength when temperature is over 150 °C.     

Microstructure evolution and nitrides precipitation in in-situ Ti2AlN/TiAl composites during isothermal aging at 900 °C

Microstructures of Ti₂AlN/TiAl composites prepared by in-situ method were characterized in in-situ and aging treatment conditions and the nitride precipitation was investigated in Ti₂AlN/TiAl composites aged at 900 °C for 24 h after being heat treated at 1400 °C for 0.5 h. The in-situ composites consist of γ+α₂ lamellar colonies, equiaxed γ grains and Ti₂AlN reinforcements. Matrix with nearly fully lamellar structure formed after solution and subsequently aging treatment. With the increase of Ti₂AlN content, the nearly fully lamellar structure becomes instable for the aged composites. According to TEM study, fine Ti₂AlN precipitates are found to distribute at the grain boundaries of lamellar colony. Needle-like Ti₃AlN precipitates arrange in line with growing axis parallel to [001] direction of the γ-TiAl matrix and another needle-like Ti₃AlN precipitates with lager size distribute at the dislocations.     

Effect of Thermal Aging on Coarsening Kinetics of γ′ in Alloy 617

The effect of thermal aging on coarsening kinetics of alloy 617, a candidate material for heat exchanger of the very high temperature reactor, was experimentally studied at 750 and 950 °C for up to 5300 h. Formation of various precipitates such as μ-phase, M₂₃C₆ and γ′ phases and significant coarsening of the γ′ phase have been observed in the microstructure of the aged samples. Experimental observation was compared to alloy thermodynamic calculation and γ′-phase precipitation kinetics simulation. Thermal aging effect on the microstructural evolution and mechanical behavior of alloy 617 was then discussed based on experimental and microstructural modeling results.     

Precipitation process and its effects on properties of aging Cu–Ni–Be alloy

The precipitation process of aged Cu-Ni-Be alloy was investigated by X-ray diffraction （XRD）, trans- mission electron microscopy （TEM）, and high-resolution transmission electron microscopy （HRTEM）. The tensile strength, yield strength, and electronic conductivity of this alloy after aging were also studied. The precipitation sequence of the C17510 alloy aged at 525 ℃ is supersat-urated solid solution→G.P zones→ γ″-γ′→ γ. This transformation can be achieved by the accumulation of Be-atom layers. The G.P zones are composed of disk-shaped monolayers of Be atoms, which are formed on （001） matrix planes. The intermediate γ″ precipitate is nucleated in the G.P zones. The γ″ and γ′ precipitates have the same orientation relationship with matrix, e.g., （110）p｜｜（100）M,[001]p｜｜[001]M. The tensile strength of specimen shows a maximum during the aging process and then continuously decreases if the specimen is over aged. The strengthening effect of γ′ phase precipitated in aging at 525 ℃ for 4 h is calculated to be 436 MPa according to the Orowan strengthening, which is quite consistent with the experimental data.     

Influence of Li addition on mechanical property and aging precipitation behavior of Al-3.5Cu-1.5Mg alloy

The influence of Li addition on mechanical property and aging precipitation behavior of Al-3.5Cu-1.5Mg alloy was investigated by tensile test, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM). The results show that the tensile strength can be significantly improved with the slightly decreased ductility and the form of fracture morphology is converted from ductile fracture into ductile/brittle mixed fracture by adding 1.0% Li. Besides, the peak aging time at 185 °C is delayed from 12 to 24 h and the main precipitation phase S'(Al₂CuMg) is converted into S' (Al₂CuMg)+δ'(Al₃Li), while the formation of S'(Al₂CuMg) is delayed.     

The Mechanism of η Phase Precipitation in A286 Superalloy During Heat Treatment

In this research, the mechanism of eta (η-Ni₃Ti) phase precipitation in iron-nickel-based A286 superalloy was assessed during aging heat treatment in the temperature range between 650 and 900 °C for the times of 1-30 h. Optical microscopy, scanning electron microscopy, differential thermal analysis, and x-ray diffractometry were used to describe the η phase transformation. The results showed that the major precipitates at temperatures below 840 °C were γ′ and η. The η phase started to precipitate at the expense of the γ′ phase after prolonged aging. The η phase existed in the samples aged at temperature higher than 760 °C with cellular morphology. The η volume fraction increased with increasing heat treatment time. In addition, when the aging temperature was increased from 760 to 820 °C, the η volume fraction increased and then decreased after 840 °C. The η phase morphology also changed from cellular to Widmanstätten-type during aging. The time-temperature-precipitation diagrams of these morphologies are presented. The results indicated the differences in precipitation mechanisms of η phase at 840 and 860 °C.     

Microstructure and properties characteristic during interrupted multi-step aging in Al-Cu-Mg-Ag-Zr alloy

The effects of interrupted multi-step aging on the microstructure and properties of Al-Cu-Mg-Ag-Zr alloy were studied by tensile,hardness,electrical conductivity tests and transmission electron microscopy(TEM).Interrupted multi-step aging delayed the peak aging time compared to one-step aging and kept the same levels of hardness,electrical conductivity,ultimate tensile strength(UTS),yield strength(YS) and elongation as those of the T6 temper alloy while increased the fracture toughness notably.Ω phase and a little θ’ phase precipitated and grew simultaneously in the process of one-step aging at 160℃.During the second-step aging at 65℃ of interrupted multi-step aging,no TEM characteristic of Ω precipitates could be found.During the third step of interrupted multi-step aging,Ω began to dominate the microstructure like what happened in the process of one-step aging.The difference of properties between the T6 temper and the interrupted multi-step aged alloys might be related to the different precipitation sequences in the process of the two heat treatment technologies.     

Mechanical properties and microstructural evolution of modified 9Cr-1Mo steel after long-term aging for 50,000 h

The mechanical properties and microstructural evolution of modified 9Cr-1Mo steel have been studied to investigate steel property changes after long-term isothermal aging at 600 °C for 50,000 h. The microhardness and strength were maintained constantly after aging but the impact energy was dramatically reduced by 62 % during the aging period. From the viewpoint of microstructural evolution after the aging process, Cr-enrichment and Fe-depletion took place within the M₂₃C₆-type precipitates in the as-aged steel and V-depletion also happened within the VX-type precipitates after aging. In addition, the precipitates of the M₂Mo-type Laves phase and the segregation of the impurity atoms would be formed during the long-term aging period. It was considered that the sharp reduction of the impact energy could be related to the formation of the Laves phases and the impurity segregation after aging at 600 °C. The phase stability was also verified by the specific heat results up to 950 °C from a DSC test. It was concluded from this study that the modified 9Cr-1Mo steel would keep its microstructural stability at 600 °C during the long-term aging period of 50,000 h, which was equivalent to the in-service life of the SFR fuel cladding.     

Stability of γ′ precipitates in a PWA1480 alloy

In the nickel-based superalloy PWA1480, 5–10-nm diameter γ precipitates form during heat treatment at 871 °C within the primary cuboidal γ′ precipitates that formed at high temperature (1079 °C). The stability of these γ precipitates during extended aging at 871 °C was investigated by atom probe tomography, computational thermodynamics, and diffusion-controlled growth kinetic modeling. The results show that these γ precipitates are not in thermodynamic equilibrium in the alloy and dissolve during extended aging. Thermodynamic and kinetic calculations indicate that the formation of these non-equilibrium precipitates is related to local concentration fluctuations in the primary γ′ phase that developed during diffusion-controlled growth of γ′ at high temperature.     

GTD-111高温合金瞬时液相连接过程中显微组织的变化

研究连接温度和时间对瞬时液相连接GTD-111高温合金显微组织的影响.连接过程采用BNi-3填料,在1080、1120和1160℃下,分别进行195、135和90 min的等温凝固,再进行均匀化热处理.结果表明,在冷却过程中,接头区域连续形成富Ni硼化物、Ni?B?Si三元化合物和共晶γ的金属间化合物和共晶化合物.随着...     

Microstructural evolution of new type Al–Zn–Mg–Cu alloy with Er and Zr additions during homogenization

A comprehensive study on the microstructural evolution of a new type Al–Zn–Mg–Cu–Er–Zr alloy during homogenization was conducted by optical microscope, scanning electron microscope, transmission electron microscopy and X-ray diffraction analysis. The results show that serious segregation exists in as-cast alloy, and the primary phases are T(AlZnMgCu), S(Al₂CuMg) and Al₈Cu₄Er, which preferentially locate in the grain boundary regions. The soluble T(AlZnMgCu) and S(Al₂CuMg) phases dissolve into the matrix gradually during single-stage homogenized at 465 °C with prolonging holding time, but the residual Al₈Cu₄Er phase cannot dissolve completely. Compared with the single-stage homogenization, both a finer particle size and a higher volume fraction of L1₂-structured Al₃(Er, Zr) dispersoids can be obtained in the two-stage homogenization process. A suitable homogenization scheme for the present alloy is (400 °C, 10 h)+(465 °C, 24 h), which is consistent with the results of homogenization kinetic analysis.