Effect of alloying elements on fracture behavior of Fe-18Ni-2Ti-(8Co) alloys

Effect of alloying elements on fracture behavior of Fe-18Ni-2Ti-(8Co) alloys has been investigated in the light of grain boundary segregation of alloying elements. During isothermal aging at 540°C, hardness level in the cobalt-bearing alloy was higher than that in the cobalt-free alloy, which is attributed to the solid solution hardening of cobalt, and the bigger hardness difference after overaging was due to the relatively fine precipitates in the cobalt-bearing alloy. The grain boundary segregation behavior of the elements was similar in two alloys, regardless of the cobalt addition. In two alloys, the fracture behavior before and after the maximum tensile strength is mainly governed by two factors: titanium at the grain boundaries and over-aging. Up to the aging time corresponding to the maximum tensile strength, the intergranular fracture strength was roughly inversely proportional to the titanium level at the grain boundaries, and after the time the tensile strength decreased with restoring ductility. Nickel or cobalt was not effective on the grain boundary strengthening of the iron-based alloys.     

时效温度对SLM 18Ni300马氏体时效钢显微组织和力学性能的影响

采用激光选区熔化(SLM)技术制备了18Ni300马氏体时效钢,结合拉伸试验、硬度测试和显微组织表征等手段,研究了时效温度(390, 490, 590℃)对SLM 18Ni300马氏体时效钢显微组织和力学性能的影响。结果表明,SLM成形试样主要由Fe-Ni马氏体基体和胞状亚结构组成,经时效处理后,试样微观组织发生显著变化。随着时效温度的升高,胞状亚结构逐渐分解,马氏体逆转变成为奥氏体,Σ3晶界占比下降。同时,Ni₃X(X=Ti, Al, Mo)纳米相弥散析出,并在590℃时粗化。随着时效温度的升高,SLM 18Ni300马氏体时效钢的强度和硬度均先增加后下降,伸长率先降低后增加。其中,490℃时效的SLM马氏体时效钢兼具超高强度和较好塑性,这与其基体中弥散分布的纳米析出相、适量的奥氏体含量和较低的Σ3晶界占比有关。     

Microstructure and Precipitate's Characterization of the Cu-Ni-Si-P Alloy

Microstructure of the Cu-Ni-Si-P alloy was investigated by transmission electron microscopy (TEM). The alloy had 551 MPa tensile strength, 226 HV hardness, and 36% IACS electrical conductivity after 80% cold rolling and aging at 450 °C for 2 h. Under the same aging conditions, but without the cold rolling, the strength, hardness, and electrical conductivity were 379 MPa, 216 HV, and 32% IACS, respectively. The precipitates identified by TEM characterization were δ-Ni₂Si. Some semi-coherent spherical precipitates with a typical coffee bean contrast were found after aging for 48 h at 450 °C. The average diameter of the observed semi-coherent precipitates is about 5 nm. The morphology of the fracture surface was observed by scanning electron microscopy. All samples showed typical ductile fracture. The addition of P refined the grain size and increased the nucleation rate of the precipitates. The precipitated phase coarsening was inhibited by the small additions of P. After aging, the Cu-Ni-Si-P alloy can gain excellent mechanical properties with 804 MPa strength and 49% IACS conductivity. This study aimed to optimize processing conditions of the Cu-Ni-Si-P alloys.     

Effects of heat treatment on microstructure and mechanical properties of Mg-6Zn-4Sm-0.4Zr alloy

A new rare earth magnesium alloy(Mg-6 Zn-4 Sm-0.4 Zr, wt.%) was prepared by permanent mould casting. The microstructure and mechanical properties of the alloy sample in as-cast and various heat treatment situations were characterized with an optical microscope(OM), X-ray diffractometer(XRD), scanning electron microscope(SEM) equipped with energy dispersive spectroscope(EDS), transmission electron microscope(TEM) and mechanical tests at room temperature, respectively. The experimental results show that the as-cast alloy mainly consists of α-Mg, eutectic Mg_2Zn_3, MgZnSm and Mg_(41)Sm_5. These eutectic phases with continuous or semicontinuous morphology principally distribute along grain boundaries. Almost all the eutectic compounds dissolve in α-Mg and the grains have no obvious growth trend after optimum solution treatment at 490 °C for 18 h. Meanwhile, the ultimate tensile strength(UTS) of 229 MPa and elongation(EL) to rupture of 9.78% can be achieved through the optimal solution treatment, which increase by 37 MPa and 57.74%, respectively, compared with that of the as-cast alloy. Further aging treatments at 200 °C for different durations lead to the conspicuous increment of mechanical properties and prominent age-hardening response. Peak-aged alloy(treated at 200 °C for 12 h) reveals better mechanical properties(UTS 258 MPa, EL 9.42%, hardness 73.4 HV) compared with the same alloy treated in other aging conditions, which is mainly ascribed to precipitated Mg_2Zn_3 and MgZn_2 phases. Fracture analysis demonstrates that the as-cast alloy belongs to inter-granular and cleavage fracture patterns, while the solutionized alloy(treated at 490 °C for 18 h) reveals trans-granular and quasi-cleavage fracture modes. For the peak-aged alloy, the fracture pattern obeys the mixture of trans-granular and cleavage modes.     

Role of chromium on mechanical properties of Fe-Mn-Ni-Mo-Ti maraging steels

This experiment investigated the role of chromium in the mechanical properties of Fe-5Mn-9Ni-5Mo-1.5Ti maraging steels containing up to 3% chromium. Remarkable age-hardening responses were observed in the Fe-5Mn-9Ni-5Mo-1.5Ti and Cr-bcaring alloys. A ductile-brittle-ductile transition occurred in the Cr-bearing alloys during isothermal aging below 510°C. This was due to the segregation of titanium and manganese to prior austenite grain boundaries and their subsequent desegregation into the matrix. The addition of chromium to the base alloy considerably improved its ductility after aging at 520°C. From microstructure and AES analyses, it is suggested that chromium addition augments the volume fractions of (Fe,Mn)₂Mo and η-Ni₃Ti precipitates in the Fe-5Mn-9Ni-5Mo-Cr alloys, which act as sinks of manganese and titanium in the matrices. This resulted in the reduction of the alloying elements concentration in the matrix, which is followed by the reduction in the segregation level of the elements at prior austenite grain boundaries, and consequently enhanced intergranular fracture strength. The optimum combination of strength and ductility was obtained in the Fe-5Mn-9Ni-5Mo-3Cr-l.5Ti alloy aged at 520°C for 2 hr. and was σ_{0 2}=1721 MPa, σ_LS=1756 MPa. and ε,= 10.2%.     

固溶温度对超纯净18Ni(350)马氏体时效钢断裂韧性及微观组织的影响

研究了在1083—1483K温度范围内，固溶温度对超纯净18Ni(350)马氏体时效钢断裂韧性(KIC)的影响．通过透射电镜(TEM)研究了马氏体时效钢微观组织的变化，结合相变曲线和断口扫描电镜(SEM)观察，探讨了固溶温度对断裂韧性的影响机理．结果表明：超纯净马氏体时效钢的断裂韧性(KIC)随着固溶温度的升高或再结晶晶粒尺寸的长大而增加，不存在常见的Ti(C，N)在晶界偏聚而引起的“热脆”现象．固溶态马氏体时效钢由单一的马氏体板条组成，其形貌、间距以及位错密度不受固溶温度的影响．在时效过程中，随着固溶温度的升高或再结晶晶粒的粗化，Ni3(Mo，Ti)等时效析出相在晶界或板条界的偏聚程度逐渐加重并导致基体软化，合金元素Ni，Mo的富集诱发了逆转变奥氏体形成．这使裂纹尖端易于钝化而表现出韧窝状穿晶断裂和保持较高的断裂韧性．     

Thermal stability evaluation of nanostructured Al6061 alloy produced by cryorolling

Grain growth of nanostructured Al6061 produced by cryorolling and aging process was investigated during isothermal heat treatment in 100–500 °C temperature range. Transmission electron microscopy (TEM) observations demonstrate that after cryorolling and aging at 130 °C for 30 h, the microstructure contains 61 nm grains with dispersed 50–150 nm precipitates and 0.248% lattice strain. In addition, an increase in tensile strength up to 362 MPa because of formation of fine strengthening precipitation and nano-sized grains was observed. Thermal stability investigation within 100–500 °C temperature range showed release of lattice strain, dissolution of precipitates and grain growth. According to the X-ray diffraction (XRD) analysis, Mg₂Si precipitates disappeared after annealing at temperatures higher than 300 °C. According to the results, due to the limited grain growth up to 200 °C, there would be little decrease in mechanical properties, but within 300–500 °C range, the grain growth, dissolution of strengthening precipitates and decrease in mechanical properties are remarkable. The activation energies for grain growth were calculated to be 203.3 kJ/mol for annealing at 100–200 °C and 166.34 kJ/mol for annealing at 300–500 °C. The effect of precipitation dissolution on Al lattice parameter, displacement of Al6061 (111) XRD peak and Portevin–LeChatelier (PLC) effect on stress–strain curves is also discussed.     

Effect of Ti addition on tensile properties of Cu-Ni-Si alloys

This study examines how varying Ni and Si contents and the addition of Ti affect the tensile behavior of Cu-Ni-Si alloys with different aging conditions. Cu-3Ni-0.7Si and Cu-6Ni-1.4Si alloys, both with and without the addition of Ti, were prepared by solution-heat treatment at 950 °C for 2 h, then aged at 500 °C for 1/ 6 h, 1/3 h, 1/2 h, 1 h, 3 h and 6 h, before tensile tests were conducted. Doubling the Ni and Si contents in Cu-Ni-Si alloys greatly increased the tensile strength and grain refinement, while marginally reducing the tensile elongation. Meanwhile, adding Ti to Cu-Ni-Si alloys reduced the grain size and greatly increased the tensile elongation. The aging response was also significantly accelerated by the addition of Ti. However, the expected improvement in tensile strength was not obtained by adding Ti, addition due to the agglomeration of coarse Ni₂Si precipitates and the accelerated lamellar structure formation. Finally, we discuss the microstructural changes that result from variations in aging time, different Ni and Si contents and the addition of Ti on Cu-Ni-Si alloys based on detailed optical, scanning electron microscope (SEM) and transmission electron microscope (TEM) micrographic observations and SEM fractographic analysis.     

Distribution and evolution of aging precipitates in Al-Cu-Li alloy with high Li concentration

The evolution and distribution of the aging precipitates in 1460 Al-Li alloy with high Li concentration (2.14%, mass fraction) during T6 aging and two-step T8 (4% predeformation) aging were investigated through TEM. The aging precipitates include δ′ (Al₃Li) and T1 (Al₂CuLi) phases, of which the δ′ phases are formed first in grain interiors. A lot of δ′/GPI/δ′ composite precipitates in which GPI zones are flanked with a pair of δ′ phases, are formed at 145 °C of T6 aging, which are thermally stable. At 160 °C and 175 °C of T6 aging, many T1 phases nucleate first at subgrain boundaries and grain boundaries, and then form and grow within grains. As to the T8 aging, the δ′/GPI/δ′ composite precipitates are formed during the first-step aging at 130 °C for 20 h, which are thermally stable during the second-step aging at 160 °C. The plastic predeformation accelerates T1 nucleation within grains during the second-step aging at 160 °C.     

Effect of step-quenching on microstructure of aluminum alloy 7055

The effect of step-quenching on the microstructure of aluminum alloy 7055 after artificial aging was studied by hardness testing and transmission electron microscopy (TEM). Step-quenching leads to decomposition of solid solution and heterogeneous precipitation of equilibrium phase mainly on dispersoids and at grain boundaries; thus lower hardness after aging. Prolonging isothermal holding at 415 ℃ results in coarser and more spaced η phase particles at grain boundaries with wider precipitates free zone, and lower density of larger η′ hardening precipitates inside grains after aging. Isothermal holding at 355 ℃ results in heterogeneous precipitation of η phase both on dispersoids and at grain boundaries. Isothermal holding at 235 ℃ results in heterogeneous precipitation of η phase first, and then S phase. Precipitates free zones are created around these coarse η and S phase particles after aging. Prolonging isothermal holding at these two temperatures leads to fewer η′ hardening precipitates inside grains, larger and more spaced η phase particles at grain boundaries and wider grain boundary precipitates free zone after aging.     

Effect of Retrogression Heat Treatment Time on Microstructure and Mechanical Properties of AA7010

The effect of retrogression time during retrogression and re-aging (RRA) treatment of AA7010 is evaluated by performing tensile tests and characterizing the microchemistry of the grain boundary precipitates (GBPs) using transmission electron microscope coupled with the energy-dispersive spectroscopy. Retrogression time is evaluated so that the ultimate tensile strength of the RRA-treated sample is equal to that of the T6-treated sample and the grain boundary microstructure similar to that of the over-aged (T7451) condition. The investigation reveals that the sample retrogressed at 200 °C for 20 min has UTS of 586 MPa which is equivalent to that of the T6 sample and 11.5% higher than that of the T7451 condition. The fracture toughness of the RRA-treated sample was 41 MPa√m. Microstructure of the RRA-treated sample is similar to T7451, along the grain boundaries and in the grain interior similar to that of the T6-treated sample. Energy-dispersive spectroscopy confirmed the increment of Cu content on the GBP’s with increase in the retrogression time, which is expected to improve the stress corrosion cracking resistance of the alloy.     

Tensile properties and fracture toughness of 18Ni (250 grade) maraging steel weldments

Abstract

Gas tungsten arc welding was performed on 18Ni (250 grade) maraging steel sheet using two different filler wires: one of the same composition as the base material and the other containing more cobalt and aluminium and less molybdenum and titanium. Weld specimens were then aged under four different sets of conditions. After metallographic characterisation, mechanical properties including hardness, tensile strength and ductility, and fracture toughness were evaluated. Results showed that use of the matching filler material led to lower strength but higher ductility than in the other case; this was attributed to the presence of reverted austenite in the former (caused by segregation, especially of molybdenum) at the fusion zone substructure boundaries. In both types of weld metal, a re-solution treatment followed by aging at 480°C resulted in optimum tensile properties. Fracture toughness of the aged weldments was in general close to that of the parent material aged at 480°C; some deterioration occurred only when welds with pronounced segregation were made at high temperature.     

Super304H钢600℃时效的组织和韧性

利用金相显微镜、扫描电子显微镜和X射线衍射仪,通过冲击试验,研究了Super304H钢在600℃时效条件下的微观组织和韧性。结果表明,Super304H钢在600℃时效条件下基体组织均是单一奥氏体相,析出相主要由Nb(C,N)、富Cu相和M23C6组成;时效时间不同,析出相的数量、形态和分布不同。其中M23C6沿晶析出是引起时效Super304H钢发生脆化的主要原因,且冲击吸收能量下降幅度与晶界上M23C6数量有关。     

Effect of precipitation from tweed matrix on the mechanical properties in a metastable beta alloy of Ti-15-3

The effects of aging treatment on the microstructures and mechanical properties of a metastable beta titanium alloy Ti-15-3 (Ti−15V−3Al−3Sn-3Cr) have been investigated with hardness measurements, tensile test, and optical and electron microscopy. Precipitate-free beta structure with average grain size of about 56 μm was obtained after solution treatment at 800°C for 15 min followed by air cooling. Solution treated specimens were aged up to 800 h in the temperature range between 350 and 600°C. The morphology of the precipitates was varied significantly, depending on the aging temperature. The fine aggregates of α precipitates were dominant above 450°C. Peak hardness values were maintained up to 800 h at 500°C, which showed the superior thermal stability of α precipitates. Tensile strength increased up to 1600 MPa along with the decrease of elongation after aging at 350 and 400°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.     

Microstructure of two-phase Al–1.7 at% Cu alloy deformed by equal-channel angular pressing

The microstructural changes of an aged Al–1.7 at% Cu alloy associated with severe plastic deformation have been studied by transmission electron microscopy (TEM) and energy-filtered transmission electron microscopy (EF-TEM). θ′ precipitates are almost completely dissolved after eight passes of equal-channel angular (ECA) pressing, and nearly single-phase α with a fine grain size of approximately 500 nm is obtained. When a severely deformed sample is aged for 24 h at 100°C, precipitation of equiaxed θ phase is observed along the grain boundaries, whereas only GP zones are formed in the undeformed sample. The dissolution and precipitation processes in severely deformed Al–1.7 at% Cu alloy have been examined by TEM and energy-filter mapping.     

固溶-时效处理对Ti-Ni-Cr形状记忆合金拉伸性能和显微组织的影响

用拉伸试验和透射电子显微镜研究了固溶时效处理对Ti-50.8Ni-0.3Cr形状记忆合金拉伸性能和显微组织的影响。800℃固溶淬火态合金的塑性优于时效态。随时效时间tag延长,300和400℃时效态合金的抗拉强度Rm300和Rm400先急剧增大后趋于稳定,且Rm300相似文献   

Characterization of Impact Deformation Behavior of a Thermally Aged Duplex Stainless Steel by EBSD

The effect of thermal aging on phase transformation and impact toughness of an as-cast duplex stainless steel was investigated at room temperature. After long-term thermal aging, the impact toughness decreases significantly and the cracks initiate and propagate more easily. The plastic deformation ability of the ferrite phase decreases after thermal aging,which leads to the degradation of impact toughness. High stress concentration occurs on the grain boundaries of the austenite phase in the aged materials. Meanwhile, high-stress concentration areas are also observed in the austenite phase near the grain boundaries. After long-term thermal aging, pinned dislocations in ferrite and along phase boundaries lead to the high stress concentration. Micro-cracks preferentially initiate in the ferrite phase and propagate via separation of phase boundaries. The blocking influences of spinodal decomposition precipitates and G-phase precipitates are stronger than the effect of grain boundaries and phase boundaries on the dislocation movement.     

Microstructure and elevated-temperature tensile properties of differential pressure sand cast Mg-4Y-3Nd-0.5Zr alloy

The microstructures of an Mg-4Y-3Nd-0.5Zr alloy by differential pressure casting were investigated using scanning electron microscopy(SEM) and transmission electron microscopy(TEM), and its tensile deformation behavior was measured using a Gleeble1500 D themo-simulation machine in the temperature range of 200 to 400 °C at initial strain rates of 5×10-4 to 10-1 s-1. Results show that the as-cast microstructure consists of primary α-Mg phase and bone-shaped Mg5 RE eutectic phase distributed along the grain boundary. The eutectic phase is dissolved into the matrix after solution treatment and subsequently precipitates during peak aging. Tensile deformation tests show that the strain rate has little effect on stress under 300 °C. Tensile stress decreases with an increase in temperature and the higher strain rate leads to an increase in stress above 300 °C. The fracture mechanism exhibits a mixed quasi-cleavage fracture at 200 °C, while the fracture above 300 °C is a ductile fracture. The dimples are melted at 400 °C with the lowest strain rate of 10-4 s-1.     

Effect of cooling rate on microstructure and tensile properties of powder metallurgy Ni-based superalloy

The effects of size distribution, morphology and volume fraction of γ′ phase and grain size on tensile properties of powder processed Ni-based superalloy were investigated by using two different quenching methods. Oil quenching and air cooling were adopted with cooling rate of 183 °C/s and 4–15 °C/s, respectively. The experimental results show that the average size of the secondary γ′ after oil quenching is 24.5 nm compared with 49.8 nm under air cooling, and corresponding volume fractions of γ′ are 29% and 34%, respectively. Meanwhile, the average grain size remains nearly equivalent from both oil-quenching and air-cooling specimens. The tensile strength at room temperature is higher for the oil-quenched specimen than the equivalent from the air-cooled specimen, but the difference approaches each other as the temperature increases to 650 °C. The fractography clearly demonstrates that transgranular fracture governs the failure process at ambient temperature, in contrast to the intergranular fracture at 650 °C or even higher temperature. These two mechanical responses indicate the strengthening effects of γ′ precipitates and grain boundary for polycrystalline Ni-based superalloys at different temperatures.