低层错能镍基单晶高温合金压缩行为和变形组织的温度相关性（英文）

在室温至1000℃的范围内，研究温度对一种低层错能镍基单晶高温合金压缩行为和变形组织的影响。研究结果表明，压缩行为和变形组织均表现出温度相关性。室温下该合金具有较高屈服强度，600℃时屈服强度有所下降；随后，随着温度的升高，屈服强度持续增加，并在800℃时达到最大值；在800℃以上时，屈服强度迅速降低。通过透射电子显微镜观察揭示合金变形机制。位错缠结和位错对塞积是室温下屈服强度较高的主要原因。在600℃时，变形机制从反相畴界切割向堆垛层错切割转变，这导致屈服强度略有下降。在800℃时，变形机制以堆垛层错切割为主，而Lomer-Cottrell锁和不同方向堆垛层错之间的反应导致最大的屈服强度。在900℃及以上时，虽然仍存在一些层错，但主要变形机制为位错绕过机制。最后，讨论变形机制和压缩行为的温度依赖性。     

Microstructural Evolution and Deformation Behavior of a Hot-Rolled and Heat Treated Fe-8Mn-4Al-0.2C Steel

The microstructural evolution following tensile deformation of a hot-rolled and heat treated Fe-8Mn-4Al-0.2C steel was studied. Quenching in the range of 750-800 °C followed by tempering at 200 °C led to a ferrite-austenite mixed microstructure that was characterized by excellent combination of tensile strength of 800-1000 MPa and elongation of 30-40%, and a three-stage work hardening behavior. During the tensile deformation, the retained austenite transformed into martensite and delayed the onset of necking, thus leading to a higher ductility via the transformation-induced plasticity (TRIP) effect. The improvement of elongation is attributed to diffusion of carbon from δ-ferrite to austenite during tempering, which improves the stability of austenite, thus contributing to enhanced tensile ductility.     

Effects of Pack Rolling Temperature on Microstructure and Mechanical Properties of Powder Metallurgical Ti-45Al-7Nb-0.3W Alloy

Powder metallurgical Ti-45Al-7Nb-0.3W (at.%) alloys were pack rolled at temperatures of 1240°C, 1255°C, 1270°C, and 1285°C. The microstructures were investigated by scanning electron microscopy (SEM) and transmission electron microscopy. The tensile properties were tested at room temperature and 800°C. After rolling, the sheets exhibited duplex microstructures with refined grains. The tensile test results showed the sheet rolled at 1270°C displayed excellent room temperature tensile properties with an ultimate tensile strength (UTS) of 782 MPa and an elongation of 1.95%. When tested at 800°C, all sheets showed UTS of over 600 MPa and elongations of around 50%. The dislocation movements and mechanical twinning played important roles at the initial stage of rolling deformation. However, during the subsequent deformation process, the deformation mechanism should mainly be the result of dynamic recrystallization.     

Performance of tungsten fibers for Wf/W composites under cyclic tensile load

Toughness improved tungsten-based composites are one of the currently considered material option for future fusion reactors capable to withstand both high heat flux and irradiation induced embrittlement. Today, fiber-reinforced composites (W_f/W) are being intensively studied as risk-mitigation materials to replace bulk tungsten which is susceptible to neutron irradiation embrittlement especially below 800 °C. Operation of a material as an element of a plasma facing component (i.e. divertor monoblock or first wall armour) implies not only high heat flux exposure but also thermal cyclic fatigue caused by repetitive oscillations of the heat loads due to the nature of the plasma and the limitations on the capacity of its confinement. In this work, we assessed the performance of potassium doped tungsten fibers under cyclic loading applied in tensile mode. Stress-controlled fatigue tests were performed at room temperature, 300 °C and 500 °C increasing the load from 50% of the yield strength up to the ultimate tensile strength of the studied fibers. It is revealed that significant cyclic hardening emerges as the fatigue stress limit exceeds the yield strength already within a few cycles. Despite the noticed cyclic hardening, the wire can sustain few hundreds of cycles without any detectable damage unless the cycle stress is increased to reach the value above the mean ultimate tensile strength. Given this observation, we have studied the impact of the cyclic stress (σ_C) on the rupture strength and total elongation of the wires exposed to twenty loading cycles varying test temperature in the range 23–500 °C. At room temperature, the rupture stress after cyclic deformation progressively increases with σ_C and saturates at 2.7 GPa with a moderate reduction of the total elongation, while the nominal ultimate tensile strength of the wire is 2.5 GPa. Thus, the strength of the wire is increased by 200 MPa, on average. At elevated temperature, the rupture stress after the cyclic deformation increases by more than 300 MPa.     

Twin-Induced Plasticity of an ECAP-Processed TWIP Steel

The TWIP steels show high strain hardening rates with high ductility which results in high ultimate tensile strength. This makes their processing by equal channel angular pressing very difficult. Up to now, this has only been achieved at warm temperatures (above 200 °C). In this paper, a FeMnCAl TWIP steel has been processed at room temperature and the resulted microstructure and mechanical properties were investigated. For comparison, the material has also been processed at 300 °C. The TWIP steel processed at room temperature shows a large increase in yield strength (from 590 in the annealed condition to 1295 MPa) and the ultimate tensile strength (1440 MPa) as a consequence of a sharp decrease in grain size and the presence within the grains of a high density of mechanical twins and subgrains. This dense microstructure results also in a loss of strain hardening and a reduction in ductility. The material processed at 300 °C is more able to accommodate deformation and has lower reduction in grain size although there is a significant presence of mechanical twins and subgrains produced by dislocation activity. This material reaches an ultimate tensile strength of 1400 MPa with better ductility than the room temperature material.     

Effect of rolling in the intercritical region on the tensile properties of dual-phase steel

A steel containing 0.088 wt% C, 1.2 wt% Mn, and 0.78 wt% Cr was rolled at intercritical temperature (790 °C) and quenched to produce dual-phase microstructure. Rolling caused anisotropic increase in tensile strength and little change in ductility. The results suggest that rolling increased strength by a combination of strengthening of the ferrite and an increase in the stress transferred to the martensite. Up to 20% rolling reduction strengthened the ferrite by work hardening, larger reductions then reduced the strength of ferrite, anisotropically, due to increased recovery. Subgrains in ferrite were observed after rolling in the intercritical region which can contribute to the ultimate strength of the rolled material.     

Tensile behavior of a new single-crystal nickel-based superalloy (CMSX-4) at room and elevated temperatures

Tensile behavior of a new single-crystal nickel-based superalloy with rhenium (CMSX-4) was studied at both room and elevated temperatures. The investigation also examined the influence of γ′ precipitates (size and distribution) on the tensile behavior of the material. Tensile specimens were prepared from single-crystal CMSX-4 in [001] orientation. The test specimens had the [001] growth direction parallel to the loading axis in tension. These specimens were given three different heat treatments to produce three different γ′ precipitate sizes and distributions. Tensile testing was carried out at both room and elevated temperatures. The results of the present investigation indicate that yield strength and ultimate tensile strength of this material initially increases with temperature, reaches a peak at around 800 °C, and then starts rapidly decreasing with rise in temperature. Both yield and tensile strength increased with increase in average γ′ precipitate size. Yield strength and temperature correlated very well by an Arrhenius type of relationship. Rate-controlling process for yielding at very high temperature (T ≥ 800 °C) was found to be the dislocation climb for all three differently heat-treated materials. Thermally activated hardening occurs below 800 °C whereas above 800 °C thermally activated softening occurs in this material.     

Tensile behavior of a new single crystal nickel-based superalloy (CMSX-4) at room and elevated temperatures

Tensile behavior of a new single-crystal nickel-based superalloy with rhenium (CMSX-4) was studied at both room and elevated temperatures. The investigation also examined the influence of γ′ precipitates (size and distribution) on the tensile behavior of the material. Tensile specimens were prepared from single-crystal CMSX-4 in [001] orientation. The test specimens had the [001] growth direction parallel to the loading axis in tension. These specimens were given three different heat treatments to produce three different γ′ precipitate sizes and distributions. Tensile testing was carried out at both room and elevated temperatures. The results of the present investigation indicate that yield strength and ultimate tensile strength of this material initially increases with temperature, reaches a peak at around 800 °C, and then starts rapidly decreasing with rise in temperature. Both yield and tensile strength increased with increase in average γ′ precipitate size. Yield strength and temperature correlated very well by an Arrhenius type of relationship. Rate-controlling process for yielding at very high temperature (T ≥ 800 °C) was found to be the dislocation climb for all three differently heat-treated materials. Thermally activated hardening occurs below 800 °C whereas above 800 °C thermally activated softening occurs in this material.     

Strain Hardening Associated with Dislocation,Deformation Twinning,and Dynamic Strain Aging in Fe–20Mn–1.3C–(3Cu) TWIP Steels

The effects of Cu on stacking fault energy,dislocation slip,mechanical twinning,and strain hardening in Fe–20Mn–1.3C twinning-induced plasticity(TWIP) steels were systematically investigated.The stacking fault energy was raised with an average slope of 2 mJ/m2 per 1 wt% Cu.The Fe–20Mn–1.3C–3Cu steel exhibited superior tensile properties,with the ultimate tensile strength reached at 2.27 GPa and elongation up to 96.9% owing to the high strain hardening that occurred.To examine the mechanism of this high strain hardening,dislocation density determination by XRD was calculated.The dislocation density increased with the increasing strain,and the addition of Cu resulted in a decrease in the dislocation density.A comparison of the strain-hardening behavior of Fe–20Mn–1.3C and Fe–20Mn–1.3C–3Cu TWIP steels was made in terms of modified Crussard–Jaoul(C–J) analysis and microstructural observations.Especially at low strains,the contributions of all the relevant deformation mechanisms—slip,twinning,and dynamic strain aging—were quantitatively evaluated.The analysis revealed that the dislocation storage was the leading factor to the increase of the strain hardening,while dynamic strain aging was a minor contributor to strain hardening.Twinning,which interacted with the matrix,acted as an effective barrier to dislocation motion.     

Microstructure and mechanical properties of TiC nanoparticle-reinforced Mg−Zn−Ca matrix nanocomposites processed by combining multidirectional forging and extrusion

TiC nanoparticle-reinforced Mg−4Zn−0.5Ca matrix nanocomposites were processed by combining multidirectional forging (MDF) and extrusion (EX). The grain size of the nanocomposite after MDF+EX multi-step deformation was significantly decreased compared with that processed only by MDF. The average size of the recrystallized grains gradually increased after EX with increasing the number of MDF passes at 270 °C. However, the grain size significantly decreased by MDF processing at 310 °C. Both fine and coarse MgZn₂ phases appeared in the (MDF+EX)-processed nanocomposites, and their volume fractions gradually increased with increasing the number of MDF passes before EX. Ultrahigh tensile properties (yield strength of ~404 MPa, ultimate tensile strength of ~450.3 MPa and elongation of ~5.2 %) were obtained in the nanocomposite after three MDF passes at 310 °C followed by EX. This was attributed to the refinement of the recrystallized grains, together with the improved Orowan strengthening provided by the precipitated MgZn₂ particles that were generated by MDF+EX multi-step deformation.     

Microstructural Characteristic and Mechanical Behavior of Nodular Silicon Hypereutectic Al-Si Alloys

The microstructure and mechanical properties of Al-Si-Cu-Mg alloys containing 12 wt.% to 30 wt.% Si are discussed. The eutectic and primary silicon particles are nodulized by a designed modification practice followed by a solution heat treatment of 6 h to 8 h at 510°C to 520°C. Metallographic analysis was used to measure structural characteristics of the Si-rich structures. Spheroidization of silicon phase leads to an increase in tensile strength and ductility of alloys at room temperature and 300°C compared with commercial Al-Si alloy. Increasing Si concentration causes the ultimate tensile strength and elongation at room temperature to fall due to the appearance of coarse silicon particles, but the ultimate tensile strength at 300°C remains unchanged.     

Subcritical Spheroidization of Medium-Carbon 50CrV4 Steel

An investigation on subcritical spheroidization anneal of the cold-rolled 50CrV4 steel at 720?°C has been carried out. During spheroidization anneal, the lamellar cementite was gradually broken down and changed to spherical shape. With prolonging of soaking time, the amount of lamellar cementite decreases gradually, and that of the spheroidized cementite particle increases gradually. The relationship of the spheroidization ratio versus soaking time for the steel can be described by a typically sigmoid curve. Additionally, the cold rolling deformation accelerates the breakup of lamellar cementite and the formation of spheroidal cementite particles during spheroidization anneal of the steel. The more severe the deformation is, the more rapidly the spheroidization occurs. From the results of tensile and hardness test, the yield strength, ultimate tensile strength, and hardness decrease and the percentage elongation to failure increases rapidly during the first 2?h of spheroidization. Between 2 and 8 h, the yield strength, ultimate tensile strength, hardness nearly keep a constant, which are independent of the soaking time, whereas the percentage elongation to failure firstly increases and then decreases with prolonging of soaking time.     

Reaction synthesis of Ni/Ni3Al multilayer composites using Ni and Al foils: High-temperature tensile properties and deformation behaviour

The effects of temperature on the tensile properties and deformation behaviour of Ni/Ni₃Al multilayer composites have been systematically investigated. With increasing tensile test temperature from room temperature to 600 °C, the ultimate tensile strength decreased. The ‘abnormal’ strengthening of Ni₃Al gave rise to a reduction in the capability for cooperative deformation between the Ni and the Ni₃Al layers at elevated temperatures. As a result, the ultimate tensile strength of the multilayer composites decreased at elevated temperatures. A mixture of transgranular cleavage and intergranular failure at relatively low temperatures, and an intergranular failure at elevated temperatures were observed in the fracture surfaces of the Ni₃Al layers in the multilayer composites. The splitting of the coarse precipitates along the {0 0 1} planes at 800 °C resulted from the differences in solubility of Al in Ni between room temperature and 800 °C.     

Temperature dependence of the flow stress of Fe–18Mn–0.6C–xAl twinning-induced plasticity steel

The tensile properties of Fe–18Mn–0.6C with Al-alloying additions of 0, 1.5 and 2.5 wt.% were investigated in the temperature range from 213 K (?60 °C) to 413 K (140 °C). The addition of Al resulted in an increase of the yield strength due to solid solution hardening and a decrease of the work hardening due to the suppression of deformation twinning. Both the decrease of the deformation temperature and the addition of Al suppressed the dynamic strain aging, clearly indicating an interaction between the stacking fault region of the mobile dislocation and Mn–C point defect complexes. A constitutive model for the temperature dependence of the flow stress, taking into account the thermally activated dislocation glide, Al solid solution hardening and the dynamic Hall–Petch effect caused by deformation twinning, was developed.     

低温退火加工硬化Al-10% Mg合金的显微组织特点和力学行为

研究了冷轧减薄率为75%的Al-10% Mg（质量分数）合金在75~150 ℃下退火的显微组织特点和力学行为。轧制态和退火态Al-10% Mg合金的特征是晶粒细长,位错密度高,Al₃Mg₂相含量极低,且无弥散分布的Al₃Mg₂相。随着退火温度的升高,细长晶粒的宽度增加,位错密度减小。在75~150 ℃退火后,相比于轧制态合金,其屈服强度降低8%~33%,极限抗拉伸强度降低1%~12%,延伸率增加16%~83%。此外,分析了各种强化机制对屈服强度的贡献,以及原有位错和Mg溶质对塑性的贡献。     

Heat treatment of 319.2 aluminium automotive alloy Part 2, Ageing behaviour

In this part of a two-part study of the heat treatment of 319.2 aluminium alloy, the hardening behaviour upon artificial ageing in the temperature range 155‐220 °C for periods of up to 24 h has been investigated. The test bars used were solution heat treated for 8 h at 515 °C (see Part 1). The results show that peak-ageing is achieved after 24 h at 155 °C or 5 h at 180 °C, giving 253 MPa yield strength, 403 MPa ultimate tensile strength, and 1.2% elongation. Inclusions and oxides have a marginal effect on the yield strength; however, they deteriorate both the ultimate tensile strength and % elongation to levels below those obtained in the as-cast condition. Quality index against ageing time relationships have been discussed in terms of the ageing conditions.     

Influence of cooling rate after homogenization on microstructure and mechanical properties of aluminum alloy 7050

The influence of cooling rate (0.009?C220 °C/s) after homogenization on the microstructure and mechanical properties of high strength aluminum alloy 7050 was investigated by tensile testing, optical microscope, X-ray diffraction, scanning electron microscope, and transmission electron microscope. A lower cooling rate after homogenization resulted in lower mechanical properties after aging. The drop in strength was significant when the cooling rate was decreased from 0.5 °C/s to 0.1 °C/s. A lower cooling rate gave rise to a larger amount of remnant S(Al₂CuMg) phase and a higher fraction of recrystallization after solution heat treatment. Consequently, the increase in strength after aging due to precipitation hardening and substructure hardening was less significant in the case of slow cooling. This was supposed to be responsible for the lower mechanical properties due to a lower cooling rate after homogenization.     

微量TiC/ZrC对TZM合金室温及高温性能与组织的影响

采用粉末冶金方法制备Mo-Ti-Zr-TiC/ZrC合金,研究微量TiC/ZrC对合金性能与组织的影响。结果表明,添加微量TiC/ZrC后合金性能得以明显提高,TiC/ZrC添加量为0.4%(质量分数,下同)时,合金室温抗拉强度分别达到最高值。同时,微量TiC/ZrC显著提高了合金的高温强度,添加的微量碳化物粒子促进了合金高温拉伸过程中韧窝的形成,使合金高温拉伸由穿晶解理和韧窝断裂的混合断裂模式向韧窝断裂转变。     

纳米银粉体新法制备及其表征

采用均匀沉淀和高温热分解相结合的方法制备了纳米银粉体,分析了粉体形成机理,研究了pH值、烧结温度和时间对粉体粒径和形貌的影响;采用X射线衍射仪、扫描电子显微镜表征了纳米银粉体结构、组成、大小和形貌.结果表明,pH=7、烧结温度300℃、烧结2h的条件下可得到分散性好、颗粒均匀,粒径50 nm的粉体.本制备方法原料易得、成本低、设备及工艺简单,反应副产物易回收且可用作肥料,整个过程满足清洁生产的要求.     

Effects of Neodymium and Calcium on the Thermal Stability of AZ71 Magnesium Alloys

The effects of an addition of 0–2 wt% Nd on thermal stability of 0–3 wt% Ca-containing modified AZ71 magnesium alloys was investigated. The ignition temperature was found to increase from that of AZ71, 574, to 825 °C with the addition of 0.5 wt% Ca and 1 wt% Nd. The ignition temperature was further increased to 1114 °C when 3 wt% Ca was added. The Ca- and Nd-added AZ71 was isothermally maintained at a temperature of 500 °C in air for 12 h. The MgO–CaO–Nd₂O₃ formed on the surface to improve the thermal stability of the AZ71–xCa–yNd alloys. While both the tensile strength and ductility decreased with the Ca concentration in the alloy, an addition of 1 wt% Nd was found able to alleviate the degradation effects of Ca on the tensile strength and ductility at 170 °C. Both solid solution formation and precipitation strengthening contributed to the increase in toughness. AZ71 containing 0.5–2 wt% Ca and 1 wt% Nd provides the optimum combination of ignition resistance and mechanical properties.