Microstructure Characterization and Fracture Toughness of Laves Phase-Based Cr–Nb–Ti Alloys

Three Laves phase-based alloys with nominal compositions of Cr2Nb–x Ti(x = 20,30,40,in at%) have been prepared through vacuum non-consumable arc melting.The results show that the microstructures of Cr2Nb-(20,30) Ti alloys are composed of the primary Laves phase C15–Cr2(Nb,Ti) and bcc solid solution phase,while the microstructure of Cr2Nb–40Ti alloy is developed with the eutectic phases C15–Cr2(Nb,Ti)/bcc solid solution.The measured fracture toughness of ternary Laves phase C15–Cr2(Nb,Ti) is about 3.0 MPa m1/2,much larger than 1.4 MPa m1/2for binary Laves phase Cr2 Nb.Meanwhile,the fracture toughness of Cr2Nb–x Ti(x = 20,30,40) alloys increases with increasing Ti content and reaches 10.6 MPa m1/2in Cr2Nb–40Ti alloy.The eutectic microstructure and addition of Ti in Cr2 Nb are found to be effective in toughening Laves phase-based alloys.     

铌对焊缝氢致裂纹敏感性及显微组织的影响

本文利用G-BOP 试验方法探讨了铌含最对低合金钢手工电弧焊焊缝氢裂纹敏感性及其显微组织的影响。结果表明,增加焊缝中的铌含量会增大焊缝氢致裂纹的敏感性,但当铌含量低于0.03%时,看不到这种有害倾向.降低焊缝中的氢含量能有效地降低焊缝的氢裂倾向。但随铌含量增高,这种效应减弱。铌能抑制焊缝中晶界铁素体生长,促进铁素体与第二相(即魏氏组织)形成。本文讨论了其机理,并提出了新的设想.通过回归分析得出焊缝氢致裂纹率Pc 与铌含量、焊接线能量E、焊缝含氢量H_(?w)以及母材预热温度T_0之间的关系式。     

On the determination of coating toughness during nanoindentation

Nanoindentation tests have been widely used to evaluate fracture toughness of brittle materials. For the thin coatings, energy based models have been shown to be effective. In this study, a new energy based method is proposed based on the analysis of unloading curve at the start and end points of the crack induced pop-in. The semi-analytical generalized expressions have been presented to determine the fracture toughness of coatings under both load and displacement control. This provides valuable theoretical guideline to determine fracture toughness from the energy point of view.     

ZG40Cr25Ni20炉管服役后断裂韧性试验分析及断口微观形貌研究

将断裂力学原理应用到炉管寿命预测中.利用MTS810试验机,对服役3万h的炉管和未服役炉管的断裂韧性进行了试验分析,并利用扫描电镜进行了断口的形貌分析.研究结果表明:服役后炉管的断裂韧性与未服役炉管相比显著下降,抵抗裂纹扩展能力减弱;服役后的炉管断裂面没有裂纹稳态扩展区和宏观塑形变形,对应于断裂韧性的下降,断裂面微观形貌呈典型的脆性断裂特征.     

TC4钛合金惯性摩擦焊接头组织及断裂韧性与裂纹扩展特性分析

文章采用了XRD、SEM、EBSD等显微表征技术分析了焊态及焊后热处理态下焊接接头各区域的微观组织特征,并研究了焊接接头的断裂韧性和疲劳裂纹扩展性能。结果表明,焊缝区以再结晶组织为主,热力影响区等轴状初生α_p相转变为棒状结构,热影响区组织与母材基本相同,热力影响区与热影响区的原始β晶粒内部分区域形成了取向差角度约为60°的针状马氏体α′相,热处理促进了残余亚稳态β相分解,在片状α_s相间形成了大量断续分布组织。焊缝区α晶粒内大量的平行或交叉分布的片状α相和复杂的相界面结构可有效阻碍裂纹的扩展并改变裂纹的扩展路径,提高焊接接头的断裂韧性及抗疲劳裂纹扩展能力。     

Near-nano WC–Co hardmetals: Will they substitute conventional coarse-grained mining grades?

Development of nanostructured hardmetals is a task of great importance. Nevertheless, in spite of some “euphoria” with respect to nanograined hardmetals, their potential application ranges are yet not clear. In some works, near-nano and nano hardmetals are believed can potentially substitute conventional medium- and coarse-grained WC–Co grades. In the present work near-nano hardmetals with WC mean grain size of nearly 200 nm and Co contents of 10–33 wt.% were produced and examined with respect to their hardness, fracture toughness, transverse rupture strength and wear-resistance. The near-nano hardmetal with 10% Co having a hardness of 20 GPa and fracture toughness of 9.5 MPa m^1/2 is characterised by exceptionally high wear-resistance obtained by use of the ASTM B611 test in comparison with an ultra-fine grade with 10%. The wear-resistance of the near-nano hardmetals in the ASTM B611 test significantly decreases with increasing the Co content and the wear rates of the difference between the wear rates of the grades with 10% and 33% Co is equal to nearly 44 times. The near-nano hardmetals with 25%, 28% and 33% Co having a moderate hardness and high fracture toughness corresponding to conventional coarse and ultra-coarse-grained mining grades have a very low wear-resistance in laboratory tests on concrete-cutting, granite-cutting and percussion drilling of quartzite. A number of grades with the very similar hardness of 13 ± 0.2 GPa, WC mean grain sizes varying from 0.2 to 4.8 μm and Co contents varying from 3% to 25% were produced and examined by use of the ASTM B611 test. The wear-resistance of the near-nano grade with 25% Co is found to be lower by more than three times compared to the coarsest grade with 3% Co at almost the same hardness. In this case, in spite of the very similar hardness of all the samples, the proportion of the soft binder phase on the surface subjected to abrasive particles when performing the test is significantly higher for the near-nano grade compared to the coarse- and ultra-coarse grained hardmetals. Thus, near-nano and presumably nano hardmetals are expected to never substitute conventional medium- and coarse-grained mining grades. The only application range, where near-nano and nano hardmetals can potentially substitute conventional grades, is an application range of hardnesses of above 18 GPa.     

Investigations on the microstructure and room temperature fracture toughness of directionally solidified NiAl–Cr(Mo) eutectic alloy

The microstructures and room temperature fracture toughness of directionally solidified NiAl-xCr-6Mo (x = 28, 32 and 36 at%) alloys were investigated. Fully eutectic microstructure could be obtained in the alloys over a wide composition range. High temperature gradient could increase the planar/cellular transition rate and expand the eutectic coupled growth zone. The volume fraction of Cr(Mo) strengthening phase increased with the increasing content of Cr, accordingly, the fracture toughness of NiAl–Cr(Mo) alloys also gradually increased. The fracture toughness of 26.15 MPa m^1/2 was obtained in the NiAl-36Cr-6Mo hypereutectic alloy solidified at withdrawal rate of 10 μm/s and temperature gradient of 600 K/cm, which is the highest value in the NiAl–Cr–Mo alloy system until now. Well-aligned microstructure was beneficial to the enhancement of the fracture toughness, while the existence of primary phase seriously deteriorated the toughness. All the directionally solidified NiAl–Cr(Mo) alloy failed as brittle quasi-cleavage fracture. Some toughening mechanisms, such as crack bridging, crack nucleation, crack blunting, crack deflection, interface debonding and shear ligament toughening as well as linkage of microcracks were observed. In addition, mobile dislocation generated from the interface also had significant influence on the toughness.     

Microstructural characteristics and impact toughness in YS690MPa steel weld metal for offshore structures

The fine-grained mixed microstructure of acicular ferrite (AF) and bainite in YS690MPa steel weld metal contributes to attain high-impact toughness. The morphology and evolutionary mechanism of fine-grained mixed microstructure in this weld metal were investigated. Single or multiple AF grains were nucleated on complex inclusions by forming Mn-depleted zones, where Mn spontaneously diffused into Ti oxide inclusions due to the cation vacancies. It is in good agreement with the theoretical calculation by first principle. The bainite nucleated on austenite grain boundary and then assisted the pre-formed AF to partition the austenite grain into small and separate regions. Furthermore, the later formed ferrite nucleated on the broad surface of pre-formed ferrite plates and grew in those small regions with limited grain size. All of them resulted in the formation of fine-grained mixed microstructure, which provided excellent impact toughness in this weld metal with dimples and quasi-cleavage fracture surface combination.     

Sinter and hot isostatic pressing (HIP) of multi-wall carbon nanotubes (MWCNTs) reinforced ZTA nanocomposite: Microstructure and fracture toughness

This work describes the microstructure and fracture toughness of zirconia toughened alumina (ZTA) nanocomposite in which multi-wall carbon nanotubes (MWCNTs) and nanosized ZrO₂ particles were used as reinforcement. The ZTA nanocomposites with additions of 0, 0.005, and 0.01 wt.% MWCNTs and 2 wt.% nanosized ZrO₂ particles were pressureless sintered in an anti-oxidant sagger with graphite powder bed at 1520 °C during 1 h in air and then HIPed at 1475 °C in argon atmosphere 1 h at a pressure of 150 MPa. Relative densities ranging 94–98% were reached. In HIPed composites the hardness and fracture toughness values were increased up to ∼17% and ∼37%, respectively, compared to the “as sintered” composites free of carbon nanotubes. A combined fracture mode, crack deflection, pull-outs of a small amount of carbon nanotubes, and bridging effect were the mechanisms leading to the improvement in fracture toughness.     

Fatigue strength of tungsten inert gas-repaired weld joints in airplane critical structures

In this work the effect of Gas Tungsten Arc Welding (GTAW) repairs on the axial fatigue strength of an AISI 4130 steel welded joint used in airframe critical to the flight-safety was investigated. Fatigue tests were performed at room temperature on 0.89 mm thick hot-rolled plates with constant amplitude and load ratio of R = 0.1, at 20 Hz frequency. Monotonic tensile tests, optical metallography and microhardness, residual stress and weld geometric factors measurements were also performed. The fatigue strength decreased with the number of GTAW repairs, and was related to microstructural and microhardness changes, as well as residual stress field and weld profile geometry factors, which gave origin to high stress concentration at the weld toe.     

Study of the effects of Zr addition on the microstructure and properties of Nb-Ti-Si based ultrahigh temperature alloys

The effects of alloying with Zr on the microstructure, mechanical and oxidation properties of Nb-Ti-Si based ultrahigh temperature alloys have been investigated in this study. The microstructures of the all alloys were comprised of primary γ(Nb,X)₅Si₃ blocks, Nbss and eutectic colonies, and the additions of Zr do not affect the microstructure and phase constituents of Nb-Ti-Si based alloys. Zr improves both the room-temperature toughness and the high-temperature strength. The alloy with addition of 8 at.% Zr shows the highest fracture toughness of 15.01 Mpa·m^1/2. The compressive strengths of the alloys are improved to 278.89–293.08 MPa for the Zr-containing alloys when compare with the Zr-free alloy (194.23 MPa). The oxidation resistance of the alloys was also obviously ameliorated with Zr addition, showing a reduced weight gain with the increase of Zr content.     

Improvement of mechanical properties of ferritic stainless steel weld metal by ultrasonic vibration

Authors investigated the effect of ultrasonic vibration on the solidification microstructure and mechanical properties of the weld metal of ferritic stainless steel by introducing directly ultrasonic vibration into the weld molten pool using ultrasonically vibrating filler metal. The main results obtained in this study are as follow.     

Roles of microstructure and carbides in fatigue crack propagation in high V-Cr-Ni cast irons

This paper describes the roles of microstructure and carbides during fatigue crack propagation (FCP) in high V-Cr-Ni cast irons, with varying C and V contents from 1 to 3% and 3.5 to 10%, respectively. FCP tests have been performed using CT specimens in laboratory air at ambient temperature, and FCP behaviour and fracture mechanisms were discussed on the basis of crack closure, crack path profile and fracture surface analysis. In the materials with non-spheroidal vanadium carbide (VC), the effect of C and V contents, i.e. microstructure, was seen when the data were characterized in terms of nominal stress intensity factor range. Also in the materials with spheroidal VC, the effect of microstructure was recognized. After allowing for both crack closure and elastic modulus, the intrinsic FCP resistance still became lower with increasing C and V contents, particularly remarkable in the materials with 3% C and 10% V regardless of VC morphology. In these materials, there existed the mutually competitive mechanisms: one was that accelerated FCP rates such as preferential growth into VCs and VC fracture, and the other was that decelerated FCP rates such as crack deflection, crack closure, secondary cracks, and uncracked-ligament bridging. The former exerted much larger influence on the overall FCP rates than the latter did, thus resulting in the lower intrinsic FCP resistance.     

Effect of Aging Treatment on Precipitation Behavior and Mechanical Properties of a Novel Aluminum–Lithium Alloy

In this work, the microstructure and precipitation phases were primarily characterized by transmission electron microscopy. The mechanical properties were evaluated by tensile and tear test. The results indicated that the samples aged at 145 °C for 45 h or 155 °C for 30 h possessed a preferable combination of strength and plasticity, owing to the precipitation of well-dispersed T1(Al2Cu Li) phases(diameter 150 nm). However, aging at more than 165 °C caused an obvious size growth of T1 plates, leading to the quick reduction in plasticity and toughness. Furthermore, the high Cu/Li ratio resulted in distinct precipitation features, including a shorter incubation time of T1 phase and the aggravated precipitate-free zones.