Effect of post-weld heat treatment on fracture toughness and fatigue crack growth behaviour of electron beam welds of a titanium (α+β) alloy

The effects of a post-weld heat treatment on the fracture toughness and fatigue crack growth behaviour of electron beam welds of an α + β titanium alloy, Ti–6.5Al–1.9Zr–0.25Si have been studied. Welds in the stress-relieved condition exhibited poor fracture toughness due to poor energy absorbing capacity of the thin α and α' phases. Post-weld heat treatment which resulted in the decomposition of α' to α + β and the coarsening of intragranular and intergranular α resulted in improved toughness. This improvement in the toughness is related to improved ductility leading to crack blunting, crack path deviation at the thick intragranular and intergranular α phase. Fatigue crack growth resistance of welds was superior to the base metal in the α + β heat-treated condition. The superior crack growth resistance of the welds is due to the acicular α microstructure which results in a tortuous crack path and possible crack closure arising from crack path tortuosity.     

The influences of sulphur and phosphorus additions on the creep cavitation characteristics in type 304 stainless steels

The effects of impurities on creep cavitation characteristics in type 304 stainless steels with and without additions of sulphur and/or phosphorus have been studied using four experimental heats. Over a limited range of stress at 1000 K, the change in size distribution of creep cavities and carbide precipitates, and the level of impurity segregation with the amount of addition have been investigated. It is found that phosphorus accelerates the nucleation of creep cavities, but retards the growth of them. However, sulphur had little effect on both nucleation and growth of the cavities. It has also been found that ageing prior to testing inhibits creep cavitation. The effects of impurities on cavitation are analysed and discussed from the viewpoint of the change in the segregation of impurities and the precipitation of carbides due to impurity additions.     

Grain boundary precipitation strengthening in high temperature creep of Fe15Cr25Ni alloys

Grain boundary carbide precipitation gives rise to a dramatic decrease in creep rates compared with those of a single phase alloy. The high stress exponent (n=7) and creep activation energy (Q_C > Q_sd) have been found due to these intergranular carbides, indicating that they both increase creep resistance and change creep characteristics of alloys. The model present here rationalizes the observed behavior in single phase and two phase alloys with grain boundary carbides by a unified power law equation involving boundary obstacle stress. The predictions of this model are in close agreement with experimental results.     

Microstructural Evolution of Ti-5.6Al-4.8Sn-2Zr-1Mo-0.35Si-0.7Nd Titanium Alloy with Carbon Additions

The effect of carbon addition on microstructural evolution was studied in a near-α titanium alloy(Ti-5.6Al-4.8Sn-2Zr-1Mo-0.35Si-0.7Nd). It was found that flake and ribbon titanium carbides with a NaCl crystal structure formed in the as-cast alloys with carbon additions of over 0.17 wt pct. Flake carbide particles are the product of eutectic transformation and precipitate from the high-temperature β phase. The ribbon carbide particles are primary phases formed prior to the nucleation of any metallic phases. The as-cast alloys with carbide precipitation after heat-treatment atβt-30℃ followed by water quenching showed the spheroidization of α lamellae and partial dissolution of carbide particles. After annealing at βt 15℃, carbide particles are mostly distributed at the grain boundary and spheroidized through mixed grain boundary plus bulk diffusions.     

Effect of boron addition on the microstructure and mechanical properties of K4750 nickel-based superalloy

The effect of boron addition at 0,0.007 wt.% and 0.010 wt.% on the microstructure and mechanical properties of K4750 nickel-based superalloy was studied.The microstructure of the as-cast and heat-treated alloys was analyzed by SEM,EPMA,SIMS and TEM.Lamellar M₅ B₃-type borides were observed in boroncontaining as-cast alloys.After the full heat treatment,boron atoms released from the decomposition of M₅ B₃ borides were segregated at grain boundaries,which inhibited the growth and agglomeration of M₂₃C₆ carbides.Therefore,the M₂₃C₆ carbides along grain boundaries were granular in boron-containing alloys,while those were continuous in boron-free alloys.The mechanical prope rty analysis indicated that the addition of bo ron significantly improved the tensile ductility at room tempe rature and stress rupture properties at 750℃/430 MPa of K4750 alloy.The low tensile ductility at room temperature of 0 B alloy was attributed to continuous M₂₃C₆ carbides leaded to stress concentration,which provided a favorable location for crack nucleation and propagation.The improvement of the stress rupture properties of boron-containing alloys was the result of the combination of boron segregation increased the cohesion of grain boundaries and granular M₂₃C₆ carbides suppressed the link-up and extension of micro-cracks.     

Influence of yttrium on purification and carbide precipitation of superalloy K4169

The effect of reactive element yttrium on purification and carbide precipitation of superalloy K4169 has been studied through vacuum induction melting.The results show that yttrium has a strong deoxidizing and desulfurizing ability,and the impurity elements of sulfur and oxygen in melting K4169 are greatly reduced to 2-5 ppm and 4-5 ppm,respectively.The metallurgical reaction product is mainly Y2O3 and yttrium containing sulfides have not been observed.With the increase of the content of yttrium from 0.005 wt.％to 0.033 wt.％,the morphology and size of MC carbides in casting are significantly influenced due to that the yttrium changes the diffusion of the carbide forming elements and solid fraction during solidification.In the alloys without yttrium,only isolated blocky or strip carbides exist,while skeleton-like carbides are observed in the alloys with yttrium.Both the average size and the number density of skeleton carbides decrease monotonously with yttrium content increasing.After yttrium increasing to 0.033 wt.％,a phase of hexagonal Ni17Y2 is observed to precipitate in the form of symbiosis with MC carbide and matrix.The addition of yttrium is not beneficial for the properties of K4169 alloys at the room temperature,which is quite related to the MC carbides with a large skeleton-like morphology through the analysis of tensile fracture.     

纳米ZrO2与微米Al2O3复合陶瓷的断裂模式

研究了两种微米Al2O3与纳米ZrO2复合陶瓷的裂纹扩展过程与显微结构的关系.结果表明,Al2O3晶粒内部形成纳米级或亚微米级ZrO2颗粒,是复合陶瓷的断裂模式从沿晶断裂向穿晶断裂转化的主因.ZrO2含量较低有利于Al2O3晶界迁移包裹纳米ZrO2形成内晶结构;而ZrO2含量较高使主晶相长大受到抑止,不利于形成内晶结构,趋向于沿晶断裂.裂纹穿晶扩展需要的驱动力比沿晶断裂大,故裂纹扩展阻力曲线的上升趋势更加显著.裂纹穿晶扩展路径主要取决于内晶颗粒产生的弹性应力场的性质.     

<Emphasis Type="Italic">In-situ</Emphasis> SEM observation on fracture behaviors of Sn-based solder alloys

The effects of displacement rate on fracture behaviors of 100Sn solder and 63Sn37Pb solder alloys have been investigated by SEM in-situ testing. It was found that for the 100Sn solder, grain boundary sliding was the dominant deformation mechanism at lower tensile rate regime, while a large area of slip lines crossing grains were detected at the surface of specimens at higher tensile rate regime. For the 63Sn37Pb eutectic alloy, however, because of existence of the second phase, fracture behavior depended on the growth and linkage of cavities ahead of crack tip, and the crack paths changed from intergranular to transgranular with the increasing of loading rate.     

Creep Deformation and Rupture in Smooth and Notched Specimens in HK40 Steel

The effects of the eutectic carbides randsecondary carbides on creep deformation andrupture in smooth bars and CT specimens havebeen studied. The results show that the resistanceof the eutectic carbides of skeleton shape tocrack growth is larger than that of the blockyshape carbides. The dendritic segregation ofsecondary carbides promotes the creep ductility.As the secondary carbide particles become coarser,the creep ductility increases and the crackgrowth rate decreases. However, if the sizeof secondary carbide is too large, the creepstrength decreases too much and therefore crackgrowth rate increases.     

Carbide Formation Process in Directionally Solidified MAR-M247 LC Superalloy

1.IntroductionSeveralphasesforminthesolidificationandcool-.ingprocessofcastsuperalloys['].MCcarbidesand7'precipitatesarethemainphasestoprovidehightemperaturestrengthinthesuperalloys.TheMCcarbidesstrengthengrainboundariesandsub-grainboundariesatelevatedtemperature.Theirvolumefraction,sizeandmorphologyhaveaveryimpor-tanteffectonthemechanicalpropertiesofNi-basesuperalloys['l.Considerable..,..,.h.s[3~14]havebeencarriedoutfromslowtorapidsolidificationconditionstocharacterizethesequenceofformation…     

Improving high-temperature mechanical properties of cast CrFeCoNi high-entropy alloy by highly thermostable in-situ precipitated carbides

The CrFeCoNi high-entropy alloy (HEA) exhibits excellent mechanical properties at lower temperatures due to its low stacking-fault energy,however,its medium-and high-temperature strengths are still insufficient.In consideration of the potential diversified applications,more strengthening approaches except for the previously proposed L12 phase hardening deserve further exploration due to its rapid coarsening tendency at high temperatures.Here,we achieved significant high-temperature strengthening of the cast CrFeCoNi HEA by in-situ precipitation of highly thermostable carbides.Alloys with 0.5 at.％ and 1 at.％ niobium and carbon were prepared by simple casting processes,i.e.drop cast,solute solution and aging.A highly thermostable microstructure was formed,which comprises very coarse grains accompanied with extensive thermostable carbide precipitates embedded,including submicrometer coherent NbC particles in grain interiors and intergranular coherent M23C6 carbides.This high thermostability of microstructure,which is beneficial for the high-temperature loading,is ascribed to the synergy of lacking growth driving force and Zenner pinning effect by the carbides.Tensile properties tested at 673,873 and 1073 K show that the yield strength and ultimate tensile strength are significantly increased by Nb/C doping,along with the elongation escalation at higher temperatures.The strengthening is mainly due to the precipitation hardening of carbide particles.     

Microstructure evolution and stress rupture properties of K4750 alloys with various B contents during long-term aging

The relationship among B content,microstructure evolution and stress rupture properties of K4750 alloy during long-term aging were investigated.After aging at 800 ℃ for 1000 h,the decomposition degree of MC carbides of K4750 alloys with 0B,0.007 wt.％ B and 0.010 wt.％ B were basically identical,which indicated that B has no inhibition on MC carbide decomposition during long-term aging.The MC carbide decomposition was accompanied by the formation of M23C6 carbides and a small number of η phases,which was controlled by the outward diffusion of C and Ti combined with the inward diffusion of Ni and Cr from the γ matrix.In addition,M23C6 carbides in boron-free alloy were in continuous chain and needle-like η phases were precipitated near them,while M23C6 carbides in boron-containing alloys remained in granular distribution and no η phases precipitation around them.Adding B could delay the agglomeration and coarsening of M23C6 carbides during long-term aging,which was because the segregation of B at grain boundary retarded the diffusion of alloy elements,thus weakened the local fluctuation of chemical composition near grain boundary.The stress rupture samples of K4750 alloys with various B contents after aging at 800 ℃ for 1000 h were tested at 750 ℃/380 MPa.The results indicated that the stress rupture properties of boron-containing alloys were significantly better than that of boron-free alloy,which could be attributed to the increase of grain boundary cohesion strength and the optimization of M23C6 carbide distribution due to the addition of B.     

Effects of high-temperature ageing on the creep-rupture properties of cobalt-base L-605 alloys

Effects of high-temperature ageing on the creep-rupture properties of cobalt-base L-605 alloys were investigated at 1089 and 1311 K in air. The specimens with serrated grain boundaries and those with normal straight grain boundaries were aged for 1080ksec at 1273 or 1323 K to cause the matrix precipitates of tungsten-rich b c c phase and M₆C carbide. The creep-rupture strength of both specimens were improved by the high-temperature ageing. The rupture strength at 1311 K was the highest in the specimens with serrated grain boundaries aged at 1273 K, while the specimens with straight grain boundaries aged at 1273 K of the highest matrix hardness had the highest rupture strength at 1089 K. The high-temperature ageing did not decrease the rupture ductility of specimens. The ruptured specimens with serrated grain boundaries exhibited a ductile grain-boundary fracture surface which consisted of dimple patterns and steps, regardless of whether high-temperature ageing was carried out. The fracture mode of the specimens with straight grain boundaries was changed from the brittle grainboundary fracture to the ductile one similar to that of the specimens with serrated grain boundaries by high-temperature ageing, since large grain-boundary precipitates which gave nucleation sites of dimples were formed during the ageing. The grain-boundary cracks initiated in the early stage of creep (transient creep regime) in both non-aged and aged specimens of L-605 alloys in creep at 1089 and 1311 K, although the time to crack initiation is shorter in the specimens with straight grain boundaries than in those with serrated grain boundaries. Thus, the period of crack growth and linkage occupied most of the rupture life. The strengthening mechanisms of the aged specimens were also discussed.     

Effects of microstructure evolution on the deformation mechanisms and tensile properties of a new Ni-base superalloy during aging at 800℃

Tensile properties at room temperature of a new casting Ni-base superalloy during aging at 800℃ for0-1000 h were investigated.During aging,granular M23C6 carbides presented at grain boundaries and kept growing from dispersed particles to continuous networks.The γ’ phase significantly coarsened,with the morphology of some γ’ phase changed from spherical to rounded cubic shape after 1000 h.Three deformation mechanisms in relation to the γ’ diameter(dγ’) were identified:(ⅰ) weakly coupled dislocations(WCD) connected by anti-phase boundary(APB) traveled in pair across the γ/γ’ structure when dγ’ was small in the under-aged alloys;(ⅱ) strongly coupled dislocations(SCD) with reduced spacing compared to(ⅰ) sheared γ’ phase when dγ’ increased in the over-aged alloys;(ⅲ) dislocations occasionally by-passed γ’ phase when dγ’ was larger than 97 nm after aging for more than 300 h.The alloy obtained the peak strength when 20 h-aged with dγ’=44 nm which was in the transition between(ⅰ) and(ⅱ).The aginginduced variation in yield strength was correlated to the coarsening of γ’ phase using a theoretical model of precipitation strengthening in terms of the formation of APB.The calculated results suggested that the γ’ phase with a volume fraction of 23% contributed more than 61% of the peak-aged yield strength.Observation after fracture revealed that the alloys usually fractured at grain boundaries.High stress concentration around carbides resulted in cracks by carbides self-cracking and the initiation of cavities.The undesirable agglomeration of M23C6 at grain boundaries was harmful to the properties of the overaged alloys.     

Mechanism of intergranular reheating crack in butt-welded joint of 12Cr1MoV tube served for 40,000 hours

Cracking failure of butt-welded joint of 12Cr1MoV tube was comprehensively studied. Results show that both of initiation and propagation of the primary crack were circumferentially intergranular in the coarse-grained heat affect zone (CGHAZ). Many isolated and intergranular micro-cracks and cavities were observed near the primary crack．Neither oxide or corrosion products were observed in the isolated cavities or micro-cracks. According to the microstructure, location, propagation mode and morphology of the crack,the primary crack in butt-welded joint is concluded to be the intergranular reheating cracking (IRC). The crack failure is mainly due to poor welding quality, characterised by high residual stress and coarsened grain size. Mechanisms on the IRC based on previous laboratory research were studied on the failed tube sample, and results showed that the IRC is accumulation of high-stress induced creep damage, such as cavities along prior austenite grain boundaries (PAGBs). Neither segregation of alloys elements nor trace impurities were detected.     

Fatigue crack propagation in aluminum nitride ceramics under cyclic compression

Room temperature fatigue crack growth characteristics under cyclic compressive loads were investigated in pure and 3 wt % yttria doped hot pressed aluminum nitride ceramics. A single edge-notch specimen geometry was used to induce a stable Mode I fatigue crack under cyclic compressive loads. The fatigue crack growth occurred in three stages, where the first stage is dominated by microcrack nucleation, coalescence and slow growth within the notch root. During the second stage, the crack growth is accelerated and finally, the crack growth deceleration and arrest occurred in third stage. The fatigue crack growth occurred predominantly by intergranular fracture. Insights gained from the experimental results and microscopic observations are discussed.     

Hydrogen embrittlement,thermal aging,and role of carbides in fatigue of high strength steel

Abstract

The influences of thermal aging and environment on Paris-Erdogan regime fatigue crack growth rates and mechanisms in a high strength Cr–Ni steel (300M) have been investigated. Crack growth rates were measured in inert (vacuum) and aggressive (hydrogen) environments, for quenched and tempered material before and after thermal aging at 500°C. Aging induced an acceleration of crack growth in vacuum at low values of ?K, but a retardation at high values of ?K. The crack path was transgranular throughout, and followed carbide/matrix interfaces. These effects were associated with the embrittlement of carbide/matrix interfaces by phosphorus segregation, which facilitated transgranular crack growth at low ?K, but produced crack tip shielding by voids surrounding the crack at high ?K. Hydrogen acted synergistically with phosphorus to embrittle carbide/matrix interfaces, but it also localised slip deformation at crack tips, reducing void formation in surrounding material and associated crack tip shielding. This produced an enhancement of crack growth rates at all values of ?K significantly above the threshold regime. The role of carbide/matrix interface embrittlement constitutes a third mechanism of hydrogen embrittlement and temper embrittlement interaction in fatigue crack growth, additional to those of carbide precipitation and grain boundary embrittlement previously reported.

MST/1142     

Investigation of the interlayers between cemented carbides and titanium carbide coatings obtained by chemical vapor deposition

Data are presented on the microstructure, microhardness and chemical composition of the interlayers between cemented carbides and chemically vapor-deposited TiC coatings. The carbon: metal ratios of the cubic carbides across the interlayers were obtained by Auger depth profile analysis. The mechanisms of nucleation and diffusional growth of the η-carbide skeleton at the upper layer of the substrate are discussed. It is proposed that a cubic carbide phase acts as a carbon donor and thus inhibits the localized formation of the η-carbide skeleton and retains the integrity of the surrounding network.     

Stability of Several Oxide Dispersion Strengthened Alloys and a directionally solidified y/y′ -α eutectic alloy in a thermal gradient

Thermal gradient testing of three oxide dispersion strengthened alloys (two Ni-base alloys, MA 754 and MA 6000E, and the Fe-base MA 956) and the directionally solidified eutectic alloy, γ/γ′-α, have been conducted. Experiments were carried out with maximum temperatures up to 1200°C and thermal gradients on the order of 100°C/mm. The oxide dispersion strengthened alloys were difficult to test because the thermal stresses promoted crack nucleation and growth; thus the ability of these alloys to maintain a thermal gradient may be limited. The stability of individual fibers in γ/γ′-α was found to be excellent; however, microstructural changes were observed in the vicinity of grain boundaries. Similar structures were also observed in isothermally annealed material; therefore thermal gradients do not affect the microstructure of γ/γ′-α in any significant manner.