Intergranular Fatigue Cracking Behaviour in Aluminium

In order to understand the basic mechanism of intergranular cracking in pure metals during fatigue, stress-controlled push-pull fatigue tests were carried out with high purity aluminium. Tests were interrupted frequently so as to study the grain boundary (GB) cracking behaviour by the surface observation. The results show that crack initiation at GB was a process controlled by multi-factors, such as boundary structure, GB-slip interaction, GB sliding and so on. If these factors are varied so that the incompati- bility at a GB increased, the possibility of cyacking at the boundary will be raised. Some inteygranular cracking phenomena are not able to be explained by the GB stepping mechanism.

INFLUENCE OF RARE EARTH ON INTERGRANULAR CRACKING OF 9% Ni CAST STEEL

The intergranular cracking of 9% Ni cast steel seems mainly to relate to the segregation of contaminates H,S and P along boundaries.An addition of rare earth may eliminate the seg- regation of these contaminates along grain boundaries and improve the binding force among boundaries,so as to reduce remarkably the intergranular cracking.

Computational Modeling of Intergranular Crack Propagation in an Intermetallic Compound Layer

A micromechanical model is presented to study the initiation and propagation of microcracks of intermetallic compounds(IMCs)in solder joints.The effects of the grain aggregate morphology,the grain boundary defects and the sensitivity of the various cohesive zone parameters in predicting the overall mechanical response are investigated.The overall strength is predominantly determined by the weak grain interfaces;both the grain aggregate morphology and the weak grain interfaces control the crack configuration;the different normal and tangential strengths of grain interfaces result in different intergranular cracking behaviors and play a critical role in determining the macroscopic mechanical response of the system.

CYCLIC DEFORMATION OF COARSE GRAINED POLYCRYSTALLINE PURE Ai——Ⅰ.SLIP CHARACTERISTICS

Comparing with ordinary ploycrystalline materials sized to μm grade,the slip morphology of the coarse grained polycrystalline pure Al is characterized by:(1)several slip domains occur in a grain,and in same domain,several slip systems operate at same time or one after another intensely,a beautiful and neat slip pattern is forming on the specimen surface;(2)for high Σ-value coincident and random grain boundaries,the grain boundary affecting zone (GBAZ),bout 50—120μm wide,is favourable site to form intergranular crack at early fa- tigue life easily,and migration or slide of the boundaries were often observed.While low Σ-value near-coincident grain boundaries show a higher degree of slip continuity and strain compatibility than high Σ-value ones.Intergranular crack is not easily nucleated at low Σ-value near-coincident boundaries;and(3)due to suppression of grain boundary slip at triple grain boundary node,the high Σ-value and random grain boundary among the three boundaries of tricrystal crack easily during cyclic deformation.

Unusual W-shaped Galvanic Cell Model of ISCC of α-brass in Neutral Mattsson's Solution

Intergranular stress corrosion cracking (ISCC) of α-brass in neutral Mattsson's solution was found to be controlled by an unusual 'W'-shaped galvanic cell whose cathode is the grain boundary oxide film (G.B.0. film) and surface film and the anode is fresh metal at the cracked tip on both sides of the G.B.0. film. Redox reactions involved in the cell have been proposed here. According to this mdel, initidtion of ISCC is caused by the rupturing of surface film along grain boundaries, thus forming a galvanic cell. Propagation of ISCC resulted from alternate advances of G.B.0. film and dissolution on both sides of G.B.0. film caused by the effect of electrochemical reaction. This work developed an effective approach to investigate the embrittlement process at the tip of the crack, by increasing the length of the embrittlement region through constant strain test and distinguishing the morphology and the nature of the corrosion products by optical microscopy and scanning electron microscopy (SEH).

Improving Intergranular Stress Corrosion Cracking Resistance in a Fe–18Cr–17Mn–2Mo–0.85N Austenitic Stainless Steel Through Grain Boundary Character Distribution Optimization

The grain boundary character distribution(GBCD) optimization and its effect on the intergranular stress corrosion cracking(IGSCC) resistance in a cold-rolled and subsequently annealed Fe-18 Cr-17 Mn-2 Mo-0.85 N high-nitrogen nickel-free austenitic stainless steel were systematically explored.The results show that stacking faults and planar slip bands appearing at the right amount of deformation(lower than 10%) are beneficial cold-rolled microstructures to the GBCD optimization.The proportion of special boundaries gradually increases in the subsequent stages of recrystallization and grain growth,accompanying with the growth of twin-related domain in the experimental steel.In this way,the fraction of low ∑ coincidence site lattice(CSL) boundaries can reach as high as 82.85% for the specimen cold-rolled by 5% and then annealed at 1423 K for 72 h.After GBCD optimization,low ∑ CSL boundaries and the special triple junctions(J2,J3) of high proportion can greatly hinder the nitride precipitation along grain boundaries and enhance the capability for intergranular crack arrest,thus improving the IGSCC resistance of the experimental steel.

Multi-scale boron penetration toward stabilizing nickel-rich cathode

Nickel-rich layered oxides LiNi_(x)Co_(y)Mn_(1-x-y)O_(2)(x≥0.8)have been recognized as the preferred cathode materials to develop lithium-ion batteries with high energy density(>300 Wh kg^(−1)).However,the poor cycling stability and rate capability stemming from intergranular cracks and sluggish kinetics hinder their commercialization.To address such issues,a multi-scale boron penetration strategy is designed and applied on the polycrystalline LiNi_(0.83)Co_(0.11)Mn_(0.06)O_(2)particles that are pre-treated with pore construction.The lithium-ion conductive lithium borate in grain gaps functions as the grain binder that can bear the strain/stress from anisotropic contraction/expansion,and provides more pathways for lithium-ion diffusion.As a result,the intergranular cracks are ameliorated and the lithium-ion diffusion kinetics is improved.Moreover,the coating layer separates the sensitive cathode surface and electrolyte,helping to suppress the parasitic reactions and related gas evolution.In addition,the enhanced structural stability is acquired by strong B-O bonds with trace boron doping.As a result,the boron-modified sample with an optimized boron content of 0.5%(B5-NCM)exhibits a higher initial discharge capacity of 205.5 mAh g^(−1)at 0.1C(1C=200 mA g^(−1))and improved capacity retention of 81.7%after 100 cycles at 1C.Furthermore,the rate performance is distinctly enhanced by high lithium-ion conductive LBO(175.6 mAh g^(−1)for B5-NCM and 154.6 mAh g^(−1)for B0-NCM at 5C)

Oxidation damage zone formed in creep fatigue crack growth of GH4169 alloy at 650℃

In the creep fatigue crack growth of GH4169 alloy,oxidation is a prominent damage source,which is mainly manifested as the oxidation damage zone in front of crack tip.In order to investigate the property of the oxidation damage zone formed in the creep fatigue crack growth,crack growth tests of directly aged GH4169 alloy were conducted at 650℃ in air under various load conditions.Interrupted tests were performed to observe the damage characteristics at crack tip.Block tests were systematically executed to quantify the dependency of oxidation damage zone size on load and holding time.The crack propagation of the GH4169 alloy has a close relationship with grain boundary oxidation at 650℃.An oxidation damage zone in front of crack tip includes intergranular microcracks and oxidised but uncracked grain boundaries.Its size has been calculated from transient crack growth rate and described as a function of maximum stress intensity factor and holding time.Based on oxidation damage zone size,a novel model has been developed to predict the creep fatigue crack growth rate of the GH4169 alloy at 650℃.