A dislocation shielding model for the fracture of semibrittle polycrystals

Using a dislocation mechanics-based crack-tip shielding model, a zeroth-order estimate of the grain size and yield strength dependence of fracture toughness is made for mild steel. This model, appropriate to cleavage fracture in the lower shelf regime, is shown to predict the fracture toughness determined over a temperature regime of 100 to 240 K in both quasi-static and dynamic tests and for two different grain sizes. Two grain size terms in the proposed model result. One is associated with grain boundary blockage effects on crack-tip shielding, while the other is proposed to affect the far-field stresses and, indirectly, the local crack-tip stress field. Differences between the present approach and the classic Cottrell-Petch model are in how the friction stress and grain size affect local stresses at the crack tip rather than at a carbide or a grain boundary.     

Relationships of fracture toughness and dislocation mobility in intermetallics

An analytical method has been developed and used to compute the Peierls-Nabarro (P-N) barrier energy, U _P-N, for relevant slip systems in several intermetallics, including NiAl, FeAl, Nb-Ti-Al (B2), Ni₃Al (L1₂), TiAl (L1₀), TiCr₂, NbCr₂ (C14, C15), Nb₅Si₃ (D8 _l ), Mo₅SiB₂ (D8 _l ), and Mo₅Si₃ (D8 _m ). The P-N barrier energy and a generalized fault energy, γ _F, are combined and used as a measure of dislocation mobility. Furthermore, a fracture model has been developed to describe the process of thermally activated dislocations moving away from the crack tip and to predict the corresponding fracture resistance. A ductility index defined in terms of the ratio of γ _s/(U _P-N+γ _F), where γ _s is the surface energy, is used to correlate with the fracture toughness, K _C, of individual intermetallics. The correlation indicates that fracture toughness increases with increasing values of γ _s/(U _P-N+γ _F), in accordance with the fracture model formulated based on thermally activated slip. The use of the fracture model for predicting the effects of slip behavior, temperature, and alloy additions on fracture resistance is demonstrated for selected intermetallics including NiAl, TiAl, Laves phase, and Nb₅Si₃.     

A dislocation based criterion for the raft formation in nickel-based superalloys single crystals

A dislocation based criterion is proposed for describing the raft formation in single crystals of nickel-based superalloys subject to creep at high temperature. Those of the precipitate faces exhibiting dislocations after the onset of creep are thought to expand by directional coarsening of γ′ precipitates. The proposed criterion predicts correctly the raft morphology for creep along cristallographic directions of high and low symmetry for alloys exhibiting a positive or a negative misfit. A diffusional mechanism based on the effect of creep dislocations on the local γ phase chemical composition is suggested to account for the observed relationship between dislocation activity and directional coarsening of precipitates.     

晶粒尺寸对车轮钢解理断裂韧性的影响

通过紧凑拉伸试验研究了碳的质量分数约为0.5%的C50车轮钢解理断裂韧性K_IC(即条件断裂韧性K_Q)与晶粒尺寸的关系.结果表明,晶粒尺寸对试样的断裂韧性有明显的影响,但决定车轮钢解理断裂韧性的是组织中最大的晶粒尺寸,而不是平均晶粒尺寸,最大晶粒尺寸越大,断裂韧性越低.对于C50车轮钢,当前5%的最大晶粒平均尺寸为30~73μm时,车轮钢的条件断裂韧性K_Q与晶粒平均尺寸的对数呈线性关系.     

Quantitative fractography and dislocation interpretations of the cyclic cleavage crack growth process

Fractographic features of cyclic cleavage crack growth in Fe and Fe-binary alloys as a function of applied stress intensity and test temperature are documented. Cyclic striations, cleavage rivers and twist angle misorientations are measured and discussed in terms of geometrical and local stress considerations. It is shown that the river height displacement must account for both twist angle misorientation between adjacent grains and the crack-tip opening displacement of an advancing fatigue crack. With regards to the rate of an advancing cleavage crack, a dislocation dynamics model is derived which qualitatively gives the correct ordering of power-law slopes (da/dN vs ΔK) and growth rate magnitudes as a function of test temperature.     

Effect of pores on the cleavage of twinned boron carbide crystals

The substructure and cleavage fracture in two types of polycrystalline B₄C specimens are investigated. It is shown that in specimens hot-pressed below 2000°C twins in poreless grains are effective barriers for cleavage cracks. Cleavage fracture occurs in planes of the type {10 $\bar 1$ 1}. There are twins and faceted pores in grains of specimens sintered below 2200°C. In these grains cleavage is along the twinning planes. Pores eliminate the effect of twins as crack stoppers.     

Mechanism of detrimental effects of carbon content on cleavage fracture toughness of low-alloy steel

The variation in fracture toughness of low-alloy base steels and weld steels with carbon contents of 0.08 and 0.21 wt pct was investigated using notched and precracked specimens tested at low temperatures. The attention is focused on the mechanism associated with detrimental effects on cleavage fracture toughness resulting from increasing carbon content. Analyses reveal that, in the case of constant ferrite grain sizes with increasing carbon content, the yield stress σ _y increases and the local fracture stress σ _f remains constant for notched specimens. For precracked specimens, the σ _y increases, whereas the σ _f decreases. In both cases, the ratio σ _f /σ _y decreases; this ratio is one of the principal factors inducing the deterioration in the cleavage fracture toughness of the higher carbon steels. Analyses also reveal that the critical strain for initiating a crack nucleus, which decreases with increasing carbon content and impurity elements, appears to be another principal factor that has a negative effect on the fracture toughness in both notched and precracked specimens. The results of the fracture toughness measured for weld metal with various grain sizes further support the predominant effect of grain size on the toughness of notched specimens.