A thermodynamic model for composition and chemical driving force for nucleation of complex carbonitrides in microalloyed steel

The extent and degree of dispersion of carbonitrides in high strength low alloy (HSLA) steels is controlled by nucleation and growth processes. A thermodynamic model is presented enabling calculation of the composition of the carbonitride embryo as well as the chemical driving force for nucleation (CDF) in the austenite region of a multicomponent microalloyed steel. The analysis made for a Nb-Ti HSLA steel indicates that the critical carbonitride nucleus is richer in N and Ti than the bulk precipitate at equilibrium at a given temperature and the difference increases when the holding temperature decreases. The results also show that the Ti addition has an optimum value defined by the maximum in the ratio of bound Ti and Nb to their total content. The maximum CDF correlated with the minimum Ti content sufficient to form a carbonitride at the austenitizing temperature.     

Shear force effects on fretting fatigue behavior of Ti-6Al-4V

The present study examined the role of shear force on fretting fatigue behavior as well as its interdependence on other test variables, such as bulk stress, normal load, relative slip, and coefficient of friction, by using a fretting test system where shear force was controlled independent of other applied loads. Two contact geometries were used: cylinder-on-flat and flat-on-flat. For a given applied bulk stress and normal load condition, there is a simple relationship between shear force and relative slip up to a maximum value of shear force where contact condition changes from partial slip to gross slip. The effects of shear force and relative slip therefore can be combined together to characterize fretting behavior such as in a fretting map. Under a prescribed loading condition, fretting fatigue life decreases as shear force increases in partial slip condition. Further, the inter-relationships between shear force and other variables appear to be independent of contact geometry. In the tests where shear force is not applied independently rather when generated indirectly through the compliance of fretting setup, it is affected by the applied bulk stress and normal load, which in turn affect the relative slip range. Therefore, there is a complex interaction among various variables, and it is difficult to isolate their effects on fretting behavior in such test conditions. An independent control of relative slip in the fretting test thus provides an alternate means to characterize the variables’ effects and their interdependence.     

Behavior of acoustic emission for low-strength structural steel during fatigue and corrosion fatigue

Fatigue tests measured with acoustic emission (AE) method have been conducted on a low-strength structural steel in either air or 3.5 pct NaCl solution. The results show the relation between AE count rate and crack growth rate or stress intensity factor range. The AE activity during the fatigue process decreases in solution in comparison with that in air. Furthermore, a model of AE behavior is proposed, and the difference of AE activity between fatigue and corrosion fatigue (CF) depends upon the effects of both the decrease of plastic zone size and material embrittlement at the crack tip.     

Crack closure and residual stress effects in fatigue of a particle-reinforced metal matrix composite

A study of the influence of macroscopic quenching stresses on long fatigue crack growth in an aluminium alloy-SiC composite has been made. Direct comparison between quenched plate, where high residual stresses are present, and quenched and stretched plate, where they have been eliminated, has highlighted their rôle in crack closure. Despite similar strength levels and identical crack growth mechanisms, the stretched composite displays faster crack growth rates over the complete range of ΔK, measured at R = 0.1, with threshold being displaced to a lower nominal ΔK value. Closure levels are dependent upon crack length, but are greater in the unstretched composite, due to the effect of surface compressive stresses acting to close the crack tip. These result in lower values of ΔK_eff in the unstretched material, explaining the slower crack growth rates. Effective ΔK_th values are measured at 1.7 MPa√m, confirmed by constant K_max testing. In the absence of residual stress, closure levels of approximately 2.5 MPa√m are measured and this is attributed to a roughness mechanism.     

The effect of heat treatment on the fatigue and corrosion fatigue behavior of a CuNiCr alloy

Fatigue experiments were conducted on a CuNiCr alloy (IN838) in air and in 0.5 N NaCl solutions under conditions of free corrosion and of applied anodic currents. The alloy was heat treated to produce a solutionized structure and also to produce a precipitation hardened structure. The fatigue behavior of the solutionized alloy was unaffected by free corrosion although increased corrosion rates resulted in a decrease in fatigue resistance for small applied anodic currents. The age hardened alloy showed a decrease in fatigue resistance under free corrosion conditions and a further decrease in resistance with small applied anodic currents. In both heat treatments fatigue in air resulted in mixed transgranular-intergranular crack initiation and propagation while corrosion increased the relative amount of intergranular cracking. These results can be explained by a consideration of previously developed fatigue and corrosion fatigue models of pure copper and copper aluminum alloys. H. N. Hahn, formerly with Rensselaer Polytechnic Institute     

Numerical determination of the elastic driving force for directional coarsening in Ni-superalloys

We have developed a general methodology, in the framework of the finite element method, for locally evaluating the generalized force acting on a material interface which is work-conjugate with the normal displacement of the interface itself. This methodology has been applied to the study of directional coarsening of γ′ precipitates in Ni-superalloys. The flexibility of the proposed method has allowed us to closely model the actual microstructural morphology of the alloys and to account for the effects of applied boundary conditions, lattice misfit, elastic anisotropy and inelastic behavior of the crystals. We have positively compared the indications of our model with available experimental data for a few alloys, and a circumscribed parametric study has lead us to formulate a more general interpretation of the rafting phenomenon, which appears to give a satisfactory explanation for all the available experimental observations.     

Crack deflection: Implications for the growth of long and short fatigue cracks

The influences of crack deflection on the growth rates ofnominally Mode I fatigue cracks are examined. Previous theoretical analyses of stress intensity solutions for kinked elastic cracks are reviewed. Simple elastic deflection models are developed to estimate the growth rates of nonlinear fatigue cracks subjected to various degrees of deflection, by incorporating changes in the effective driving force and in the apparent propagation rates. Experimental data are presented for intermediate-quenched and step-quenched conditions of Fe/2Si/0.1C ferrite-martensite dual phase steel, where variations in crack morphology alone influence considerably the fatigue crack propagation rates and threshold stress intensity range values. Such results are found to be in good quantitative agreement with the deflection model predictions of propagation rates for nonlinear cracks. Experimental information on crack deflection, induced by variable amplitude loading, is also provided for 2020-T651 aluminum alloy. It is demonstrated with the aid of elastic analyses and experiments that crack deflection models offer a physically-appealing rationale for the apparently slower growth rates of long fatigue cracks subjected to constant and variable amplitude loading and for the apparent deceleration and/or arrest of short cracks. The changes in the propagation rates of deflected fatigue cracks are discussed in terms of thelocal mode of crack advance, microstructure, effective driving force, growth mechanisms, mean stress, slip characteristics, and crack closure.     

Dependence of the driving force of the sodium pump on rate of transport

The driving force for active transport of Na+ in the isolated toad bladder, ENa, was measured as the reciprocal slope of the change in conductance with change in short-circuit current after stimulation with antidiuretic hormone. The base-line short-circuit current was altered by change in ambient Na+ concentration or addition of amiloride, maneuvers which alter availability of Na+ at the site of active transport. In the absence of a chemical gradient for Na+ across the bladder, ENa was found to be inversely related to the rate of Na+ transport, a finding incompatible with the simple electrical analogue that has been proposed for the system. The results provide additional support for the view that ENa measured in this way has both energetic and kinetic components.     

Effect of testing frequency on the corrosion fatigue of a squeeze-cast aluminum alloy

The corrosion fatigue crack propagation behavior of a squeeze-cast Al-Si-Mg-Cu aluminum alloy (AC8A-T6), which had been precracked in air, was investigated at testing frequencies of 0.1, 1, 5, and 10 Hz under a stress ratio (R) of 0.1. Compact-toughness specimens were precracked about 6 mm in air prior to the corrosion fatigue test in a 3 pct saline solution. At some near-threshold conditions, these cracks propagated faster than would be predicted by the mechanical driving force. This anomalous corrosion fatigue crack growth was affected by the initial stress-intensity-factor range (ΔK _i), the precracking conditions, and the testing frequency. The initial crack propagation rate was as much as one order of magnitude higher than the rate for the same conditions in air. This rapid rate was associated with preferential propagation along the interphase interface in the eutectic structure. It is believed that a chemical reaction at the crack tip and/or hydrogen-assisted cracking produced the phenomenon. Eventual retardation and complete arrest of crack growth after this initial rapid growth occurred within a short period at low ΔK values, when the testing frequency was low (0.1 and 1 Hz). This retardation was accompanied by corrosion product-induced crack closure and could be better explained by the contributory stress-intensity-factor range (ΔK _cont) than by the effective stress-intensity-factor range (ΔK _eff).     

On calculation of Ms and driving force for martensitic transformation in Fe-C

The approximate approach for evaluation of the critical driving force is briefly introduced, i.e. ΔG^α→M = 2.1 δ + 900 J mol⁻¹, where δ is the yield strength of parent phase and 900 is the stored energy. By application of this approach M_s can directly be deducted through various method for calculation of ΔG^γ→α, i.e. LFG, KRC, Fisher-Bhadeshia and Fisher-Hsu, with ΔG_Fe^γ→α given by Kaufman el al., Orr and Chipman and Mogutnov. The results of the combination of LFG-Mogutnov, KRC-Orr and Chipman, Fisher-Bhadeshia-Orr and Chipman and Fisher-Hsu-Orr and Chipman (-Mogutnov as χ < 0.02) are all in good agreement with the experimental M_s, verifying the validity of the above equation. The driving force obtained by substitution the experimental M_s of KRC to the above equation is very close to that from calculation through LFG-Mogutnov, KRC-Orr and Chipman and Fisher-Hsu-Orr and Chipman (χ ⩾ 0.02). fairly close to Fisher-Bhadeshia-Orr and Chipman. The driving force calculated from LFG-Orr and Chipman seems too low. The driving force increases monotonically with the carbon content. The present work emphasizes that the selection of M_s is a key factor and the value of ΔG_Fe^γ→α plays an important role in the calculation of the driving force.     

Crack closure load measurements for microcracks developed during the fatigue of Al 2219-T851

A compliance technique is utilized to determine the closure load for surface microcracks of one grain size and smaller produced by fully reversed loading fatigue of Al 2219-T851. Specimens are fatigued in flexure in air at 5, 18, and 45 pct relative humidities. Scanning electron microscopy is then utilized to measure crack compliance for selected microcracks and crack closure load is inferred from the break point in the linear relationship between crack opening and applied stress. For zero applied load the microcracks are found to be partially opened and a linear relationship is found between the closure load measured for the microcracks and the zero load crack opening normalized to crack length. This relationship holds regardless of the ambient humidity during fatigue, although there are significant changes in the zero load crack openings developed with humidity. An empirical relationship between the irregularity of the microcrack propagation path as affected by humidity and crack opening at zero load is also identified, which can be used to estimate crack closure load from crack dimensional parameters measured at the specimen surface.     

An analysis of frequency and amplitude effects on corrosion fatigue crack propagation in Ti-8Al-1Mo-1V

Crack growth rates were measured in Ti-8Al-1Mo-1V, which exemplifies titanium alloys susceptible to transgranular stress-corrosion cracking (SCC). A wide stress intensity amplitude (δK) range, withR = 0, was employed at ½ to 30 cps in H₂O, 3 ½ pct NaCl solution, and methanol; at 10 to 30 cps in air; and at 20 to 30 cps in vacuum, the reference environment. Crack growth was augmented by two kinds of environmental effects. The first, Type A corrosion fatigue (cyclic SCC), was effective at low frequencies in the liquid environments when K_max≥ K_Iscc, intensified with increasing amplitude, but diminished with increasing frequency. In addition to fatigue striations, cleavage on a plane 15 deg from (0001)_α was observed. The second kind, Type B corrosion fatigue, was effective at low amplitudes and in all the environments, diminished at higher amplitudes, was unaffected by changes in cyclic frequency, and was not related to the SCC susceptibility of the alloy. Striated cleavage facets on (0001)_α, and on a plane 15 deg from (0001)_α occurred at low amplitudes, while ordinary ductile striations occurred at higher amplitudes. Fatigue in vacuum did not produce fractographic fatigue striations.     

广义热弹性扩散下非等径颗粒的烧结驱动力分析

在广义热弹性扩散理论框架下建立非等径两颗粒系统三维有限元模型,研究颗粒系统温度场和浓度场的分布规律,分析场分布对脉冲电流烧结初期迁移驱动力的影响。结果表明,颗粒颈部空位浓度梯度、温度梯度、由温度场和应力场产生的浓度梯度是颗粒颈部物质迁移的共同驱动力。烧结颈部的温度会产生两次突变,烧结过程中小颗粒一直保持高温状态;温度变化会引起浓度改变,使得颈部浓度高于边缘浓度;热扩散占总扩散通量的2/3,浓度扩散占1/3,因此烧结颈部的热扩散驱动力和浓度扩散驱动力是脉冲电流烧结过程的主导驱动力,提高热扩散能力和浓度扩散通量可显著提高烧结过程驱动力。非等径颗粒的烧结驱动力远远大于等径颗粒,为非等径颗粒的烧结比等径颗粒更为迅速提供了理论依据。     

Influence of interfacial reaction rates on the wetting driving force in metal/ceramic systems

The wetting of copper-silicon alloys of various compositions on vitreous carbon substrates at 1423 K was studied by the sessile drop method. The morphology and chemistry of products of interfacial reactions between silicon and carbon were characterized by scanning electron microscopy (SEM), electron probe microanalysis, and high-resolution optical profilometry. In addition to measurements of contact angles and spreading kinetics in the reactive Cu-Si/Cv system, similar measurements were performed for the nonreactive Cu-Si/SiC system. It was found that the reaction rate has no effect on the final contact angle, which is nearly equal to the thermodynamic contact angle of the alloy on the reaction product. These findings appear to be valid for a wide range of interfacial reaction rates and for different types of interfacial reactions.