Dislocation structures around propagating fatigue cracks in iron

Dislocation structures around the fatigue cracks for known stress intensity factor ranges were observed through an electron microscope operating at 2000 kV. To prepare thin foils for this purpose, a jet electropolishing technique has been developed, and regions in the vicinity of crack tips, as close as 0.1 μm from the sides, could be preserved in these foils. There were found 1) in front of the crack tip, substructures characterized by considerably dense dislocations in the cells just ahead of the tip and the formation of a lot of cells elongated in one direction, and 2) in the vicinity of the crack sides, a very fine cell structure with the interior of the cells showing relatively low density of dislocations. Besides cracks growing along the boundaries of cells or grains, some cracks penetrating into cells were observed. However, any crack supposed to be formed by linking of voids was not found. The misorientation between adjacent cells formed near the crack was generally a few minutes of arc, but the value as high as about 10° was sometimes observed immediately ahead of the crack tips. From the difference between the structures ahead of the tip and those in the vicinity of the sides, a brief discussion is made about structural changes produced when the crack tip has passed.     

Dislocation structures in the strain localized region in fatigued 70/30 brass and the interaction with grain boundary

Long-term fatigue tests of polycrystalline 70/30 brass were carried out under low strain amplitudes in vacuum, and dislocation structures of the strain localized regions developed were examined by means of transmission electron microscopy In the planar dislocation structures, the strain localized regions (SLRs) bounded by a pair of parallel active glide layers were frequently observed in favorably oriented grains. Where the SLRs impinge on grain boundaries (GBs), extrusion-type deformations were sometimes formed notwithstanding the restraints of the neighboring grains. On the basis of observations, the mechanism of crack initiation at the GB is discussed.     

The role of dislocation substructures in fatigue crack propagation in copper and alpha brass

The effect of dislocation substructures on fatigue crack propagation (FCP) behavior in copper and alpha brass was studied. Various dislocation substructures were obtained by prestraining in tension. Dislocation cells were formed by this prestraining in copper and 90/10 brass and when they formed the resistance to FCP at intermediate propagation rates (5×10⁻⁹ to ∼10⁻⁷ m/cycle) increased with increasing prestrain. Planar dislocation arrays were observed in 70/30 brass instead of cells, and the effect of prestraining on the FCP resistance was insignificant. From the FCP data for each material it was observed that, regardless of the difference in the dislocation substructures and grain sizes, the two constantsC andm in the Paris equation,da/dN=C(ΔK) ^m, were interrelated. Possible relations between the cyclic strain hardening exponent andm are discussed. The influence of both prestrain and grain size on threshold behavior was also studied.     

Comparison of temper-rolled and temper-annealed 70/30 brass strip

Abstract

The possibility of using temper annealing of heavily rolled strip, as an alternative to temper rolling has been investigated in 70/30 brass.

It has been found that temper annealing is an attractive alternative because (a) the combination of mechanical properties obtained is better at virtually any desired strength level, and (b) temper annealing lends itself easily to continuous annealing as well as to batch annealing. Normal mill variations in composition were found to have negligible effects on the annealing behaviour and on the properties of the finished material.

Résumé

Les auteurs ont étudié la possibilité d'effectuer un recuit ménagé sur de la tôle fortement laminée de laiton 70/30 pour obtenir les mêmes propriétés qu'après laminage superficiel.

Les essais ont montré que le recuit ménagé est une alternative intéressante parce que (a) il en résulte une meilleure combinaison de propriétés mécaniques pour presque tous les niveaux désirés de résistance à la traction, et (b) le recuit ménagé peut être réalisé facilement par recuit en continu tout aussi bien que par recuit en lots. Les variations de composition normalement acceptées en usine exercent une influence négligeable sur le comportement au recuit et sur les propriétés du produit fini.     

Near- threshold fatigue crack growth in copper and alpha- brass: Grain- size and environmental effects

The fatigue propagation rates and fatigue threshold ( ΔK _th) values were studied (R = 0.1 and frequency = 20 Hz) on copper and 70-30 α-brass of two different grain sizes in laboratory air and dry argon. With decreasing grain size, the threshold increased in copper, while it decreased in α-brass. These results suggest that in copper, crack tip plasticity considerations were more important in determining the threshold values than crack closure effects. Dry argon increased ΔK _th slightly in copper and more significantly in α-brass. A transition from completely transgranular to partially intergranular and back to completely transgranular cracking was observed with decreasing crack growth rates in both materials and environments. The growth rates for which intergranular cracking was obtained were found to be consistent with a hydrogen embrittlement mechanism, associated with adsorption of water molecules and dislocation transport of hydrogen.     

Yielding behavior of prestrained interstitial-free steel and 70/30 brass

The yielding behavior of interstitial-free (IF) steel and 70/30 brass prestrained in plane strain tension and subsequently strained in uniaxial tension has been investigated experimentally. Upon reloading in uniaxial tension, brass exhibited a negative transient (decrease in flow stress) and steel exhibited a positive transient (increase in flow stress). When the yield stress is defined by the offset method, the positive transient is difficult to model using conventional yield theories as elastic deformation is thought to occur outside the original yield or loading surface. In this work, the yield point was defined using the axial strainvs transverse strain curve as measured with biaxial resistance strain gages. The curve has an initially linear elastic portion; the slope then gradually changes until the linear plastic slope is reached. The intersection of the elastic and plastic slopes is defined as the yield point. Using this alternate definition, the yielding behavior of the prestrained metals was investigated. The yield stress for both prestrained brass and steel was found to be lower than the expected monotonic stress. Compared to previous research based on a traditional definition of yield point, this result is unexpected in prestrained steel and shows that yielding does occur inside the loading surface. The positive transient may, therefore, be modeled using conventional yield theories provided that the yield surface is defined using this alternate technique.     

Dislocation structures in the strain localized region in fatigued 85/15 brass

The correlation between the formation of extrusions and the dislocation structures in polycrystalline 85/15 brass subjected to a long term stress cycling with a low strain amplitude in vacuum was examined by means of transmission electron microscopy. The overall dislocation structures consisted of two types of structures,i.e., copper-type and 70/30 brass-type. Within them the strain localized regions bounded by two closely located active slip layers were frequently observed. In these boundary layers appeared a fringe pattern which was suggestive of intensive slip. Extrusions were formed in close association with the two closely located layers, and the initiation and growth of fatigue cracks occurred along one of them. On the basis of these observations, the mechanism of extrusion formation and of fatigue crack initiation are discussed.     

Dislocation structures of monocrystalline copper during corrosion fatigue in 0.1 M perchloric acid

We have studied the dislocation structures of single crystals of copper cycled in 0.1 M perchloric acid and under different polarization potentials. TEM samples were cut from representative specimens both after saturation and after fracture. Although much higher strain localization was observed in the crystals cycled at anodic potentials, the dislocation structures observed were very similar to those of specimens tested at cathodic potentials and in air. For a plastic shear strain amplitude of 2×10⁻³, regular loop patches and dipolar walls were observed. For a higher strain amplitude of 4×10⁻³, dipolar walls associated with secondary slip were found in addition to regular primary walls. We believe this structure to be associated with breaking the primary persistent slip bands (PSB's) into truncated groups, of which the truncations were lined up along the traces of the secondary slip plane. Dislocation structures observed after final fracture of the crystals were different from those formed by cycling just into saturation; dipolar walls formed first and cell structures developed later. Thus, we confirmed the transition of dipolar walls into cells, reported by other investigators. Moreover, the transition of loop patches into “rungs” structure, the embryo of the PSB, was also observed in the late stages of life. BENDA YAN, formerly with the University of Pennsylvania     

Transient tensile behavior of interstitial-free steel and 70/30 brass following plane-strain deformation

The transient behavior of interstitial-free (IF) steel and 70/30 brass which results from an abrupt change in strain state has been investigated experimentally and modeled analytically. After a plane-strain prestrain, reloading in uniaxial tension results in a negative stress transient for brass and a positive stress transient for IF steel. The strain behavior during the stress transient was studied by measuring the local axial and transverse strains using resistance strain gages. The monotonic data exhibited a constant plastic strain ratio, whereas the prestrain data showed decreasing plastic strain ratios with increasing axial strain for both IF steel and brass. A simple analysis of the transient was performed by modifying Hill's nonquadratic yield surface to allow variable plastic anisotropy (via r) during the transient. By choosing an appropriate variation ofr, the stress transient could be reproduced. The predicted variations in strain ratios by the model agreed qualitatively with measurements for brass but were of the opposite sign to measurements for steel. Although not conclusive, this result suggests that the normality condition is violated during a stress transient induced by an abrupt change in strain path. A.B. DOUCET, formerly A.E. Browning, Graduate Research Associate, Department of Materials Science and Engineering, The Ohio State University     

Experimental deformation textures of OFE copper and 70:30 brass from wire drawing,compression, and torsion

Inverse pole figures have been made from OFE copper and 70:30 brass samples which were deformed to a von Mises equivalent strain of 1.5 in wiredrawing, compression, and torsion. Measurements were taken from the plane normal to the axis of maximum or minimum principal strain for the cases of tension and compression, respectively. We studied three sections for the specimens deformed in torsion, namely the planes normal to the three cylindrical coordinate axes (Z, R, and π). Our results were consistent with the literature data for both wiredwaring and compression. In wiredrawing we found a duplex fiber texture of [001] and [111] orientations. The amount of the [001] texture was much less for the 70:30 brass. A [011] fiber texture developed in both materials in compression. For the OFE copper, the [011] texture spread toward [113]. In the case of the 70:30 brass there was a spread toward [111]. The inverse pole figures we obtained from the torsion samples had similar trends for both copper and brass. Data from the section taken normal to theR axis showed no distinct preferred orientation. For both materials the Z-section had an increased concentration of [111] poles and a distinct absence of [011] poles. In the case of the copper there was also a slight [001] component. Finally, we found the [011] component in the π-section. This section was also characterized by a lack of [001] or [111] orientations.     

Non-octahedral deformation activity in cold rolled 70:30 brass and its influence on the development of brass texture

A structure of elongated, wavy bands develops at high strains in both fine grained and normal 70:30 brass. Recovery twins and evidence of non-octahedral slip activity are observed in the wavy bands after further deformation. The non-octahedral process generally operates on several sets of closely spaced (0.002–0.004 μm) planes, producing substructure comprised of intersecting trace lines. The occurrence of this process cannot be explained by slip on only a single system but combined activation of {110} and {112} planes accounts for most of the traces observed. Texture changes that can be attributed to the non-octahedral process are also evident and orientations around {110}〈112〉 develop in the regions where the activity has operated. The results are in good agreement with recent simulations of rolling textures based on non-octahedral slip in which a strong brass texture is produced bythe operation of {110} + {112} slip systems.     

Existence of the coaxing effect and effects of small artificial holes on fatigue strength of an aluminum alloy and 70-30 brass

In order to study the existence of the coaxing effect and the effects of small defects on the fatigue strength of an aluminum alloy and 70-30 brass, plain specimens and special holed specimens containing one or more very small drilled holes with diameters of 40,50,100,200, and 500 ώm were prepared. Contrary to commonly accepted knowledge, the existence of a distinct coaxing effect was confirmed in the fatigue test on 2017-T4 aluminum alloy. However, the coaxing effect was not confirmed in the fatigue test on 70-30 brass, though specimens with small artificial holes could contain non-propagating cracks at the fatigue limit. It was found that the appearance of the coaxing effect depends on the endurance at a higher stress level of small cracks initiated at a lower stress level. The very small drilled holes with diameters of 40 and 50 ώm had no harmful effect on the fatigue strength of both the aluminum alloy and 70-30 brass; that is, the fatigue limits of specimens containing one or more drilled holes with diameters of 40 and 50 ώm were identical with those of the plain specimens.     

On the structures formed in eutectic alloys at high growth rates

Abstract

It is well established that dendrite-free structures can be produced in off-eutectic alloys by solidification at high growth rates. It has also been demonstrated that solidification at high growth rate can lead to the formation of a layer of supercooled liquid at the solid-liquid interface. In the present work the relation between the presence of supercooled interface liquid and the formation of dendrite-free structures is examined. Observations on tin-rich lead-tin eutectic alloys have shown that wholly lamellar structures can be produced when a supercooled layer is present, but that such a supercooled layer is not a necessary condition for the formation of dendrite-free structures.

Résumé

Il est bien connu qu'il est possible de produire des structures libres de dendrites dans les alliages quasi-eutectiques à condition d'utiliser des taux de croissance élevés. De plus, on sait que des taux étevés de solidification donnent lieu à la formation d'une zone de liquide en surfusion à l'interface solide-liquide. Les auteurs étudient ici la relation entre la présence de cette zone de liquide en surfusion et la formation des structures libres de dendrites. Ils trouvent que des structures complètement lamellaires peuvent exister en présence d'une zone de liquide en surfusion mais que la présence de cette zone n'est pas une condition nécessaire pour la formation de structures libres de dendrites.