Effect of grain size and annealing texture on the cyclic response and the substructure evolution of polycrystalline copper

In attempting to interpret the mechanical response of polycrystalline copper, for which the results in the literature show marked scatter, the effects of microstructure on the cyclic behavior and the substructure evolution of copper polycrystals have been investigated. The microstructure is described by a complex factor—grain size and texture combined. It is found that there is a very significant effect of microstructure in the cyclic response of copper at low and intermediate strain amplitudes, where dislocation structures which localize deformation are expected to be present. In general, the cyclic response of coarse-grained copper shows a much more pronounced cyclic hardening and higher saturation stresses than those for fine-grained copper. This behavior is associated with a well defined hard 〈111〉−〈100〉 fiber texture, inherited in the coarse-grained material after annealing at relatively high temperatures. The multiple slip associated with the 〈111〉−〈001〉 oriented grains homogenizes the deformation very early, resulting in strong cyclic hardening, and a faster substructure evolution into cell structure.     

Influence of precipitates on the mechanical response and substructure evolution of shock-loaded and quasi-statically deformed Al−Li−Cu alloys

The mechanical response and substructure evolution of two Al−Li−Cu alloys (Al-2.90 wt pct Li-1.00 pct Cu-0.12 pct Zr and Al-2.30 pct Li-2.85 pct Cu-0.12 pct Zr) subjected to shock-loading (strain rate έ> 10⁶ s^-1), Split-Hopkinson-Pressure-Bar compression (έ ~ 5 × 10³ s^-1), and quasi-static compression (έ ~ 1.5 × 10^-3 s^-1) were examined. The strain levels achieved in these three deformation paths were desined to be comparable,i.e., all ∼15 pct. Both alloys were either naturally or artificially aged to yield an underaged or overaged condition. Various precipitates, such as theδ' andT ₁ phase, of different sizes and volume fractions were dispersed in the matrix and at the grain boundaries. The substructure in all of the shock-loaded, Split-Hopkinson-Pressure-Bar, and quasi-static compression samples was characterized by localized slip bands and microbands with the exception of the overaged alloys. The density of dislocations and dislocation loops was higher, independent of the aging condition, in the shock-loaded specimens. Well-defined cell structures were not observed in any of the samples, independent of strain rate. The influence of precipitates, shearable or not, on the substructure development in Al−Li−Cu alloys during shock-loading was seen to be pronounced, even though the size and volume fraction of precipitates was small and low, respectively. Flow stress measurements showed that the shock-loaded samples have flow strengths 3 to 8 pct higher than the quasi-statically deformed samples. This small, but reproducible, strength increment, for alloys deformed to equivalent strains at low and high rates, indicates that the Al−Li alloys studied have a small rate sensitivity. Based upon comparison of the results of the shock-loaded and quasi-static samples, it is concluded that the fundamental deformation mechanisms and substructure evolution in all three loading paths are not drastically different, corroborating previous investigations.     

Influence of grain size on the hardness of alpha plutonium

The effect of grain size on the hardness of alpha plutonium was measured at temperatures between 77° and 373°K. The observations were: 1) fine-grained metal had the lowest hardness at 373°K, 2) fine-grained metal had the highest hardness at 77°K, and 3) fine-grained metal had the greatest temperature dependence of hardness. At room temperature, fine-grained α-plutonium (1 μm) had a hardness of 235 Dph, whereas coarse-grained metal (2000 μm) had a hardness of 310 Dph. At 373°K fine-grained metal had a hardness of 120 compared to 225 Dph for the coarse-grained metal. At 77°K the hardness values were 505 and 475 Dph, the fine-grained metal being harder at this temperature. The hardness was estimated to be independent of grain size at 140°K. Above 140°K, grain boundaries have a softening effect due to the contribution of grain boundary sliding to deformation.     

Weibull尺寸分布晶粒组织演变的仿真研究

采用一种改进的Potts模型Monte Carlo算法,对具有Weibull尺寸分布(参数β=3.47)的晶粒组织进行了3D正常晶粒长大过程的仿真研究.仿真结果表明:整个晶粒长大过程遵循抛物线长大规律,晶粒生长指数为0.501,非常接近理论值0.5.晶粒长大过程可分为过渡阶段与准稳态长大两个阶段.Weibull尺寸分布参数β由过渡阶段的3.47逐渐演变为准稳态阶段的2.76,准稳态阶段晶粒尺寸分布参数保持β=2.76不变.晶粒的平均面数〈f〉随仿真时间的增加而增大,在准稳态阶段后期趋近于稳定数值.晶粒面数分布为Lognormal分布,最高频率面数f为10,个体晶粒面数范围为3~43.     

Influence of grain size and porosity on the high-temperature friction of titanium carbide

Conclusions It was established that in friction of a TiC-TiC pair in vacuum the coefficient of friction at 250C and the wear rate at 1250C are practically independent of grain size. At higher temperatures these characteristics have an inverse relationship to grain size.It was shown that with an increase in porosity both the wear rate and the coefficient of friction increase. With an increase in temperature the influence of porosity on the wear rate decreases.With variations in porosity in the 1–10% range, in grain size in the 1–50 m range, and in temperature in the 20–1500C range the wear rate changes within limits of 10–45% and the coefficient of friction within limits of 3–35%.Translated from Poroshkovaya Metallurgiya, No. 9(297), pp. 56–61, September, 1987.     

The role of grain size and ferrite substructure in respect to mechanical properties of an HSLA steel

Several heat treatments were applied to an HSLA steel of type StE 460 (German standard) to produce four ferrite substructures of different strength and at least two different grain sizes respectively. Whereas the ferrite substructure had a strong influence on yield strength the effect of grain size was negligible though probably to some extent masked by different pearlite volume fractions. The different strength levels could be explained by regarding the arrangement of dislocations and vanadium carbonitride particles and their mutual interaction. Increasing both grain size and pearlite volume fraction leads to a remarkable shift of transition temperature T_27J which was further enhanced by increasing ferrite strength.     

Pore-boundary interactions and evolution equations for the porosity and the grain size during sintering

The retardation of grain boundary migration by a pore is analysed numerically and by an analytical approximation, which becomes exact in the limiting case of small velocities. At high velocities, the numerical solution describes separation of the pore from the boundary. Based on the analysis of a single migrating boundary facet dragging a pore, evolution equations for the grain size and the porosity are suggested for the final stage of sintering. It turns out that three-dimensional pores on two-grain junctions have an only moderate effect on the rate of grain coarsening: pores with a high mobility follow the migrating grain boundary easily, while those with a low mobility detach from the boundary. It is also shown that a two-dimensional pore does not detach and can therefore reduce the coarsening rate to very low values if the pore mobility is low. This strong dependence on the pore configuration suggests that other possible configurations need to be analysed, before final evolution equations can be formulated.     

A model for the evolution of grain size distribution during superplastic deformation

In a recent paper, we postulated that a grain size distribution in a polycrystal can result in mixed mode deformation during superplastic flow. Since diffusional flow is strongly grain size-dependent while power-law creep is not, it was inferred that large grains may deform by power-law creep, while concomitantly, the small grains deform by diffusional creep. Here, a first order model for dynamic change in the grain size distribution with strain is developed to explain the shape of the stress-strain curves obtained during superplastic deformation of aluminum alloys. The model is based on the simple assumption that regions deforming by diffusion creep suffer strain induced grain growth, while dislocation creep results in grain refinement. In spite of the approximation, the model correctly predicts the shape of the stress-strain curves. The possible significance of these concepts in classical dynamic recrystallization phenomena is also discussed.     

The effect of grain size on the shock-loading response of 304-type stainless steel

The residual microstructure and mechanical response of shock-loaded stainless steel (AISI-304) of four different grain sizes—23, 55, 85 and 187 μm-was investigated. In addition to mechanical twinning and planar dislocation arrays, transformation to both ɛ and α martensite occurred in all shock-loaded specimens but became more extensive with decreasing grain size. In comparison to the Hall-Petch behavior of yield and early flow stress observed for the material after 5.2 pet cold rolling, the strengthening efficiency of shock loading decreased with increasing grain size. Shock loading enhanced the strain-induced transformation to α martensite during subsequent tensile deformation.     

The effect of grain size on the shock-loading response of 304-type stainless steel

The residual microstructure and mechanical response of shock-loaded stainless steel (AISI-304) of four different grain sizes-23, 55, 85 and 187 Μm-was investigated. In addition to mechanical twinning and planar dislocation arrays, transformation to both e and α martensite occurred in all shock-loaded specimens but became more extensive with decreasing grain size. In comparison to the Hall-Petch behavior of yield and early flow stress observed for the material after 5.2 pct cold rolling, the strengthening efficiency of shock loading decreased with increasing grain size. Shock loading enhanced the strain-induced transformation to α martensite during subsequent tensile deformation.     

Influence of grain size and stacking-fault energy on deformation twinning in fcc metals

This article investigates the microstructural variables influencing the stress required to produce deformation twins in polycrystalline fcc metals. Classical studies on fcc single crystals have concluded that the deformation-twinning stress has a parabolic dependence on the stacking-fault energy (SFE) of the metal. In this article, new data are presented, indicating that the SFE has only an indirect effect on the twinning stress. The results show that the dislocation density and the homogeneous slip length are the most relevant microstructural variables that directly influence the twinning stress in the polycrystal. A new criterion for the initiation of deformation twinning in polycrystalline fcc metals at low homologous temperatures has been proposed as (σ _tw −σ ₀)/G=C(d/b)^A, where σ _tw is the deformation twinning stress, σ ₀ is the initial yield strength, G is the shear modulus, d is the average homogeneous slip length, b is the magnitude of the Burger’s vector, and C and A are constants determined to have values of 0.0004 and −0.89, respectively. The role of the SFE was observed to be critical in building the necessary dislocation density while maintaining relatively large homogeneous slip lengths.     

齿轮轴承钢坯模锻过程晶粒尺寸演变模拟

齿轮轴承钢坯模锻过程中不同部位的形变参数变化极不均匀,晶粒尺寸演变规律复杂多变。通过Gleeble等温热压缩试验获得材料物性参数,利用YLJ再结晶模型对有限元软件DEFORM-3D进行二次开发,进而模拟出模锻过程中齿轮轴承钢坯的晶粒尺寸演变规律。研究发现:齿轮轴承钢坯晶粒尺寸在锻造期呈脉冲变化,不同部位的晶粒在再结晶期增长幅值不一样,底部边缘和中下部晶粒尺寸易粗化,中心和中上部晶粒尺寸比较小;将锻件心部和底部边缘的晶粒形貌和晶粒尺寸与模拟结果进行对比,模拟与实际结果对应良好。提出的晶粒演变模拟方法能很好预测模锻过程复杂的晶粒尺寸演变规律。     

On the relation between grain size and grain topology

Experimental studies of the topology of grains in polycrystals have indicated that the topological complexity of a grain is related to its diameter, as opposed to its surface area or volume. This paper presents additional experimental documentation of this correlation and a theoretical derivation of the empirically observed relationship.     

Influence of titanium content and grain size on hydrogen cracking behaviour of hot-rolled steels

Hydrogen induced cracking was investigated for hot-rolled titanium steels. Aim of the present work was to observe the influence of titanium content and grain size on the cracking behaviour. Three titanium steels (0.12-0.30 % Ti; O.0057-0.0480 % C) and one non-titanium steel (0.0056 % C) were used for the investigation. Various grain sizes were generated by heat treatment at 950, 1050 and 1150 °C; furnace cooling was applied. The specimens were electrolytically charged with hydrogen at various current densities. It was found that cracks are generated at low charging current densities for the investigated steels. The titanium steels showed better performance than the non-titanium steel. It was shown that the charging current density does not correspond to the hydrogen concentration in a steel; the hydrogen concentration in steel B was 3.8 ppm at 1 mA/cm², in steel D it was found to be 15.5 ppm at 0.5 mA/cm². The total hydrogen concentration was found to be influenced by content of precipitates and grain size. It was shown that the percentage of cracked grain boundary area increases with increasing grain size. This increase is linear for the non-titanium steel whilst for the titanium steels a plateau was observed at a grain size diameter of 50 μm.     

成形压力与粉末粒径对钨铜复合材料烧结性能的影响

为进一步提高钨铜合金的致密度和简化制备工艺,研究了粉末粒度与成形压力对无压烧结制备的W-15Cu复合材料致密度的影响.发现随着球磨时间延长,钨铜粉末发生明显的细化和圆化,粉末分布更为均匀,烧结活性有较大提高,合金性能更加优异,组织结构更加良好,致密度相应提高.通过对烧结试样密度和铜含量的测定,得到不同成形压力下材料致密度和铜含量随烧结温度的变化曲线,发现随着成形压力增大,材料的烧结致密度升高,铜流失的现象得到一定的控制.     

Influence of the residual aluminum content on the grain size and the mechanical properties of 20G steel

The article presents the results of detecting the possible influence of the residual aluminum content on the grain size and the mechanical properties of the 20G steel produced at ZAO VMZ Krasny Oktyabr.     

Dislocation substructure evolution in torsion of pure copper

The evolution of dislocation substructures in pure copper during torsion deformation at strains ranging from 0 to 440 pct has been investigated using transmission electron microscopy. The study reveals that checkerboard patterns have formed and shrunk in size at strains ranging from 10 to 60 pct. This was followed by the development of laminar dislocation structures consisting of paired sheets which evolved from short double walls delineating the checkerboard patterns. Linear strain hardening was found to be maintained in the paired sheets at strains from 120 to 330 pct. Dislocation wall annihilation and microbands located along the wall of paired sheets were observed in stage IV of the work-hardening curve. At higher strains, another set of wall formation intersects with the paired sheets. The strain hardening of copper under torsional loading from the checkerboard pattern to the laminar structure is described by the mesh length theory of dislocation structures.