Observations of grain-boundary sliding and surface topography in an 8090 al alloy after uniaxial and biaxial superplastic deformation

The surface topography of an 8090 aluminum alloy was studied after uniaxial or biaxial superplastic deformation, with particular reference to grain-boundary sliding (GBS) offsets, grain rotation angles, formation of striated bands (SBs) or fibers, cavity distribution, and cavity formation mechanisms. Additionally, the contribution of GBS or grain separation to the overall strain was evaluated. Striated bands were observed and are thought to be the newly exposed faces of the grains inclined to the specimen’s surface. They were formed by sliding of grains upward and downward relative to the specimen surface. Grooves and crests inside SBs were formed from the relative motion of grainboundary defects. Fibers were observed and are thought to be the further development of the SBs resulting from the formation of elongated cavities and grain separation. More cavitation was found in equibiaxially strained regions than in other regions subjected to approximately equivalent levels of strain. About 50 pct of the total strain was contributed by GBS in the uniaxial tensile-loaded specimens, as compared to about 30 pct in the biaxial-strained specimens. The effects of grain separation, grain rotation, and secondary GBS may be the reasons for the reduction of the observed strain contribution from GBS in biaxially strained specimens.     

Effect of impurity type on boundary sliding behavior in the superplastic Zn-22 pct Al alloy

The effect of impurity type on boundary sliding behavior in the superplastic Zn-22 pct Al alloy was investigated using two grades of the alloy: Zn-22 pct Al-0.13 pct Cu (grade Cu) and Zn-22 pct Al-0.14 pct Fe (grade Fe). In the investigation, boundary sliding offset measurements in both grades were made at strain rates ranging from 5×10⁻⁷/s to 10⁻¹/s. This range of strain rate covered region I (the low strain rate region), region II (the intermediate strain rate region), and region III (the high strain rate region) of the sigmoidal plot between stress and strain rate that was previously reported for grade Fe. The experimental results show that the contributions of boundary sliding to the total strain, ξ, in the two grades of Zn-22 pct Al are about 20 and 52 pct at high (region III) and intermediate (region II) strain rates, respectively. By contrast, the experimental data reveal that ξ in grade Cu at low strain rates (52 pct) is essentially equal to that at intermediate strain rates (region II), while ξ in grade Fe at low strain rates (24 pct) is considerably lower than that at intermediate strain rates (56 pct). It is demonstrated that the difference in sliding behavior between grade Fe and grade Cu at low strain rates corresponds well with the difference in superplastic behavior between the two grades. In addition, consideration of the present and earlier data on sliding behavior in Zn-22 pct Al provides a correlation between two roles played by boundaries during superplastic deformation: the ability of boundaries to contribute to deformation through the process of boundary sliding and their ability to serve as favorable sites for the accumulation of impurities, i.e., boundary segregation.     

Roles of Friction-Induced Strain Hardening and Recrystallization in Dry Sliding Wear of AZ31 Magnesium Alloy

Dry sliding wear tests were performed on AZ31 alloy using a pin-on-disc configuration under the loads of 5–360 N and sliding speeds of 0.1–1.5 m/s. Friction and wear characteristics of AZ31 alloy were investigated as a function of the load and sliding speed. Wear mechanisms for AZ31 alloy were characterized by scanning electron microscopy. The wear behavior in mild and severe wear regimes was described in terms of plastic deformation and microstructure evolution in subsurface, and surface hardness change and temperature rise of worn surfaces. The results revealed that surface strain hardening caused by large plastic deformation played an important role in maintaining a low slope of wear rate in mild wear regime, while surface thermal softening originating from dynamic recrystallization and surface melting were responsible for a rapid wear in severe wear regime.     

Dynamic phase transformation during superplastic deformation of Nb/Nb<Subscript>3</Subscript>Al <Emphasis Type="Italic">in-situ</Emphasis> composite

Nb_ss/Nb₃Al in-situ composite with the nominal composition of Nb-16 mol pct Al-1 mol pct B, consisting of bcc niobium solid solution (Nb_ss) and A15 ordered Nb₃Al, was synthesized by arc melting, homogenization annealing, and isothermal forging, and their superplastic deformation behavior was investigated by tensile tests and microstructure observations. Maximum superplastic elongation over 750 pct was obtained at 1573 K and at a strain rate of 1.6 × 10⁻⁴ s⁻¹ for as-forged specimens. Phase transformation from Nb_ss to Nb₃Al was observed to occur during superplastic deformation. Dynamic phase transformation during superplastic deformation progresses more quickly than static phase transformation during annealing without applied stress. Dynamic phase transformation is accompanied by phase-boundary migration, which operates as an accommodation process of grain-boundary sliding. Dislocation creep dominates deformation and grain-boundary sliding is inhibited at a high strain rate, while grain-boundary sliding and cavity formation are promoted at a low strain rate because of insufficient accommodation of grain-boundary sliding arising from sluggish dynamic phase transformation. It is concluded that there exists an optimum strain rate that guarantees the grain-boundary sliding and the rapid dynamic phase transformation to achieve maximum superplastic elongation.     

On the microstructural aspects of the nonhomogeneity of superplastic deformation at the level of grain groups

The surface relief of a superplasticity deformed magensium alloy (Mg-1.5% Mn-0.3%Ce) was studied. Zones/bands of localized deformation were detected by means of vacuum etching. Between the localized deformation bands were less-deformed regions. After 20% elongation in vacuum (1.33 × 10⁻³ Pa), zones of intensive vacuum etching were observed as two intersecting bands of localized deformation oriented at 35–60° to the tensile axis. Spacing between the localized deformation bands is 6–8 grain diameters after a tensile elongation of 20% and 3–4 grain diameters after an elongation of 160%. The observed bands of strain localization are explained from the viewpoint of cooperative grain boundary sliding, i.e. shifting of grain groups as a whole unit along grain boundary surfaces oriented close to the maximum shear stress direction. It is suggested that the cooperative nature of GBS be taken into account when evaluating the real local strain rate and the real strain-rate sensitivity of grain boundary sliding process.     

The role of deformation localization in superplastic flow

The model Zn22%Al alloy is used to show superplastic deformation takes place when localized deformation bands separating the material volume into large grain conglomerates are formed and developed. The formation of these bands is due to the cooperative grain boundary sliding which can be stimulated by intragranular sliding. The number and shape of the bands, as well as their contribution to the total elongation (the technique of surface markers was used) depend on strain rate. The movement of grain in the specimen volume takes place by the mutual displacement and rotation of grain conglomerates. The data presented can serve as a basis for the development of general physical theory of superplasticity based on macroscopic and mezoscopic deformation.     

Dry sliding wear of ferrous PM materials

Abstract

Ashby's map for dry sliding wear of wrought steels has been used as a guide to interpret the dry sliding wear behaviour of PM materials. It has been shown that this map is useful in understanding the acting wear mechanisms and also the experimental wear rates. For given tribological conditions, in terms of normalised pressure and sliding velocity, the sliding wear resistance of PM materials is similar to that of wrought steels, although a closer look at the experimental results highlights the peculiar role of porosity and of a heterogeneous microstructure. In particular, materials with a porosity content of about 10% and with an homogeneous microstructure display the best performances. Since mild wear in metals can be obtained through the formation of protective oxide glazes, steam treatment turned out to be a natural way of increasing the sliding wear resistance of PM ferrous materials. The ‘surface durability’ of steam treated materials was thus shown to depend on the quality of the layer, the applied load and the nature of the counterface. The role of the counterface and the opportunity to adopt other surface treatments to increase the sliding wear resistance of PM ferrous materials are also discussed.     

The measurement of grain boundary sliding in polycrystals

Measurements on the creep strain attributable to grain boundary sliding, ∈_gb, have frequently been performed on the specimen surface. However, thus far there has been no investigation using correct techniques to show that such surface measurements are also representative of the specimen interior. Measurements of ε_gb using methods which are correct in principle for both surface and interior grain boundaries have been made for a number of copper/aluminum alloys and for type 316 stainless steel. It is shown that for these materials under the test conditions employed ε_gb, is in fact the same for both surface and interior. It is also shown that it is possible to obtain β_gb simply and easily from a single parameter such as the mean height of surface steps, •v, provided the required constant •k, relating •v and β_gb is determined by correct methods for the material being considered. Formerly graduate student in the Materials Department at Queen Mary College, London, England     

钢包滑动水口滑板多炉连用技术的研究与实践

针对130t钢包滑动水口机构设计上的缺陷和滑板质量问题,根据生产现场的工艺技术条件,对滑动水口机构的面压负荷方式、滑动行程、滑板的安装方式进行设计改造,改进滑动水口耐材的性能,优化精炼工艺等,采用综合技术,实现了滑板的4炉连用。     

Deformation mechanisms during low-and high-temperature superplasticity in 5083 Al-Mg alloy

The controlling deformation mechanisms and grain boundary sliding behavior during low-, medium-, and high-temperature superplasticity (LTSP, MTSP, and HTSP) in fine-grained 5083 Al-Mg base alloys are systematically examined as a function of strain. Grain boundary sliding was observed to proceed at temperatures as low as 200 °C. With increasing LTSP straining from the initial (ε<0.5) to later stages (ε>1.0), the strain rate sensitivity m, plastic anisotropy factor R, high-angle grain boundary fraction, grain size exponent p, and grain boundary sliding contribution all increased. During the initial LTSP stage, there was little grain size dependence and the primary deformation mechanisms were solute drag creep plus minor power-law creep. At later stages, grain size dependence increased and grain boundary sliding gradually controlled the deformation. During MTSP and HTSP, solute drag creep and grain boundary sliding were the dominant deformation mechanisms.     

A constitutive model for the tensile deformation of a binary aluminum alloy at high fractions of solid

Tensile tests were performed on alloy Al-4.5 pct Cu with a solidification unit dubbed the direct chill surface simulator (DCSS) that simulates the primary cooling conditions encountered during the direct chill (DC) casting process of sheet ingots. The curves obtained showed a near linear increase of the load with strain up to the point where nonlinearity induced by hot tearing prevented further increase of the load. A constitutive model based on the Lahaie-Bouchard stress-strain theoretical model was generalized by assuming a nonsingular channel thickness distribution. This approach considers a fully lubricated arrangement that allows grain boundary sliding as the most important mechanism of accommodation. As the strain increases, the grain boundaries become more and more subjected to friction sliding and oppose a higher resistance than a fully lubricated sliding condition. This results in a gradual increase of stress with strain. The important variables of the model include solid fraction, channel thickness, geometric standard deviation, and creep law parameters. The model seems appropriate to correlate the tensile behavior of semisolid microstructures having up to 30 pct liquid phase distributed over a large spectrum of grain morphologies.     

Substructure of the friction surface of copper and copper-graphite materials

Conclusions In friction at a sliding rate of 40 m/sec and a pressure of 0.1 MPa. a finely disperse mixture of particles of copper and its oxides forms on the surface of pure copper; these particles increase the strength of the surface layer of copper, reduce its wear, but do not prevent bonding processes from taking place.In friction of DGr-5 material at a sliding speed of 11 m/sec under a pressure of 0.1 MPa., surface films consisting of ultradisperse particles of copper and oxides form; the dimensions of the graphite particles are comparable with those of the initial particles. Since the graphite particles cannot extend completely along the prismatic planes to the friction surface and the amount of graphite is insufficient for ensuring the lubricating effect, DGr-5 material is not capable of efficient operation in these conditions.The surface films formed during friction on DGr-10 composite material are characterized by uniform distribution of the graphite particles and consist of ultradisperse copper and fineplate textured graphite with preferential orientation with the basal planes parallel to the friction surface; this results in higher values of the tribotechnical characteristics.Translated from Poroshkovaya Metallurgiya, No. 12(300), pp. 66–71, December, 1987.     

δ/γ Interface bondary sliding as a mechanism for strain accommodation during hot deformation in a duplex stainless steel

In the present work, the mechanical properties and the microstructural evolution of a duplex stainless steel in the as-cast and wrought conditions during deformation under hot-working conditions have been studied. Hot torsion tests, at strain rates of = 1 s⁻¹, have been carried out using prepolished samples on which surface parallel scratches have been practiced. The observation of the surface of the samples shows a large displacement of the scratches produced by two different mechanisms, sliding on the δ/γ interface, and shearing of the ferrite. The displacements in the as-cast condition have been found to concentrate in a reduced set of ferrite/austenite interfaces leading to the formation of cracks along them. In the wrought material, the distribution of the sliding is more homogeneous over all the ferrite/austenite interfaces, and no damage has been produced. These behavioral differences between both materials have been related in the present work, to the characteristics of the corresponding microstructures, to the spatial phase distribution, and to the nature of the ferrite/austenite interface, among others.     

Creep deformation of TD-nickel chromium

The creep behavior of thoria dispersed nickel-chromium (TD-NiCr) was examined at 1093°C (2000°F). Major emphasis was placed on 1) the effects of the material and the test related variables (grain size, temperature, stress, strain and strain rate) on the deformation characteristics, and 2) the evaluation of single crystal TD-NiCr material produced by a directional recrystallization technique. Creep activation enthalpies were found to increase with increasing grain size reaching maximum values for the single crystal TD-NiCr. Stress exponent of the steady state creep rate was also significantly higher for the single crystal material as compared to that determined for the polycrystalline TD-NiCr. The elevated temperature deformation of TD-NiCr was analyzed in terms of two parallel-concurrent processes: 1) diffusion controlled grain boundary sliding and 2) dislocation motion. The characteristics of the dislocation motion deformation mode (as observed in the single crystal TD-NiCr) suggest that strong particle-dislocation interactions are present. The relative contributions of dislocation motion and grain boundary sliding in TD-NiCr were estimated. In creep, grain boundary sliding was found to predominate for the small, equiaxed grain structures, whereas the dislocation deformation mode became significant for only the large grain TD-NiCr and the single crystal material. Formerly Graduate Assistant, Case Western Reserve University, Cleveland, Ohio 44106.     

On the spreading of grain boundary dislocations and its effect on grain boundary properties

The effect of dissociation of trapped lattice dislocations (TLDs) on the energy of grain boundaries (GBs) is calculated. Two possible effects are considered. The first is the change of energy of the GBs caused by the presence of TLDs cores. It was shown to be smaller than 5% of the grain boundary energy. The other, more important, is an increase of the energy of the strain fields connected with grain boundaries. Dissociation of TLDs causes an increase of incompatibility strains between plastically deformed grains. The energy of these strains is of the order the GB energy in equilibrium. The EGBDs strain fields provide a driving force for GB sliding and migration. In fact, EGBDs cannot leave the grain boundaries. Recovery of these dislocations can take place only by sliding and migration of grain boundaries.     

基于有限元极限平衡法的三维边坡稳定性

提出了一种基于有限元弹塑性应力场和极限平衡状态的三维边坡稳定分析方法——三维有限元极限平衡法。首先,考虑三维空间中滑动方向,提出滑动面上一点在滑动方向上的极限平衡条件,并证明滑动面上土体整体达到极限平衡状态与滑动面上土体各处在滑动方向上处于极限平衡状态等价。再通过刚体极限平衡假定计算主滑方向和滑动面上各点滑动方向。最后,定义局部安全系数为抗剪强度与滑动方向上剪应力投影的比值,基于三维边坡整体极限平衡条件将局部安全系数通过积分中值定理转变为整体安全系数。该方法计算简单,消除了剪应力比形式定义安全系数滑动面形状限制,具备合理性与有效性。算例验证结果表明,该方法滑动方向假设合理,安全系数与严格三维极限平衡法结果一致。      

Numerical models of creep and boundary sliding mechanisms in single-phase, dual-phase, and fully lamellar titanium aluminide

Finite element simulations of the high-temperature behavior of single-phase γ, dual-phase α₂+γ, and fully lamellar (FL) α₂+γTiAl intermetallic alloy microstructures have been performed. Nonlinear viscous primary creep deformation is modeled in each phase based on published creep data. Models were also developed that incorporate grain boundary and lath boundary sliding in addition to the dislocation creep flow within each phase. Overall strain rates are compared to gain an understanding of the relative influence each of these localized deformation mechanisms has on the creep strength of the microstructures considered. Facet stress enhancement factors were also determined for the transverse grain facets in each model to examine the relative susceptibility to creep damage. The results indicate that a mechanism for unrestricted sliding of γ lath boundaries theorized by Hazzledine and co-workers leads to unrealistically high strain rates. However, the results also suggest that the greater creep strength observed experimentally for the lamellar microstructure is primarily due to inhibited former grain boundary sliding (GBS) in this microstructure compared to relatively unimpeded GBS in the equiaxed microstructures. The serrated nature of the former grain boundaries generally observed for lamellar TiAl alloys is consistent with this finding.     

Image analysis for grain shape characterization in lamp filaments

Algorithms have been developed for appraising the quality of tungsten lamp filament wire microstructure with respect to its resistance to sag. They allow a numerical determination of grain aspect ratio, grain boundary contour, angle of boundary with wire diameter, and degree of grain boundary surface convolution. These values are combined algebraically to give a grain shape parameter (GSP) which reliably predicts how resistant the filament is to grain boundary sliding. Measurements are made of thermally etched grain boundaries on scanning electron microscopy (SEM) images of the surfaces on coiled filaments. Data are recorded and parameters computed by means of an image analyzer. The technique has been tested on samples made from the same wire modified in process to have varying strain after the last anneal. It has also been tested on samples purchased from various vendors. These applications indicate that the computed GSP is a sensitive predictor of filament creep resistance and reliably reflects variation in wire drawing strain after the last anneal.