Strength and high fatigue properties of ultrafine-grained titanium rods produced by severe plastic deformation

The formation of an ultrafine-grained structure (UFGS) in titanium by severe plastic deformation significantly increases its mechanical properties, among them fatigue strength. In this work, high mechanical properties of titanium long rods are achieved using a combination of equal-channel angular pressing and subsequent thermal and thermomechanical treatment. The formation of a homogeneous UFGS in a rod of commercial titanium is found to increase the fatigue limit by a factor of about 1.5. Moreover, the fatigue strength of UFGS titanium estimated on notched specimens is higher than that of coarse-grained titanium, which is important for its structural application in medicine.     

晶粒尺寸对大塑性变形的两相合金超塑性的影响

在室温下对共晶铅锡合金( Pb-62% Sn)进行了高压扭转变形( HPT)。采用不同时间的自退火制备不同晶粒尺寸的样品。最长的自退火时间为12天。通过研究这些样品在室温下不同的拉伸行为,从而获得了关于超塑性性能的结果。之后,本文通过将这些结果与等通道挤压( ECAP)获得的样品进行比较,不仅证明了所有样品都具有良好的超塑性,而且具有小晶粒尺寸的样品更容易在大应变速率的条件下获得超塑性。     

Limit of ferrite grain refinement by severe plastic deformation of austenite

High-angle grain-boundary spacing in deformed austenite is analyzed using Ni-30Fe alloy to explain the change of ferrite grain size by severe plastic deformation (SPD) of austenite in low carbon steel. It is suggested that constant high-angle grain-boundary spacing in deformed austenite resulting from dynamic recrystallization (DRX) or geometric DRX is responsible for the limit of ferrite grain refinement over a certain level of plastic deformation of austenite.     

Mechanical properties of iron processed by severe plastic deformation

In the present study, the mechanical properties of Fe processed via severe plastic deformation (equal-channel angular pressing (ECAP)) at room temperature were investigated for the first time. The grain size of annealed Fe, with an initial grain size of about 200 μm, was reduced drastically during ECAP. After eight passes, the grain size reaches 200 to 400 nm, as documented by means of transmission electron microscopy (TEM). The value of microhardness during pressing increases 3 times over that of the starting material after the first pass and increases slightly during subsequent pressing for higher-purity Fe. Examination of the value of microhardness after eight passes as a function of post-ECAP annealing temperature shows a transition from recovery to recrystallization, an observation that resembles the behavior reported for heavily deformed metals and alloys. The tensile and compression behaviors were examined. In tension, a drop in the engineering stress-engineering strain curve beyond maximum load was observed both in the annealed Fe and the ECAP Fe. This drop is related to the neck deformation. The fracture surface, examined by scanning electron microscopy (SEM), shows vein patterns, which is different from the dimples found on the fracture surface of annealed Fe. In compression, an initial strain-hardening region followed by a no-strain-hardening region was observed in the ECAP Fe. The yield strength in tension of the ECAP Fe was observed to be higher than that in compression. The strengthening mechanisms and softening behavior are discussed.     

The effect of grain size on the high temperature plastic deformation of nb3sn

A study of high temperature plastic deformation has been undertaken on 10, 20, and 60 μm grain size Nb₃Sn. The materials were produced by the hot isostatic pressing of powder blends. The 20 and 60 μm grain size material involved a stoichiometric blend of Nb and Sn powder, whereas the 10 μm grain size material involved a blend of 30.2 wt pct Sn powder and 69.8 wt pct Nb-1 Zr powder. The ZrO₂ formed during processing limits grain size and NbO formation. Through compression testing and load relaxation testing, deformation has been studied over a strain rate range from 10^-6 to 10^-2 per second and a temperature range from 1150 to 1650 °C. “Power law creep” was generally observed, although stress exponent reduction at the higher temperatures and lower strain rates suggests substructural coarsening. Analysis of stress-strain rate-temperature data projected an activation energy for creep of 400 to 500 kJ/mol. Grain size refinement clearly strengthened the polycrystals. Assuming a Hall-Petch relationship, “lattice friction stresses” and “unpinning constants” were calculated, both increasing with decreased temperature and increased strain rate. Grain size refinement from 60 to 10 μm lowered the ductile-to-brittle transition temperature for simple compression by the order of 125 °C.     

Severe plastic deformation of copper: The state of grain boundaries and their triple junctions

The results of electron microscopic studies of the structure and phase composition of nanocrystalline copper produced during severe plastic deformation by torsion under high quasi-hydrostatic pressure are presented. Particular attention was focused on grain triple junctions. It is found that the triple junctions contain partial disclinations and secondary-phase particles. The strain dependences of the fraction of these triple junctions during formation of nanocrystalline copper are measured. Phase analysis of the structure of secondary phases is performed, and the sizes volume fractions of the secondary-phase particles are determined. The torsion-curvature of the crystal lattice appearing near triple junctions is measured. The problem of screening of the long-range field of stresses forming in triple junctions is discussed.     

Reverse transformation of ferrite and pearlite to austenite in an ultrafine-grained low-carbon steel fabricated by severe plastic deformation

Reverse transformation characteristics of a low-carbon steel consisting of ultrafine-grained (UFG) ferrite and severely deformed pearlite by severe plastic deformation were investigated and compared to those of the steel having coarse-grained (CG) ferrite and undeformed pearlite by austenitization and subsequent air cooling. Coarse-grained steel exhibited two serial transformation stages, i.e., pear-lite → austenite followed by ferrite → austenite. Contrarily, UFG steel transformed with the three serial stages, i.e., probably carbon-supersaturated ferrite → austenite, not-fully-dissolved pearlite → austenite, and ferrite → austenite transformations.     

The effect of small plastic deformation and annealing on the properties of polycrystals: Part II. Theoretical model for the transformation of nonequilibrium grain boundaries

The nonequilibrium grain boundary state which has a high energy state is the result of absorption of a certain density of extrinsic grain boundary dislocations (EGBD’s). The equilibrium of such a boundary occurs by annealing at higher temperatures. A model has been proposed in this paper which assumes that the equilibrium of a nonequilibrium grain boundary involves the annihilation of EGBD’s by climbvia lattice diffusion of vacancies at the triple points. Due to the stress field of the EGBD’s, there is a vacancy concentration gradient around the triple points. The profile of the vacancy concentration gradient is derived by assuming a steady state flux of vacancies. Using this vacancy concentration profile, the expressions for the rate of climb of EGBD’s are derived. The proposed model predicts that the time required for the equilibration of nonequilibrium grain boundaries is dependent not only on the annealing temperature but also on the initial density of EGBD’s and the boundary length (which is related to the grain size). It has also been shown that the equilibrium behavior predicted by our model is in good agreement with the experimental results obtained for 316L stainless steel.     

Influence of the temperature on the plastic deformation in TiAl

A new alloy, Ti-48.6Al-1.9Cr-1.9Nb-1B, with a near-equiaxed γ microstructure and with a lamellar microstructure is investigated by compression tests between 20 and 800 °C and transmission electron microscopy (TEM). The yield stress exhibits no anomaly as a function of the temperature, while an anomaly, related to strain hardening, is found at 400 °C in the hardening rate and the activation volume for both the lamellar and nonlamellar structure. Above 700 °C, a change in the deformation mechanism occurs and the material becomes remarkably softer. The TEM micrographs highlight the importance of ordinary dislocation motion for both structures at all temperatures. The comparison with previously reported TEM observations on single-phase TiAl alloys shows definitely that the density of ordinary dislocations is higher in the investigated two-phase TiAl alloy deformed at room temperature. Also, the presence of the lamellar interfaces drastically changes the mechanical properties of the alloy and the deformation mechanism. In contrast to the nonlamellar samples, superdislocations are rare, and twinning is very frequent in the lamellar structure.     

The influence of grain boundaries on mechanical properties

The effect of interfaces on mechanical properties is considered. Elastic and plastic compatibilities at boundaries are treated. Specific influences at both low and high temperatures are discussed, with emphasis on dislocation mechanisms and the atomic scale structure of boundaries.     

Effect of severe plastic deformation on the phase and structural transformations and mechanical properties of metastable austenitic Ti-Ni alloys

The effect of deformation under pressure and severe plastic torsional deformation (SPTD) on the structure and mechanic properties of Ti-Ni alloys is studied. It is found that, as an external load is applied, metastable austenitic alloys of the Ti-Ni system undergo the B2 → B19′ transformation. The SPTD Ti-Ni alloys are characterized by high mechanical properties and high thermal stability of their nanostructured state and properties.     

Effect of temperature and deformation histories on grain coarsening in medium carbon steel

Abstract

Hot rolling of medium carbon steel bars using a prototype mill and numerical analyses were carried out to examine quantitatively the effect of temperature and strain throughout rolling on the final grain size of the product. Cross-sections of the rolled bars were subjected to optical microscopy to investigate the effect of the instantaneous decrease in temperature owing to contact with the roll groove on grain coarsening in the vicinity of the bar surface. By analysing the grain size after rolling, the temperature history throughout rolling, and the equivalent plastic strain after rolling, it was found that the instantaneous decrease in temperature owing to contact did not affect grain coarsening unless the final values of the temperature and strain fell within a narrow band of values meeting the criteria for grain coarsening. Referring to both the present results and the authors' previous studies, criteria have been established for the prevention of grain coarsening in steel bar sizing operations.     

轧制塑性变形对W-Cu复合材料组织性能影响

对不同Cu含量[20%～50%(质量分数)]的热挤压后的W-Cu合金坯料进行轧制,通过对比得出最佳轧制工艺参数,并制备出微观组织均匀、致密度高、性能优异的W-Cu合金板材。结果表明:轧制力随着道次轧下量的增加而逐渐增大,并且当一次轧下量超过60%时,轧制力急剧增加;随着轧制变形量的增加,Cu相发生变形,W相形状基本不发生变化,但W相有细化的趋势;轧制变形能在保持W-Cu合金热挤压后高组织均匀性、高致密度、高性能的基础上获得薄板甚至是箔材。     

Effect of plastic deformation on the physicomechanical properties of tungsten carbide-cobalt hard alloys

Summary A study was made of the effect of prior plastic deformation on the hardness, strength, coercive force, and electrical resistivity of tungsten carbide-cobalt hard alloys containing 10–25% cobalt. Plastic deformation decreases the hardness of the alloys. Up to a deformation of about 5–6%, all the alloys investigated showed a marked drop in hardness. Further deformation did not decrease the hardness of alloy VK25; for the alloys with lower cobalt contents, the hardness decrease was less pronounced.     

Transformation of the grain structure in a metal during plastic deformation

Statistical data processing techniques are used to study the distribution laws of the geometrical parameters of grains in steel 22GYu after sheet rolling. A method for predicting changes in the geometrical characteristics of the grain structure after deformation is developed.     

Efficiency of the strengthening of titanium and titanium alloys of various classes by the formation of an ultrafine-grained structure via severe plastic deformation

The efficiency of strengthening induced by microstructure refinement to an ultrafine-grained (UFG) state is studied on commercial purity VT1-0 titanium and two-phase VT6 and VT22 titanium alloys. An UFG structure with a grain size <0.5 μm is formed by multiaxial isothermal deformation. The refinement of a titanium microstructure to a grain size of ～0.4 μm is found to result in an almost twofold increase in the strength and the fatigue limit. The strength of a VT22 alloy with an UFG structure is equal to that of the thermally strengthened alloy. The strength and fatigue limit of a VT6 alloy with an UFG structure are higher than those of the thermally strengthened state by approximately 25%. The strengthening by microstructure refinement is found to be expedient for low and medium alloys.     

Effect of plastic deformation on the coarsening of

Particle coarsening has been studied in a rapidly solidified Al-8.8 wt pct Fe-3.7 wt pct Ce alloy subjected to isothermal annealing for various times at 425 °C. The effect of static and dynamic loading on the particle coarsening rates at the same temperature also has been examined. The dispersed particles in all specimens of the present study are the equilibrium Al¹³Fe⁴ and Al¹⁰Fe²Ce phases. They are incoherent with the matrix and constitute 23 pct of the total volume. The coarsening rate in isothermally annealed specimens is orders of magnitude greater than predicted by the modified Lifshitz-Slyozov-Wagner theory for volume diffusion controlled coarsening but can be explained using Kirchner’s model for coarsening by diffusion along grain boundaries. In the case of intragranular particles, coarsening by diffusion along dislocation cores also is likely to be significant. Creep loading is seen to cause a significant enhancement of the coarsening rate. Fatigue testing with a hold period at the maximum tensile stress also accelerates coarsening whereas continuous cycling appears initially to retard the increase in the average particle size. Dislocations connecting dispersed phase particles are observed more frequently in crept specimens and specimens fatigued with a hold period than in specimens fatigued with no hold period. The effects of plastic deformation on particle coarsening rates are discussed in terms of excess vacancy generation, short circuiting along dislocations, and fine precipitation during fatigue.     

Formation of the limiting nanostructural state in metals during plastic deformation

The mechanisms that determine the formation kinetics of the limiting states in deformed solid nanostructures are considered. These states are characterized by the minimum average nanocrystallite size. The limiting states are shown to correspond to the point of dynamic equilibrium between the deformation-induced competing processes of nanograin fragmentation and growth.     

Effect of plastic deformation on the mechanical properties and the fine-structure parameters of Pb-Ca-Sn alloys

The specific features of structure formation in Pb-Ca-Sn alloys intended for battery current leads are considered. The effect of plastic deformation on the mechanical properties and the fine-structure parameters in Pb-0.1% Ca-0.3% Sn and Pb-0.05% Ca-1.1% Sn alloys is studied. The influence of the initial structure on the mechanical properties and the fine-structure parameters of the alloys is shown, and the variation of the alloy properties under plastic deformation is demonstrated.