TEM-in situ deformation of beryllium single crystals—a new explanation for the anomalous temperature dependence of the critical resolved shear stress for prismatic slip

The critical resolved shear stress for prismatic slip within beryllium single crystals passes through a maximum in the temperature range of 170–450 K. A valid explanation has not been found till now. In the present paper the experimental results of TEM-in situ deformation of single crystal beryllium are presented. Although the samples were oriented to promote prismatic slip, combined slip mechanisms on prismatic and basal planes were observed at room temperature. However, at 170 K plastic deformation only occurred by prismatic slip. A new model explaining the anomaly of the critical resolved shear stress is proposed which is based on elementary dislocation processes, i.e. formation and motion of salient points on dislocation lines. The same mechanisms might also result for other hexagoanl metals in a comparable anomaly of the plastic behavior.     

Prismatic slip in α-titanium single crystals

Single crystals of Ti, oriented favorably for prismatic slip and containing two levels of interstitial impurities, have been deformed in tension at temperatures between 78 to 1120 K. The stress-strain curves exhibit three stages of hardening similar to those of Zr and of fcc crystals. The work hardening rate in Stage II, θ_II/G is lower in Ti than in Zr. From the strain rate dependence of the stress at the onset of Stage III, the stacking fault energy on the prism planes of Ti has been estimated as 0.145 Joules/m². The relative values of stacking fault energy for Ti and Zr are consistent with a dissociation model which is based on the hcp ai bcc transformation. The thermally activated prismatic slip below 250 K is controlled by the interaction of dislocations with interstitial solute atoms. Above 250 K the dislocation mechanism for plastic flow is not clearly understood.     

Ambient Compression–Compression Fatigue Behavior of Magnesium Single Crystal

A magnesium single crystal sample with a near $ 1 1 {\bar{{2}}{0}} $ orientation was tested at room temperature under compression–compression cyclic loading, and the microstructure was characterized to disclose the involved deformation mechanisms. No plastic deformation region appeared on the stress–strain curve during the cyclic loading. The stress–strain curve stabilized at the first cycle, the strain range for each cycle fluctuated slightly around a constant value, and the mean strain for each cycle was in a narrow range from 0.0846 to 0.0863 during the whole test. The ratcheting strain rate decreased exponentially from ~0.0003, and reached a relatively small and stable value of about zero. The observed deformation mechanisms were prismatic slip, compression twinning, and tension twinning. The prismatic dislocation slip roughened the cylindrical sample surface by forming extrusions and intrusions, and small cracks were also observed on the surface.     

Mechanical Properties and Microstructure Evolution During Deformation of Fe–Mn–C TWIP Steel

The mechanical and deformation microstructure properties of the Fe–Mn–C TWIP steel was investigated by means of tensile experiment, in situ scanning electron microscope (SEM) and transmission electron microscope (TEM).The results showed that the sample has excellent mechanical with tensile strength of the steel is about 1140 MPa and the yield strength is higher than 480 MPa, while the elongation is above 57%, the true stress–strain curve from tension tests exhibited repeated serrations and its strain‐hardening rate is constantly changing. It is found that there were different deformation mechanisms at different deformation stages result in the unique true stress–strain curve. Dislocation slip dominated the initial deformation and with the accumulation of deformation stress concentration reached the twin shear stress resulting in twin shear, which lead to TWIP effect. As the strain capacity increased continually, the parallel twins can no longer rotate and shear deformation occurred, which lead to the forming of shear bands. The intercoordination of slip deformation, twin deformation, and shear deformation mechanism make the TWIP steel show high strength and high plasticity.     

Influence of deformation conditions and texture on the high temperature flow stress of magnesium AZ31

The evolution of hot working flow stress with strain is examined in torsion, uniaxial compression and channel die compression. The flow stress was found to be strongly dependent on texture and deformation mode. At low strains this dependency accounted for a difference in flow stress of up to a factor of two. At higher strains the influence of texture and deformation mode was less marked. The stresses corresponding to an equivalent strain of 0.5 were modelled using a power law expression with an activation energy of 147 kJ/mol and a strain rate exponent of 0.15. The influence of texture and deformation mode on flow stress is rationalised in terms of the influence of prismatic slip, twinning and dynamic recrystallisation on deformation stress and structure.     

Prismatic slip in zirconium single crystals at elevated temperatures

Single crystals of Zr oriented favorably for prismatic slip have been deformed in tension over a range of strain rates at temperatures between 473 and 1113 K. A temperature independent plateau is observed between 600 and 800 K and dynamic strain aging occurs in the vicinity of 723 K. The flow stress is temperature dependent both above and below this temperature interval. Plastic flow above 850°K is represented by an equation of the form:γ = AT ⁿ e-Q/rT where y is the shear strain rate,A is a constant whose value is 680 ± 20 (MN/m²)^•4.3. The stress exponentn = 4.3 ± 0.3 and the activation energy Q = 2.05 = 0.15 eV. It is proposed that the high temperature prismatic slip in Zr is controlled by a glide-climb process where the rate of plastic flow is determined by the rate of climb of dislocations.     

Prismatic slip in zirconium single crystals at elevated temperatures

Single crystals of Zr oriented favorably for prismatic slip have been deformed in tension over a range of strain rates at temperatures between 473 and 1113 K. A temperature independent plateau is observed between 600 and 800 K and dynamic strain aging occurs in the vicinity of 723 K. The flow stress is temperature dependent both above and below this temperature interval. Plastic flow above 850°K is represented by an equation of the form: {ie1217-05} where {ie1217-06} is the shear strain rate,A is a constant whose value is 680 ± 20 (MN/m²)^−4.3. The stress exponentn = 4.3 ± 0.3 and the activation energyQ = 2.05 ± 0.15 eV. It is proposed that the high temperature prismatic slip in Zr is controlled by a glide-climb process where the rate of plastic flow is determined by the rate of climb of dislocations.     

Deformation twinning as a mode of energy accommodation

The conditions necessary for mechanical twin nucleation are considered. It is concluded that a high elastic strain energy accumulation must precede twin nucleation, and that twin formation provides an efficient means of excess energy accommodation. It is suggested that the significance of the resolved shear stresses present is limited to the slip deformation, which must precede, accompany, and follow twin formation in real crystals. The resultant shear deformation is not inherent in twinning itself, but originates in the associated slip. In the absence of slip, in a perfect crystal, twins can form by atom shuffles requiring no overall shear.     

锻造方式对Ti-6Al-4V合金组织及取向分布的影响

在快锻液压机上对Ti-6Al-4V合金进行了锻造变形,采用扫描电镜、背散射电子衍射技术以及X射线衍射技术研究了不同锻造方式下合金组织及晶粒取向的变化规律.在单向镦拔和换向镦拔两种不同锻造方式下,难变形区、小变形区及大变形区中α相及β相的分布差别不大,组织均匀性基本一致,两种变形方式下锻坯不同区域的应变稍有差别.进一步对不同变形区域形变织构的定量分析可知:在应变较小的边缘区域,变形主要以{0001}基面滑移为主,形成基面织构;在应变较大的内部区域,织构明显转向{1120}、{1010}等柱面织构;在应力集中的位置,会产生{1122}、{1011}等锥面织构.两种锻造方式均能提高Ti-6Al-4V合金中形变织构的均匀性,而且换向镦拔优于单向镦拔.      

TEM investigation of γ′ free bands in nimonic PE16 under LCF loading at room temperature

The cyclic stress strain behavior of nickel base alloy Nimonic PE16 has been studied at room temperature under constant strain rate of 4 × 10⁻³s⁻¹ and at a strain amplitude of 0.57% for different γ′ particle sizes. Fatigue softening is observed for all microstructures. Detailed TEM investigations reveal γ′ free bands in all specimens. Evolution of these bands is studied as a function of cumulative deformation the specimen has bee subjected to γ′ particles within deformation bands are initially sliced by movement of dislocations and on further deformation slices are fragmented by cross slip of dislocations within bands. A conceptual model for reduction in size of γ′ particles during fatigue is proposed on the basis of these results. Reaging studies of deformed samples reveal nucleation of very fine γ′ particles within bands. Comparison of γ′ particle size within these bands with those formed after ageing in specimens deformed in solution treated condition indicates that γ′ particles have completely dissolved in the specimen containing smallest γ′ precipitates. These results are explained on the basis of the concept of particle fragmentation under cyclic loading.     

Creep and plasticity of hexagonal polycrystals as related to single crystal slip

The role of slip on basal, prismatic and pyramidal systems of hexagonal single crystals in determining inelastic polycrystalline behavior is studied using a uniform strain-rate upper bound and a self-consistent method. Steady power-law creep is considered. Included as a limiting case is rigid-perfectly plastic behavior, for which the upper bound to the yield stress of the polycrystal coincides with the Bishop-Hill bound for these materials. When the resolved shear stress needed to produce a given level of slip on the pyramidal systems is large compared to that on the other systems the upper bound lies well above the self-consistent estimate. Scif-consistent theory indicates that overall inelastic deformation of a polycrystal is possible without pyramidal slip. Implications for hexagonal materials, including ice, are discussed.     

Shear band angles in rolled F.C.C. materials

Rolled metals and alloys exhibit a laminar structure after moderate strains and are treated in this work as laminar composites. A model for shear band propagation at angles other than the principal resolved stress is presented which allows the shear strength of a particular texture component to be evaluated in terms including an in-plane shear stress component existing between adjacent laminae. Flow localisation in the form of a shear band occurs when a reduced number of slip systems necessary to accomplish the shear coincides with the minima in shear strength. The model is derived from the behaviour of the {111} 〈112〉 rolling deformation texture formed in α-brass which shears at 35° and is applied to another alloy system, PbCaSn, which shears at ~ 30°. Theory and experiment are in reasonable agreement.     

Cyclic deformation behavior of high-purity titanium single crystals: Part I. Orientation dependence of stress-strain response

Randomly oriented single crystals of high-purity titanium were prepared by strain annealing and were subjected to multiple-step fatigue testing under strain-controlled conditions, in order to determine their cyclic stress-strain curves (CSSCs). These were found to fall into three groups, depending on orientation and the extent of slip and twinning. For those crystals oriented for single prismatic slip, a plateau was observed in the CSSCs, persistent slip bands (PSBs) occurred, and the plateau stress was 38 MPa. In a second group, oriented for prismatic slip but for which cross-slip and twinning was favored, the plateau was suppressed and the flow stresses were higher. In a third group, connected with orientations on the borders of the unit triangle, extensive hardening occurred, the CSSCs were steep, and there were multiple cases of slip and twinning. The results are interpreted in terms of maps in the stereographic projection recording the Schmid factors for the various deformation modes.     

The effect of twinning on the work-hardening behavior and microstructural evolution of hafnium

The work-hardening behavior of hexagonal-close-packed (hcp) metals, such as hafnium, is influenced by temperature, strain rate, chemistry, and texture. In the case of hafnium, while slip on the prism and pyramidal planes is dominant during deformation, the propensity of deformation twinning is known to increase with decreasing temperature and increasing strain rate. In this study, hafnium was prestrained quasi-statically in compression at liquid nitrogen temperature (77 K), creating a heavily twinned microstructure. The specimens were then reloaded in compression at room temperature (298 K). Yield stress, flow stress, and work-hardening behaviors of the prestrained specimens were higher than room-temperature compression test data typical of the as-annealed material. The microstructure of each specimen was characterized optically and using a transmission electron microscope (TEM). Texture was measured by neutron diffraction and the texture evolution due to twinning, and the interaction of slip with the twins was seen to lead to higher work-hardening rates and flow stresses in the cold prestrained specimens.     

Analysis of the Deformation Behavior in Tension and Tension-Creep of Ti-3Al-2.5V (wt pct) at 296 K and 728 K (23 °C and 455 °C) Using In Situ SEM Experiments

The deformation behavior of a Ti-3Al-2.5V (wt pct) near-α alloy was investigated during in situ deformation inside a scanning electron microscopy (SEM). Two plates with distinct textures were examined. Tensile experiments were performed at 296 K and 728 K (455 °C) (~0.4T _m), while a tensile-creep experiment was performed at 728 K (455 °C) and 180 MPa (σ/σ _ys = 0.72). The active deformation systems were identified in the α phase using electron backscattered diffraction based slip-trace analysis and SEM images of the surface. Prismatic slip deformation was the dominant slip mode observed for all the experiments in both plates, which was supported by a critical resolved shear stress (CRSS) ratio analysis. However, due to the texture of plate 1, which strongly favored the activation of prismatic slip, the percentages of prismatic slip activity for specimens from plate 1 tested at 296 K and 728 K (23 °C and 455 °C) were higher than the specimens from plate 2 under the same testing conditions. T1 twinning was an active deformation mode at both 296 K and 728 K (23 °C and 455 °C), but the extent of twinning activity decreased with increased temperature. T1 twinning was more frequently observed in specimens from plate 2, which exhibited a higher fraction of twinning systems favoring activation at both 296 K and 728 K (23 °C and 455 °C). The tension-creep experiment revealed less slip and more grain boundary sliding than in the higher strain rate tensile experiments. Using a previously demonstrated bootstrapping statistical analysis methodology, the relative CRSS ratios of prismatic, pyramidal 〈a〉, pyramidal 〈c+a〉, and T1 twinning deformation systems compared with basal slip were calculated and discussed in light of similar measurements made on CP Ti and Ti-5Al-2.5Sn (wt pct).     

Fe-25Mn-3Si-3AlTWIP钢等温压缩流变应力的研究

在Gleeble-1500D热模拟试验机上采用等温压缩试验研究了高锰奥氏体Fe-25Mn-3Si-3AlTWIP钢在变形温度为900～1100℃,变形速率为0．01～1s。条件下的热变形行为。研究结果表明,Fe-25Mn-3Si-3Al钢热变形流变应力曲线呈现明显的动态再结晶特征,出现了一个明显的流变应力峰值,峰值之后...     

Implementation of an Associative Flow Rule Including Hydrostatic Stress Effects into the High Strain Rate Deformation Analysis of Polymer Matrix Composites

A previously developed analytical formulation has been modified in order to more accurately account for the effects of hydrostatic stresses on the nonlinear, strain rate dependent deformation of polymer matrix composites. State variable constitutive equations originally developed for metals have been modified in order to model the nonlinear, strain rate dependent deformation of polymeric materials. To account for the effects of hydrostatic stresses, which are significant in polymers, the classical J2 plasticity theory definitions of effective stress and effective inelastic strain, along with the equations used to compute the components of the inelastic strain rate tensor, are appropriately modified. To verify the revised formulation, the shear and tensile deformation of a representative polymer are computed across a wide range of strain rates. Results computed using the developed constitutive equations correlate well with experimental data. The polymer constitutive equations are implemented within a strength of materials based micromechanics method to predict the nonlinear, strain rate dependent deformation of polymer matrix composites. The composite mechanics are verified by analyzing the deformation of a representative polymer matrix composite for several fiber orientation angles across a variety of strain rates. The computed values compare well to experimentally obtained results.     

Enhanced deformation mechanisms by anisotropic plasticity in polycrystalline Mg alloys at room temperature

This article presents room-temperature deformation mechanisms in polycrystalline Mg alloys. Dislocation slip of basal 〈a〉 and prismatic 〈a〉 types are shown to occur nearly at the same ease when the basal planes are tilted in such a way that the Schmid-factor ratio (equivalent to the critically resolved shear stress (CRSS) ratio) of prismatic 〈a〉 to basal 〈a〉 slip is larger than a value ranging from 1.5 to 2.0, depending on the initial texture distribution and grain size. Grain-boundary sliding (GBS) also occurs at room temperature up to 8 pct of total strain, enhanced by plastic anisotropy as well as by the increasing number of grain-boundary dislocations. Twinning plays an important role in both flow and fracture behaviors. Twins are induced mostly by stress concentrations caused by the anisotropic nature of dislocation slip. Twins can be classified into two types based on their shape: a wide lenticular type and a narrow banded type. The wide twins are twins appearing in the early stage of deformation and accompany little change of surface height. The narrow twins are or appearing in the late stage of deformation and accompany a substantial change in surface height. The formation of the narrow twins seems to give rise to highly localized shear deformation within the twin, leading to strain incompatibility and to final failure. This article is based on a presentation made in the symposium entitled “Phase Transformations and Deformation in Magnesium Alloys,” which occurred during the Spring TMS meeting, March 14–17, 2004, in Charlotte, NC, under the auspices of ASM-MSCTS Phase Transformations Committee.