Cyclic deformation behavior and dislocation structure of Ti-2 at. Pct Al single crystals oriented for double prism slip

Cyclic deformation behavior and dislocation substructure in the single crystals of Ti-Al alloy containing 2.0 pct Al (by atom) with double prism slip at different cyclic strain amplitudes were examined. The testing results showed that Ti-2Al displayed an initial slight cyclic hardening followed by a striking softening period, and a saturation stage was reached finally. Secondary cyclic hardening was observed for most specimens before failure, specifically at the high cyclic strain amplitudes. The cyclic resolved shear stress-strain curve (CSSC) was observed to contain a plateau in the testing strain range with a saturation plateau stress of about 55 MPa. Trace analysis on the surface of specimens with an optical microscope shows that the (10 0) and (1 00) double prism slips can be distinguished from the traces on the (0001) surface of the fatigued specimens. The dislocation substructure in the fatigued specimens was examined using a transmission electron microscope (TEM). The typical dislocation configuration is the saturated dislocation bundles, which are tangled on the (10 0) and (1 00) slip planes and arranged parallel to the [0001] direction.     

The effect of twinning and slip on the bauschinger effect of hadfield steel single crystals

The Bauschinger effect (BE) in single crystals of Hadfield manganese steel (Fe, 12.3Mn, 1.0C in wt pct) was studied for three crystallographic orientations, , and [001]. Both forward tension-reverse compression (FT/RC) and forward compression-reverse tension (FC/RT) loading schemes were used to investigate the role of deformation history on the BE. The evolution of stress-strain response and a dimensionless Bauschinger parameter were used to study the BE. The BE stems from long-range back stress generated by the dislocation pileups at the twin and localized slip boundaries. Twinning boundaries present a strong obstacle and lead to a strong BE. If localized slip followed twinning, permanent softening was evident, such as in the case of the FT/RC scheme. Localized slip and multiple slip in the forward loading provided a transient effect in the stress-strain response without a significant permanent softening. Hadfield steel single crystals have demonstrated a high BE for orientations conducive to combined twinning/slip deformation. The BE increased with increasing prestrain, then saturated and started to decrease, in contrast with precipitation-hardened alloys. A unique strain-hardening approach along with the back stress calculation was introduced into a viscoplastic self-consistent (VPSC) formulation. The strain-hardening formulation incorporates length scales associated with spacing between twin lamellae. The simulations correctly predicted the BE and the stress-strain response for both forward and reverse loading.     

The effect of twinning and slip on the bauschinger effect of hadfield steel single crystals

The Bauschinger effect (BE) in single crystals of Hadfield manganese steel (Fe, 12.3 Mn, 1.0C in wt pct) was studied for three crystallographic orientations, , and [001]. Both forward tensionreverse compression (FT/RC) and forward compression-reverse tension (FC/RT) loading schemes were used to investigate the role of deformation history on the BE. The evolution of stress-strain response and a dimensionless Bauschinger parameter were used to study the BE. The BE stems from long-range back stress generated by the dislocation pileups, at the twin and localized slip boundaries. Twinning boundaries present a strong obstacle and lead to a strong BE If localized slip followed twinning, permanent softening was evident, such as in the case of the FT/RC scheme. Localized slip and multiple slip in the forward loading provided a transient effect in the stress-strain response without a significant permanent softening. Hadfield steel single crystals have demonstrated a high BE for orientations conducive to combined twinning/slip deformation. The BE increased with increasing prestrain, then saturated and started to decrease, in contrast with precipitation-hardened alloys. A unique strain-hardening approach along with the back stress calculation was introduced into a viscoplastic self-consistent (VPSC) formulation. The strain-hardening formulation incorporates length scales associated with spacing between twin lamellae. The simulations correctly predicted the BE and the stress-strain response for both forward and reverse loading. IBRAHIM KARAMAN, formerly Research Associate, Department of Mechanical and Industrial Engineering, University of Illinois     

Influence of secondary precipitates and crystallographic orientation on the strength of single crystals of a Ni-based superalloy

The effect of crystallographic orientation and aging heat treatment at 850 °C on the creep rupture strength of single crystals of a nickel-based superalloy was examined at 700 °C in detail. Initial tensile orientations were selected over a wide range on the standard stereographic triangle. The {111}〈112〉-type slip systems were found to be operative during the creep deformation. The creep behavior was found to be greatly influenced by the additional aging at 850 °C for 20 hours. It was found that the effect of the aging at 850 °C was quite different between orientations favored for the slip system and those favored for the (111) slip system and that the creep deformation mechanisms of these two slip systems were different. In the orientations favored for slip systems, in the single-aged specimens, a small mean surface-to-surface spacing due to hyperfine γ′ precipitates in the matrix channel promoted the slip and the primary creep. As a result of the additional aging at 850 °C, the hyperfine γ′ precipitates were dissolved into the matrix, and the resultant large mean surface-to-surface spacing between the cuboidal precipitates inhibited extensive shearing of the γ-γ′ structure by the slip system. As a result, the creep strengths of these orientations were increased in double-aged specimens; however, the low ductility associated with the difficulty of secondary noncoplanar slip did not enlarge rupture lifetime in the double-aged [001] specimen. In the orientations favored for the (111) slip system, creep deformation occurred by twinning shear through γ and γ′ precipitates, and a distinct effect of the aging at 850 °C was not observed. In the multiple orientation of the {111} -type slip systems, i.e., the and orientations, hyperfine precipitates improved creep strength because they prevented dislocations from gliding in the matrix channel in the single-aged specimens.     

Crystallographic details of precipitates in Fe-22Cr-21Ni-6Mo-(N) superaustenitic stainless steels aged at 900 °C

Crystallographic features of second phases and the effect of nitrogen addition on the microstructural evolution in superaustenitic Fe-22Cr-21Ni-6Mo-(N) (all in wt pct) stainless steels during isothermal aging at 900 °C were investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Both alloys with and without nitrogen contained sigma phase and M₂₃C₆ carbides in the solution treated condition. While four phases (sigma, M₂₃C₆, M₆C, and chi) of intermetallics and carbides appeared sequentially as a function of aging time in the nitrogen-free alloy, two nitrides (Cr₂N and AlN) were additionally observed after long time aging of the nitrogen-containing alloy. The addition of nitrogen into Fe-22Cr-21Ni-6Mo steel promoted a finer and more uniform distribution of precipitates during isothermal aging. The exact identification and crystallography of various second phases were confirmed from the analyses of selected area diffraction patterns from various orientations, stereographic projection, and energy dispersive spectroscopy. The orientation relationships between the precipitates and austenite matrix can be summarized as follows: (1) two carbides (M₂₃C₆ and M₆C): cube-on-cube orientation relationship; (2) chi phase: Kurdjumov-Sachs (K-S) orientation relationship; (3) two nitrides (Cr₂N and (AlN): (11 0)_nitrides //(211) _γ and [0001]_nitrides //[ 11] _γ ; and (4) sigma phase: (1) ( 11) _γ //(00 ) _σ and [ 0] _γ //[ 0] _σ or (2) ( 10) _γ //( 10) _σ and [ 2] _γ //[113] _σ . For the sigma phase, the former orientation relationship was predominant throughout aging, and the latter orientation relationship was occasionally observed under limited aging conditions.     

Deformation of a Burgers oriented bimetallic bicrystal of alpha-beta (Ti-13Mn)

The deformation behavior of a Burgers oriented α-β-Ti-13Mn bimetallic bicrystal was studied at two plastic strains, 0.52 and 2.08 pct. Two single crystals, α and β, each corresponding to the orientation of its respective bicrystal component were also investigated. The stress axes were and [1218]_β. The interface planes were and and lay in the x’-z’ plane. The deformation behavior of the a component differed from that of the a single crystal because of plastically induced stresses,T _y’z’ ,T _x’z’ ,T _x’y’ , and σ _x’x’ . Prismatic slip and twinning were found in the single crystal α whereas the bicrystal revealed additionally pyramidal andc + a slip, the latter at the interface. Slip on the front and back surfaces was different and both thec + a and twinning systems acted to maintain compatibility. Slip in the β single crystal and the β bicrystal component were quite similar. However, there were differences in the intensity and amount of primary slip, (231) , on the front and back surfaces. The diminished amount of (231) slip on the back surface was due to plastically induced stresses, and on the front surface the primary slip cross slipped to slip which triggeredc + a slip in α. On the back surface the dominant slip system was which acted in response to the plastically induced stresses. An approximate calculation revealed that the interface deformation zone had about twice the flow stress of the average bicrystal stress. Formerly a Graduate Student in the Department of Physical and Engineering Metallurgy at Polytechnic Institute of New York, Brooklyn, NY     

Twins as barriers to basal slip in hexagonal-close-packed metals

The boundary structure of {10 1}, {10 2}, {11 1} and, {11 2} twins in hexagonal-close-packed (hcp) metals and the interaction of crystal dislocations with the first two twin types have been studied previously using atomic-scale computer simulation. The interaction of crystal dislocations with {11 1} and {11 2} twin boundaries is described here and compared with the results for {10 1} and {10 2} twins. These four twins are found to create barriers to the motion of crystal dislocations gliding on the basal plane, and the strength of the barrier depends in a relatively complex manner on crystallographic parameters and details of the atomic structures of the interfaces. In some circumstances, crystal dislocations can be transmitted through the twin boundary, thereby creating twinning dislocations. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Investigation of the annealing texture evolution in hafnium

This article presents a study of the evolution of the annealing texture in hafnium, as measured by electron backscattering diffraction patterns (EBSPs). It was found that the annealing texture of asreceived extruded rod depended on the annealing temperature. After low-temperature recrystallization, the deformation axis was parallel to or and the basal planes were approximately parallel to the deformation axis. These orientations were deduced by the position of the points in the standard stereographic triangle used to produce the inverse pole figure. As the annealing temperature was raised to 1700 °C, the direction parallel to the rolling direction changed to and the grain size increased. It appeared that the increase in grain size occurred by a process of abnormal grain growth, and this abnormal grain growth appeared to be the cause of the change in the texture. Texture was also examined in samples that had been warm rolled to thickness reductions between 10 and 90 pct and then annealed at 1700 °C. In these samples, the main feature of the texture was that the basal plane became parallel to the rolling plane as the amount of rolling increased. The maximum grain size was observed in samples that had been rolled to a reduction in thickness of 50 pct.     

Evolution of microstructure and texture in Mg-Al-Zn alloys during electron-beam and gas tungsten arc welding

The evolution of microstructure and texture in the AZ-series Mg alloys subjected to electron-beam welding and gas tungsten arc welding are examined. Electron-beam welding is demonstrated to be a promising means of welding delicate Mg plates, bars, or tubes with a thickness of up to 50 mm; gas tungsten arc welding is limited to lower-end thin Mg sheets. The grains in the fusion zone (FZ) are nearly equiaxed in shape and ∼8 μm or less in size, due to the rapid cooling rate. The as-welded FZ microhardness and tensile strength are higher than the base metals due to the smaller grain size. The weld efficiency, defined as the postweld microhardness or tensile strength at the mid-FZ region divided by that of the unwelded base metal, is around 110 to 125 pct for electron-beam welding and 97 to 110 pct for gas tungsten arc welding. There are three main texture components present in the electron-beam-welded (EBW) FZ, i.e., (with TD// ), (with ND_∧ ∼15 deg), and (with WD_∧ ∼30 deg), where TD, ND, and WD are the transverse, normal, and welding directions, respectively. The crystal growth tends to align toward the most closed-packed direction, . The texture in gas tungsten arc welded (GTAW) specimens is more diverse and complicated than the EBW counterparts, due to the limited and shallow FZ and the lower cooling rate. The cooling rates calculated by the three-dimensional (3-D) and two-dimensional (2-D) heat-transfer models are considered to be the lower and upper bounds. The cooling rate increases with decreasing Al content, increasing weld speed, and increasing distance from the weld top surface. The influences of the FZ location, welding speed, and alloy content on the resulting texture components are rationalized and discussed.     

The role of twinning in the cavitation erosion of cobalt single crystals

The deformation characteristics contributing to the superior cavitation erosion properties of HCP cobalt single crystals have been determined. Results indicate that its erosion response is highly orientation sensitive. A homogeneous distribution of and glide occurs in {0110} crystals, whereas slip in the (0001) crystals is much more heterogeneous and consists mainly of dislocations. Continued exposure to cavitation nucleates a large number of twins, predominantly on the and planes in the and (0001) crystals respectively. The former twins are finer and more needle-like than the latter. The crystals are also significantly more erosion resistant than the (0001) crystals. The twin density increases continuously with cavitation exposure until a dense network of twins spans the entire exposed area. This fine-scale twinning is considered responsible for the superior erosion resistance of the metal.     

A constitutive description for aluminum-0.1 pct magnesium alloy under hot working conditions

The constitutive behavior of an aluminum 0.1 wt pct Mg alloy deformed in the temperature range of 573 to 823 K at strain rates between 0.001 and 100 s⁻¹ is analyzed on the basis of the concept of the mechanical threshold stress (MTS), , taking into consideration the contributions from the different strengthening mechanisms that could be present in this alloy, , which include one component that arises from the interaction between dislocations and solute atoms, , and another contribution from the interaction between mobile and forest dislocations, . The evolution of is described in terms of a generalized form of an exponential-saturation equation, whereas the characterization of the ratio, s _i ( , T), between the flow stress at any strain rate and temperature, and the two components is carried out by means of the phenomenological model advanced by Kocks and co-workers. It is shown that the experimental values of the flow stress as well as the work-hardening rate can be accurately described following this approach and that the maximum difference between the experimental and calculated values of such a parameter is less than ±4 MPa. The analysis conducted from continuous stress-strain curves determined at constant temperature and strain rate indicates that the relaxation strain in the generalized form of the Sah et al. relationship displays a significant strain rate dependence. The inclusion of such a dependence into the analysis by means of a simple parametric relationship leads to an improvement in the accuracy of the prediction of the model.     

Twins as barriers to basal slip in hexagonal-close-packed metals

The boundary structure of , , , and twins in hexagonal-close-packed (hcp) metals and the interaction of crystal dislocations with the first two twin types have been studied previously using atomic-scale computer simulation. The interaction of crystal dislocations with and twin boundaries is described here and compared with the results for and twins. These four twins are found to create barriers to the motion of crystal dislocations gliding on the basal plane, and the strength of the barrier depends in a relatively complex manner on crystallographic parameters and details of the atomic structures of the interfaces. In some circumstances, crystal dislocations can be transmitted through the twin boundary, thereby creating twinning dislocations. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.     

Dislocation structures in zirconium and zircaloy-4 fatigued at different temperatures

The development of characteristic dislocation structures in pure zirconium and zircaloy-4 fatigued under pull-push strain control as the testing temperature and the cyclic strain range varied was examined using a thin-foil transmission electron microscopy (TEM) technique. The slip planes and the twinning planes were determined by a standard stereographic trace analysis technique. The first-order prismatic slip {10 0} is the primary deformation mode in zirconium and zircaloy-4 fatigued from room temperature (RT) to 873 K. The pyramidal sli { 2 } is activated at 673 K and at high cyclic strain ranges, whereas the basal slip {0001} only appears in those specimens fatigued at 873 K. The {10 2}, {11 1}, and {11 2} types of twins were detected in specimens fatigued at RT. Twinning becomes less frequent as the testing temperature increases. The schematic map of the cyclic deformation modes as a function of the plastic strain range and the test temperature is described. The dislocation configurations in fatigued pure zirconium specimens evolve from a planar arrangement to a cell structure as the test temperature and the strain range increase. For zircaloy-4, the fatigued dislocation structure is parallel dislocation lines at RT, cells at 673 K, and two sets of approximate mutually perpendicular dislocation bands at 873 K, respectively. Finally, the fatigued dislocation-structure evolution map with the cyclic strain range and the test temperature are qualitatively established for zirconium and zircaloy-4, respectively. The effect factors on the fatigue mechanism and the thermodynamic and dynamic criteria of the dislocation-pattern evolution are discussed.     

An electron-backscattered diffraction study of the texture evolution in a coarse-grained AZ31 magnesium alloy deformed in tension at elevated temperatures

Electron-backscattered diffraction (EBSD) has been used to investigate the texture evolution during tensile deformation at temperatures between 673 and 773 K of a coarse-grained commercial AZ31 magnesium alloy. A weak (0001) fiber texture was initially present in the hot-rolled magnesium alloy plate. The [0001] directions of the grains spread 0 to 45 deg around the normal direction (ND) of the magnesium alloy plate. This pre-existing weak texture evolved during tensile deformation into a strong texture close to the {0001} 〈1 00〉. The [0001] directions of the grains rotated toward the orientations perpendicular to the tension axis of the samples, indicating that the 〈11 0〉 slip system appeared to be the most active slip system, especially in the early stages of deformation. The EBSD Schmid-factor analysis revealed that, however, with an increase in strain and the rotation of the (0001) slip plane, the {11 2} 〈11 〉 slip system appeared to be more favorable. The {1 00} 〈11 0〉 and {1 01} 〈11 0〉 slip systems remained favored throughout the strains investigated, indicating that {1 00} and {1 01} are two important slip planes for cross slip using the 〈11 0〉 slip vector. It is found that the misorientation across one coarse grain (as high as 38.2 deg) is accommodated by low-angle grain boundaries (LAGBs). The formation of these LAGBs may be an intermediate stage of the coarse grain refinement that occurred during deformation. This article is based on a presentation made in the symposium entitled “Processing and Properties of Structural Materials,” which occurred during the Fall TMS meeting in Chicago, Illinois, November 9–12, 2003, under the auspices of the Structural Materials Committee.     

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.     

Anisotropic grain growth based on the atomic adsorption model in WC-25 pct Co alloy

The process of triangular prism formation and abnormal grain growth of WC was modeled using pseudo-Monte-Carlo simulation based on atomic adsorption and the coalescence mechanism. Grains of WC evolved into a triangular prism shape due to {10 0} and {1 10} planes of fast growth rate. Coalescence of {10 0} and {1 00} planes subsequent to the anisotropic evolution was the main reason for the abnormal grain growth. The probability of coalescence computed by the Monte-Carlo method agreed well with a theoretical prediction. Experimental evaluation of the computational model was made in sintered WC-25 wt pct Co alloy. The experimental alloy was made with WC powder of different particle size, 0.8 μm and −325 mesh, respectively, and with two different sintering conditions: solid-phase sintering and liquid-phase sintering. The sample made from the coarse powder (−325 mesh) showed the same morphological characteristics as those of the original milled state, whereas the sample made from the fine powder (0.8 μm) assumed a triangular prism shape quickly during solid-phase sintering. The anisotropic growth process of the latter sample could be explained by using the adsorption and coalescence model.     

Effect of solidification cooling rate on the morphology and number per unit volume of primary Mg<Subscript>2</Subscript>Si particles in a hypereutectic Al-Mg-Si alloy

Growth morphology and number per unit volume have been determined vs withdrawal velocity V over the range 0.1 to 4 mm/s for primary Mg₂Si in a Bridgman-solidified hypereutectic Al-Mg-Si alloy. Primary Mg₂Si shows a transition from irregular or regular polyhedral to dendritic with increasing V, and increases with solidification cooling rate according to the relationship . These results are compared with corresponding ones for Al-Mg-Si alloy wedge castings and for primary silicon in hypereutectic Al-Si alloys.     

Deformation twinning and texture variation in Zr-2.5 pct Nb alloy during rolling

Recently, it was proven that delayed hydride cracking (DHC) is accompanied by deformation twinning through the texture analysis of a fractured surface. Thus, in order to understand the operation of deformation twinning, the texture variations by rolling were investigated using Zr-2.5 pct Nb alloy with {11 0} 〈10 0〉 texture. It was observed that deformation twinning was operated predominantly in the range of a 5 to 15 pct strain. The basal poles were rotated in the normal direction of a rolling plane with the strain, and the (0002) texture was fully reversed after 15 pct strain. This finding was established to be due to the operation of the {10 2} and {11 1} twinning systems through the analysis of the inverse pole figure. It appeared that the degree and easiness of the twinning operation was affected by changing the direction of compression during rolling with respect to the initial {11 0} 〈10 0〉 texture. The contribution of deformation twinning to strain was quantitatively calculated using the change in the basal pole components. This article is based on a presentation made in the symposium entitled “Processing and Properties of Structural Materials,” which occurred during the Fall TMS meeting in Chicago, Illinois, November 9–12, 2003, under the auspices of the Structural Materials Committee.     

Microstructure and mechanical properties of a 2000 MPa Co-free maraging steel after aging at 753 K

The precipitation kinetics at the aging temperature of 753 K in a 2000 MPa grade Co-free maraging steel (Fe-18.9Ni-4.1Mo-1.9Ti, wt pct) has been studied. Under the peak-aged condition at 753 K, Ni₃Ti precipitates of moderate size were uniformly distributed in the martensite matrix, leading to optimal combination of strength (2000 MPa of yield strength) and fracture toughness (70 MPa ). The ultra-high strength of the maraging steel subjected to long time aging at 753 K is attributed to the high resistance to coarsening of precipitates. The orientation relationship between martensite and Ni₃Ti was observed as and . The Orowan mechanism is the dominant strengthening mechanism.