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 influence of nitrogen and orientation on the rolling deformation mechanisms of austenitic single crystals

The effect of nitrogen content and initial orientation on the deformation mechanisms of austenitic single crystals was investigated. From a nitrogen alloyed and a nitrogen free single crystal two initial orientations were cut out and cold rolled up to 90% thickness reduction, the corresponding {111} pole figures were measured and the slip traces were analyzed from the longitudinal section. In one of the nitrogen alloyed austenitic single crystals massive microcrack formation was observed. This observation was discussed in terms of the planarity of slip caused by the interstitially solved alloying element nitrogen. Nitrogen has a strong affinity to dislocations and thus alters the slip character from homogeneous to planar and the slip distribution from single slip to coarse slip. The present investigation proves that the coarse planar slip caused by the alloying element nitrogen in dependency of the initial orientation is responsible for the observed crack formation during rolling. The planarity of slip is also reflected in the measured pole figures. The {111} poles of the nitrogen alloyed single crystals show less scattering and higher maximum intensities than the poles of the nitrogen free single crystals. The differences were attributed to the influence of the nitrogen content on deformation mechanisms.     

Orientation dependence of dislocation structures in cyclically deformed Cu-16 At. Pct Al alloy single crystals

The dislocation structures induced by the cyclic deformation of Cu-16 at. pct Al alloy single crystals oriented, typically, as for single slip and [023] and for double slip, were studied by transmission electron microscopy (TEM) and compared with the results of Cu single crystals. Completely unlike the dislocation structures of Cu single crystals of corresponding orientations, the dislocation structures of these oriented Cu-16 at. pct Al alloy single crystals show a typical planar morphology. As the applied-plastic-stain amplitude increases, the dislocation configuration changes, on the whole, from multipolar arrays to dislocation tangles in the primary slip plane and from low-density planar slip bands to well-developed persistent Lüder’s bands (PLBs) in the planes normal to the primary slip plane, respectively. Secondary slips can be clearly observed to activate from very low plastic-strain amplitudes in all three Cu-16 at. pct Al single crystals investigated. Interestingly, the crystallographic orientation has almost no effect on the dislocation structure of Cu-16 at. pct Al single crystals.     

Dual slip in germanium under compression

Germanium single crystals, oriented for both single and dual slip, were deformed in compression at various strain rates and temperatures. The deformed crystals were studied by interference microscopy, optical microscopy, and electron transmission microscopy. On the basis of observations on slip lines and on dislocation structures a model is proposed to explain the observed stress-strain behavior.     

表面机械研磨工业纯钛表面纳米化研究

表面机械研磨处理可以使工业纯钛形成纳米表面层, 通过扫描电镜、透射电镜和高分辨电镜观察SMAT处理后的工业纯钛表层组织, 并研究了工业纯钛表面纳米化机制. 工业纯钛表面纳米化机制为: 孪晶的形成和孪晶的交割使得原始晶粒尺寸减小, 同时使晶格取向发生改变, 有利于位错滑移; 孪晶通过自身交割, 以及位错密度增加及其相互作用, 形成了细小的孪晶与胞状组织; 胞状组织转变为多边形亚晶; 亚晶不断吸收位错形成大角度晶界, 亚晶以及取向不同的细小孪晶逐渐转变为随机取向的纳米晶.     

A crystallographic study of fatigue damage in titanium

Large grain specimens with average grain size of 0.0127 m made from commercial purity titanium were subjected to a torsional cyclic strain at two different amplitudes: ±0.008 and =0.003. Fatigue damage was studied by scanning electron microscopy and crystal orientations were determined by X-ray diffraction and surface trace analysis. It was found that cyclic strain amplitude influenced the deformation mode and the nature of the macroscopic crack propagation. At high strain amplitudes the normal slip processes were observed and microcracking was observed on the (0001), and {1100} slip planes. The macroscopic crack propagation was dominated by the Stage I shear mode; however, some Stage II tensile mode propagation was observed after extensive Stage I propagation. At low strain amplitude twin plane cracking was observed on the {1011}, {1010}, and {1123} planes in addition to normal slip plane cracking, and the macroscopic crack propagation was dominated by the Stage II tensile mode. However, microscopic examination showed the macroscopic tensile mode cracks to be composed of microscopic shear mode cracks along slip planes and twin planes. At both low and high strain amplitudes cracking was observed on the {1120} plane which is neither a slip or twin plane in titanium. It is proposed that this cracking mode was a result of a dislocation reaction forming sessile dislocations on the {1120} plane.     

Some effects of high rates of strain on the deformation of aluminum single crystals

Specimens of single crystals of commercially pure (99.5 pct) aluminum were prepared in various orientations. The specimens were tested to destruction in an explosively operated impact mechanism. Overall and local strain distributions were measured together with lattice rotation. Some selected specimens were examined under a scanning electron microscope. It was found that while velocity effects are more pronounced in the vicinity of the fracture, the general slip characteristics were largely insensitive to the impact velocity. The lattice rotation, however, showed a considerable dependence on impact velocity. It is proposed here that the difference in the dynamic behavior of edge and screw dislocations in commercially pure aluminum may be the responsible mechanism for the reported anomaly in lattice rotation. Presently on leave at The Pennsylvania State University, University Park, Pa., U.S.A.     

Latent hardening in cyclic deformation of copper single crystals

In order to explore latent hardening produced by cyclic deformation, single crystals of copper oriented for single slip were first cycled at strain amplitudes corresponding to those of the plateau in the cyclic stress-strain curve, and subsequently sectioned for compression testing. The specimens were cycled enough to form persistent slip bands, and orientations in the secondary test were chosen to excite selectively coplanar or non-coplanar slip systems. Coplanar systems were found to be similar in hardening to the primary system, but non-coplanar systems showed a latent hardening ratio of about 1.25. If the plastic shear strain amplitude of the initial cycling was greater than 2×10⁻³ (the threshold for producing secondary slip), then the hardening rate in the secondary test was high. If the cyclic strain amplitude was less than 2×10⁻³, the latent yield stress was high because of the frictional effect of the loop patches, but the hardening rate was found to be low.     

The influence of orientation on the stress rupture properties of nickel-base superalloy single crystals

The influence of orientation on the stress rapture properties of MAR-M247 single crystals was studied. Stress rupture tests were performed at 724 MPa and 774 °C where the effect of anisotropy is prominent. The mechanical behavior of the single crystals was rationalized on the basis of the Schmid factors for the operative slip systems and the lattice rotations which the crystals underwent during deformation. The stress rupture lives at 774 °C were found to be greatly influenced by the lattice rotations required to produce intersecting slip, because second-stage creep does not begin until after the onset of intersecting slip. Crystals which required large rotations to become oriented for intersecting slip exhibited a large primary creep strain, a large effective stress level at the onset of steady-state creep, and consequently, a short stress rupture life. Those crystals having orientations within about 25° of the [001] exhibited significantly longer lives when their orientations were closer to the [001]-[011] boundary of the stereographic triangle than to the [001]-[1l 1] boundary, because they required smaller rotations to produce intersecting slip and the onset of second-stage creep. Thus, the direction off the [001], as well as the number of degrees off the [001], has a major influence on the stress rapture lives of single crystals in this temperature regime. REBECCA A. MacKAY, formerly Graduate Assistant, Case Western Reserve University, Cleveland, OH RALPH D. MAIER, formerly Assistant Professor, Case Western Reserve University     

Plastic flow of Ni3(Si,Ti) Single crystals

The flow stress of L1₂-type Ni₃(Si, Ti) by the compression test was measured as a function of temperature, strain rate, orientation and alloy composition. The critical resolved shear stress (CRSS) increased from 77 K, reached a peak, and then decreased rapidly with increasing temperature. The single crystals with orientations close to [011] and [1̄11] showed the operations of octahedral slips (111) [1̄01] below the peak temperature and of cube slips (001) [1̄10] above the peak temperature, whereas those with orientation close to [001] showed the operations of octahedral slips {111} over the entire temperatures. The CRSS depended on the orientation and alloy composition below the peak temperature but on the orientation and the strain rate above the peak temperature. The peak temperature was dependent on the orientation but almost independent of the strain rate and alloy composition. It is suggested that the deformation mechanisms for Ni₃(Si, Ti) single crystals are quite similar to those for other Ni-based L1₂ single crystals such as Ni₃Al, Ni₃Ga and Ni₃Ge; at low temperatures the dislocation movement of the superpartials dissociated on (111) plane with APB became sessile by micro cross slip to (001) plane. At high temperatures, two deformation modes were operative, depending on the orientation. The deformation of the single crystals with orientations far from [001] is due to the Peierls-Nabarro mechanism of (001) [1̄10] slip, whereas the deformation of the single crystals with orientations close to [001] is due to the intrusion of the “diffusive” process of (111) [1̄01] slip.     

Fracture of single crystals of the nickel-base superalloy PWA 1480E in helium at 22 °C

The fracture behavior of single crystals of the PWA 1480E nickel-base superalloy was studied using both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. Notched single crystals with seven different crystal growth orientations near [100], [110], [111], [013], [112], [123], and [223] were tensile tested at 22 °C in a helium atmosphere at 34 MPa. Gamma prime particles were orderly and closely aligned with the cube edges along the [100], [010], and [001] directions of theγ matrix. The cuboid morphology of theγ’ precipitate was not influenced by the crystal growth orientation. The specimen with the [110] orientation was the strongest, while the crystal with the [100] orientation was the weakest. A stereoscopic technique, combined with the use of planary’ morphologies, was applied to identify the cleavage plane orientation. All specimens failed predominately by {lll}-type cleavage which originated from combined slip on various {111} planes. In most cases, deformation was found to occur inhomogeneously in intense slip bands lying on {111} planes and aligned parallel to the different slip directions. Both SEM and TEM studies indicated that {lll}-type slip was the controlling factor during cleavage fracture of single crystals of the PWA 1480E nickel-base superalloy. Formerly Graduate Student, Auburn University     

Influence of Microtexture on Early Plastic Slip Activity in Ti-6Al-4V Polycrystals

Microtextured regions are known to influence the fatigue performance of titanium alloys. Previous studies revealed that crack initiation, accounting for most of the fatigue life, is triggered by slip activity. The influence of microtextured regions on the early plastic slip activity was presently investigated by means of an in situ tensile test performed inside a scanning electron microscope on a bimodal Ti-6Al-4V polycrystalline specimen. A slip trace analysis was carried out in several regions with different crystallographic textures to highlight potentially different deformation behaviors. Significant stress heterogeneities were revealed through an early slip activation in microtextured regions with a predominant [0001] orientation. This point was shown to be related to a locally increased resolved shear stress. Consequences on behavior under cyclic loadings are finally discussed.     

TWIP钢室温静态拉伸下的组织形态和变形机制

在室温下对退火Fe-24Mn-1Si-1.5Al-0.045CTWIP钢进行了不同程度的拉伸变形,采用JEM-2100透射电子显微镜对变形后的组织形貌进行表征和分析。研究结果表明:在变形初期,晶粒内存在着大量位错,它们相互缠结,呈胞状结构。在此阶段,位错滑移为主要变形机制。随着变形量的增加,形变孪晶在晶界等处形成,孪生机制被激活,孪生和滑移机制相互竞争。双孪生系统在大多数晶粒内先后被激活,孪生和滑移机制相互交割,起到动态细化晶粒的作用,使强度显著提高。在变形后期,试验钢的变形机制主要是TRIP效应,以及孪生与滑移的相互作用而诱发了去孪生机制,层状组织出现,孪晶特征减弱,从而导致样品的局部变形和失效。     

Non-octahedral deformation activity in cold rolled 70:30 brass and its influence on the development of brass texture

A structure of elongated, wavy bands develops at high strains in both fine grained and normal 70:30 brass. Recovery twins and evidence of non-octahedral slip activity are observed in the wavy bands after further deformation. The non-octahedral process generally operates on several sets of closely spaced (0.002–0.004 μm) planes, producing substructure comprised of intersecting trace lines. The occurrence of this process cannot be explained by slip on only a single system but combined activation of {110} and {112} planes accounts for most of the traces observed. Texture changes that can be attributed to the non-octahedral process are also evident and orientations around {110}〈112〉 develop in the regions where the activity has operated. The results are in good agreement with recent simulations of rolling textures based on non-octahedral slip in which a strong brass texture is produced bythe operation of {110} + {112} slip systems.     

Observations of Facet Formation in Near-<Emphasis Type="Italic">α</Emphasis> Titanium and Comments on the Role of Hydrogen

Faceted features are frequently observed on the fracture surfaces of titanium alloys that have failed by static loading, continuous cycling, dwell fatigue loading, and stress corrosion cracking (SCC). Although the facets formed under different loading conditions seem qualitatively similar, there are significant differences in the spatial and crystallographic orientations of the facets as well as subtle differences in facet surface topography. The current study compares and contrasts facets for various loading conditions (cyclic, creep, SCC, and dwell) in the Ti-8Al-1Mo-1V alloy with the primary motivation being to understand the mechanisms of crack initiation and faceted growth during dwell fatigue. The spatial and crystallographic orientations of the facets were determined using quantitative tilt fractography and electron backscatter diffraction, whereas facet topography was examined using ultra-high-resolution scanning electron microscopy. Collectively, the experimental observations suggest that hydrogen may play an important role in facet formation and accelerating small crack growth rates during dwell fatigue loading.     

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.     

Orientation and temperature dependence of slip in iron single crystals

The orientation and temperature dependence of slip in ZrH₂-purified iron crystals have been investigated at strains up to about 15 pct. Both {110} and {112} slip planes were observed at all temperatures investigated (295°, 250°, 195°, and 143°K) when the orientation of a crystal was such that its maximum resolved shear stress plane (MRSSP) corresponded to {110} or {112}. At 195° and 143°K {110} and {112} slip planes were also observed for orientations where the MRSSP deviated from {110} or {112} slip planes were also observed for orientations where the MRSSP deviated from {110} or {112}. At 295° and 250°K, the slip planes of crystals whose MRSSP was not {110} or {112} deviated from the MRSSP toward {110}. The variation in the intensity and waviness of the slip traces with orientation suggested that cross slip was easiest for orientations that slipped on a {112} plane in the twinning sense (near [001]), and hardest for orientations that slipped on a {112} plane in the antitwinning sense (near [011]). This appears to be in accord with observations of easier dislocation multiplication for orientations near [001] than for orientations near [011]. At 143°K the critical resolved shear stress law was not obeyed; the resolved shear stress was about 14 pct lower for slip on a {112} plane when the sense of the applied stress was favorable for twinning than when it was unfavorable. The slip line observations and the asymmetry of slip on a {112} plane appear to be qualitatively explainable in terms of dissociated dislocation models that are based on differences in the ease with which slip and cross slip can occur on {110} and {112}.     

Dislocation substructure in NiAl single crystals cyclically deformed at ambient temperature

Dislocation structures have been characterised in cyclically deformed NiAl single crystals tested at room temperature. The primary slip system <001>{110} was found to be in operation for the <112> ‘soft’ orientation. The dislocation structure was found to have a high density of dipoles and point defect clusters. Dislocation cross grids that accommodate the misfit between PSBs (persistent slip bands) and the matrix were observed. Possible cyclic hardening mechanisms at room temperature are discussed. The density of the dipoles was not uniform and was found to be modulated on the slip plane, unlike Ni₃Al which shows a homogeneous distribution. Computer simulation of the microstructure has been carried out, based on the mechanism of dipole diffusion in a stress gradient.     

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.     

Cyclic Deformation Behaviors of $$ [\bar{5}79] $$-Oriented Al Single Crystals

During the cyclic deformation of [[`5]79] [\bar{5}79] , Al single crystals with a high stacking fault energy produced slip bands that were characterized by wavy slip. Its cyclic stress response curve demonstrated that the specimen experienced hardening–softening–secondary hardening in sequence with repeated fluctuation stresses usually less than 10 MPa, which is far lower than those of Cu, Ni, and Ag single crystals. Finally, the whole surface of the Al single crystals was covered with intense intrusion and extrusion, and the cell structure is the most typical dislocation arrangement. These cells mainly comprise loose clusters of dislocations, which move more freely. In the center of the cell, the dislocation density is relatively low, and most dislocations concentrate in the cell wall. At room temperature, compared with cyclically deformed Cu, Ni, and Ag single crystals, the cyclic deformation behaviors of Al single crystals show significant differences, which are highlighted in this study.