Tensile behavior of a free-standing Pt-aluminide (PtAl) bond coat

The tensile behavior of a high activity stand-alone Pt-aluminide (PtAl) bond coat was evaluated by the micro-tensile test method at various temperatures (room temperature to 1100 degrees C) and strain rates (10(-5) s(-1)-10(-1) s(-1).) At all strain rates, the stress strain behavior of the stand-alone coating was significantly affected by the variation in temperature. The stress strain response was linear, indicating brittle behavior, at temperatures below the brittle ductile transition temperature (BDTT). The coating exhibited appreciable ductility (up to 2%) above the BDTT. The strength (both yield stress and ultimate tensile strength) of the coating decreased and its ductility increased with increasing temperature above the BDTT. The tensile behavior of the coating was sensitive to strain rate in the ductile regime, with its strength increasing with increasing strain rate at any given temperature. The BDTT of the coating was found to increase with increasing with increasing strain rate. The coating exhibited two distinct mechanisms of deformation above the BDTT. The transition temperature for the change of deformation mechanism also increased with increasing strain rate. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Effect of shotpeening on sliding wear and tensile behavior of titanium implant alloys

Titanium has good biocompatibility and so its alloys are used as implant materials, but they suffer from having poor wear resistance. This research aims to improve the wear resistance and the tensile strength of titanium alloys potentially for implant applications. Titanium alloys Ti–6Al–4V and Ti–6Al–7Nb were subjected to shotpeening process to study the wear and tensile behavior. An improvement in the wear resistance has been achieved due to surface hardening of these alloys by the process of shotpeening. Surface microhardness of shotpeened Ti–6Al–4V and Ti–6Al–7Nb alloys has increased by 113 and 58 HV(0.5), respectively. After shotpeening, ultimate tensile strength of Ti–6Al–4V increased from 1000 MPa to 1150 MPa, higher than improvement obtained for heat treated titanium specimens. The results confirm that shotpeening pre-treatment improved tensile and sliding wear behavior of Ti–6Al–4V and Ti–6Al–7Nb alloys. In addition, shotpeening increased surface roughness.

Mechanical behavior of irregular fibers part I : modeling the tensile behavior of linear elastic fibers

Fiber irregularities are inherent to textile fibers, natural fibers in particular. This series of papers examines the impact of fiber irregularity on the mechanical behavior of textile fibers. In the first part, the effect of fiber dimensional irregularities on the tensile behavior of linear elastic fibers is examined, using the finite element method (FEM). Fiber dimensional irregularities are simulated with sine waves of different magnitude and frequency. The results indicate that increasing the level or magnitude of irregularity will decrease the breaking load, breaking elongation and method Young’s modulus of the fiber, while increasing the frequency of irregularity will decrease the breaking load and method Young’s modulus, but the breaking elongation will increase. Fiber dimensional irregularity and the gauge length effect are also simulated in this study.

Mechanical behavior of irregular fibers part II : non-linear tensile behavior

To further our study of the linear tensile behavior of irregular fibers, in this paper we examine the nonlinear tensile behavior of irregular fibers. As before, we simulate the fiber dimensional irregularities with sine waves of different magnitude and frequency, and report results on the tensile behavior and gauge length effect of the simulated fibers.

Weibull analysis of the tensile behavior of fibers with geometrical irregularities

This paper further develops the conventional Weibull/weakest-link model by incorporating the within-fiber diameter variation. This is necessary for fibers with considerable geometrical irregularities, such as the wool and other animal fibers. The strength of wool fibers has been verified to follow this modified Weibull/weakest-link distribution. In addition, the modified Weibull model can predict the gauge length effect more accurately than the conventional model.

Modeling the tensile behavior of fibers with geometrical and structural irregularities

Virtually all fibers exhibit some dimensional and structural irregularities. These include the conventional textile fibers, the high-performance brittle fibers and even the newly developed nano-fibers. In recent years, we have systematically examined the effect of fiber dimensional irregularities on the mechanical behavior of the irregular fibers. This paper extends our research to include the combined effect of dimensional and structural irregularities, using the finite element method (FEM). The dimensional irregularities are represented by sine waves with a 30 % magnitude of diameter variation while the structural irregularities are represented by longitudinal and horizontal cavities distributed within the fiber structure. The results indicate that fiber geometrical or dimensional variations have a marked influence on the tensile properties of the fiber. It affects not only the values of the breaking load and extension, but also the shape of the load-extension curves. The fiber structural irregularities simulated in this study appear to have little effect on the shape of the load-extension curves.

Numerical modeling of the dynamic tensile behavior of irregular fibers

Most fibers are irregular, and they are often subjected to rapid straining during mechanical processing and end-use applications. In this paper, the effect of fiber dimensional irregularities on the dynamic tensile behavior of irregular fibers is examined, using the finite element method (FEM). Fiber dimensional irregularities are simulated with sine waves of different magnitude (10%, 30% and 50% level of diameter variation). The tensile behavior of irregular fibers is examined at different strain rates (333%/sec, 3,333%/sec and 30,000%/sec). The breaking load and breaking extension of irregular fibers at different strain rates are then calculated from the finite element model. The results indicate that strain rate has a significant effect on the dynamic tensile behavior of an irregular fiber, and that the position of the thinnest segment along the fiber affects the simulation results markedly. Under dynamic conditions, an irregular fiber does not necessarily break at the thinnest segment, which is different from the quasi-static results.

Modeling the tensile behavior of fiber bundles with irregular constituent fibers

In this paper, the effect of fiber dimensional irregularities on the tensile behavior of fiber bundles is modeled, using the finite element method (FEM). Fiber dimensional irregularities are simulated with sine waves of different magnitude. The specific stress-strain curves of fiber bundles and the constituent single fibers are obtained and compared. The results indicate that fiber diameter irregularity along fiber length has a significant effect on the tensile behavior of the fiber bundle. For a bundle of uniform fibers of different diameters, all constituent fibers will break simultaneously regardless of the fiber diameter. Similarly, if fibers within a bundle have the same pattern and level of diameter irregularity along fiber length, the fibers will break at the same time also regardless of the difference in average diameter of each fiber. In these cases, the specific stress and strain curve for the bundle overlaps with that of the constituent fibers. When the fiber bundle consists of single fibers with different levels of diameter irregularity, the specific stress-strain and load-elongation curves of the fiber bundle exhibit a stepped or “ladder” shape. The fiber with the highest irregularity breaks first, even when the thinnest section of the fiber is still coarser than the diameter of a very thin but uniform fiber in the bundle. This study suggests that fiber diameter irregularity along fiber length is a more important factor than the fiber diameter itself in determining the tensile behavior of a fiber bundle consisting of irregular fibers.

Laser Shock Processing influence on local properties and overall tensile behavior of friction stir welded joints

Based on laser beam intensities above 109 W/cm2 with pulse energy of several Joules and duration of nanoseconds, Laser Shock Processing (LSP) is capable of inducing a surface compressive residual stress field. The paper presents experimental results showing the ability of LSP to improve the mechanical strength and cracking resistance of AA2024-T351 friction stir welded (FSW) joints. After introducing the FSW and LSP procedures, the results of microstructural analysis and micro-hardness are discussed. Video Image Correlation was used to measure the displacement and strain fields produced during tensile testing of flat specimens; the local and overall tensile behavior of native FSW joints vs. LSP treated were analyzed. Further, results of slow strain rate tensile testing of the FSW joints, native and LSP treated, performed in 3.5% NaCl solution are presented. The ability of LSP to improve the structural behavior of the FSW joints is underscored.

Oxidation and tensile behavior of ferritic/martensitic steels after exposure to lead-bismuth eutectic

Two ferritic/martensitic steels, T91 steel and newly developed SIMP steel, were subject to tensile test after being oxidized in the liquid lead-bismuth eutectic (LBE) at 873 K for 500 h, 1000 h and 2000 h. Tensile tests were also carried out on the steels only thermally aged at 873 K. The result shows that thermal aging has no effect. Exposure to LBE at 873 K leads to a slight decrease in strength, but a large decrease in elongation when tested at 873 K. When tested at 873 K after 2000 h exposure, the tensile strength of T91 decreases slightly, and elongation from 39% to 21%. For SIMP, the decreases are slightly and from 44% to 28%, for tensile strength and elongation, respectively. The room temperature strength has slightly larger percentage reductions after the LBE exposure, but the elongation changes little.

In situ synchrotron X-ray diffraction studies of the effect of microstructure on tensile behavior and retained austenite stability of thermo-mechanically processed transformation induced plasticity steel

Transmission electron microscopy and in situ synchrotron high-energy X-ray diffraction were used to investigate the martensitic transformation and lattice strains under uniaxial tensile loading of Fe-Mn-Si-C-Nb-Mo-Al Transformation Induced Plasticity (TRIP) steel subjected to different thermo-mechanical processing schedules. In contrast with most of the diffraction analysis of TRIP steels reported previously, the diffraction peaks from the martensite phase were separated from the peaks of the ferrite-bainite α-matrix. The volume fraction of retained γ-austenite, as well as the lattice strain, were determined from the diffraction patterns recorded during tensile deformation. Although significant austenite to martensite transformation starts around the macroscopic yield stress, some austenite grains had already experienced martensitic transformation. Hooke's Law was used to calculate the phase stress of each phase from their lattice strain. The ferrite-bainite α-matrix was observed to yield earlier than austenite and martensite. The discrepancy between integrated phase stresses and experimental macroscopic stress is about 300 MPa. A small increase in carbon concentration in retained austenite at the early stage of deformation was detected, but with further straining a continuous slight decrease in carbon content occurred, indicating that mechanical stability factors, such as grain size, morphology and orientation of the retained austenite, played an important role during the retained austenite to martensite transformation.

Influence of martensite content and morphology on the toughness and fatigue behavior of high-martensite dual-phase steels

A series of high-martensite dual-phase (HMDP) steels exhibiting a 0.3 to 0.8 volume fraction of martensite (V m ), produced by intermediate quenching (IQ) of a vanadium and boron-containing microalloyed steel, have been studied for toughness and fatigue behavior to supplement the contents of a recent report by the present authors on the unusual tensile behavior of these steels. The studies included assessment of the quasi-static and dynamic fracture toughness and fatigue-crack growth (FCG) behavior of the developed steels. The experimental results show that the quasi-static fracturetoughness (K ICV ) increases with increasing V m in the range between V m =0.3 and 0.6 and then decreases, whereas the dynamic fracture-toughness parameters (K ID , K D , and J ID ) exhibit a significant increase in their magnitudes for steels containing 0.45 to 0.60 V m before achieving a saturation plateau. Both the quasi-static and dynamic fracture-toughness values exhibit the best range of toughnesses for specimens containing approximately equal amounts of precipitate-free ferrite and martensite in a refined microstructural state. The magnitudes of the fatigue threshold in HMDP steels, for V m between 0.55 and 0.60, appear to be superior to those of structural steels of a similar strength level. The Paris-law exponents (m) for the developed HMDP steels increase with increasing V m , with an attendant decrease in the pre-exponential factor (C).

Micromechanisms of fracture and strengthening in free-standing Pt-aluminide bond coats under tensile loading

Free-standing Pt-aluminide (PtAl) bond coats exhibit a linear stress strain response under tensile loading and undergo brittle cleavage fracture at temperatures below the brittle-to-ductile transition temperature (BDTT). Above the BDTT, these coatings show yielding and fail in a ductile manner. In this paper, the various micromechanisms affecting the tensile fracture stress (FS) below the BDTT and yield strength (YS) above the BDTT in a PtAl bond coat have been ascertained and quantified at various temperatures. The micromechanisms have been identified by carrying out microtensile testing of stand-alone PtAl coating specimens containing different levels of Pt at temperatures between room temperature and 1100 degrees C and correlation of the corresponding fracture mechanisms with the deformation substructure in the coating. An important aspect of the influence of Pt on the tensile behavior, slip characteristics, FS/YS and BDTT in the PtAl coating has also been examined. The addition of Pt enhances the FS of the coating by Pt solid solution strengthening and imparts a concomitant increase in fracture toughness and yet causes a significant increase in the BDTT of the coating. Published by Elsevier Ltd. on behalf of Acta Materialia Inc.