Dislocation Structures in a Cyclically Deformed Single-Slip-Oriented Fe-35?wt?pct Cr Alloy Single Crystal Containing Fine Cr-Rich Precipitates

The dislocation structures induced by the cyclic deformation of a $ [\bar{1}49] $ single-slip-oriented Fe-35?wt?pct Cr alloy single crystals containing fine Cr-rich precipitates have been studied by transmission electron microscopy (TEM) over the plastic strain amplitude ?? _pl range of 5?×?10^?4 to 5?×?10^?3. Persistent slip bands (PSBs) with different structures, such as ladder-like structure, irregular ladders, elongated cells, etc., were observed to form at plastic strain amplitudes ranging from 5.0?×?10^?4 to 2.5?×?10^?3, and the volume fraction of PSBs increases with increasing ?? _pl. As ?? _pl is as high as 5.0?×?10^?3, dislocation cells dominate the microstructure, even though a small amount of irregular PSB ladder structures still exists and they tend to evolve as labyrinth-like structures. The instability of Cr-rich precipitates during cyclic straining was believed to facilitate the formation of PSBs and thus promote some similarities of cyclic deformation characteristics between the current body-centered cubic (bcc) Fe-Cr single crystals and face-centered cubic (fcc) metal crystals. Whatever the internal structure of PSBs is, they could always carry the majority of the plastic strain in the course of cyclic deformation, thus causing the occurrence of a stress plateau region in the cyclic stress?Cstrain (CSS) curve of Fe-Cr alloy single crystals.     

Cyclic deformation of AISI-310 stainless steel—I. Cyclic stress-strain responses

Cyclic responses of AISI-310 austenitic stainless steel with coarse grains are investigated in the plastic strain amplitude (ε_pl) range 1 × 10⁻⁴–9 × 10⁻³. With respect to the cyclic hardening behaviour, three ranges of ε_pl may be distinguished. ε_pl < 8 × 10⁻⁴: neither cyclic hardening nor cyclic softening is observed; 8 × 10⁻⁴ < ε_pl < 6 × 10⁻³: a cyclic softening follows a cyclic hardening before the saturation is approached; ε_pl > 8 × 10⁻³: a cyclic hardening leads directly to saturation. The cyclic stress-strain (CSS) curve exhibits three regimes of strain dependence of saturation stress (σ_s): in regime I, σ_s increases almost unnoticeably with increasing ε_pl; in regime II, σ_s increases smoothly with increasing ε_pl; in regime III, σ_s increases dramatically with increasing ε_pl. The three regimes of the CSS curve fall into line with the different ranges of ε_pl of different cyclic hardening behaviours.     

Cyclic deformation of AISI-310 stainless steel—II. Saturation dislocation structures

Saturation dislocation configurations of AISI-310 stainless steel cyclically deformed at constant plastic strain amplitudes (ε_pl) in the range 1 × 10⁻⁴–9 × 10⁻³ are investigated with transmission electron microscopy (TEM). At low ε_pl range, dislocations are generated in many slip systems and distributed more or less uniformly. At intermediate ε_pl range, various dislocation structures, such as dislocation bundles, veins or {110} walls, form. Different structures may be distributed in separate layers with boundaries parallel to {111} at the up part of this ε_pl range. When ε_pl ⩾ 6 × 10⁻³, {100} labyrinth walls and misoriented dislocation cells form. The dislocation configurations found at low ε_pl cases are also developed in some areas of the specimens deformed at high ε_pl. The three ε_pl ranges of different characteristics of saturation dislocation configurations coincide well with the three regimes of different cyclic behaviours of this material reported in Part I [ Acta metall. mater.38, 2135 (1990)].     

Effect of Cyclic Predeformation on the Uniaxial Tensile Deformation Behavior of [017] Cu Single Crystals Oriented for Critical Double Slip

The effect of cyclic predeformation at different plastic strain amplitudes γ _pl on the uniaxial tensile behavior of the [017] critical double-slip-oriented copper single crystal was investigated. A cyclic predeformation at a low γ _pl of 7.0 × 10^?4 was found able to enhance the tensile strength of the [017] crystal at nearly no expense of the decrease in plasticity. However, as the γ _pl for the prefatigue increases to a higher value of 3.0 × 10^?3, the tensile strength of the prefatigued [017] crystal decreases to be slightly less than that of the unfatigued crystal, and simultaneously its tensile plasticity decreases markedly. Therefore, a cyclic predeformation at an appropriate plastic strain amplitude might bring about an obvious strengthening effect for metallic single crystals.     

Mechanical and microstructural aspects of the high temperature plastic deformation of yttria-stabilized zirconia polycrystals

The high-temperature plastic deformation of 6 mol% Y₂O₃-stabilized ZrO₂ polycrystals with grain sizes of 1.8, 3.4 and 6.3 μm has been studied in compression between 1350 and 1450 °C in air at constant strain rate (between 1 × 10⁻⁵ and 2 × 10⁻⁴s⁻¹) and under constant load (between 5 and 90 MPa). Two mechanical behaviours were observed depending on strain rate or stress levels: grain boundary sliding controlled by cation bulk diffusion, with an activation energy of 560 kJ/mol, and intergranular cavitation without plastic deformation of the grains.     

Cyclic deformation of ferritic steel—I. Stress-strain response and structure evolution

The complete CSS-curve has been established for a low alloyed structural steel tested under plastic strain control. The curve which can be divided into three separate regimes has a plateau region between cyclic strain levels of 10⁻⁴ and 8·10⁻⁴. PSBs are formed on the specimen surface when the plastic strain range corresponds to the plateau regime. The PSBs are sites for crack nucleation. The substructure evolution as seen going along the CSS-curve and with accumulating numbers of cycles is documented in detail and includes: dislocation loops, veins, walls including the ladder-like walls usually associated with PSBs, labyrinths, cells, subgrains, banded cells and subgrains. At low and intermediate plastic strain ranges the surface grains contain a more fine-scaled substructure and develop features which appear in the interior at higher plastic strain ranges. At larger cyclic strain levels microbands and noncrystallographic deformation bands become dominating features. Heavily displaced and serrated grain boundaries are observed at intermediate and high plastic strains both in interior- and surface grains containing microbands, banded cells or banded subgrains.     

Plastic-Strain-Amplitude Dependence of Dislocation Structures in Cyclically Deformed 〈112〉-Oriented Cu-7 at. pct Al Alloy Single Crystals

Dislocation structures in \( [\overline{1} 12] \) Cu-7 at. pct Al alloy single crystals cyclically deformed at different plastic strain amplitudes were investigated by transmission electron microscope (TEM) and compared with the results of \( [\overline{1} 12] \) Cu single crystals. It is found that the plastic strain amplitude γ _pl has an obvious effect on the slip deformation mode, and consequently on the cyclic hardening behavior of \( [\overline{1} 12] \) Cu-7 at. pct Al alloy single crystals with an intermediate stacking fault energy. For instance, a high slip planarity (i.e., only formation of planar-slip bands) contributes to the occurrence of a gentle cyclic hardening with a much lower saturation stress at a low γ _pl of 4.5 × 10^?4. A mixed planar/wavy-slip mode (e.g., persistent Lüder’s bands/wall-like microstructures) at an intermediate γ _pl of 2.2 × 10^?3 causes an obvious cyclic hardening up to a comparable saturation stress to that for the \( [\overline{1} 12] \) Cu single crystal. In contrast, the deformation mode is dominated by wavy slip (e.g., ill-defined dislocation cells and walls) at the highest γ _pl of 7.2 × 10^?3, causing that its cyclic hardening curve is quite similar to that for the \( [\overline{1} 12] \) Cu single crystal; in this case, a slightly higher saturation stress level than that for the Cu single crystal is reached due to the additional solid solution strengthening.     

The effect of grain size on low-cycle fatigue behavior of Al-2024 polycrystalline alloy

Two different sets of fatigue specimens were heat treated at different times or temperatures to investigate the effect of grain size on the low-cycle fatigue behavior of Al-2024 polycrystalline alloy. Strain-controlled low-cycle fatigue testing with a strain rate of 1×10⁻⁴ s⁻¹ was conducted at room temperature. The fatigue response of the alloy was evaluated macroscopically in terms of cyclic stress strain response and microscopically in terms of appearance of cyclic slip bands. The cyclic stress strain response of Al-2024 polycrystalline alloy exhibited a definite plateau region where saturation stress remained constant with plastic strain. It was found that the smaller the grain size, the lower the saturation stress and the longer the plateau, whereas the larger the grain size, the higher the saturation stress and shorter the plateau (i.e., reverse grain size effect). Microscopic observations using scanning electron microscope revealed that persistent slip bands (PSBs) were observed at 45 deg orientations from the grain boundary. The volume fraction of PSBs was higher in small-grained Al-2024 polycrystalline alloy as compared to large-grained Al-2024 polycrystalline alloy. This article is based on a presentation given in the symposium “Dynamic Deformation: Constitutive Modeling, Grain Size, and Other Effects: In Honor of Prof. Ronald W. Armstrong.” March 2–6, 2003, at the 2003 TMS/ASM Annual Meeting, San Diego, California, under the auspices of the TMS/ASM Joint Mechanical Behavior of Materials Committee.     

The influence of strain amplitude on the transient current behaviour in corrosion fatigue of Fe-26Cr-1Mo

Corrosion fatigue damage and its electrochemical characteristics of high purity ferritic stainless steel (Fe-26Cr-1Mo) under symmetrical tension and compression strain control have been investigated in 1M H₂SO₄ and 3.5 % NaCl solution by using three-electrode technique at imposed passive potential. The tests were carried out at the total strain amplitudes of 4 × 10^?3, 8 × 10^?3, 1.0 × 10^?2 and 1.2 × 10^?2. The effects of the strain amplitude on the electrochemical dissolution and plastic deformation on the surface were studied particularly. At low strain amplitude (Δε_t = 4 × 10^?3), the passivity-maintaining current was small and stable because of the small slip activity. The maximum current at the tensile half cycle, l_t^p, was always bigger than l_c^p, or sometimes equal to l_c^p, the maximum current at the compressive half cycle in 3.5 % NaCl. At high strain amplitude (Δε_t = 1.2 × 10^?2), the enhanced deformation on the surface induced the increasing dissolution. The increasing anodic current reflects the breakdown of the surface film, and only one current peak occurs within one cycle. At intermediate strain amplitude (Δε_t = 8 × 10^?3), the current behaviour was very stable after cyclic hardening in 3.5%, NaCl, but periodical jumpings of current during cycling were observed in the saturation region in 1 M H₂SO₄.     

A new strain rate change technique for distinguishing between pure metal and alloy type creep behavior

A new technique involving strain rate changes has been developed for distinguishing between pure metal and alloy type creep behavior. Both positive and negative strain rate changes were performed on aluminum and Al-5.8 at.% Mg (pure metal and alloy type material respectively) using an Instron electromechanical testing machine. Tests were conducted at 573 K with initial total strain rates of either 4 × 10⁻⁵ or4 × 10⁻⁴s⁻¹. Immediately following an order of magnitude change in total strain rate, the plastic strain rate was monitored as a function of stress. The observed transient response for both pure aluminum and Al-5.8 Mg was found to agree with predicted behavior, indicating that the strain rate change test can be used to distinguish between pure metal and alloy type creep behavior. The strain rate change test was also found to be a promising single specimen technique for studying constant structure deformation. The quality of the constant structure data obtained using this technique is shown to depend on the accuracy with which plastic strain rate can be determined. A procedure is described for determining the plastic strain rate with sufficient accuracy to allow the strain rate change test to be used in place of multiple stress reduction tests to study constant structure deformation.     

High-temperature deformation of MnO

Steady-state flow stresses have been measured for {100}-oriented MnO single crystals for temperatures from 900 to 1400°C and strain rates from 2 × 10⁻⁶to 2 × 10⁻³s⁻¹. The crystals were equilibrated in oxygen partial pressures ranging from 10¹¹ to 10²Pa, which induced deviations from stoichiometry of 4 × 10⁻⁴to 9 × 10⁻². Deformation rates are controlled by oxygen diffusion. Pipe diffusion along dislocation cores is predominant for T ≤ 1000°C; volume diffusion is predominant for T ≥ 1200dgC. In general, the primary diffusing oxygen species are singly charged vacancies for low Po₂ and neutral interstitials for high Po₂. At 1400°C, ionization states decrease for the oxygen point defects.     

A model for the unloading in the saturation fatigue stage of pure polycrystalline aluminium

Stiffness measurements are frequently used for determining the fatigue damage of materials. Other mechanisms, such as interactions between dislocations and structural defects, can also affect the stiffness value. The aim of this study is to evaluate their contribution to the stiffness change. This study is concerned with pure polycrystalline aluminium cycled in push-pull mode at room temperature with a plastic strain amplitude between 10⁻⁴ and 1.5 × 10⁻³. Stiffness is determined all along the unloading curve. Effects of both temperature and strain rate on stiffness evolutions are investigated. A model for the unloading phenomena is derived from the theory of the thermally activated deformation. A contribution due to changes in the curvature of dislocations is also included in the model. Moreover a long range internal stress profile is introduced by taking into account the cellular nature of dislocations in the cycled aluminium. The analysis of experimental results from the present approach shows that the proposed model is very convenient to explain the main phenomena involved in the unloading.     

Effect of Deoxidation Process on Inclusion and Fatigue Performance of Spring Steel for Automobile Suspension

55SiCrA spring steel was smelted in a vacuum induction levitation furnace. The liquid steel was treated by Si deoxidation, Al modification with Ca treatment and Al modification, and the steel samples were obtained with deformable Al₂O₃-SiO₂-CaO-MgO inclusions closely contacted with steel matrix, Al₂O₃-CaO-CaS-SiO₂-MgO inclusions surrounded by small voids or Al₂O₃(> 80 pct)-SiO₂-CaO-MgO inclusions surrounded by big voids, respectively. Effect of three types of inclusions on steel fatigue cracks was studied. The perpendicular and transverse fatigue cracks around the three types of inclusions leading to fracture were found to vary in behavior. Under the applied stress amplitude of 775 MPa, the fatigue lives of the three spring steels decreased from 4.0 × 10⁷ to 3.8 × 10⁷, and to 3.1 × 10⁷ cycles. For the applied stress amplitude of 750 MPa, the fatigue lives of the three spring steels decreased from 5.2 × 10⁷ to 4.1 × 10⁷, and to 3.4 × 10⁷ cycles. Based on the voids around inclusions, the equivalent size of initial fatigue crack has been newly defined as \( \sqrt {\frac{{{\text{area}}_{\text{inclusion}} }}{{(1 - {\text{CC}})}}} \), where the contraction coefficient CC of inclusion was introduced. A reliable forecast model of the critical size of inclusion leading to fracture was established by the incorporation of actual width b_inclusion or diameter d_inclusion of internal inclusion; the model prediction was found to be in agreement with experimental results.     

Enhancing the Damping Behavior of Dilute Zn-0.3Al Alloy by Equal Channel Angular Pressing

The effect of grain size on the damping capacity of a dilute Zn-0.3Al alloy was investigated. It was found that there was a critical strain value (≈1 × 10⁻⁴) below and above which damping of Zn-0.3Al showed dynamic and static/dynamic hysteresis behavior, respectively. In the dynamic hysteresis region, damping resulted from viscous sliding of phase/grain boundaries, and decreasing grain size increased the damping capacity. While the quenched sample with 100 to 250 µm grain size showed very limited damping capacity with a loss factor tanδ of less than 0.007, decreasing grain size down to 2 µm by equal channel angular pressing (ECAP) increased tanδ to 0.100 in this region. Dynamic recrystallization due to microplasticity at the sample surface was proposed as the damping mechanism for the first time in the region where the alloy showed the combined aspects of dynamic and static hysteresis damping. In this region, tanδ increased with increasing strain amplitude, and ECAPed sample showed a tanδ value of 0.256 at a strain amplitude of 2 × 10⁻³, the highest recorded so far in the damping capacity-related studies on ZA alloys.

     

Low-cycle fatigue and cyclic stress-strain behavior of incoloy 800

Strain-controlled low-cycle fatigue tests of solution-annealed Incoloy 800 were performed at temperatures of 538°, 649°, 704°, and 760°C using axial strain rates of 4 × 10^-3 and 4 × 10^-4 sec^-1. A few hold-time tests were also performed to indicate a noticeable reduction in fatigue life at hold times of 10 and 60 min. A comparison of these fatigue data with similar results for AISI 304 stainless steel indicates essentially identical behavior. An extensive study is made of the cyclic stress-strain behavior of Incoloy 800 and the relationship between the cyclic strain-hardening exponent and fatigue behavior is confirmed. Exponents onN _f in the elastic and plastic strain range terms of the total strain range equation are identified and compared with those used in the Universal Slopes equation.     

Effects of nitrogen implantation on low cycle fatigue behavior of ferritic Fe-24Cr-4Al stainless alloy

The effects of nitrogen implantation on cyclic deformation response, near-surface dislocation sub-structure, surface slip band formation, crack initiation, and fatigue life under low cycle fatigue of the ferritic Fe-24Cr-4Al stainless alloy were investigated. Implantation was carried out at an energy of 65 keV and at a fluence of 2 × 10¹⁷ ions/cm². Nitrogen implantation resulted in a substantial cyclic hardening in the alloy. Homogeneous planar dislocation arrangements were formed in the near-surface region of implanted specimens after fatigue, while dislocation loop debris and patches were developed in the un-implanted specimens. Moreover, formation of persistent slip bands (PSBs) was greatly suppressed in the surface of the implanted specimens. Nitrogen implantation also resulted in an alteration of the crack initiation mode from the grain boundary to the surface penetration of the PSBs nucleated below the surface layer. Fatigue life improvements after nitrogen implantation could only be obtained when the PSBs were not only suppressed but also homogenized in the implanted surface layer.     

Influence of cyclic deformation on surface microstructure and hardness of ion-implanted nickel

The effects of cyclic deformation on near-surface dislocation structure and hardness of ion-implanted nickel were characterized and correlated with the evolution of fatigue damage. Polycrystalline nickel fatigue specimens were implanted at 220 °C with 350 keV and 3 MeV nickel ions to a fluence of 1 × 10¹⁶ ions/cm² or at 220 °C and 500 °C with 350 keV aluminum ions to a fluence of 5 × 10¹⁷ ions/cm². Both the self-implantations and aluminum implantations approximately doubled the hardness of the near-surface region. During cyclic deformation, the near-surface regions of self-implanted specimens cyclically softened and formed clear channels through which subsurface persistent slip bands (PSBs) penetrated the implanted region. The near-surface regions of aluminum-implanted specimens maintained a high hardness during cyclic deformation and effectively suppressed the evolution of fatigue damage and extended fatigue life. The results of the study indicate that the cyclic stability of implantation-induced surface hardening is a key factor in the ability of an ion implantation treatment to suppress the evolution of fatigue damage. Formerly with The University of Michigan-Ann Arbor. Formerly with The University of Michigan-Ann Arbor.     

Fatigue Crack Growth Analysis Under Spectrum Loading in Various Environmental Conditions

The fatigue process consists, from the engineering point of view, of three stages: crack initiation, fatigue crack growth, and the final failure. It is also known that the fatigue process near notches and cracks is governed by local strains and stresses in the regions of maximum stress and strain concentrations. Therefore, the fatigue crack growth can be considered as a process of successive crack increments, and the fatigue crack initiation and subsequent growth can be modeled as one repetitive process. The assumptions mentioned above were used to derive a fatigue crack growth model based, called later as the UniGrow model, on the analysis of cyclic elastic–plastic stresses–strains near the crack tip. The fatigue crack growth rate was determined by simulating the cyclic stress–strain response in the material volume adjacent to the crack tip and calculating the accumulated fatigue damage in a manner similar to fatigue analysis of stationary notches. The fatigue crack growth driving force was derived on the basis of the stress and strain history at the crack tip and the Smith–Watson–Topper (SWT) fatigue damage parameter, D = σ_maxΔε/2. It was subsequently found that the fatigue crack growth was controlled by a two-parameter driving force in the form of a weighted product of the stress intensity range and the maximum stress intensity factor, ΔK ^p K _max ^1?p . The effect of the internal (residual) stress induced by the reversed cyclic plasticity has been accounted for and therefore the two-parameter driving force made it possible to predict the effect of the mean stress including the influence of the applied compressive stress, tensile overloads, and variable amplitude spectrum loading. It allows estimating the fatigue life under variable amplitude loading without using crack closure concepts. Several experimental fatigue crack growth datasets obtained for the Al 7075 aluminum alloy were used for the verification of the proposed unified fatigue crack growth model. The method can be also used to predict fatigue crack growth under constant amplitude and spectrum loading in various environmental conditions such as vacuum, air, and corrosive environment providing that appropriate limited constant amplitude fatigue crack growth data obtained in the same environment are available. The proposed methodology is equally suitable for fatigue analysis of smooth, notched, and cracked components.     

Diffusion measurements in the Fe82B18 amorphous alloy by rutherford backscattering spectrometry

Diffusion of Au, and Pb in the as-quenched metallic glass Fe₈₂B₁₈ was studied in the temperature range 575–645 K by employing the technique of Rutherford Backscattering Spectrometry (RBS). The diffusion coefficients (D) were obtained by fitting an error function type of solution of the diffusion equation to the tail region of the concentration versus depth profiles. In the case of Pb, where such a solution was not applicable, the equation was solved numerically. The D values were found to lie in the range ~ 1 × 10⁻²¹−4 × 10⁻²⁰ m² s⁻¹. Diffusion measurement of Au were also carried out on relaxed and pre-crystallized specimens of the alloy. The results indicated an insignificant effect of the relaxation treatment on diffusivity values, while significantly higher diffusion rates were observed in the case of pre-crystallized specimens. The temperature dependence of D could be fitted to an Arrhenius relation yielding the values of the frequency factor, D₀, and the activation energy, Q.     

Superplastic-like behavior in as-extruded AlZnMg alloy matrix composites reinforced with Si3N4 whiskers

For as-extruded AlZnMg alloy without reinforcement of Si₃N_4ω, a maximum elongation is 140%, which was measured near a relatively high strain rate 2 × 10⁻²s⁻¹ at 818 K. On the other hand, a maximum elongation of 160% for α-Si₃N_4ω/AlZnMg composites, whose grain size was less than 4 μm, was obtained at 798 K and at the highest end in strain rate range of 8 × 10⁻¹s⁻¹ evaluated in this work, and for β-Si₃N_4ω/AlZnMg composites with grain size of about 4 μm, a maximum elongation of 230% was obtained at 818 K and a high strain rate of 2 × 10⁻¹s⁻¹. In these AlZnMg system alloys the composites reinforced with α-Si₃N_4ω exhibited a high superplastic elongation at higher strain rates than the composites reinforced with β-Si₃N_4ω.