Influence of Strain Rate and Temperature on Tensile Deformation and Fracture Behavior of Type 316L(N) Austenitic Stainless Steel

Tensile tests were performed at strain rates ranging from 3.16 × 10^?5 to 3.16 × 10^?3 s^?1 over the temperatures ranging from 300 K to 1123 K (27 °C to 850 °C) to examine the effects of temperature and strain rate on tensile deformation and fracture behavior of nitrogen-alloyed low carbon grade type 316L(N) austenitic stainless steel. The variations of flow stress/strength values, work hardening rate, and tensile ductility with respect to temperature exhibited distinct three temperature regimes. The steel exhibited distinct low- and high-temperature serrated flow regimes and anomalous variations in terms of plateaus/peaks in flow stress/strength values and work hardening rate, negative strain rate sensitivity, and ductility minima at intermediate temperatures. The fracture mode remained transgranular. At high temperatures, the dominance of dynamic recovery is reflected in the rapid decrease in flow stress/strength values, work hardening rate, and increase in ductility with the increasing temperature and the decreasing strain rate.     

Diffusion studies in the metallic glass Zr61 Ni39 by Auger electron spectroscopy

The diffusion coefficients (D) of Cu and Al in the metallic glass Zr₆₁Ni₃₉ were measured in the temperature range 551–621 K by using the Auger depth profiling technique. The D values for Cu were observed to be higher than the corresponding values for Al by about an order of magnitude in this temperature range. The faster diffusion of Cu was consistent with the observed dependence, in this amorphous alloy, of diffusivity on the atomic size of the diffusing species. The values of the activation energy for diffusion (Q), evaluated on the basis of the observed Arrhenius type temperature dependence of D, were found to be 1.33 ± 0.17 eV for Cu and 1.68 ± 0.13 eV for Al. The corresponding values of the pre-exponential factor (D₀) were 10^{−7.57 ± 1.46} m² s⁻¹ and 10^{−5.35 ± 1.15} m² s⁻¹ respectively. No significant effect of structural relaxation of the glass on diffusivity could be observed from measurements on specimens subjected to a relaxation heat treatment prior to diffusion annealing.     

A SIMS investigation of impurity diffusion in amorphous Fe40Ni40B20

The diffusion coefficients of Be and Si in the amorphous alloy Fe₄₀Ni₄₀B₂₀ were measured in the temperature range 575–643 K by using the technique of secondary ion mass spectrometry (SIMS). The measurements were carried out on the pre-relaxed alloy specimens and the diffusion coefficients D were found to lie in the range 1.0 × 10⁻²³−2.0 × 10⁻²⁰ m² s⁻¹. The temperature dependence of the measured diffusion coefficients was found to be Arrhenius in nature yielding the values of the activation energy Q as 2.16 ± 0.19 eV atom⁻¹ and 3.39 ± 0.30 eV atom⁻¹ for the diffusion of Be and Si respectively. The corresponding values of the pre-exponential factors (D₀) were 9.4×10^{(−4.0±1.57)}m²s⁻¹ and 7.0 × 10^{5.0 ± 2.42} m² s⁻¹. An overall comparison of all the available data on diffusion in amorphous Fe₄₀Ni₄₀B₂₀ clearly shows that the previously reported correlations for the diffusion coefficient D, the activation energy Q, and the pre-exponential factor D₀ for amorphous alloys are seen in this alloy also. It is suggested that the mechanism of diffusion possibly involves a cooperative movement of a group of atoms.     

Rate effects in metal-ceramic interface sliding from the periodic film cracking technique

The periodic film cracking technique [Agrawal and Raj, Acta metall.37, 1265 (1989)] was used to characterize sliding mechanisms at the copper-silica interface of silica films on a copper substrate at temperatures up to 550°C and at strain-rates ranging from 1.7 × 10⁻³ to 1.6 × 10⁻⁵s⁻¹. Two regimes of behavior were observed. The sliding was strongly rate sensitive in the high temperature/low strain-rate regime, with a power law stress component of n = 1.00 ± 0.04. At low temperatures and/or high strain-rates, the crack-spacing was strain-rate independent. These results, when analyzed in terms of the global mechanisms of deformation in crystalline materials, lead to the following interesting ideas: (a) the diffusional accomodation mechanism of sliding which assumes that diffusional transport can be applied to sliding at a wavy metal-ceramic interface shape, is consistent with the phenomenology of the strain-rate sensitive regime; (b) the non-linear power law creep mechanism (n ≈ 4.5) is not observed for deformation near the interface, presumably because the width of the deformation zone near the interface is smaller than the characteristic subgrain size required in power law creep; (c) in the low temperature regime, interface sliding occurs by dislocation slip in the metal at or near the interface. We extend the concept of “deformation mechanism map” first introduced for bulk crystalline materials by Ashby [Acta metall.20, 887 (1972)] to sliding at metal-ceramic interfaces.     

Dynamic Strain Aging of Nickel-Base Alloys 800H and 690

The objective of the current investigation is to characterize the dynamic strain aging (DSA) behavior in alloys 800H and 690. Constant extension rate tests were conducted at strain rates in the range of 10^?4 s^?1 to 10^?7?s^?1and temperatures between 295?K and 673?K (22?°C and 400?°C), in an argon atmosphere. Maps for the occurrence of serrated flow as a function of strain rate and temperature were built for both alloys. The enthalpy of serrated flow appearance of alloy 800H was found to be 1.07?±?0.30?eV.     

Creep and oxygen diffusion in magnetite

The creep rate of polycrystalline Fe₃O₄ has been measured as a fonction of stress and oxygen partial pressure in the temperature range 480–1100°C. A regime of power law creep is found at high stress, with a stress exponent of ≈- 3.1 and an activation energy of 264 kJ/mol. A regime of diffusional flow is found at lower stresses and is interpreted as Nabarro-Herring creep. The data for the two regimes are combined to deduce an oxygen diffusion coefficient of ≈-10⁻⁵ exp(−264 kJ/mol/RT) m²s⁻¹, with oxygen vacancies suggested as the mobile species.     

Low temperature volume diffusion of zinc in aluminium

The low temperature volume diffusion coefficients of zinc in aluminium have been measured over the temperature range from 180° to 235° C by determining the zinc concentraton profiles across grain boundaries more or less perpendicular to the surface of the extending across thick foils of aluminium which were chemically coatted with zinc and anneleed for times greater than that necessary to attain a steady state flow of zinc along the boundaries from one side of the foils to the other. The zinc concentrations were determined by energy dispersive X-ray analysis of thin foils in a transmission electron microscope and were fitted with the constant source of the diffusion equation in order to determine the diffusion coefficients. The diffusivities obtained agreed well with an extrapolation of the high temperature values of Hilliard et al. and yielded values of 0.018 cm²·s⁻¹ and 112 kJ·mol⁻¹ for the preexponential factor D₀ and the activation energy Q, respectively.     

Hydrogen Decrepitation and Recycling of NdFeB-type Sintered Magnets

With the rapid growth in the use of NdFeB-type magnets and with the growing environmental need to conserve both energy and raw materials, the recycling of these magnets is becoming an ever important issue. In this paper it is demonstrated that hydrogen could play a vital role in this process. Fully dense, sintered NdFeB-type magnets have been subjected to the hydrogen decrepitation (HD) process. The resultant powder has been subsequently processed in one of two ways in order to produce permanent magnets. Firstly, the powder was subjected to a vacuum degassing treatment over a range of temperatures up to 1000°C in order to produce powder that would be suitable for the production of anisotropic bonded or hot pressed magnets. Secondly, the HD-powder has been used to produce fully dense sintered magnets; in which case optimisation of the milling time, sintering temperature and time was carried out. The optimum degassing temperature for coercive powder was found to be 700°C, giving powder with a remanence (B_r) of ∼1350mT (±10 mT) and an intrinsic coercivity (H_cj) of ∼750kAm⁻¹ (±10 kAm⁻¹). The best sintered magnet was produced by very lightly milling the powder (30 min, roller ball mill), aligning, pressing and vacuum sintering at 1080°C for 1 hour. The magnetic properties of this magnet were: (BH)_max = 290 kJm⁻³ (±5 kJm⁻³), Sr = 1240mT (±5 mT) and H_cj = 830 kAm⁻¹ (±5 kAm⁻¹); representing decreases of 15%, 10%, and 20% respectively, from the properties of the initial magnet.     

Influence of Temperature and Strain Rate on Tensile Deformation and Fracture Behavior of P92 Ferritic Steel

Tensile tests were performed at strain rates ranging from 3.16 × 10^?5 to 1.26 × 10^?3 s^?1 over a temperature range of 300 K to 923 K (27 °C to 650 °C) to examine the effects of temperature and strain rate on tensile deformation and fracture behavior of P92 ferritic steel. The variations of flow stress/strength values, work hardening rate, and tensile ductility with respect to temperature exhibited distinct three temperature regimes. The fracture mode remained transgranular. The steel exhibited serrated flow, an important manifestation of dynamic strain aging, along with anomalous variations in tensile properties in terms of peaks in flow stress/strength and work hardening rate, negative strain rate sensitivity, and ductility minima at intermediate temperatures. At high temperatures, the rapid decrease in flow stress/strength values and work hardening rate, and increase in ductility with increase in temperature and decrease in strain rate, indicated the dominance of dynamic recovery.     

A critical assessment of flow and cavity formation in a superplastic yttria-stabilized zirconia

Tensile tests were performed on specimens of high purity 3Y-TZP (tetragonal ZrO₂ stabilized with ∼ 3 mol% of Y₂O₃) at temperatures from 1623 to 1803 K. Superplastic-like flow was achieved with elongations of up to >400%. The experiments show that the stress exponent, n, is ∼ 3.0–3.4 and the activation energy for flow, Q, is ∼ 602±20 kJ mol⁻¹. Internal cavities developed in all specimens during flow, and quantitative measurements were taken of the cavity shapes and sizes over a range of experimental conditions. The results demonstrate that there is an increase in the extent of cavitation with (i) decreasing temperature at constant strain rate and (ii) increasing strain rate at constant temperature. The influence of strain rate on cavitation in 3Y-TZP is the opposite of most superplastic metals, and the difference arises because of an inhibition in the diffusion growth process in 3Y-TZP.     

Effects of temperature and strain rate on tensile properties and activation energy for dynamic strain aging in alloy 625

Alloy 625 ammonia cracker tubes were service exposed for 60,000 hours at 873 K. These were then subjected to a solution-annealing treatment at 1473 K for 0.5 hours. The effects of temperature and strain rate on the tensile properties of the solution-annealed alloy were examined in the temperature range of 300 to 1023 K, employing the strain rates in the range of 3×10⁻⁵ s⁻¹ to 3×10⁻³ s⁻¹. At intermediate temperatures (523 to 923 K), various manifestations of dynamic strain aging (DSA) such as serrated flow, peaks, and plateaus in the variations of yield strength (YS) and ultimate tensile strength (UTS) and work-hardening rate with temperature were observed. The activation energy for serrated flow (Q) was determined by employing various methodologies for T<823 K, where a normal Portevien-Le Chatelier effect (PLE) was observed. The value of Q was found to be independent of the method employed. The average Q value of 98 kJ/mol was found to be in agreement with that for Mo migration in a Ni matrix. At elevated temperatures (T≥823 K), type-C serrations and an inverse PLE was noticed. The decrease in uniform elongation beyond 873 K for 3×10⁻⁵ s⁻¹ and 3×10⁻³ s⁻¹ and beyond 923 K for 3×10⁻⁴ s⁻¹ strain rates seen in this alloy has been ascribed to reduction in ductility due to precipitation of carbides and δ phase on the grain boundaries.     

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.     

Self-diffusion in α-iron: The influence of dislocations and the effect of the magnetic phase transition

Self-diffusion in dislocated α-iron single crystals has been studied with the radioactive tracer ⁵⁹Fe in the temperature range 754–1163 K. Serial sectioning was performed either by microtome cutting or by sputtering. Based on systematic studies of diffusion along dislocations in the same material by the present authors [in DIMETA 88—Diffusion in Metals and Alloys (edited by F. J. Kedves and D. L. Beke). Defect and Diffusion Forum 66–69, 591 (1989)] lattice diffusion coefficients have been selected by considering the influence of dislocations on the apparent diffusivities measured in dislocated crystals. The temperature dependence of the lattice self-diffusion coefficient shows a deviation from the Arrhenius behaviour due to the magnetic order-disorder transition. The general relations and several semi-empirical models which describe this magnetic diffusion anomaly are discussed. From fits of the respective models to a set of lattice diffusion coefficients activation parameters for self-diffusion in the ferromagnetic state (D_f⁰ and Q_f) and in the completely disordered paramagnetic state (D_p⁰ and Q_p) are obtained: D_f⁰ = 6.8–27.7 · 10⁻⁴ m² s⁻¹, Q_f = 2.95–3.10 eV, D_p⁰ = 6.8–12.3 · 10⁻⁴ m² s⁻¹, and Q_p = 2.57–2.68 eV. Independent of the details of these models the parameters fall within fairly narrow limits.     

CaTiO3–Sm0.9Nd0.1AlO3 ceramics with a moderate dielectric constant and a high quality factor prepared by sol–gel auto combustion method

CaTiO₃–LnAlO₃ (Ln = La, Nd, Sm) is a perovskite-type microwave material system characterized by a moderate dielectric constant ε_r, a high quality factor Q × f, and a small temperature coefficient of resonant frequency τ_f, making this system promising for microwave devices. However, its high synthesis temperature and sintering temperature limit its industrial applications. In our work, single phase 0.7CaTiO₃–0.3Sm_0.9Nd_0.1AlO₃ (7CT–3SNA) was synthesized via the sol–gel auto combustion method using citric acid as fuel at a relatively low temperature. After being calcined at 600 °C for 2 h, well-crystallized 7CT–3SNA powders with 30–50 nm average particle size were achieved, suggesting good sintering activity. The new and narrow order band at about 800 cm⁻¹ in the Raman spectra indicates a high ordering degree in the B site of 7CT–3SNA solid solution. Compared with the solid–state reaction method and co-precipitation method, the 0.7CT–0.3SNA ceramics fabricated by the current method possess a much lower calcination temperature, a similar ε_r value, and an improved Q × f value. The optimum microwave dielectric properties of ε_r = 43.54, Q × f = 54375 GHz, and τ_f = −6.3 × 10⁻⁶/°C are obtained for the CTSA ceramics derived from the sol–gel auto combustion process. Therefore, the 7CT–3SNA ceramics prepared in this study are potential dielectric materials for microwave applications, indicating that the sol–gel auto combustion method is a good alternative strategy for the fabrication of CaTiO₃–LnAlO₃ ceramics.     

Cryogenic S–N Fatigue and Fatigue Crack Propagation Behaviors of High Manganese Austenitic Steels

In the current study, the S–N fatigue and the fatigue crack propagation (FCP) behaviors of high manganese austenitic steels, including Fe24Mn and Fe22Mn, were studied, and the results were compared with STS304 (Fe-1Si-2Mn-20Cr-10Ni). The S–N fatigue tests were conducted at 298 K and 110 K (25 °C and ?163 °C), respectively, and at an R ratio of 0.1 under a uniaxial loading condition. The FCP tests were conducted at 298 K and 110 K (25 °C and ?163°C), respectively, and at R ratios of 0.1 and 0.5, respectively, using compact tension specimens. The resistance to S–N fatigue of each specimen increased greatly with decreasing temperature from 298 K to 110 K (25 °C to ?163 °C) and showed a strong dependency on the flow stress. The FCP behaviors of the austenitic steels currently studied substantially varied depending on testing temperature, applied ΔK (stress intensity factor range), and R ratio. The enhanced FCP resistance was observed for the Fe24Mn and the Fe22Mn specimens particularly in the near-threshold ΔK regime, while the enhancement was significant over the entire ΔK regimes for the STS304 specimen, with decreasing temperature from 298 K to 110 K (25 °C to ?163 °C). The S–N fatigue and the FCP behaviors of high manganese austenitic steels are compared with STS304 and discussed based on the fractographic and the micrographic observations.     

Flow Stress Behaviors and Microstructure Evolution of 300M High Strength Steel under Isothermal Compression

The compressive deformation behaviors of 300M high strength steel were investigated over a wide range of temperatures （850- 1200 C） and strain rates （0. 001- 10 s^- 1 ） on a Gleeble-3800 thermo-mechanical simulator. The measured flow stress was modified by the corrections of the friction and the temperature compensations, which nicely reflect negative effects of the friction and temperature on the flow stress. The corrected stress-strain curves were the dynamic recrystallization type on the conditions of higher deformation temperature and lower strain rate. Flow stress increases with the increase of strain rate at the same deformation temperature and strain. By contrast, flow stress decreases with the increase of temperature at the same strain rate and strain. Dependence of the peak stress on temperature and strain rate for 300M steel is described by means of the conventional hyperbolic sine equation. By re gression analysis, the activation energy （Q） in the whole range of deformation temperature is determined to be 367. 562 kJ/mol. The effects of the temperature and the strain rate on mierostructural evolution are obvious. With the increase of the deformation temperature and the decrease of the strain rate, the original austenite grain sizes of 300M steel increase. At the same time, the corrected flow stress curves more accurately determine the evolution of the microstrueture.     

The Portevin-Le Chatelier Effect in hydrogenated nickel

The macroscopic activation energy for the initiation of the serrations which are characteristic of the Portevin-Le Chatelier Effect in hydrogenated nickel, was found to be 0.59 ev at 0.21 at. pct H. This differs markedly from the 0.33 ev previously reported by earlier workers.¹ Also, it differs from activation energy for hydrogen diffusion in nickel,Q _D , which is reported to be 0.42 ev.¹³ The difference between the macroscopic activation energy for the disappearance (Q _U ) and initiation of the serrations (Q _L ) was found to be approximately equal to the binding energy of hydride to dislocations (ΔE), of 0.14 ev. The concentration dependence for the initiation and disappearance of serrations was found to approach a saturation level at approximately 0.08 at. pct H. The dominant dislocation locking mechanism responsible for the serrated yielding is thought to be one involving the drift flow of hydrogen to dislocations under an electrical driving force and the ultimate precipitation of the hydride. Electron microscopy has confirmed that an enhanced dislocation multiplication accompanies serrated yielding in hydrogenated nickel.     

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.     

Influence of hot isostatic pressing on the structure and properties of an innovative low-alloy high-strength aluminum cast alloy based on the Al–Zn–Mg–Cu–Ni–Fe system

Hot isostatic pressing (HIP) is applied for treatment of castings of innovative low-ally high-strength aluminum alloy, nikalin ATs6N0.5Zh based on the Al–Zn–Mg–Cu–Ni–Fe system. The influence of HIP on the structure and properties of castings is studied by means of three regimes of barometric treatment with different temperatures of isometric holding: t ₁ = 505 ± 2°C, p ₁ = 100 MPa, τ₁ = 3 h (HIP1); t ₂ = 525 ± 2°C, p ₂ = 100 MPa, τ₂ = 3 h (HIP2); and t ₃ = 545 ± 2°C, p ₃ = 100 MPa, τ₃ = 3 h (HIP3). It is established that high-temperature HIP leads to actually complete elimination of porosity and additional improvement of the morphology of second phases. Improved structure after HIP provides improvement properties, especially of plasticity. In particular, after heat treatment according of regime HIP2 + T4 (T4 is natural aging), the alloy plasticity is improved by about two times in comparison with the initial state (from ~6 to 12%). While applying regime HIP3 + T6 (T6 is artificial aging for reaching the maximum strength), the plasticity has improved by more than three times in comparison with the initial state, as after treatment according to regimes HIP1 + T6 and HIP2 + T6 (from ~1.2 to ~5.0%), which are characterized by a lower HIP temperature.     

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.