Effect of nitrogen on the dynamic strain ageing behaviour of type 316L stainless steel

The dynamic strain ageing (DSA) behaviours of type 316L stainless steels containing different nitrogen contents (0.01–0.15 wt% N) were studied in tension under varying strain rates (1 × 10^–2–2 × 10^–4s^–1) and the test temperatures (R.T.–1023 K). The temperature range for DSA was moved to higher temperature for increasing nitrogen contents. The critical strain, _c for the onset of serration increased with nitrogen content at 773 K and then became almost constant at 873 K. Type A and B serrations were observed at 873 K with the value of the strain required to effect the transition from type A to type B serration increasing for nitrogen contents upto 0.1 wt% and then becoming saturated. The activation energy for DSA was 23.4–26.2 kcal mol^–1 (97.8–109.5 kJ mol^–1) at the onset and 65.0–76.6 kcal mol^–1 (271–320.2 kJ mol^–1) at the end of serration. The lower activation energy was related to vacancy diffusion and the higher activation energy was attributed to the diffusion of chromium to dislocations. The activation energy for DSA was slightly increased with nitrogen addition. DSA was retarded by an increase in the nitrogen content since nitrogen reduced the chromium diffusion to dislocations due to a strong interaction between the nitrogen and chromium.     

Mechanism of the fatigue life increment due to dynamic strain ageing during hold time

The effect of hold time on the low-cycle fatigue behaviour of AlSl 4140 steel at 303 to 473 K was investigated in air and argon gas. All tests were conducted under a constant total strain amplitude of 0.5, 0.65, 1.0, 1.3 and 1.6×10^–2 with zero mean level. Experimental results show that dynamic strain ageing is occurring during the hold time period and that piled-up dislocations near the carbides are locked by the Cottrell atmosphere. Formation of the Cottrell atmosphere around dislocations near the carbides induces the redistribution of carbon or carbides in the matrix. These randomly dispersed grown up carbides behave like the increased number of dislocation pile-up sites from which fatigue cracks may initiate. Therefore, as the hold time is applied in fatigue cycles under constant strain amplitude the dislocation density at each site will be lowered to retard the fatigue damage or improve the fatigue life.     

Work hardening of mild steel within dynamic strain ageing temperatures

An investigation has been performed on the plastic behaviour of a mild steel within the region of dynamic strain ageing. For this purpose tension tests have been performed on annealed XC18 steel within a range of temperatures, from 305–776 K, and a range of strain rates, from 1.0×10^–4–1.85×10^–1s^–1. An analysis of experimental results is presented using a model for plastic deformation based on dislocation multiplications.     

The effect of thermal ageing on low cycle fatigue behaviour of 316 stainless steel welds

It is well known that welds are the weak links in any structure. Therefore, it is of out most importance to characterize the mechanical properties of welds. Moreover, the changes in the microstructure that occur in welds on exposure to high temperatures affect the mechanical properties and must be studied by ageing the welds at high temperature. In this paper the low cycle fatigue behaviour of thermally aged 316 stainless steel weld metal is presented. Weld pads with single V configuration were prepared by the shielded metal arc welding process using 316 electrodes. Thermal ageing was done for 10,000 h at 823 and 873 K. Total strain controlled low cycle fatigue tests were conducted at a constant strain rate of 3 × 10^?3 s^?1 with strain amplitudes in the range ±0.25% to ±0.6% at 823 and 873 K. Weld metal exhibited initial hardening followed by cyclic softening prior to failure. The aged samples exhibited higher stress response as compared to the unaged samples. At both the temperatures and all strain amplitudes fatigue life was inferior to that of unaged samples. The metallography of the aged and tested material was studied through optical, scanning and transmission electron microscopy. The effect of transformation of δ-ferrite to sigma phase and carbides in the weld metal on low cycle fatigue behaviour was evaluated.     

Effect of rapid deformation and temperature on the strength and plasticity characteristics of chromium-nickel-molybdenum steel

The results of mechanical tests of chromium-nickel-molybdenum steel in uniaxial tension in the strain-rate range of 10^–3–510⁴ sec^–1 and at temperatures ranging from 77 to 523 K are presented. The increase in strength and plasticity characteristics with increasing strain rate is confirmed. The results of the investigations conducted contradict the hypothesis of equivalence of the effect of a temperature reduction and increase in strain rate on the mechanical characteristics of metals.Translated from Problemy Prochnosti, No. 9, pp. 17–19, September, 1991.     

The temperature and strain rate dependence of the flow stress of tantalum

The temperature and strain rate dependence of the flow stress of tantalum was studied between 78 to 800 K at strain rates from 10^–5 to 2×10⁴ sec^–1. The effect of temperature and strain rate on the lower yield stress can be explained by a model incorporating the combined operation of the Peierls mechanism and dislocation drag processes. The general behaviour of the stress—strain curve at various strain rates and temperatures is analysed in terms of a rate—temperature parameter.     

Stress corrosion cracking of Al-Zn-Mg alloy AA-7039 by slow strain-rate method

The stress corrosion behaviour of Al-Zn-Mg alloy AA-7039 in an aqueous 3.5 wt % Nad solution (pH=1) was studied with the specimens under constant strain rate as function of ageing state and cold working. The tests were carried out at temperatures of 30 and 45°C and strain rates between 7.6×10^–7 and 7.6×10^–6 sec^–1 and the apparent activation energy for mechanical deformation in oil and stress corrosion cracking (SCC) process in NaCl solution were determined. The fracture energy in NaCl solution under constant strain rate, as compared with experiments in oil, was lowered in the overaged specimens and markedly lowered in sequence of the peak-aged and the underaged specimens. The values of fracture energy for peak-aged and overaged specimens were nearly similar to those in oil at the higher strain rate of 7.6×10^–6 sec^–1. The relative fracture energy was comparatively lowered in the fine-grained specimens, as compared to coarsegrained specimens. The apparent activation energy for mechanical processes in oil was found to be 103 kJ mol^–1 in the peak-aged and 96.5 kJ mol^–1 in the overaged specimens. Those for SCC processes in NaCl solution were 47.5 kJ mol^–1 in the peak-aged and 51.5 kJ mol^–1 in the overaged specimens. The results suggest that stress corrosion (SC) cracks are initiated by electrochemical dissolution of grain boundaries (gbs) and propagated by mechanical processes such as creep.     

Yield stress of a copper single crystal containing B2O3 particles

The yield stress of a copper single crystal containing vitreous B₂O₃ particles was measured as a function of temperature (77–1073 K) and strain rate (5.6 × 10^–6–5.6 × 10^–4sec^–1). Although the B₂O₃ particles in the copper matrix are a plastically non-deformable solid at low temperatures, they became liquid-like at high temperatures, above approximately 550 K. The yield stress of the Cu-B₂O₃ alloy at low temperatures was explained by the Orowan mechanism and the modulus-corrected yield stress of the Cu-B₂O₃ alloy at 1073 K was about four-fifths of the values at low temperatures. It was found that the liquid B₂O₃ particles could be the effective hardening centres even at high temperatures. At 873 and 1073 K, the yield stresses of the Cu-B₂O₃ alloy varied with the logarithm of the strain rates.     

Cavitation and fracture of mechanically alloyed aluminium at high homologous temperatures

The tensile behaviour of mechanically alloyed (dispersion strengthened) IN90211 was characterized at strain rates between 0.0001 and 340 sec^–1 at temperatures between 425 and 475 ° C, At strain rates above 0.1 sec^–1, superplastic elongations were obtained (maximum elongation 525% at 475 ° C, 2.5sec^–1. Large elongations were possible due to the lack of cavitation, even though the strain-rate sensitivity was lower (m 0.25) than usually found in superplasticity. Cavitation was precluded by the morphology of the platelet-shaped grains in which low-angle subgrain boundaries were predominantly perpendicular to the tensile axis. Grain-boundary sliding was observed along high-angle grain boundaries which were generally parallel to the tensile axis. At the high homologous testing temperatures (0.76 to 0.81), concurrent grain-boundary sliding and lattice slip was made possible by the rapid lattice diffusivity and easy climb of lattice dislocations over dispersions in the matrix and grain boundaries.     

Assessment of fatigue response of thermally aged reduced activation ferritic-martensitic steel based on finite element analysis

Abstract

In the present investigation, effect of thermal ageing on low cycle fatigue (LCF) behaviour of Reduced Activation Ferritic Martensitic steel has been assessed by finite element analysis. The steel was thermally aged at 873 K for 3000 hour. Low cycle fatigue tests were carried out on both the as-received and thermally aged material at strain rate of 3×10^?3 s^?1 at 823 K, over strain amplitudes in the range of ± 0.25 to ± 0.8%. Continuous cyclic softening till final failure, except for initial few cycles especially at relatively lower strain amplitudes, was observed in both the material conditions. Thermal ageing resulted in marginally higher cyclic stress response accompanied by lower fatigue life. The differences in fatigue responses have been attributed to the coarsening of precipitates on thermal ageing. Finite element analysis has been carried out considering combined isotropic and kinematic hardening as material model to estimate the effect of thermal ageing on the response of material under LCF loading. Thermal ageing was found to decrease both the isotropic and kinematic hardening with appreciable effect on isotropic hardening. The predicted cyclic stress response and hysteresis loops were found to be in good agreement with the experimental data. The LCF life of the steel has been estimated based on the hysteresis energy approach.     

Grain growth in copper and alpha-brasses

The wires of 99.999% copper and alpha-brasses containing 12, 20, 30 and 35 at % Zn have been annealed in vacuum for 30 to 240 min at 873, 923, 973 and 1023 K. The grain-growth data obtained are well encompassed by the relationD ², —D ₀ ² , =Kt exp(-H/kT), whereD is the instantaneous mean grain diameter at the time,t, of isothermal anneal andD ₀ refers to the initial mean grain diameter. In alpha-brasses the activation energy for grain-boundary self-diffusion,H, and the pre-exponential factor,K, depends on the zinc concentration,c, asH = (H ₀ — 1.1c) eV andK =K ₀ exp(-10.7c) cm² sec^–1. The values ofH ₀ andK ₀, referred to the base metal are respectively 0.87 eV and 3.0 × 10^–4 cm² sec^–1, which are in good agreement with those (0.85 eV and 3.6 × 10^–4 cm² sec^–1) found for copper.     

Deformation of CoTi polycrystals

Deformation behaviour of polycrystalline B2-type CoTi intermetallic compounds was investigated in relation to testing temperature (77 to 1073 K), alloy composition (50 to 48 at.% Ti) and strain rate (4.2 x 10^–4 to 4.2 x 10^–3 sec^–1). Four distinct temperature regions were distinguished. A low-temperature region where the yield stress rapidly decreased with temperature; an intermediate temperature region where the yield stress increased with temperature, accompanied by very high strain-hardening rate, serration on the stress-strain curves and kink deformation; a high temperature region where the yield stress again rapidly decreased with temperature, and a very high temperature region where the yield stress slowly decreased with temperature. Deviation from stoichiometry resulted in increased yield stresses over whole temperature regions. Positive strain-rate dependence of the yield stress was regarded in the two higher temperature regions. Slip traces were observed at all temperatures. Possible explanations are given in comparison with other B2-type intermetallic compounds.     

The yield and deformation behaviour of some polycarbonate blends

Polycarbonate and its blends with PE and MBS have been tested to investigate the impact modification mechanism. These materials have been tested in tension over the speed range 10^–2 to 10² in. sec^–1 (2.5×10^–2 to 2.5×10² cm sec^–1). The tensile deformation behaviour of these materials is similar except for the magnitude of the yield stresses. The yield stress versus log curves have identical slopes. Based on Eyring's equation for the flow of viscous materials, these materials have identical activation volumes, implying that the mechanical behaviour modification is not due to changes in molecular mechanisms. The modifier particles probably change only the stress state of the matrix material. Three-point bending tests on notched bars of these materials have also been performed over the speed range 10^–2 to 10² in. sec^–1 (2.5×10^–2 to 2.5×10² cm sec^–1). The areas under the load-deflection curves have been measured as the total energy absorbed during the deformation. It was found that both geometric constraint and rate of deformation can bring about ductile-brittle transitions. However, the thickness sensitivity of the blends is less than that of the pure material. Scanning electron micrographs show that the matrix material voids and flows extensively around the modifier particles before the ductilebrittle transition speed is reached. This voiding probably relieves plane strain. However, at higher speeds, the modifier particles cannot relax sufficiently rapidly, and they lose this plane strain relieving capability.     

Development of substructure in Zircaloy-4 during LCF at 873 K

Transmission electron microscopy techniques were used to study the substructure developed in Zircaloy-4 during strain controlled Low Cycle Fatigue (LCF) test at 873 K. A complete microstructural analysis on samples cycled with a total strain range of 0.01 and total strain rate 2×10^–3s^–1 was made at a different number of cycles. This study revealed that the dislocations arranged themselves into a band structure which remains essentially constant since the first cycle up to fracture. This substructure stability agrees with the stress response observed of this alloy at 873 K. A mechanism to account for the observed structure is proposed.     

Influence of strain rate on the temperature rise of specimens in static tests of a titanium alloy in the 293–4.2 K range

The influence of two strain rates, 1.10^–1 and 2.10^–2 sec^–1, on the temperature rise of specimens of -titanium alloys in static tests in the 290- 4.2 K range is investigated. It is established that at room temperature conditions (290 K) the temperature rise of the specimens is nonuniform over the length and is 14 K, in liquid nitrogen (77 K) it is more than 0.5 K, and in liquid helium (4.2 K) the temperature depends upon the strain rate and reaches 46 K. It is shown that the temperature rise of the specimens in liquid helium in strain at a rate of 2.10^–2 sec^–1 reduces the tensile strength but does not influence the yield strength of the material.Translated from Problemy Prochnosti, No. 12, pp. 70–78, December, 1992.     

Cyclic surface deformation behaviour of ZrH2-purified iron: Effects of solute carbon,strain amplitude,strain rate and environment

The surface deformation behaviour in ZrH₂-purified interstitial-free iron was studied during fatigue in the push-pull plastic strain control mode under various combinations of plastic strain amplitude (5×10^–4, 5×10^–3), strain rate (5×10^–4, 3× 10^–2s^–1) and environment (ultrahigh vacuum, oxygen). Comparative tests of vacuum-melted commercially pure iron (CPI) containing 170 p.p.m. C were also conducted to investigate the effect of interstitials. At a plastic strain amplitude of 5×10^–4, the environmental effect is clearly exhibited regardless of the strain rate and the presence of interstitials. Fatigue in ultrahigh vacuum produces very fine slip lines not only in interstitial-free iron but also in vacuum-melted CPI. In the presence of oxygen, fatigue produces prominent slip lines, but those developed in CPI are more intense and coarser than those developed in interstitial-free iron. At the higher plastic strain amplitude of 5×10^–3, the gaseous environmental effect on the cyclic surface deformation is insignificant. The surface deformation behaviour is discussed in terms of the environmental effect and the basic mechanisms that govern the cyclic plasticity of iron.     

Oxygen self-diffusion in rutile under hydrothermal conditions

Oxygen self-diffusion coefficients have been determined for synthetic and natural rutile single crystals under hydrothermal conditions at 100 MPa total pressure and in the temperature range 873–1373 K. The diffusion coefficients are lower than the results from dry gas exchange studies would predict. Between 973 and 1373 K the results can be characterized by two linear Arrhenius relationships. D=1.14×10^–11 exp(–168.8 kJ mol^–1/RT) m^–2s^–1 for the natural rutile, and D=2.41×10^–12 exp(–172.5 kJ mol^–1/RT) m²s^–1 for the synthetic crystal. The results have been interpreted in terms of a defect model involving the dissolution of water in rutile as substitutional hydroxyl defects on oxygen lattice sites, with a solution enthalpy in the range 81–106 kJmol^–1.     

Effects of temperature on the low cycle fatigue behaviour of nitrogen alloyed type 316L stainless steel

Strain-controlled low cycle fatigue tests have been conducted in air between 298–873 K to ascertain the influence of temperature on LCF behaviour of nitrogen-alloyed type 316L stainless steel. A strain amplitude of ± 0.60% and a symmetrical triangular waveform at a constant strain rate of 3 × 10⁻³ s⁻¹ were employed for all tests. Crack initiation and propagation modes were evaluated, and the deformation and damage mechanisms which influence the cyclic stress response and fatigue life identified. The cyclic stress response at all temperatures was characterized by an initial hardening to the maximum stress, followed by gradual softening prior to attaining saturation. Temperature dependence of fatigue life showed a maximum in the intermediate temperature range. The drastic reduction in fatigue life at elevated temperatures has been ascribed primarily to the combined influence of dynamic strain ageing effects and oxidation-enhanced crack initiation, while the lower life at room temperature is attributed to detrimental effects associated with deformation-induced martensite.     

The tensile properties of a superplastic aluminium bronze

The high temperature tensile properties of a micrograin Cu-9.5% Al-4% Fe alloy, which is superplastic at 800° C, have been determined. Elongations at fracture of greater than 700% are achieved when the nominal strain-rate is in the range 3.9×10^–2 min^–1 to 7.9×10^–2 min^–1. The nature of plastic instability in superplastic materials is considered and it is shown that the amount of strain at the onset of plastic instability is inversely related to the applied strain-rate and is relatively independent of the strain-rate sensitivity exponent, m. The onset of plastic instability during a tensile test results in an increase of local strain-rate at the point of minimum cross-section and this, together with the existence of a triaxial stress state in the necked region, may produce errors in the m versus strain-rate plot if m is determined by the change-rate method. The initial strain-rate for maximum elongation is lower than the strain-rate for maximum m. This may be ascribed either to the influence of plastic instability or the formation of cavities at the higher strain-rates.     

Labyrinth structures in Zircaloy-4 as a consequence of strain ageing effects on fatigue

Dislocation structures in Zircaloy-4 samples fatigued at 623, 713 and 823 K were studied by transmission electron microscopy techniques. The samples, cycled with a total strain rate 2×10^–3s^–1, show a linear cyclic hardening rate which is maximum at temperatures where stronger strain ageing effects were reported. A two-wall structure similar to the labyrinth structure observed in fcc metals was found. This structure is attributed to the enhanced dislocation activity produced by strain ageing phenomena.