Low cycle fatigue behaviour of Inconel® alloy 783

Low cycle fatigue (LCF) tests were performed on Inconel® Alloy 783 at a strain rate of 3×10^?3s^?1 and a strain amplitude of ±0.6%. The tests were conducted in the temperature range, 573 to 923 K besides room temperature. Further, influence of strain amplitude on LCF behaviour was studied at a constant temperature of 923 K using strain amplitudes in the range, ±0.3 to ±0.8%. Creep-fatigue interaction tests were conducted at a strain amplitude of ±0.4%, at 923 K. The material generally showed a stable stress response followed by a region of continuous softening upto failure. Also, the alloy was seen to exhibit dynamic strain ageing (DSA) in the temperature range, 573 to 723 K as evidenced by the observed peaks in the half-life stress in the above temperature range. Dynamic strain ageing was seen to offset the softening exhibited by the alloy outside the above temperature range. Fractography/EDS analysis of the failed samples showed numerous brittle Nb-rich precipitates that were seen to influence crack propagation. A continuous reduction in the LCF life was noticed with increase in the test temperature. The observed material behaviour has been correlated with the damage mechanisms through microstructural observations.     

Activation energy for serrated flow in a 15Cr-5Ni Ti-modified austenitic stainless steel

Serrated flow in a 15Cr-15Ni titanium-modified austenitic stainless steel has been investigated at a wide range of temperatures (300–1023 K) and strain rates (10⁻⁵−10⁻²s⁻¹). Three regimes of serrated flow have been identified as low temperature type A (LT-A), high temperature type A (HT-A) and high temperature type C (HT-C). Different methods suggested in the literature for the determination of activation energy, Q were employed and Q was obtained aas 115 ± 9, 140 ± 7 and 178 ± 7 kJ/mol for LT-A, Ht-A and HT-C regimes respectively. Based on the Q values, the mechanisms responsible for the three regimes of serrated flow were identified as vacancy migration for LT-A, C (or N) as controlling diffusing species to form a pair with vacancy causing Schoeck-Seeger locking for HT-A regime and interaction of Ti with C (or N) for HT-C regime. Titanium was found to influence the serrated flow in all regimes.     

Lüders bands formation in a rapidly solidified Ni3al alloy ribbon

The phenomenology of Lüders bands formation in a rapidly solidified Ni-20Al-12Cr-1.8Mo intermetallic alloy ribbon in the temperature range of 300-770 K is discussed. It was observed that strength and Lüders bands aspect on the specimen were irrespective of temperature. The flow characteristics in the Lüders region of the load-elongation curve were, however, very temperature sensitive. At low temperatures (<470 K), a flat plastic region with few instabilities was seen; but at higher temperatures (>470 K), a clear serrated behavior was manifested and the amplitude of serration increased with temperature. It is suggested that yielding occurs by dislocation generation at grain boundaries and that the stress required for dislocation generation (σ_eff) is athermal. A temperature dependent stress originated by the dynamic pile-up of dislocations at grain boundaries (dynamic stress) is, however, introduced as rate controlling for Lüders front motion and responsible for serration appearance.     

An assessment of cold work effects on strain-controlled low-cycle fatigue behavior of type 304 stainless steel

The influence of prior cold work (PCW) on low-cycle fatigue (LCF) behavior of type 304 stainless steel has been studied at 300, 823, 923, and 1023 K by conducting total axial strain-controlled tests in solution annealed (SA, 0 pct PCW) condition and on specimens having three levels of PCW, namely, 10, 20, and 30 pct. A triangular waveform with a constant frequency of 0.1 Hz was employed for all of the tests performed over strain amplitudes in the range of ±0.25 to ± 1.25 pct. These studies have revealed that fatigue life is strongly dependent on PCW, temperature, and strain amplitude employed in testing. The SA material generally displayed better endurance in terms of total and plastic strain amplitudes than the material in 10, 20, and 30 pct PCW conditions at all of the temperatures. However, at 300 K at very low strain amplitudes, PCW material exhibited better total strain fatigue resistance. At 823 K, LCF life decreased with increasing PCW, whereas at 923 K, 10 pct PCW displayed the lowest life. An improvement in life occurred for prior deformations exceeding 10 pct at all strain amplitudes at 923 K. Fatigue life showed a noticeable decrease with increasing temperature up to 1023 K in PCW state. On the other hand, SA material displayed a minimum in fatigue life at 923 K. The fatigue life results of SA as well as all of the PCW conditions obeyed the Basquin and Coffin-Manson relationships at 300, 823, and 923 K. The constants and exponents in these equations were found to depend on the test temperature and prior metallurgical state of the material. A study is made of cyclic stress-strain behavior in SA and PCW states and the relationship between the cyclic strain-hardening exponent and fatigue behavior at different temperatures has been explored. The influence of environment on fatigue crack initiation and propagation behavior has been examined.     

Flow Behaviour of Modified 9Cr–1Mo Steel at Elevated Temperatures

In this investigation, the flow behaviour of modified 9Cr–1Mo steel at elevated temperatures is reported. To understand the flow behaviour of the steel, tensile tests were performed at nominal strain rate of 3 × 10^?3 s^?1 and temperatures in the range of 300–823 K. The yield strength and ultimate tensile strength were found to decrease with increase in temperature with a plateau in a intermediate temperature regime (523–673 K). Serrations were also observed in the tensile curve at intermediate temperatures which caused plateau/peak in tensile behaviour of material and was a typical manifestation of dynamic strain ageing. An attempt was made to represent the flow behaviour of the material using different constitutive equations viz., Hollomon, Ludwik, Swift, Ludwigson and Voce. It was observed that the Voce equation could describe the experimental flow curve at different temperatures quite well. Instantaneous work hardening rate with respect to flow stress exhibited two stages of hardening especially at relatively lower temperatures.     

Tensile flow and work-hardening behavior of a Ti-modified austenitic stainless steel

The flow-stress data of a 15Cr-15Ni-2.2Mo-Ti modified austenitic stainless steel in the temperature range 300 to 1023 K was analyzed in terms of Ludwigson and Voce equations. The parameters of these equations were critically examined with respect to the effect of Ti/C ratio and test temperature. It was found that the Ludwigson equation described the flow behavior adequately up to the test temperature of 923 K, whereas the Voce equation could be employed in the full temperature range. The peaks/plateaus observed in the variation of these parameters as a function of temperature in the intermediate temperature range have been identified as one of the manifestations of dynamic strain aging (DSA). Also, the variation of these parameters with temperature clearly could bring out the different domains of DSA observed in this alloy. The work-hardening analysis of the flow-stress data revealed that in the DSA regime, the onset of stage III hardening is athermal.     

Study on the Deformation Behavior of Mg-3.6% Er Magnesium Alloy

The deformation behaviour of a casting Mg-3.6% Er magnesium alloy after T6 treatment was studied in tensile tests from room temperature to 450 ℃ under different strain rates ranging from 1.0 ×10^-4 to 6.0 × 10^-3 S^-1 Obtained local plateau in the temperature dependence of the ultimate strength （σb） and yield strength （σ0.2） under constant strain rate indicated the presence of dynamic strain ageing （DSA）. Serrated flow was observed at the temperature of 200, 250, and 300 ℃. The observed negative strain rate sensitivity suggested that the serrated flow behavior arose from DSA. The temperature and strain rate dependence of the critical strain for the onset of serrated flow was analyzed using a phenomenological DSA equation, and the apparent activation energy Q for the serrated flow was obtained by calculation.     

Phase transformations during tempering of the Fe-15Cr-1Mo martensites containing nitrogen or carbon

Hardness measurements, dilatometry, internal friction measurements, Mössbauer spectroscopy and transmission electron microscopy are utilized in order to study the effect of tempering on the microstructure of a stainless martensitic steel containing 15% Cr, 1% Mo and 0.6% N. A similar carbon steel containing 15% Cr, 1% Mo and 0.6% C is used for comparison. Tempering of alloy Fe-15Cr-1Mo-0.6N in the low temperature range of 353-473 K leads to formation of hexagonal ?-nitride (Fe,Cr)₂N, which is followed by precipitation of the orthorombic ?-nitride (Fe,Cr)₂N at temperatures of 573-773 K. The hexagonal nitride Cr₂N is precipitated at 923 K and preferably formed at grain boundaries. The alloy Fe-15Cr-1Mo-0.6C shows the expected tempering behaviour. ?-carbide (Fe,Cr)₂C and cementite (Fe,Cr)₃C are precipitated during low temperature ageing, followed by the formation of Cr₇C₃ carbides after the temperature has risen to 873 K. With a similar interstitial content the amount of retained austenite in the nitrogen martensite is nearly twice as high as in the carbon one. Furthermore, the thermal stability of the retained austenite of the nitrogen alloy is substantially higher than that of the carbon steel.     

Anomalous strain rate dependence of the serrated flow in NiH and NiCH alloys

Serrated flow in NiH, NiCH, and NiC alloys was studied over a wide range of temperature and strain rates. The results for C related serrated flow in NiC or NiCH alloys were in excellent agreement with previous results and were consistent with dislocation pinning by C solutes at the dislocation cores. Hydrogen related serrated yielding was observed in NiH and NiHC alloys. Solute C had only a small effect on the temperature range of this H related serrated flow. The results could be interpreted on the basis of hydride formation at the dislocation cores and diffusion of H in these hydrides.     

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.     

The grain size dependence of flow and fracture in a Cr-Mn-N austenitic steel from 300 to 1300K

The influence of polycrystal grain size in the range 18 μm to 184 μm on the tensile behavior of an austenitic stainless steel containing by wt pct 21 Cr, 14 Mn, 0.68 N and 0.12 C has been investigated over the temperature range 298 to 1273 K. Decreasing grain size has been shown to increase the flow stress at small strains in accordance with the Hall-Petch relationship at temperatures up to 873 K. The variation of the Hall-Petch constants with temperature is influenced by dynamic strain ageing between 575 and 775 K. Above 875 K, especially at low strain-rates a reversal of the Hall-Petch correlation occurs and the flow stress decreases with decreasing grain size. The relationship between ductility and temperature is marked by a minimum ductility at about half the absolute melting temperature and intergranular cavitation is observed. A decrease in grain size generally enhanced the ductility in this temperature regime whilst at fine grain sizes this trend was reversed. These results are explained in terms of a combination of a Griffith-Orowan type fracture criterion and an intergranular void sheet mechanism of fracture.     

Mechanical Behaviour of Aged High Nitrogen Austenitic Stainless Steels

The present investigation is aimed at studying the effect of ageing on mechanical behaviour of two high nitrogen austenitic stainless steels as these steels are exposed to high temperatures in their applications. Two steels with nitrogen contents greater than 0.6% were given solution treatment and were aged at 500–900°C for 1–100 h. It was found that the steels exhibit superior mechanical properties in solution treated condition while ageing has a deleterious effect on properties due to weak interfaces of depleted austenite matrix and Cr₂N lamellae. The plastic flow behaviour can be modeled using modified Ludwik equation in these steels.     

Influence of Temperature on Tensile Stress–Strain and Work Hardening Behaviour of Forged Thick Section?9Cr–1Mo Ferritic Steel

True stress (??)?Ctrue plastic strain (??) behaviour examined for temperatures ranging from 300 to 873?K suggested that a combination of Ludwigson and Hollomon equations describes ??C?? behaviour most accurately in forged thick section?9Cr?C1Mo steel. Alternatively, the flow behaviour for temperature range 300?C873?K can be described adequately using single Voce equation. Good correlations have been obtained between work hardening parameters evaluated using Voce equation and the respective tensile properties. The variations of instantaneous work hardening rate (???=?d??/d??) and ??? with stress displayed two-stage behaviour characterised by a rapid decrease at low stresses (transient stage) followed by a gradual decrease at high stresses (stage-III). The variations of work hardening parameters, ?? versus ?? and ??? versus ?? with temperature exhibited three distinct temperature regimes along with signatures of dynamic strain ageing at intermediate temperatures and dominance of dynamic recovery at high temperatures. Comparison of work hardening parameters obtained using Voce equation and Kocks?CMecking approach suggested an analogy between the two for the steel.     

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.     

Effects of prior cold working on low cycle fatigue behavior of stainless steels, titanium alloy timetal 834 and superalloy IN 718: A review

This paper gives a brief review of the efforts made to study the effects of cold rolling on low cycle fatigue (LCF) behavior of stainless steels, titanium alloy Timetal 834 and Ni-Fe based Super alloy IN 718 at different temperatures and different strain amplitudes. Low Cycle Fatigue tests on cold worked 316L stainless steel at various temperatures from room temperature to 923K have been reported. In all tests the effect of 20% prior cold work(PCW) on LCF behavior of type 316L (N) stainless steel had been studied at 873K under total axial strain controlled mode in air at strain amplitudes from ±0.25% to ±1.0%. Fatigue life in the solution annealed condition was similar to that of the PCW material at higher strain amplitudes (≥0.5%) while at lower strain amplitudes the PCW material exhibited longer life. The influence of PCW on LCF behavior of type 304 and AISI 304LN stainless steel was studied at various temperatures and it was observed that the fatigue life was strongly dependent on prior cold work, temperature, and strain amplitude employed. Cold rolling of the titanium alloy Timetal 834 and age hardenable Ni-Fe-based superalloy IN 718 has been found to cause marked enhancement in LCF life at low strain amplitude and eliminate the bilinearity from the Coffin-Manson(C-M) relationship. Work carried on this aspect, however is very limited particularly in the case of Titanium alloys and Ni-Fe based Superalloy IN 718     

The influence of preceding cryoforming and testing temperature on the mechanical properties of austenitic stainless steels

The TRIP effect in austenitic stainless steels leads to temperature dependent mechanical properties. As this is caused by stress or strain induced austenite/martensite transformation a predeformation at low temperatures (cryoforming) will change the microstructure and the transformation behaviour of the remaining austenite constituent. The mechanical properties in tensile tests and the J‐integral of the chromium and nickel alloyed steels 1.4301 and 1.4571 have been tested in the temperature range from 123 to 323 K in the as‐industrially supplied condition and after 10 % cryoforming at 77 K. The temperature dependence of the elongation values and the strain hardening behaviour of the undeformed steels is much more pronounced than of the yield and tensile strength. The mechanical behaviour can be explained by differences in response to the ?‐, the α_e'‐ and the α_g'‐martensite transformation. A cryoforming changes the mechanical properties of the examined austenitic stainless steels.     

Creep strengthening of low carbon grade type 316LN stainless steel by nitrogen

Nitrogen-alloyed 316LN stainless steel is used as a structural material for high temperature fast breeder reactor components. With a view to increase the design life of the components up to 60 years and beyond, studies are being carried out to develop nitrogen alloyed 316LN stainless steel with superior tensile, creep and low cycle fatigue properties. This paper presents the results from studies on the influence of nitrogen on the high temperature creep properties of this material. The influence of nitrogen on the creep behaviour of 316LN stainless steel has been studied at nitrogen levels of 0.07, 0.11, 0.14 and 0.22 wt%. Creep tests were carried out at 923 K at stress levels 140, 175, 200 and 225 MPa. Creep rupture strength increased substantially with increase in nitrogen content. The variation of steady state creep rate with stress showed a power law relationship. The power law exponent varied between 6.4 and 13.7 depending upon the nitrogen content. Rupture ductility was generally above 40% at all the test conditions and for all the nitrogen contents. It was observed that the internal creep damage and surface damage decreased with increase in nitrogen content. Fracture mode was found to generally shift from intergranular failure to transgranular failure with increasing nitrogen content.     

Ti-5Mo-5V-2Cr-3Al合金热压缩变形行为

对Ti-5Mo-5V-2Cr-3Al钛合金进行等温压缩实验,变形温度范围为923～1123 K,应变速率为0.001～1 s<'-1>.分析表明该材料的流变应力对温度与应变速率敏感:当变形温度为923～1023 K时,流变应力曲线呈现动态再结晶曲线特征;当变形温度为1073 K时,低应变速率(0.001s<'-1>)的流变应力曲线呈现动态再结晶曲线特征,高应变速率(0.01-1 s<'-1>)的流变应力曲线呈现动态回复曲线特征;当变形温度为1123 K时,流变应力曲线呈现动态回复曲线特征;峰值流变应力随着变形温度的升高而下降,且下降速率随着温度升高而降低;峰值流变应力随着应变速率的升高而升高,升高速率在923～1023 K范围内随着应变速率升高而下降,在1073 K时随着应变速率升高而升高,在1123 K时随着应变速率升高无变化.Ti-5Mo-5V-2Cr-3Al钛合金在等温压缩变形时的流变行为可用包含Zener-Holomon参数的Arrhenius本构方程描述,变形激活能为789 kJ·mol-1.     

On the Role of Grain Boundary Serration in Simulated Weld Heat-Affected Zone Liquation of a Wrought Nickel-Based Superalloy

The effects of grain boundary serration on boron segregation and liquation cracking behavior in a simulated weld heat-affected zone (HAZ) of a wrought nickel-based superalloy 263 have been investigated. The serrated grain boundaries formed by the developed heat treatment were highly resistant to boron segregation; the serrated sample contained 41.6 pct grain boundaries resistant to boron enrichment as compared with 14.6 pct in the unserrated sample. During weld thermal cycle simulation, liquated grain boundaries enriched with boron were observed at the peak temperature higher than 1333 K (1060 °C) in both unserrated and serrated samples; however, serrated grain boundaries exhibited a higher resistance to liquation. The primary cause of liquation in this alloy was associated with the segregation of the melting point depressing element boron at grain boundaries. The hot ductility testing result indicated that the serrated grain boundaries showed a lower susceptibility to liquation cracking; the grain boundary serration led to an approximate 15 K decrease in the brittle temperature range. These results reflect closely a significant decrease in interfacial energy as well as a grain boundary configuration change by the serration.     

Effect of strain rate and temperature on hot workability and flow behaviour of duplex stainless steel

The flow behaviour and processing map of a duplex stainless steel were studied via hot compressive tests in a temperature range of 1223–1473?K and a strain rate range of 0.01–30?s^??1. The effect of strain rate and temperature on the hot workability, strain partitioning and dominant flow behaviour of the alloy was systematically investigated. It is found that the softening mechanism of each constituent phase differs from each other. The ferrite is softened by dynamic recovery and continuous dynamic recrystallisation (CDRX), while the austenite is softened only by the limited discontinuous dynamic recrystallisation (DDRX). At lower strain rates (0.01 and 0.1?s^??1), the strain is mainly accommodated by ferrite due to its excellent softening capability, which causes the apparent activation energy Q_p to decline continuously with the increase in true strain. In this case, plastic deformation of the austenite rarely occurs, and at this time, DDRX of austenite is not observed. When the strain rate increases, CDRX of ferrite is weakened at a relative low temperature, which prompts the strain transfer into austenite and induces the strain hardening due to its restricted softening. Accordingly, interactions between the strain hardening in austenite and weakened softening of ferrite leads to one or more platforms of Q formed at the medium stage of deformation (1–30?s^??1). The processing map shows that two flow instability regions appear at high strain rate due to the lack of sufficient response time for dynamic restoration at the early deformation stage. As the strain increases, dynamic softening mechanism is activated at a higher temperature, resulting in a gradually narrowed flow instability region. Differently, a decrease in temperature suppresses dynamic softening of the alloy with a high strain rate, which deteriorates the hot workability of the alloy and induces microcrack formation after straining of 0.8.