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1.
G. V. Prasad Reddy R. Sandhya S. Sankaran M. D. Mathew 《Metallurgical and Materials Transactions A》2014,45(11):5044-5056
In this study, the influence of cyclic strain amplitude on the evolution of cyclic stress–strain response and the associated cyclic deformation mechanisms in 316LN stainless steel with varying nitrogen content (0.07 to 0.22 wt pct) is reported in the temperature range 773 K to 873 K (500 °C to 600 °C). Two mechanisms, namely dynamic strain aging and secondary cyclic hardening, are found to strongly influence the cyclic stress response. Deformation substructures associated with both the mechanisms showed planar mode of deformation. These mechanisms are observed to be operative over certain combinations of temperature and strain amplitude. For strain amplitudes >0.6 pct, wavy or mixed mode of deformation is noticed to suppress both the mechanisms. Cyclic stress–strain curves revealed both single and dual-slope behavior depending on the test temperature. Increase in nitrogen content is found to increase the tendency toward planar mode of deformation, while increase in strain amplitude leads to transition from planar slip bands to dislocation cell/wall structure formation, irrespective of the nitrogen content in 316LN stainless steel. 相似文献
2.
The relationship between cyclic deformation, slip-mode and dislocation structures is investigated in 316LN stainless steel (with 0.07–0.22 wt% Nitrogen) subjected to low cycle fatigue at temperatures in the range 300–873 K and at a 0.6 % strain amplitude. Irrespective of the nitrogen content, cyclic softening/saturation occupied a large fraction of fatigue life at temperatures <773 K. The end-of-life dislocation structures (e.g. dislocation cells, planar slip-bands) characterizing the cyclic softening/saturation belong to wavy/mixed/planar slip-modes of deformation. On the other hand at temperatures ≥773 K, similar dislocation structures are noticed to be associated with significant cyclic strengthening with limited softening. The differences in the above deformation behavior is found to be controlled not by the nature of slip-mode but by the consequences of dynamic strain aging occurrence (e.g. significant cyclic strengthening and pronounced serrations) which are noticed to vary in the temperature range 573–873 K. Maximum fatigue life is observed at 0.11–0.14 wt% N that induced mixed mode of deformation. 相似文献
3.
G. V. Prasad Reddy R. Sandhya S. Sankaran M. D. Mathew 《Metallurgical and Materials Transactions A》2014,45(11):5057-5067
Influence of nitrogen content on low cycle fatigue life and fracture behavior of 316LN stainless steel (SS) alloyed with 0.07 to 0.22 wt pct nitrogen is presented in this paper over a range of total strain amplitudes (±0.25 to 1.0 pct) in the temperature range from 773 K to 873 K (500 °C to 600 °C). The combined effect of nitrogen and strain amplitude on fatigue life is observed to be complex i.e., fatigue life either decreases/increases with increase in nitrogen content or saturates/peaks at 0.14 wt pct N depending on strain amplitude and temperature. Coffin–Manson plots (CMPs) revealed both single-slope and dual-slope strain-life curves depending on the test temperature and nitrogen content. 316LN SS containing 0.07 and 0.22 wt pct N showed nearly single-slope CMP at all test temperatures, while 316LN SS with 0.11 and 0.14 wt pct N exhibited marked dual-slope behavior at 773 K (500 °C) that changes to single-slope behavior at 873 K (600 °C). The changes in slope of CMP are found to be in good correlation with deformation substructural changes. 相似文献
4.
M. Valsan D. Sundararaman K. Bhanu Sankara Rao S. L. Mannan 《Metallurgical and Materials Transactions A》1995,26(5):1207-1219
A comparative evaluation of the low-cycle fatigue (LCF) behavior of type 316LN base metal, 316 weld metal, and 316LN/316 weld
joints was carried out at 773 and 873 K. Total strain-controlled LCF tests were conducted at a constant strain rate of 3 ×
10−3 s−1 with strain amplitudes in the range ±0.20 to ±1.0 pct. Weld pads with single V and double V configuration were prepared by
the shielded metal-arc welding (SMAW) process using 316 electrodes for weld-metal and weld-joint specimens. Optical microscopy,
scanning electron microscopy (SEM), and transmission electron microscopy (TEM) of the untested and tested samples were carried
out to elucidate the deformation and the fracture behavior. The cyclic stress response of the base metal shows a very rapid
hardening to a maximum stress followed by a saturated stress response. Weld metal undergoes a relatively short initial hardening
followed by a gradual softening regime. Weld joints exhibit an initial hardening and a subsequent softening regime at all
strain amplitudes, except at low strain amplitudes where a saturation regime is noticed. The initial hardening observed in
base metal has been attributed to interaction between dislocations and solute atoms/complexes and cyclic saturation to saturation
in the number density of slip bands. From TEM, the cyclic softening in weld metal was ascribed to the annihilation of dislocations
during LCF. Type 316LN base metal exhibits better fatigue resistance than weld metal at 773 K, whereas the reverse holds true
at 873 K. The weld joint shows the lowest life at both temperatures. The better fatigue resistance of weld metal is related
to the brittle transformed delta ferrite structure and the high density of dislocations at the interface, which inhibits the
growth rate of cracks by deflecting the crack path. The lower fatigue endurance of the weld joint was ascribed to the shortening
of the crack initiation phase caused by surface intergranular crack initiation and to the poor crack propagation resistance
of the coarse-grained region in the heat-affected zone. 相似文献
5.
G. V. Prasad Reddy R. Sandhya M. Valsan K. Bhanu Sankara Rao S. Sankaran 《Transactions of the Indian Institute of Metals》2010,63(2-3):505-510
In this paper, Low cycle fatigue (LCF) behavior of 316LN austenitic stainless steel alloyed with 0.078 and 0.22 wt% nitrogen, designated as N078 and N022 steels respectively, is compared in the temperature range 300–873 K by strain controlled fatigue tests at ± 0.6% strain amplitude. Interestingly, N022 steel showed continuous decrease in fatigue life with temperature in contrast to N078 steel which showed maximum in fatigue life at 573 K. Drastic reduction in fatigue life is observed in both the steels in the temperature range 673–873 K and has been attributed to the occurrence of dynamic strain aging. Both steels exhibited manifestations (for ex.: decrease in plastic strain and anomalous stress response with increase in temperature) corresponding to the occurrence of Dynamic Strain Ageing (DSA) in the above temperature range. Under all testing conditions, fracture surfaces revealed transgranular crack initiation and transgranular crack propagation. 相似文献
6.
H. Tian P. K. Liaw D. E. Fielden C. R. Brooks M. D. Brotherton L. Jiang B. Yang H. Wang J. P. Strizak L. K. Mansur 《Metallurgical and Materials Transactions A》2006,37(1):163-173
The high-cycle fatigue behavior of type 316 low-carbon, nitrogen-added (LN) stainless steel (SS), the prime-candidate target-container
material for the spallation neutron source (SNS), was investigated in air and mercury. Test frequencies ranged from 0.2 to
10 Hz with an R ratio of −1, and 10 to 700 Hz with an R ratio of 0.1. During tension-compression fatigue studies, a significant increase in the specimen temperature was observed
at 10 Hz in air, which decreased the fatigue life of the 316 LN SS relative to that at 0.2 Hz. Companion tests in air were
carried out, while cooling the specimen with nitrogen gas at 10 Hz in air. In these experiments, fatigue lives were comparable
at 10 Hz in air with nitrogen cooling and at 0.2 Hz in air. During tension-tension fatigue studies, a higher specimen temperature
was observed at 700 than at 10 Hz. After cooling the specimen, comparable fatigue lives were found at 10 and at 700 Hz. The
frequency effect on the fatigue life in mercury was found to be much less than that in air, due to the fact that mercury acts
as an effective coolant during the fatigue experiment. Striation spacing on the fracture surface at different test frequencies
was closely examined, relative to calculated ΔK values, during fatigue of the 316 LN SS. Specimen self-heating has to be considered in understanding fatigue characteristics
of 316 LN SS in air and mercury. 相似文献
7.
V. Ganesan M. D. Mathew P. Parameswaran K. Bhanu Sankara Rao 《Transactions of the Indian Institute of Metals》2010,63(2-3):417-421
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. 相似文献
8.
采用径向应变控制研究了Z3CN20-09M奥氏体不锈钢在室温和350℃高温下的低周疲劳行为.Z3CN20-09M不锈钢表现为先硬化后软化的循环特性,但硬化的程度取决于温度和应变幅.随着应变幅的增加,Z3CN20-09M钢的低周疲劳循环寿命逐渐减短,而相同循环次数下应力幅也随之提高.温度对Z3CN20-09M钢的低周疲劳行为影响较大,与室温相比高温下的循环硬化程度更高,相同应变幅下高温的低周疲劳寿命也高于常温下的寿命.通过疲劳实验的原位观察发现,奥氏体内的滑移面、夹杂物及奥氏体和铁素体两相的界面是疲劳裂纹可能的形核位置,奥氏体和铁素体两相的不协调变形使相界处产生应力集中,导致疲劳裂纹容易沿两相界面扩展. 相似文献
9.
Hyong Jik Lee Chong Soo Lee Young Won Chang 《Metallurgical and Materials Transactions A》2005,36(4):967-976
The role of nitrogen in the cyclic deformation behavior of duplex stainless steels (DSS) has been studied under fully reversed
total-strain amplitude. The cyclic hardening-softening curves show that cyclic stress levels become lower with increasing
nitrogen content. The cyclic softening becomes more evident with increasing nitrogen content. It can be attributed to the
greater strength of austenite than that of ferrite as plastic strain is accumulated beyond the critical strain. This is achieved
by a higher strain hardening of austenite than that of ferrite with increasing nitrogen content. In this regard, the higher
austenite volume fraction is also responsible for higher cyclic softening, resulting from much stronger strain partitioning
in ferrite. Dislocation-structure observations reveal that severe strain localization in ferrite causes greater cyclic softening
in the alloys with higher nitrogen content. The cyclic stress-strain response can be described in terms of two regimes with
low and high plastic-strain amplitudes. In the former regime, the cyclic strain-hardening rates (CSHRs) become higher with
increasing nitrogen content because austenite dominantly takes part in plastic deformation, being more strain hardened due
to the higher nitrogen content in austenite. On the contrary, those in the high-plastic-strain-amplitude regime hardly change
because ferrite, more dominantly accommodating plastic strain, rarely shows a change of strain-hardening behavior due to the
similar nitrogen content in ferrite. 相似文献
10.
T. H. Sanders R. E. Frishmuth G. T. Embley 《Metallurgical and Materials Transactions A》1981,12(6):1003-1010
Recent developments in the area of water cooled gas turbine design have created a need for low cycle fatigue test data for
alloy 718 in the temperature range of 204 to 649 °C. To support this need, data were generated in the room temperature to
649 °C range. As noted by previous investigators, there was a crossover in fatigue lives at low strain depending on temperature.
At high strain ranges the lowest fatigue life was exhibited at the higher temperatures. However, in the low strain, long life
regime this trend reversed with the fatigue life at a given strain range exhibiting a peak at some intermediate temperature.
Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) studies were conducted on the fatigue specimens
to determine the nature of the cyclic deformation process as a function of strain range and temperature, the principal mode
of deformation was by mechanical twinning. However, at the two highest temperatures, the primary process for deformation was
slip. The principal difference between the strain-life behavior of the specimens cycled at 538 and 649 °C, and those cycled
at the three lower temperatures (204, 316, 427 °C) is interpreted in light of this change in deformation process with temperature. 相似文献
11.
《Acta Metallurgica》1981,29(2):425-436
The fatigue hardening response of LiF single crystals subjected to uniaxial cyclic deformation, at various strain amplitudes and strain-rates, has been related to the monotonic hardening rate. Cumulative strain in fatigue is obtained through Kocks' flow stress theory on a statistical distribution of obstacles. The difference between monotonie and cyclic deformation is in the relative rate of accumulation of short and long range internal stresses. In the case of LiF at room temperature, fatigue hardening is essentially due to the increase in short range stresses. LiF crystals which are susceptible to cleavage fracture, can withstand large tensile stresses in fatigue when compared to monotonie deformation. Activation volume analysis indicates that vacancy and jog concentrations determine the dislocation mobility and fatigue hardening of LiF single crystals. Microscopic examination reveals the existence of very fine slip bands. In the case of crystals subjected to large number of fatigue cycles, surface irregularities and dislocation banding is observed. Optical and electron microscopy suggests dynamic recovery in the crystal studied. 相似文献
12.
13.
Commercially pure niobium (CPNb) and a niobium-1 pct zirconium (Nb-lZr) alloy were tested under low-cycle fatigue conditions
at plastic strain amplitudes in the range of 0.02 pct ≤Δεpl/2≤ 0.7 pct. At low temperatures, the cyclic deformation response of body-centered cubic (bcc) metals is strongly dependent
on strain rate. Thus, it was necessary to test at slow (2 x 10su-4 s-1) and fast (2 x 10-2 s-1) strain rates in order to fully characterize the cyclic deformation at ambient temperature. Only cyclic hardening was observed
for both metals under all testing conditions. As expected, higher cyclic stresses were recorded at the fast strain rate compared
to the slow strain rate. The Nb-lZr alloy was always stronger than CPNb, although both metals had the same cyclic life at
equal plastic strain amplitudes. Further, the strain rate had no effect on the cyclic life. At the fast strain rate, intergranular
cracking occurred, and a microplastic plateau was observed in the cyclic stress-strain (CSS) curve for CPNb. At the slow strain
rate, no definitely intergranular cracks were detected, and a microplastic plateau was not observed for CPNb. The results
of these experiments are interpreted in terms of the influence of strain rate and solute content on the relative mobilities
of edge and screw dislocations. 相似文献
14.
采用轴向应变幅控制的低周疲劳试验研究了总应变幅对4Cr5MoSiV1热作模具钢700 ℃低周疲劳行为的影响,包括循环应力响应行为、循环应力应变行为、循环迟滞回线和应变疲劳寿命行为等。结果表明:随着总应变幅从0.2%增大到0.6%,4Cr5MoSiV1钢在700 ℃时循环应力响应均表现为先循环硬化再循环软化的特性,并且应力幅最大值从220 MPa增大到308 MPa。同时,随着总应变幅的增大,4Cr5MoSiV1钢在700 ℃下的低周疲劳寿命由6750循环周次降低到210循环周次,且其过渡寿命约为1313循环周次。疲劳断口形貌分析结果显示,高温低周疲劳过程中裂纹主要萌生于试样表面处,且随着应变幅增大,裂纹源逐渐增多,疲劳条纹间距变宽,其断裂方式由韧性断裂转变为脆性断裂。透射电镜分析结果显示,循环软化可能与板条结构转变为胞状结构、基体发生位错湮灭、碳化物的析出和粗化有关。 相似文献
15.
The deformation and failure mechanisms under cyclic deformation in an 8090 Al-Li alloy reinforced with 15 vol pct SiC particles
were studied and compared to those of the unreinforced alloy. The materials were tested under fully reversed cyclic deformation
in the peak-aged and naturally aged conditions to obtain the cyclic response and the cyclic stress-strain curve. The peak-aged
materials remained stable or showed slight cyclic softening, and the deformation mechanisms were not modified by the presence
of the ceramic reinforcements: dislocations were trapped by the S′ precipitates and the stable response was produced by the mobile dislocations shuttling between the precipitates to accommodate
the plastic strain without further hardening. The naturally aged materials exhibited cyclic hardening until failure, which
was attributed to the interactions among dislocations. Strain localization and slip-band formation were observed in the naturally
aged alloy at high cyclic strain amplitudes, whereas the corresponding composite presented homogeneous deformation. Fracture
was initiated by grain-boundary delamination in the unreinforced materials, while progressive reinforcement fracture under
cyclic deformation was the main damage mechanism in the composites. The influence of these deformation and damage processes
in low-cycle fatigue life is discussed. 相似文献
16.
K. Bhanu Sankara Rao M. G. Castelli G. P. Allen John R. Ellis 《Metallurgical and Materials Transactions A》1997,28(2):347-361
The low-cycle fatigue (LCF) behavior of a wrought cobalt-base superalloy, Haynes 188, has been investigated over a range of
temperatures between 25 °C and 1000 °C employing a triangular waveform and a constant strain amplitude of ±0.4 pct. Correlations
between macroscopic cyclic deformation and fatigue life with the various microstructural phenomena were enabled through scanning
electron microscopy (SEM) and transmission electron microscopy (TEM), detailing the crack initiation and propagation modes,
deformation substructure, and carbide precipitation. Cyclic stress response varied as a complex function of temperature. Dynamic
strain aging (DSA) was found to occur over a wide temperature range between 300 °C and 750 °C. In the DSA domain, the alloy
exhibited marked cyclic hardening with a pronounced maximum at 650 °C. Dynamic strain aging has been documented through the
occurrence of serrated yielding, inverse temperature dependence of maximum cyclic stress, and cyclic inelastic strain developed
at half of the fatigue life. Additionally, the alloy also displayed a negative strain rate sensitivity of cyclic stress in
the DSA regime. These macroscopic features in the DSA domain were accompanied by the substructure comprised of coplanar distribution
of dislocations associated with the formation of pileups, stacking faults, and very high dislocation density. Toward the end
of the DSA domain, dislocation pinning by M23C6 precipitates occurred predominantly. The deformation behavior below and above the DSA domain has also been investigated in
detail. The temperature dependence of LCF life showed a maximum at ≈300 °C. The drastic reduction in life between 300 °C and
850 °C has been ascribed primarily to the deleterious effects of DSA on crack initiation and propagation, while the lower
life at temperatures less than 200 °C has been attributed to the combined influence of low ductility and larger cyclic response
stress. 相似文献
17.
Bending fatigue studies of low carbon steel after gaseous nitrocarburizing have shown that the fatigue strength is increased
by about 140 pct relative to annealed material. X-ray diffraction and electron microscopy studies have demonstrated that fine
scale precipitation occurs in the outermost 400 μ region of the diffusion zone and, beyond this, both nitrogen and carbon
essentially remain in solid solution. Substantial cyclic hardening has been observed during the fatigue of nitrocarburized
material. The higher the applied stress the greater the depth of hardening and at high applied stresses the depth of hardening
extends beyond the diffusion zone into the core of the material. Fading of surface residual stresses during fatigue is observed,
however subsurface compressive stresses are present throughout the life of the material. At low applied stresses subsurface
plastic deformation occurs within the nitrocarburized material. Under long life fatigue conditions the amount of bulk plastic
deformation required to cause fracture of the nitrocarburized material is significantly greater than for annealed material,
but for short lives, the reverse is observed. 相似文献
18.
Effect of boron on the low-cycle fatigue behavior and deformation structure of INCONEL 718 at 650 °C
L. Xiao M. C. Chaturvedi D. L. Chen 《Metallurgical and Materials Transactions A》2004,35(11):3477-3487
Symmetrical push-pull low-cycle fatigue (LCF) tests were performed on INCONEL 718 (IN718) containing 12, 29, 60, and 100 ppm
B at 650 °C. The results showed that all the alloys experienced a relatively short period of initial cyclic hardening at low
strain amplitudes, followed by a regime of saturation or slightly continuous cyclic softening. The initial cyclic hardening
phase decreased with increasing strain amplitudes, and disappeared at the high strain amplitudes. A serrated flow was observed
in the plastic regions of cyclic stress-strain hysteresis loops. The saturated cyclic stress amplitude at a given strain amplitude
was highest for the alloy with 60 ppm B, and lowest for the alloy with 12 ppm B. The LCF lifetime increased with increasing
B concentration up to 60 ppm, and then decreased as the B content increased from 60 to 100 ppm. Fractographic analysis suggested
that the fracture mode changed from intergranular to transgranular cracking as the B concentration increased. The characteristic
deformation microstructures produced by LCF tests at 650 °C, examined via transmission electron microscopy, were regularly spaced arrays of planar deformation bands on {111} slip planes in all four
alloys. A ladderlike structure was observed in some local regions in the alloy with 12 ppm B. Heavily deformed planar deformation
bands were observed in the fatigued specimens with 100 ppm B. The mechanism of improvement in the LCF life of IN718 due to
B addition is discussed. 相似文献
19.
试验的316LN钢(/%:0.011C,0.49Si,1.52Mn,0.016P,0.001S,16.65Cr,13.43Ni,2.18Mo,0.153N)为7.2 t铸锭初轧开坯并锻造成的Φ180 mm圆钢。利用Gleeble-3800热模拟试验机研究了316LN钢在应变速率0.1~50 s-1、1050~1200℃时的热变形行为和组织演变规律,并通过线性回归分析建立了316LN钢的本构方程。结果表明,变形温度升高有利于动态再结晶的形核,并在热变形过程中促进动态再结晶扩展;热变形过程材料常数α为0.0061,应力指数n为5.5436,表观激活能550.512 kJ/mol,动态再结晶晶粒数量随着变形温度的增加而增多,到1 200℃变形时发生了完全的动态再结晶。 相似文献
20.
Low-cycle fatigue behavior of INCONEL 718 superalloy with different concentrations of boron at room temperature 总被引:1,自引:0,他引:1
L. Xiao M. C. Chaturvedi D. L. Chen 《Metallurgical and Materials Transactions A》2005,36(10):2671-2684
Symmetrical push-pull low-cycle fatigue (LCF) tests were performed on INCONEL 718 superalloy containing 12, 29, 60, and 100
ppm boron (B) at room temperature (RT). The results showed that all four of these alloys experienced a relatively short period
of initial cyclic hardening, followed by a regime of softening to fracture at higher cyclic strain amplitudes (Δɛ
t
/2≥0.8 pct). As the cyclic strain amplitude decreased to Δɛ
t
/2≤0.6 pct, a continuous cyclic softening occurred without the initial cyclic hardening, and a nearly stable cyclic stress
amplitude was observed at Δɛ
t
/2=0.4 pct. At the same total cyclic strain amplitude, the cyclic saturation stress amplitude among the four alloys was highest
in the alloy with 60 ppm B and lowest in the alloy with 29 ppm B. The fatigue lifetime of the alloy at RT was found to be
enhanced by an increase in B concentration from 12 to 29 ppm. However, the improvement in fatigue lifetime was moderate when
the B concentration exceeded 29 ppm B. A linear relationship between the fatigue life and cyclic total strain amplitude was
observed, while a “two-slope” relationship between the fatigue life and cyclic plastic strain amplitude was observed with
an inflection point at about Δɛ
p
/2=0.40 pct. The fractographic analyses suggested that fatigue cracks initiated from specimen surfaces, and transgranular
fracture, with well-developed fatigue striations, was the predominant fracture mode. The number of secondary cracks was higher
in the alloys with 12 and 100 ppm B than in the alloys with 29 and 60 ppm B. Transmission electron microscopy (TEM) examination
revealed that typical deformation microstructures consisted of a regularly spaced array of planar deformation bands on {111}
slip planes in all four alloys. Plastic deformation was observed to be concentrated in localized regions in the fatigued alloy
with 12 ppm B. In all of the alloys, γ″ precipitate particles were observed to be sheared, and continued cyclic deformation reduced their size. The observed cyclic
deformation softening was associated with the reduction in the size of γ″ precipitate particles. The effect of B concentration on the cyclic deformation mechanism and fatigue lifetime of IN 718 was
discussed. 相似文献