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1.
Y. G. Ko C. S. Lee D. H. Shin S. L. Semiatin 《Metallurgical and Materials Transactions A》2006,37(2):381-391
The low-temperature superplasticity of ultra-fine-grained (UFG) Ti-6Al-4V was established as a function of temperature and
strain rate. The equiaxed-alpha grain size of the starting material was reduced from 11 to 0.3 μm (without a change in volume
fraction) by imposing an effective strain of ∼4 via isothermal, equal-channel angular pressing (ECAP) at 873 K. The ultrafine microstructure so produced was relatively stable
during annealing at temperatures up to 873 K. Uniaxial tension and load-relaxation tests were conducted for both the starting
(coarse-grained (CG)) and UFG materials at temperatures of 873 to 973 K and strain rates of 5 × 10−5 to 10−2 s−1. The tension tests revealed that the UFG structure exhibited considerably higher elongations compared to those of the CG
specimens at the same temperature and strain rate. A total elongation of 474 pct was obtained for the UFG alloy at 973 K and
10−4 s−1. This fact strongly indicated that low-temperature superplasticity could be achieved using an UFG structure through an enhancement
of grain-boundary sliding in addition to strain hardening. The deformation mechanisms underlying the low-temperature superplasticity
of UFG Ti-6Al-4V were also elucidated by the load-relaxation tests and accompanying interpretation based on inelastic deformation
theory. 相似文献
2.
A. Venugopal Reddy G. Sundararajan 《Metallurgical and Materials Transactions A》1987,18(13):1043-1052
The objective of this work was to understand the influence of grain size on solid impingement erosion behavior characterized by deformation at high strain rates and large strains. Experiments were carried out at a velocity of 40 m/s, impact angle of 90 deg with 300 to 450 μm steel shot as erodent on iron, copper, and titanium with varying grain sizes. The results indicate that the erosion rate is independent of grain size in iron and copper while it is apparently grain size dependent in titanium. The results are rationalized in terms of the negligible contribution of the Hall-Petch component to the flow stress at large strains in the case of copper and iron. The decreasing erosion rate in titanium with increasing grain size was due to the increased interstitial content picked up during thermal treatment and consequent increase in strain hardening and strain rate hardening and not due to increased grain sizeper se. Adiabatic shear bands were observed in coarse-grained iron under actual erosion conditions. 相似文献
3.
The objective of this work was to understand the influence of grain size on solid impingement erosion behavior characterized
by deformation at high strain rates and large strains. Experiments were carried out at a velocity of 40 m/s, impact angle
of 90 deg with 300 to 450 μm steel shot as erodent on iron, copper, and titanium with varying grain sizes. The results indicate
that the erosion rate is independent of grain size in iron and copper while it is apparently grain size dependent in titanium.
The results are rationalized in terms of the negligible contribution of the Hall-Petch component to the flow stress at large
strains in the case of copper and iron. The decreasing erosion rate in titanium with increasing grain size was due to the
increased interstitial content picked up during thermal treatment and consequent increase in strain hardening and strain rate
hardening and not due to increased grain sizeper se. Adiabatic shear bands were observed in coarse-grained iron under actual erosion conditions. 相似文献
4.
Yulong Zhang Li Wang Kip O. Findley John G. Speer 《Metallurgical and Materials Transactions A》2017,48(5):2140-2149
With an aim to elucidate the influence of temperature and grain size on austenite stability, a commercial cold-rolled 7Mn steel was annealed at 893 K (620 °C) for times varying between 3 minutes and 96 hours to develop different grain sizes. The austenite fraction after 3 minutes was 34.7 vol pct, and at longer times was around 40 pct. An elongated microstructure was retained after shorter annealing times while other conditions exhibited equiaxed ferrite and austenite grains. All conditions exhibit similar temperature dependence of mechanical properties. With increasing test temperature, the yield and tensile strength decrease gradually, while the uniform and total elongation increase, followed by an abrupt drop in strength and ductility at 393 K (120 °C). The Olson–Cohen model was applied to fit the transformed austenite fractions for strained tensile samples, measured by means of XRD. The fit results indicate that the parameters α and β decrease with increasing test temperature, consistent with increased austenite stability. The 7Mn steels exhibit a distinct temperature dependence of the work hardening rate. Optimized austenite stability provides continuous work hardening in the temperature range of 298 K to 353 K (25 °C to 80 °C). The yield and tensile strengths have a strong dependence on grain size, although grain size variations have less effect on uniform and total elongation. 相似文献
5.
6.
Jun Hwan Park Kyung-Tae Park Yong Shin Lee Won Jong Nam 《Metallurgical and Materials Transactions A》2005,36(5):1365-1368
The compression behaviors of well-annealed coarsegrained (CG) and ultrafine-grained (UFG) 5083 Al alloys at 77 and 298 K were
compared. For the CG alloy, stage II and III strain hardening were dominant at 77 and 298 K, respectively, depending on the
completeness of dislocation cell formation. The UFG alloy exhibited the elastic-near perfectly plastic behavior without distinctive
dislocation cell formation at both temperatures. For both alloys, the flow stress at 77 K was much higher than that at 298
K.
This work was supported by the Basic Research Program (Grant No. R01-2003-000-10202-0) of the Korea Science & Engineering
Foundation. 相似文献
7.
Farghalli A. Mohamed Mohamed M. I. Ahmed Terence G. Langdon 《Metallurgical and Materials Transactions A》1977,8(6):933-938
The maximum attainable ductility in the superplastic Zn-22 pct Al eutectoid depends critically on the imposed strain rate,
the testing temperature, and the initial grain size. High ductilities are observed at intermediate strain rates, and there
is a decrease at both higher and lower rates of strain. It is shown that i) the maximum ductility occurs at higher strain
rates as the temperature is increased and/or the initial grain size is decreased, and ii) the maximum attainable ductility
increases with increasing temperature and/or decreasing initial grain size. For specimens tested at different temperatures,
similar macroscopic fracture characteristics are observed in specimens exhibiting a similar maximum flow stress. The experimental
trends are qualitatively explained by relating maximum ductility to the maximum strain rate sensitivity and examining the
influence of cavitation on the time to rupture. 相似文献
8.
Zhenhua Wang Wantang Fu Shuhua Sun Hui Li Zhiqing Lv Deli Zhao 《Metallurgical and Materials Transactions A》2010,41(4):1025-1032
The hot deformation behavior of a high nitrogen CrMn austenitic stainless steel in the temperature range 1173 to 1473 K (900
to 1200 °C) and strain rate range 0.01 to 10 s−1 was investigated using optical microscopy, stress-strain curve analysis, processing maps, etc. The results showed that the work hardening rate and flow stress decreased with increasing deformation temperature and decreasing
strain rate in 18Mn18Cr0.5N steel. The dynamic recrystallization (DRX) grain size decreased with increasing Z value; however, deformation heating has an effect on the DRX grain size under high strain rate conditions. In the processing
maps, flow instability was observed at higher strain rate regions (1 to 10 s−1) and manifested as flow localization near the grain boundary. Early in the deformation, the flow instability region was at
higher temperatures, and then the extent of this unstable region decreased with increasing strain and was restricted to lower
temperatures. The hot deformation equation as well as the quantitative dependence of the critical stress for DRX and DRX grain
size on Z value was obtained. 相似文献
9.
A comparative study of the embrittlement of monel 400 at room temperature by hydrogen and by mercury
Slow strain rate tensile tests were performed at room temperature on Monel 400 specimens of grain sizes 35 to 500 μm, in the
environments of air, mercury, and electrolytically generated hydrogen. Specimens of grain size 250 μm were tested at a range
of strain rates in the three environments. It was found that cracks initiated easiest in hydrogen but propagated easiest in
mercury; consequently the embrittlement was usually more severe in mercury. The embrittlement decreased with increasing strain
rate, and with increasing grain size in hydrogen. Embrittlement in mercury was a maximum at intermediate grain sizes. A fracture
sequence of intergranular to transgranular to microvoid coalescence was common. The intergranular and transgranular fractures
are interpreted in terms of the reduced cohesive stress and enhanced shear models of embrittlement, respectively. 相似文献
10.
11.
Clarissa A. Yablinsky Ellen K. Cerreta George T. GrayIII Donald W. Brown Sven C. Vogel 《Metallurgical and Materials Transactions A》2006,37(6):1907-1915
The work-hardening behavior of hexagonal-close-packed (hcp) metals, such as hafnium, is influenced by temperature, strain
rate, chemistry, and texture. In the case of hafnium, while slip on the prism and pyramidal planes is dominant during deformation,
the propensity of deformation twinning is known to increase with decreasing temperature and increasing strain rate. In this
study, hafnium was prestrained quasi-statically in compression at liquid nitrogen temperature (77 K), creating a heavily twinned
microstructure. The specimens were then reloaded in compression at room temperature (298 K). Yield stress, flow stress, and
work-hardening behaviors of the prestrained specimens were higher than room-temperature compression test data typical of the
as-annealed material. The microstructure of each specimen was characterized optically and using a transmission electron microscope
(TEM). Texture was measured by neutron diffraction and the texture evolution due to twinning, and the interaction of slip
with the twins was seen to lead to higher work-hardening rates and flow stresses in the cold prestrained specimens. 相似文献
12.
Aashish Rohatgi Kenneth S. Vecchio George T. Gray III 《Metallurgical and Materials Transactions A》2001,32(1):135-145
The role of stacking fault energy (SFE) in deformation twinning and work hardening was systematically studied in Cu (SFE ∼78
ergs/cm2) and a series of Cu-Al solid-solution alloys (0.2, 2, 4, and 6 wt pct Al with SFE ∼75, 25, 13, and 6 ergs/cm2, respectively). The materials were deformed under quasi-static compression and at strain rates of ∼1000/s in a Split-Hopkinson
pressure bar (SHPB). The quasi-static flow curves of annealed 0.2 and 2 wt pct Al alloys were found to be representative of
solid-solution strengthening and well described by the Hall-Petch relation. The quasi-static flow curves of annealed 4 and
6 wt pct Al alloys showed additional strengthening at strains greater than 0.10. This additional strengthening was attributed
to deformation twins and the presence of twins was confirmed by optical microscopy. The strengthening contribution of deformation
twins was incorporated in a modified Hall-Petch equation (using intertwin spacing as the “effective” grain size), and the
calculated strength was in agreement with the observed quasi-static flow stresses. While the work-hardening rate of the low
SFE Cu-Al alloys was found to be independent of the strain rate, the work-hardening rate of Cu and the high SFE Cu-Al alloys
(low Al content) increased with increasing strain rate. The different trends in the dependence of work-hardening rate on strain
rate was attributed to the difference in the ease of cross-slip (and, hence, the ease of dynamic recovery) in Cu and Cu-Al
alloys. 相似文献
13.
V. Kutumba Rao D. M. R. Taplin P. Rama Rao 《Metallurgical and Materials Transactions A》1975,6(1):77-86
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. 相似文献
14.
15.
Woei-Shyan Lee Tao-Hsing Chen Chi-Feng Lin Wen-Zhen Luo 《Metallurgical and Materials Transactions A》2012,43(11):3998-4005
The impact response and microstructural evolution of 316L stainless steel are examined at strain rates ranging from 1?×?103 to 5?×?103?s?1 and temperatures between 298?K and 1073?K (25?°C and 800?°C) using a split Hopkinson pressure bar and transmission electron microscopy (TEM). The results show that the flow behavior, mechanical strength, and work-hardening properties of 316L stainless steel are significantly dependent on the strain rate and temperature. The TEM observations reveal that the dislocation density increases with increasing strain rate but decreases with increasing temperature. Moreover, twinning occurs only in the specimens deformed at 298?K (25?°C), which suggests that the threshold stress for twinning is higher than that for slip under impact loading. Finally, it is found that the volume fraction of transformed ???? martensite increases with increasing strain rate or decreasing temperature. Overall, the results suggest that the increased flow stress observed in 316L stainless steel under higher strain rates and lower temperatures is determined by the combined effects of dislocation multiplication, twin nucleation and growth, and martensite transformation. 相似文献
16.
For ultra low carbon (ULC) and low carbon steel (LC), the influence of heating rate, annealing temperature, and holding time on the recrystallisation behaviour and the resulting grain size was investigated. For ULC smallest grain sizes of about 9 μm were obtained at the lowest heating rate whereas for LC significant smaller grain sizes of about 5 μm were determined at the highest heating rate. Furthermore, the evolution of the grain size distribution with varying heating rate, annealing temperature, and holding time was studied in dependence of the rolling and normal direction. The state of the as‐hot rolled microstructure as well as the precipitation state exert a strong influence on the development of the recrystallised microstructure along the different directions for both steel grades. The inherent prolonged microstructure due to the cold rolling process is still obvious just after recrystallisation. With ongoing annealing and grain growth, the aspect ratio approaches the equiaxed state. This change proceeds faster for the ULC steel grade. With increasing annealing temperature, the bimodal character of the grain size distribution disappears and the distribution becomes more homogeneous. 相似文献
17.
B. K. Choudhary 《Metallurgical and Materials Transactions A》2014,45(1):302-316
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. 相似文献
18.
Tensile tests using coarse grained zirconium specimens were conducted at two strain rates, differing by 3 orders of magnitude,
between 77° and 1032°K. At each strain rate, peaks were observed when the flow stress was plotted against the temperature.
The temperature corresponding to a given peak was observed to rise with increasing strain rate. A pronounced minimum in the
strain rate sensitivity of zirconium near 675°K can be explained in terms of the strain rate dependence of these peaks. At
each strain rate, the zirconium tensile specimens also showed a minimum elongation at the hardening peak temperature. Since
the reduction in area did not pass through a corresponding minimum, the elongation minima do not reflect a true ductility
loss. What actually takes place is an increased tendency to neck at the hardening peak temperature. This tendency to promote
a neck can be rationalized in terms of variations in the strain rate sensitivity caused by dynamic strain aging.
Former Graduate Student now Post Doctoral Worker and Professor 相似文献
19.
This work identifies the influence of strain rate, temperature, plastic strain, and microstructure on the strain rate sensitivity of automotive sheet steel grades in crash conditions. The strain rate sensitivity m has been determined by means of dynamic tensile tests in the strain rate range 10?3–200 s?1 and in the temperature range 233–373 K. The dynamic flow curves have been tested by means of servohydraulic tensile testing. The strain rate sensitivity decreases with increasing plastic strain due to a gradual exhausting of work hardening potential combined with adiabatic softening effects. The strain rate sensitivity is improved with decreasing temperature and increasing strain rate, according to the thermally activated deformation mechanism. The m‐value is reduced with increasing strength level, this decrease being most pronounced for steels with a yield strength below 400 MPa. Solid solution alloying with manganese, silicon, and especially phosphorous elements lowers the strain rate sensitivity significantly. Second phase hardening with bainite and martensite as the second constituent in a ferritic matrix reduces the strain rate sensitivity of automotive sheet steels. A statistical modeling is proposed to correlate the m‐value with the corresponding quasistatic tensile flow stress. 相似文献
20.
S. Kajiwara 《Metallurgical and Materials Transactions A》1986,17(10):1693-1702
In order to elucidate roles of dislocations and grain boundaries in martensite nucleation, the transformation temperature
(Ms) of specimens austenitized at various temperatures and subjected to prestrain has been measured, using Fe-Ni, Fe-Ni-C, and
Fe-Cr-C alloys. It is concluded that the plastic accommodation, in austenite, of the shape strain of the transforming martensite
is a vital step in the nucleation event. Any factors impeding such plastic accommodation, such as the lack of dislocations,
work hardening, and grain refinement, suppress the transformation. Contrary to the general belief, dislocations themselves
do not act as favorable nucleation sites. Grain boundaries provide nucleation site, but only certain types of grain boundaries
are qualified to be potential nuclei. A quantitative analysis shows that the increasing difficulty for the plastic accommodation
with decreasing grain size is the main factor to depress Ms in fine-grained specimens. 相似文献