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1.
This article presents a study of fatigue-crack propagation behavior in Nitinol a 50Ni-50Ti (at. pct) superelastic/shape-memory
alloy, with particular emphasis on the effect of the stress-induced martensitic transformation on crack-growth resistance.
Specifically, fatigue-crack growth was characterized in stable austenite (at 120°C), superelastic austenite (at 37°C), and
martensite (at—65°C and—196°C). In general, fatigue-crack growth resistance was found to increase with decreasing temperature,
such that fatigue thresholds were higher and crack-growth rates slower in martensite compared to stable austenite and superelastic
austenite. Of note was the observation that the stress-induced transformation of the superelastic austenite structure, which
occurs readily at 37°C during uniaxial tensile testing, could be suppressed during fatigue-crack propagation by the tensile
hydrostatic stress state ahead of a crack tip in plane strain; this effect, however, was not seen in thinner specimens, where
the constraint was relaxed due to prevailing plane-stress conditions.
A.L. McKELVEY, formerly Graduate Student, Department of Materials Science and Mineral Engineering, University of California 相似文献
2.
A. I. Popelyukh A. A. Bataev A. M. Teplykh A. Yu. Ognev E. D. Golovin 《Steel in Translation》2011,41(4):361-364
Heat treatment with mixed martensite-bainite transformation of austenite is optimized in terms of strength and fatigue-crack
resistance. For the manufacture of components in impact mechanisms, the best combination of strength, hardness, and crack
resistance is obtained for steel in which 40% martensite is formed on intermediate cooling, while isothermal heating converts
the residual supercooled austenite to lower bainite. This increases the strength by 200–250 N/mm2 in relation to quenching + tempering + isothermal quenching (with comparable crack resistance) and increases the resistance
to fatigue-crack propagation by a factor of 1.3–2 (with analogous strength). 相似文献
3.
K. T. Venkateswara Rao J. C. McNulty R. O. Ritchie 《Metallurgical and Materials Transactions A》1993,24(10):2233-2245
Aluminum-lithium alloys are currently being considered for applications at moderately elevated temperatures; accordingly,
a study has been made on the effects of prolonged (100 and 1000 hours overaging) thermal exposure at 149 °C and 260 °C on
the mechanical properties of a peakaged Al-Li-Cu-Mg-Zr alloy 8090-T8771. In the as-received T8771 temper, the alloy exhibits
an excellent combination of strength (˜500 MPa) and toughness (35 MPa√m) with moderate tensile elongation (4 pct). Overaging
at 149 °C results in a ˜50 pct reduction in ductility and toughness, primarily associated with the growth of equilibrium phases
along grain/subgrain boundaries, resulting in formation of solute-depleted precipitate-free zones and coarsening of matrix8' andS precipitates; strength levels and fatigue-crack growth rates, however, remain largely unchanged. Thermal exposures at 260
°C, conversely, lead to dramatic reductions in strength (by ˜50 to 80 pct), toughness (by ˜30 pct) and fatigue-crack propagation
resistance; crack-growth rates at all ΔK levels above ~5 MPa√m are 2 to 3 orders of magnitude faster. Microstructurally, this was associated with complete dissolution
of δ′, severe coarsening ofS andT
2
precipitates in the matrix, and formation of equilibrium Cu- and Mg-rich intermetallic phases in the matrix and along grain
boundaries. The resulting lack of planar-slip deformation and low yield strength of 8090 following overaging exposures at
260 °C increase the cumulative crack-tip damage per cycle and reduce the tendency for crack-path deflection, thereby accelerating
fatigue-crack growth rates. Despite this degradation in properties, the 8090-T8771 alloy has better strength retention and
generally superior fatigue-crack growth properties compared to similarly overaged Al-Li-Cu-Zr 2090 and Al-Cu-Zn-Mg 7150 alloys.
formerly with the University of California,
formerly with the University of California, 相似文献
4.
Yasuharu Sakuma David K. Matlock George Krauss 《Metallurgical and Materials Transactions A》1992,23(4):1233-1241
Stress-strain behavior and deformation-induced transformation of retained austenite were studied for intercritically annealed
and isothermally transformed Si-Mn and Ni steels as a function of testing temperature between −80 °C and 120 °C. Rapid decrease
of retained austenite at small strains dominates at low-temperature testing and in microstructures containing martensite.
The austenite transformation in microstructures without martensite shifts to larger strains with increasing testing temperature.
The accompanying increase of strain-hardening rates at larger strains deters the onset of necking and improves ductility.
The benefits of the austenite transformation lead to a peak in ductility between 20 °C and 70 °C in the Si-Mn steel and at
70 °C in the Ni steel. The peaks are dependent on the nature of the dispersed microconstituents produced in the ferrite during
isothermal transformation. Higher testing temperatures enhance the mechanical stability of the austenite and result in lower
ductility. 相似文献
5.
C. X. Huang G. Yang C. Wang Z. F. Zhang S. D. Wu 《Metallurgical and Materials Transactions A》2011,42(7):2061-2071
The technique of equal-channel angular pressing (ECAP) was used to refine the microstructure of an AISI 301 austenitic stainless
steel (SS). An ultrafine-grained (UFG) microstructure consisting mainly of austenite and a few martensite was achieved in
301 steel after ECAP processing for four passes at 523 K (250 °C). By submitting the as-ECAP rods to annealing treatment in
the temperature range from 853 K to 893 K (580 °C to 620 °C) for 60 minutes, fully austenitic microstructures with grain sizes
of 210 to 310 nm were obtained. The uniaxial tensile tests indicated that UFG 301 austenitic SS had an excellent combination
of high yield strength (>1.0 GPa) and high elongation-to-fracture (>30 pct). The tensile stress–strain curves exhibited distinct
yielding peak followed by obvious Lüders deformation. Measurements showed that Lüders elongation increased with an increase
in strength as well as a decrease in grain size. The microstructural changes in ultrafine austenite grains during tensile
deformation were tracked by X-ray diffraction and transmission electron microscope. It was found that the strain-induced phase
transformation from austenite to martensite took place soon after plastic deformation. The transformation rate with strain
and the maximum strain-induced martensite were promoted significantly by ultrafine austenite grains. The enhanced martensitic
transformation provided extra strain-hardening ability to sustain the propagation of Lüders bands and large uniform plastic
deformation. During tensile deformation, the Lüders bands and martensitic transformation interacted with each other and made
great contribution to the excellent mechanical properties in UFG austenitic SS. 相似文献
6.
7.
In an attempt to understand the role of retained austenite on the cryogenic toughness of a ferritic Fe-Mn-AI steel, the mechanical
stability of austenite during cold rolling at room temperature and tensile deformation at ambient and liquid nitrogen temperature
was investigated, and the microstructure of strain-induced transformation products was observed by transmission electron microscopy
(TEM). The volume fraction of austenite increased with increasing tempering time and reached 54 pct after 650 °C, 1-hour tempering
and 36 pct after 550 °C, 16-hour tempering. Saturation Charpy impact values at liquid nitrogen temperature were increased
with decreasing tempering temperature, from 105 J after 650 °C tempering to 220 J after 550 °C tempering. The room-temperature
stability of austenite varied significantly according to the(α + γ) region tempering temperature;i.e., in 650 °C tempered specimens, 80 to 90 pct of austenite were transformed to lath martensite, while in 550 °C tempered specimens,
austenite remained untransformed after 50 pct cold reductions. After tensile fracture (35 pct tensile strain) at -196 °C,
no retained austenite was observed in 650 °C tempered specimens, while 16 pct of austenite and 6 pct of e-martensite were
observed in 550 °C tempered specimens. Considering the high volume fractions and high mechanical stability of austenite, the
crack blunting model seems highly applicable for improved cryogenic toughness in 550 °C tempered steel. Other possible toughening
mechanisms were also discussed.
Formerly Graduate Student, Seoul National University. 相似文献
8.
Influence of the amount and morphology of retained austenite on the mechanical properties of an austempered ductile iron 总被引:1,自引:0,他引:1
J. ARanzabal I. Gutierrez J. M. Rodriguez-Ibabe J. J. Urcola 《Metallurgical and Materials Transactions A》1997,28(5):1143-1156
High Si contents in nodular cast irons lead to a significant volume fraction of retained austenite in the material after the
austempering treatment. In the present work, the influence of the amount and morphology of this phase on the mechanical properties
(proof stress, ultimate tensile strength (UTS), elongation, and toughness) has been analyzed for different austempering conditions.
After 300 °C isothermal treatments at intermediate times, the austenite is plastically stable at room temperature and contributes,
together with the bainitic ferrite, to the proof stress and the toughness of the material. For austenite volume fractions
higher than 25 pct, the proof stress is controlled by this phase and the toughness depends mainly on the stability of γ. In these conditions (370 °C and 410 °C treatments), the present material exhibits a transformation-induced plasticity (TRIP)
effect, which leads to an improvement in ductility. It is shown that the strain level necessary to initiate the martensitic
transformation induced by deformation depends on the carbon content of the austenite. The martensite formed under TRIP conditions
can be of two different types: “autotempered” plate martensite, which forms at room temperature from an austenite with a quasi-coherent
epsilon carbide precipitation, and lath martensite nucleated at twin boundaries and twin intersections. 相似文献
9.
Martensite formation,strain rate sensitivity,and deformation behavior of type 304 stainless steel sheet 总被引:1,自引:0,他引:1
The strain and strain rate dependence of the deformation behavior of Type 304 stainless steel sheet was evaluated by constant
temperature tensile testing in the temperature range of −80 °C to 160 °C. The strain rate sensitivity, strain hardening rate,
and ductility reflected the compctition of two strengthening mechanisms: strain-induced transformation of austenite to martensite
and dislocation substructure formation. At low temperatures, the strain rate sensitivity and strain hardening rate correlated
with the strain-induced transformation rate. A maximum in total ductility occurred between 0 °C and 25 °C, and the contributions
of strain rate sensitivity and strain hardening to independent maxima with temperature of the uniform and post-uniform strains
are discussed.
Formerly Visiting Scientist, Department of Metallurgical Engineering, Colorado School of Mines. 相似文献
10.
A. L. McKelvey K. T. Venkateswara Rao R. O. Ritchie 《Metallurgical and Materials Transactions A》2000,31(5):1413-1423
A study has been made of the effect of temperature (between 25 °C and 800 °C) on fracture toughness and fatigue-crack propagation
behavior in an XD-processed, γ-based titanium aluminide intermetallic alloy, reinforced with a fine dispersion of ∼1 vol pct TiB2 particles. It was found that, whereas crack-initiation toughness increased with increasing temperature, the crack-growth
toughness on the resistance curve was highest just below the ductile-to-brittle transition temperature (DBTT) at 600 °C; indeed,
above the DBTT, at 800 °C, no rising resistance curve was seen. Such behavior is attributed to the ease of microcrack nucleation
above and below the DBTT, which, in turn, governs the extent of uncracked ligament bridging in the crack wake as the primary
toughening mechanism. The corresponding fatigue-crack growth behavior was also found to vary inconsistently with temperature.
The fastest crack growth rates (and lowest fatigue thresholds) were seen at 600 °C, while the slowest crack growth rates (and
highest thresholds) were seen at 800 °C; the behavior at 25 °C was intermediate. Previous explanations for this “anomalous
temperature effect” in γ-TiAl alloys have focused on the existence of some unspecified environmental embrittlement at intermediate temperatures or
on the development of excessive crack closure at 800 °C; no evidence supporting these explanations could be found. The effect
is now explained in terms of the mutual competition of two processes, namely, the intrinsic microstructural damage/crack-advance
mechanism, which promotes crack growth, and the propensity for crack-tip blunting, which impedes crack growth, both of which are markedly enhanced by increasing temperature. 相似文献
11.
Liu Cheng C. M. Brakman B. M. Korevaar E. J. Mittemeijer 《Metallurgical and Materials Transactions A》1988,19(10):2415-2426
The aging behavior of iron-carbon martensite (1.13 wt Pct C) between -190 °C and 450 °C was investigated by quantitative analysis
of the corresponding changes in volume and enthalpy. A method to determine activation energies of the occurring solid-state
transformations by performing non-isothermal measurements of some physical property of the specimen has been described. Martensitic
specimens were prepared by carburizing pure iron and quenching in brine and liquid nitrogen. The dilatometric and calorimetric
experiments were supplemented with microhardness measurements. At least five different stages of structural change can be
distinguished, which are quantitatively analyzed in terms of their effects on volume and enthalpy: (i) transformation of retained
austenite into martensite (between −180 and −100 °C); (ii) redistribution of carbon atoms (below 100 °C); (iii) precipitation
of transition carbide (between 80 and 200 °C); (iv) decomposition of retained austenite (between 240 and 320 °C); and (v)
conversion of transition carbide into cementite (between 260 and 350 °C). 相似文献
12.
Xiang-Mingh He Li-Jian Rong Yi-Yi Li De-Sheng Yan Zhi-Min Jiang 《Metallurgical and Materials Transactions A》2004,35(9):2783-2788
The shape-memory characteristics in the Ni41.3Ti38.7Nb20 alloy have been investigated by means of cryogenic tensile tests and differential scanning calorimetry measurement. The martensite
start temperature M
s
could be adjusted to around the liquid nitrogen temperature by controlling the cooling condition. The reverse transformation
start temperature A′
s
rose to about 70 °C after the specimens were deformed to 16 pct at different temperatures, where the initial states of the
specimens were pure austenite phase, martensite phase, or duplex phase. The shape-memory effect and the reverse transformation
temperatures were studied on the specimens deformed at (M
s
+30 °C). It was found that once the specimens deformed to 16 pct, a transformation hysteresis width around 200 °C could be
attained and the shape recovery ratio could remain at about 50 pct. The Ni41.3Ti38.7Nb20 alloy is a promising candidate for the cryogenic engineering applications around the liquid nitrogen temperature. The experimental
results also indicated that the transformation temperature interval of the stress-induced martensite is smaller by about one
order of magnitude than that of the thermal-induced martensite. 相似文献
13.
A systematic study of stress-induced and thermal-induced transformation of retained austenite in carburized 4320 steel with
an initial retained austenite of 35 pct has been conducted. The transformation was monitored by recording the change in volume
of smooth fatigue specimens. Stress-induced transformation was studied by conducting monotonic and cyclic tests at temperatures
in the range from 22 °C to 150 °C. The volumetric transformation strain was as large as 0.006 at 22 °C. The anisotropy of
the transformation was such that the axial transformation strain component exceeded the diametral transformation strain component
by a factor of 1.4. Thermal-induced transformation was investigated with temperature stepup tests in the range from 150 °C
to 255 °C at constant stress (-500 MPa, 0 MPa, and 500 MPa) and with static tests where temperature was held constant at zero
load. The maximum thermal-induced volumetric transformation strain of 0.006 was independent of stress. However, the anisotropy
of the transformation strain components was dependent on stress direction and magnitude. An axial tensile stress increased
the axial transformation strain relative to the diametral transformation strain. The influence of low-temperature creep(T = 150 °C) on the anisotropy of strains is noted. The differences between stress-induced and thermal-induced transformation
mechanisms are discussed. Thermal-induced transformation primarily occurred at temperatures between 100 °C and 200 °C, with
the rate of transformation increasing with temperature, while the stress-induced transformation primarily occurred at 22 °C,
with the rate of transformation decreasing with increasing temperature. There was no stress-induced transformation above 60
°C. 相似文献
14.
Influence of tempering on the microstructure and mechanical properties of HSLA-100 steel plates 总被引:5,自引:0,他引:5
The influence of tempering on the microstructure and mechanical properties of HSLA-100 steel (with C-0.04, Mn-0.87, Cu-1.77,
Cr-0.58, Mo-0.57, Ni-3.54, and Nb-.038 pct) has been studied. The plate samples were tempered from 300 °C to 700 °C for 1
hour after austenitizing and water quenching. The transmission electron microscopy (TEM) studies of the as-quenched steel
revealed a predominantly lath martensite structure along with fine precipitates of Cu and Nb(C, N). A very small amount of
retained austenite could be seen in the lath boundaries in the quenched condition. Profuse precipitation of Cu could be noticed
on tempering at 450 °C, which enhanced the strength of the steel significantly (yield strength (YS)—1168 MPa, and ultimate
tensile strength (UTS)—1219 MPa), though at the cost of its notch toughness, which dropped to 37 and 14 J at 25 °C and −85
°C, respectively. The precipitates became considerably coarsened and elongated on tempering at 650 °C, resulting in a phenomenal
rise in impact toughness (Charpy V-notch (CVN) of 196 and 149 J, respectively, at 25 °C and −85 °C) at the expense of YS and
UTS. The best combination of strength and toughness has been obtained on tempering at 600 °C for 1 hour (YS-1015 MPa and UTS-1068
MPa, with 88 J at −85 °C). 相似文献
15.
H. Choe J. H. Schneibel R. O. Ritchie 《Metallurgical and Materials Transactions A》2003,34(2):225-239
The need for structural materials with high-temperature strength and oxidation resistance coupled with adequate lower-temperature
toughness for potential use at temperatures above ∼1000 °C has remained a persistent challenge in materials science. In this
work, one promising class of intermetallic alloys is examined, namely, boron-containing molybdenum silicides, with compositions
in the range Mo (bal), 12 to 17 at. pct Si, 8.5 at. pct B, processed using both ingot (I/M) and powder (P/M) metallurgy methods.
Specifically, the oxidation (“pesting”), fracture toughness, and fatigue-crack propagation resistance of four such alloys,
which consisted of ∼21 to 38 vol. pct α-Mo phase in an intermetallic matrix of Mo3Si and Mo5SiB2 (T2), were characterized at temperatures between 25 °C and 1300 °C. The boron additions were found to confer improved “pest”
resistance (at 400 °C to 900 °C) as compared to unmodified molybdenum silicides, such as Mo5Si3. Moreover, although the fracture and fatigue properties of the finer-scale P/M alloys were only marginally better than those
of MoSi2, for the I/M processed microstructures with coarse distributions of the α-Mo phase, fracture toughness properties were far superior, rising from values above 7 MPa √m at ambient temperatures to almost
12 MPa √m at 1300 °C. Similarly, the fatigue-crack propagation resistance was significantly better than that of MoSi2, with fatigue threshold values roughly 70 pct of the toughness, i.e., rising from over 5 MPa √m at 25 °C to ∼8 MPa √m at 1300 °C. These results, in particular, that the toughness and cyclic
crack-growth resistance actually increased with increasing temperature, are discussed in terms of the salient mechanisms of
toughening in Mo-Si-B alloys and the specific role of microstructure. 相似文献
16.
Javad Mola Christiane Ullrich Buxiao Kuang Reza Rahimi Qiuliang Huang David Rafaja Roman Ritzenhoff 《Metallurgical and Materials Transactions A》2017,48(3):1033-1052
The high-temperature austenite phase of a high-interstitial Mn- and Ni-free stainless steel was stabilized at room temperature by the full dissolution of precipitates after solution annealing at 1523 K (1250 °C). The austenitic steel was subsequently tensile-tested in the temperature range of 298 K to 503 K (25 °C to 230 °C). Tensile elongation progressively enhanced at higher tensile test temperatures and reached 79 pct at 503 K (230 °C). The enhancement at higher temperatures of tensile ductility was attributed to the increased mechanical stability of austenite and the delayed formation of deformation-induced martensite. Microstructural examinations after tensile deformation at 433 K (160 °C) and 503 K (230 °C) revealed the presence of a high density of planar glide features, most noticeably deformation twins. Furthermore, the deformation twin to deformation-induced martensite transformation was observed at these temperatures. The results confirm that the high tensile ductility of conventional Fe-Cr-Ni and Fe-Cr-Ni-Mn austenitic stainless steels may be similarly reproduced in Ni- and Mn-free high-interstitial stainless steels solution annealed at sufficiently high temperatures. The tensile ductility of the alloy was found to deteriorate with decarburization and denitriding processes during heat treatment which contributed to the formation of martensite in an outermost rim of tensile specimens. 相似文献
17.
Ductility and strain-induced transformation in a high-strength transformation-induced plasticity-aided dual-phase steel 总被引:7,自引:0,他引:7
Koh-Ichi Sugimoto Mitsuyuki Kobayashi Shun-Ichi Hashimoto 《Metallurgical and Materials Transactions A》1992,23(11):3085-3091
The influence of forming temperature and strain rate on the ductility and strain-induced transformation behavior of retained
austenite in a ferritic 0.4C-1.5Si-1.5Mn (wt pct) dual-phase steel containing fine retained austenite islands of about 15
vol pct has been investigated. Ex- cellent combinations of total elongations (TELs), about 48 pct, and tensile strength (TS),
about 1000 MPa, were obtained at temperatures between 100 °C and 200 °C and at a strain rate of 2.8 X 10-4/s. Under these optimum forming conditions, the flow curves were characterized by intensive serrations and increased strain-hardening
rate over a large strain range. The retained austenite islands were mechanically the most stable at temperatures between 100
°C and 200 °C, and the retained austenite stability appeared to be mainly controlled by strain-induced martensite and bainite
transformations (SIMT and SIBT, respectively), with deformation twinning occur- ring in the retained austenite. The enhanced
TEL and forming temperature dependence of TEL were primarily connected with both the strain-induced transformation behavior
and retained aus- tenite stability. 相似文献
18.
Dual phase steels are characterized by a microstructure consisting of ferrite, martensite, retained austenite, and/or lower
bainite. This microstructure can be altered by tempering with accompanying changes in mechanical properties. This paper examines
such changes produced in a vanadium bearing dual phase steel upon tempering below 500 °C. The steel mechanical properties
were minimally affected on tempering below 200 °C; however, a simultaneous reduction in uniform elongation and tensile strength
occurred upon tempering above 400 °C. The large amount of retained austenite (≅10 vol pct) observed in the as-received steel
was found to be essentially stable to tempering below 300 °C. On tempering above 400 °C, most of the retained austenite decomposed
to either upper bainite (at 400 °C) or a mixture of upper bainite and ferrite-carbide aggregate formed by an interphase precipitation
mechanism (at 500 °C). In addition, tempering at 400 °C led to fine precipitation in the retained ferrite. The observed mechanical
properties were correlated with these microstructural changes. It was concluded that the observed decrease in uniform elongation
upon tempering above 400 °C is primarily the consequence of the decomposition of retained austenite and the resulting loss
of transformation induced plasticity (TRIP) as a contributing mechanism to the strain hardening of the steel.
B. V. N. RAO, formerly Senior Research Engineer, Analytical Chemistry Department, General Motors Research Laboratories 相似文献
19.
Austenitic specimens of Fe-15 wt pct Ni-0.8 wt pct C were tested in tension at strain rates of 10−4 s−1 and 10−1 s−1 over the temperature range −20°C to 60 °C. The influence of strain rate and temperature on the deformation behavior depended
on whether stress-assisted or strain-induced martensitic trans-formation occurred during testing. Under conditions of stress-assisted
transformation, the ductility was low and independent of strain rate. However, when strain-induced transformation occurred,
the duc-tility increased significantly and the higher strain rate resulted in greater ductility and more transfor-mation.
Although the ductility increased continuously with temperature, the amount of strain-induced transformation decreased and
no martensite was observed above 40 °C. Microstructural examination showed that the martensite was replaced by intense bands
and that these bands contained very fine (111) fcc twins. The twinning resulted in enhanced plasticity by providing an additional
mode of deformation as slip became more difficult due to dynamic strain aging at the higher temperature. This study confirms
that the substructure following deformation will depend on the proximity of the deformation temperature to theM
s
σ
temperature. At temperatures much greater thanM
s
σ
, austenite twinning will occur, while at temperatures close toM
s
σ
, bcc martensite will form. 相似文献
20.
I. Karaman H. Ersin Karaca Z. P. Luo H. J. Maier 《Metallurgical and Materials Transactions A》2003,34(11):2527-2539
A Ti-49.8 at. pct Ni alloy was severely deformed at three different temperatures using equal-channel angular extrusion (ECAE).
Three deformation temperatures—room temperature (below the martensite finish temperature), 50 °C (below the austenite start
temperature), and 150 °C (above the austenite finish temperature)—were selected such that the initial deforming phase (B2
austenite or B19’ martensite) and the initial governing deformation mechanism (martensite reorientation, stress-induced martensitic
transformation, or dislocation slip in martensite) would be different. The X-ray analysis results revealed that all processed
samples mostly contained a deformed martensitic phase, regardless of the initial deforming phase and the deformation mechanism.
Although the martensite start temperature did not change, the austenite start temperature decreased significantly in all deformation
conditions, probably because of the effect of the internal stress field caused by the deformed microstructure. All deformation
conditions led to an increase in the strength levels and some deterioration of shape-memory characteristics. However, a subsequent
low-temperature annealing treatment significantly improved pseudoelastic strain levels while preserving the ultrahigh strength
levels. The sample deformed at room temperature followed by the low-temperature annealing resulted in the most promising strength
and shape-memory characteristics under compression, such that a 5.3 pct shape-memory strain at a 2200 MPa strength level and
a 3.3 pct pseudoelastic strain at a 1900 MPa strength level were achieved. The differences between the strength levels and
the shape-memory characteristics after severe deformation at different temperatures were attributed to the different amounts
of plastic deformation and the resulting deformation textures, since at each deformation temperature the deformation mechanism
was different. It is concluded that the severe marforming using ECAE could easily improve strength levels of NiTi alloys while
preserving the shape-memory and pseudoelasticity (PE) characteristics and, thus, improve the thermomechanical fatigue behavior.
However, lower deformation temperatures are necessary to hinder formation of macroshear bands, and ECAE angles larger than
90 deg should be used to reduce the amount of strain applied in one pass. 相似文献