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1.
Step-aging programs, based on principles of particle-dislocation interactions, were developed systematically to obtain increases
in the high-temperature strength and ductility properties of Ti-7 at. pct Mo-Al alloys. A triple-step aging program applied
to Ti-7 Mo-16 Al produced a yield stress σ 0.2 = 1,500 MN/m 2, elongation to fracture ε
F
= 4 pct at room temperature, and σ 0.2 = 900 MN/m 2, ε
F
= 12 pct at 600°C. A two-step aging program resulted in σ 0.2 = 1,350 MN/m 2, ε
F
= 5 pct at room temperature; σ 0.2 = 800 MN/m 2, ε
F
= 20 pct at 600°C.
Formerly Assistant Research Professor, Materials Research Laboratory, Rutgers University 相似文献
2.
The thermal cycling of an Fe-17 wt pct Mn alloy between 303 and 573 K was performed to investigate the effects of thermal
cycling on the kinetics of the γ → ε martensitic transformation in detail and to explain the previous, contrasting results of the change in the amount of ε martensite at room temperature with thermal cycling. It was observed that the shape of the γ → ε martensitic transformation curve (volume fraction vs temperature) changed gradually from a C to an S curve with an increasing number of thermal cycles. The amount of ε martensite of an Fe-17 wt pct Mn alloy at room temperature increased with thermal cycling, in spite of the decrease in the
martensitic start ( M
s) temperature. This is due to the increase in transformation kinetics of ε martensite at numerous nucleation sites introduced in the austenite during thermal cycling. 相似文献
3.
Strain controlled low cycle fatigue tests have been conducted in air to ascertain the influence of strain rate(ε = 4 × 10 -6'to 4 × 10 -3 s -1) and temperature (T = 750/850/950 °C) on LCF behavior of Alloy 617. A strain range of 0.6 pct and a symmetrical triangular wave form were employed
for all the tests. Crack initiation and propagation modes were studied. Microstructural changes that occurred during fatigue
deformation were evaluated and compared with the results obtained on isothermal aging. Deformation and damage mechanisms which
influence the endurance have been identified. A reduction in fatigue life was observed with decreasing ε at 850 °C and with
increasing temperature at ε = 4 × 10 -5 s -1. Cyclic stress response varied as a complex function of temperature and strain rate. Fatigue deformation was found to induce
cellular precipitation of carbides at 750 and 850 ‡. Dynamic strain aging characterized by serrated flow was observed at 750
°C (ε = 4 × 10 -5 s -1) and in the tests at higher ε at 850 °C. Strengthening of the matrix due to dynamic strain aging of matrix dislocations by
precipitation of M 23C 6 carbides led to fracture of grain boundary carbide films formed at 750 °C, producing brittle intergranular crack propagation.
At 850 °C transgranular crack propagation was observed at the higher strain rates ε≥4× 10 -4 s -1. At 850 and 950 °C even at strain rates of 4 × 10 -5 s -1 or lower, life was not governed by intergranular creep rupture damage mechanisms under the symmetrical, continuous cycling
conditions employed. Reduction of endurance at lower strain rates is caused by increased inelastic strain and intergranular
crack initiation due to oxidation of surface connected grain boundaries.
formerly Guest Scientist at the De-partment for Reactor Materials of the Nuclear Research Centre, Juelich (IRW/KFA), 相似文献
4.
Martensite reversion treatment was utilized to obtain ultrafine grain size in Fe-18Cr-12Mn-N stainless steels containing 0 to 0.44 wt pct N. This was achieved by cold rolling to 80 pct reduction followed by reversion annealing at temperatures between 973 K and 1173 K (700 °C and 900 °C) for 1 to 10 4 seconds. The microstructural evolution was characterized using both transmission and scanning electron microscopes, and mechanical properties were evaluated using hardness and tensile tests. The steel without nitrogen had a duplex ferritic-austenitic structure and the grain size refinement remained inefficient. The finest austenitic microstructure was achieved in the steels with 0.25 and 0.36 wt pct N following annealing at 1173 K (900 °C) for 100 seconds, resulting in average grain sizes of about 0.240 ± 0.117 and 0.217 ± 0.73 µm, respectively. Nano-size Cr 2N precipitates observed in the microstructure were responsible for retarding the grain growth. The reversion mechanism was found to be diffusion controlled in the N-free steel and shear controlled in the N-containing steels. Due to a low fraction of strain-induced martensite in cold rolled condition, the 0.44 wt pct N steel displayed relatively non-uniform, micron-scale grain structure after the same reversion treatment, but it still exhibited superior mechanical properties with a yield strength of 1324 MPa, tensile strength of 1467 MPa, and total elongation of 17 pct. While the high yield strength can be attributed to strengthening by nitrogen alloying, dislocation hardening, and slight grain refinement, the moderate strain-induced martensitic transformation taking place during tensile straining was responsible for enhancement in tensile strength and elongation. 相似文献
5.
Superplastic behavior of two commercial grade white cast irons, eutectic Fe-C and Ni-Cr white cast irons, was investigated
at intermediate temperatures (650 to 750 °C). For this purpose, rapidly solidified powders of the cast irons were fully consolidated
by compaction and rolling at about 650 °C. The volume fractions of cementite in the eutectic cast iron and in the Ni-Cr cast
iron were 64 pct and 51 pct, respectively, and both cast irons consisted of fine equiaxed grains of cementite (1 to 2 μm)
and ferrite (0.5 to 2 μm). The cast iron compacts exhibited high strain-rate sensitivity (strain-rate-sensitivity exponent
of 0.35 to 0.46) and high tensile ductility (total elongation of 150 pct to 210 pct) at strain rates of 10 -4 to 10 -3 s -1 and at 650 °C to 750 °C. Microstructure evaluations were made by TEM, SEM, and optical microscopy methods. The equiaxed grains
in the as-compacted samples remained unchanged even after large tensile deformation. It is concluded that grain boundary sliding
( e.g., along cementite grain boundaries in the case of the eutectic cast iron) is the principal mode of plastic deformation in
both cast irons during superplastic testing conditions.
Formerly with the Department of Materials Science and Engineering, Stanford University
Formerly Visiting Scholar, Department of Materials Science and Engineering, Stanford University 相似文献
6.
Five carbon steels, AISI 1008, 1020, 1035, renitrogenized 1010 and 1522, were dynamically strain aged in the temperature range
100 to 600°C (373 to 873 K), with strains from 3 to 9 pct and strain rates from 2 × 10 -4 to 2 × 10 -2/s. Following this, changes in tensile, notch impact and fatigue properties were determined. The data obtained indicate that
for a commercial combined straightening and strengthening operation on the cooling beds of a bar mill, the temperature during
straining should be between 200 and 400° C, the strain should be less than 5 pct and the strain rate is unimportant, up to
at least 10 -1/s. After such treatment the tensile properties will be:
相似文献
7.
Heat treatments were performed using an isothermal bainitic transformation (IBT) temperature compatible with continuous hot-dip galvanizing on two high Al–low Si transformation induced plasticity (TRIP)-assisted steels. Both steels had 0.2 wt pct C and 1.5 wt pct Mn; one had 1.5 wt pct Al and the other had 1 wt pct Al and 0.5 wt pct Si. Two different intercritical annealing (IA) temperatures were used, resulting in intercritical microstructures of 50 pct ferrite ( α)-50 pct austenite ( γ) and 65 pct α-35 pct γ. Using the IBT temperature of 465 °C, five IBT times were tested: 4, 30, 60, 90, and 120 seconds. Increasing the IBT time resulted in a decrease in the ultimate tensile strength (UTS) and an increase in the uniform elongation, yield strength, and yield point elongation. The uniform elongation was higher when using the 50 pct α-50 pct γ IA temperature when compared to the 65 pct α-35 pct γ IA temperature. The best combinations of strength and ductility and their corresponding heat treatments were as follows: a tensile strength of 895 MPa and uniform elongation of 0.26 for the 1.5 pct Al TRIP steel at the 50 pct γ IA temperature and 90-second IBT time; a tensile strength of 880 MPa and uniform elongation of 0.27 for the 1.5 pct Al TRIP steel at the 50 pct γ IA temperature and 120-second IBT time; and a tensile strength of 1009 MPa and uniform elongation of 0.22 for the 1 pct Al-0.5 pct Si TRIP steel at the 50 pct γ IA temperature and 120-second IBT time. 相似文献
8.
Grain coarsening in a Ti-5 Al-2.5 Sn titanium alloy, deformed in tension to 13 pct uniform elongation and then heated to 1144
K (1600°F) for one h, was investigated. The influence of deformation temperature (77 to 598 K), grain size (10.7, 11.8, and
22.5 μm), and strain rate (2.67 × 10 -2, 6.67 × 10 -4, 2.67 × 10 -5 s -1) was also studied. Critical elongation and work input values for maximum grain coarsening varied with deformation temperature.
The critical elongation value increased from 9 to 12 pct as the temperature decreased from 598 to 367 K and decreased from
12 to 9 pct as temperature decreased from 367 to 77 K. The critical work energy input increased linearly with decreasing temperature. 相似文献
9.
By thermally cycling through their transformation temperature range, coarse-grained polymorphic materials can be deformed
superplastically, owing to the emergence of transformation mismatch plasticity (or transformation superplasticity) as a deformation
mechanism. This mechanism is presently investigated under biaxial stress conditions during thermal cycling of unalloyed titanium,
Ti-6Al-4V, and their composites (Ti/10 vol. pct TiC
p
, Ti-6Al-4V/10 vol. pct TiC
p
, and Ti-6Al-4V/5 vol. pct TiB
w
). During gas-pressure dome bulging experiments, the dome height was measured as a function of forming time. Adapting existing
models of biaxial doming to the case of transformation superplasticity where the strain-rate sensitivity is unity, we verify
the operation of this deformation mechanism in all experimental materials and compare the biaxial results directly to new
uniaxial thermal cycling results on the same materials. Finally, existing thickness distribution models are compared with
experimentally measured profiles. 相似文献
10.
The reason why thermal cycling decreases the martensite start ( M
s
) temperature of an Fe-17 wt pct Mn alloy was quantitatively investigated, based on the nucleation model of ε martensite and a thermodynamic model for a martensitic transformation. The M
s
temperature decreased by about 22 K after nine cycles between 303 and 573 K, due to the increase in shear-strain energy (Δ G
sh
) required to advance the transformation dislocations through dislocation forests formed in austenite during thermal cycling.
The Δ G
sh
value increased from 19.3 to 28.8 MJ/m 3 due to the increase in austenite dislocation density from 1.5 × 10 12 to 3.8 × 10 13/m 2 with the number of thermal cycles (in this case, up to nine cycles). The austenite dislocation density increased rapidly
for up to five thermal cycles and then increased gradually with further thermal cycles, showing a good agreement with the
increase in austenite hardness with the number of thermal cycles. 相似文献
11.
Eight medium manganese steels ranging from 10 to 15 wt pct Mn have been produced with varying levels of aluminum, silicon, and carbon to create steels with varying TRIP (transformation-induced plasticity) character. Alloy chemistries were formulated to produce a range of intrinsic stacking fault energies (ISFE) from − 2.2 to 13.3 mJ/m2 when calculated at room temperature for an austenitic microstructure having the nominal alloy composition. Two-stage TRIP behavior was documented when the ISFE of the γ-austenite phase was 10.5 mJ/m2 or less, whereas an ISFE of 11.9 mJ/m2 or greater exhibited TWIP (twin-induced plasticity) with single-stage TRIP to form α-martensite. Properties were measured in both hot band (hot rolled) and batch annealed (hot rolled, cold rolled, and annealed) conditions. Hot band properties were influenced by the Si/Al ratio and this dependence was related to incomplete recovery during hot working for alloys with Si/Al ratios greater than one. Batch annealing was conducted at 873 K (600 °C) for 20 hours to produce ultrafine-grained microstructures with mean free slip distances less than 1 μm. Batch-annealed materials were found to exhibit a Hall–Petch dependence of the yield strength upon the mean free slip distance measured in the polyphase microstructure. Ultimate tensile strengths ranged from 1450 to 1060 MPa with total elongations of 27 to 43 pct. Tensile ductility was shown to be proportional to the sum of the products of volume fraction transformed times the volume change associated for each martensitic transformation. An empirical relationship based upon the nominal chemistry was derived for the ultimate tensile strength and elongation to failure for these batch-annealed steels. Two additional alloys were produced based upon the developed understanding of these two-stage TRIP steels and tensile strengths of 1150 MPa with 58 pct total elongation and 1400 MPa and 32 pct ductility were achieved. 相似文献
12.
Superplastic deformation behavior of a fine grain 5083 Al sheet (Al-4.2 pct Mg-0.7 pct Mn, trade name FORMALL 545) has been
investigated under uniaxial tension over the temperature range of 500 °C to 565 °C. Strain rate sensitivity values >0.3 were
observed over a strain rate range of 3 × 10 −5 s −1 to 1 × 10 −2 s −1, with a maximum value of 0.65 at 5 × 10 −4 s −1 and 565 °C. Tensile elongations at constant strain rate exceeded 400 pct; elongations in the range of 500 to 600 pct were
obtained under constant crosshead speed and variable strain rates. A short but rapid prestraining step, prior to a slower
superplastic strain rate, provided enhanced tensile elongation at all temperatures. Under the two-step schedule, a maximum
tensile elongation of 600 pct was obtained at 550 °C, which was regarded as the optimum superplastic temperature under this
condition. Dynamic and static grain growth were examined as functions of time and strain rate. It was observed that the dynamic
grain growth rate was appreciably higher than the static growth rate and that the dynamic growth rate based on time was more
rapid at the higher strain rate. Cavitation occurred during superplastic flow in this alloy and was a strong function of strain
rate and temperature. The degree of cavitation was minimized by superimposition of a 5.5 MPa hydrostatic pressure during deformation,
which produced a tensile elongation of 671 pct at 525 °C.
R. VERMA, formerly Visiting Scientist, Department of Materials Science and Engineering, University of Michigan 相似文献
13.
The isothermal and non-isothermal oxidation kinetics of a converter vanadium slag in the presence of calcium oxide was studied using thermal analysis. The isothermal experimental data for the whole oxidation process are described in terms of the equation [1? (1? α) 2/3] = kt with Ea = 20.42 kJ mol –1 at lower temperatures of 400-500 °C, and described by [(1? α) –1/3?1] 2 = kt with Ea = 227.66 kJ mol –1 at temperature higher than 500 °C. In the nonisothermal oxidation study, heating rate greatly affects the oxidation process. Using a heating rate of 3 °C min –1 results in overlapping oxidations of vanadium spinel and augite over temperature range of 608-959 °C, which is described by the 3/2 order reaction. Increasing the heating rate to 5 °C min –1 or 10 °C min –1, only oxidation of vanadium spinel takes place in temperature range of 657-914 °C and 691-954 °C respectively, both described by the third order chemical reaction. As the slag particle decreases from 250 µm to 48 µm, the kinetic equation for describing the overlapping oxidation process changes from the Anti–Zhuravlev equation with internal diffusion controlling to reaction limiting equations. 相似文献
14.
Low-temperature superplasticity (LTSP) at 250 °C and 1 × 10 −3 s −1 was observed in a 5083 Al-Mg base alloy after thermomechanical treatments (TMTs). With a higher TMT rolling strain, the fraction
of high-angle grain boundaries increased, which was favorable for the further operation of grain-boundary sliding (GBS) and
LTSP. The near-brass {110}〈112〉, S {123}〈634〉, and Cu {112}〈111〉 texture components in the as-thermomechanically treated specimens
gradually evolved into a random orientation distribution during LTSP straining from 30 to 100 pct. Static annealing at 250
°C itself could not alter the existing texture. The grain-misorientation distribution curves also showed that, after 100 pct
LTSP elongation, the misorientation angles approached the random distribution. In the latter case, the low-, medium-, and
high-angle boundaries each would partition around 10, 20, and 70 pct, respectively. When the LTSP elongation was greater than
150 pct, the macrodeformation anisotropy ( R) ratio would reach a plateau value of ∼0.8. During the initial stage, a group of over 60 grains proceeded cooperative grain-boundary
sliding (CGBS); most individual grain boundaries started to slide at the later stage. It seems that it is the high-angle boundaries,
not the special coincidence-site lattice (CSL) boundaries, which could govern the LTSP performance. 相似文献
15.
The microtexture and grain boundary misorientation distributions ( i.e., mesotexture) of the superplastic alloy Al-5 pct Ca-5 pct Zn have been investigated in the as-processed condition, after
annealing at 520 °C (for times ranging from 7 minutes to 90 hours) and after tensile straining in the transverse direction
(TD). Three different superplastic straining conditions were considered: 550 °C/10 −2 s −1, 550 °C/10 −1 s −1, and 400 °C/10 −2 s −1. Microtexture data were obtained by means of computer-aided electron backscatter diffraction analysis methods. The retention
of the deformation texture of the as-received material and the development of an increasingly bimodal grain boundary misorientation
distribution following static annealing are consistent with the occurrence of recovery and continuous recrystallization. During
superplastic straining, deformation texture components are also retained, but with a change in the grain boundary misorientation
distribution toward random, indicating that grain switching occurs during grain boundary sliding (GBS). At the midlayer, however,
a change from an initial texture component near the Cu-type texture component toward the Brass texture component, {011}〈211〉,
was observed even as the misorientation distribution became more random. This change in texture component is associated with
the occurrence of single slip during superplastic flow. 相似文献
16.
The designed steel of Fe-0.25C-1.5Mn-1.2Si-1.5Ni-0.05Nb (wt pct) treated by a novel quenching-partitioning-tempering (Q-P-T)
process demonstrates an excellent product of strength and elongation (PSE) at deformed temperatures from 298 K to 573 K (25 °C
to 300 °C) and shows a maximum value of PSE (over 27,000 MPa pct) at 473 K (200 °C). The results fitted by the exponent decay
law indicate that the retained austenite fraction with strain at a deformed temperature of 473 K (200 °C) decreases slower
than that at 298 K (25 °C); namely, the transformation induced plasticity (TRIP) effect occurs in a larger strain range at
473 K (200 °C) than at 298 K (25 °C), showing better mechanical stability. The work-hardening exponent curves of Q-P-T steel
further indicate that the largest plateau before necking appears at the deformed temperature of 473 K (200 °C), showing the
maximum TRIP effect, which is due to the mechanical stability of considerable retained austenite. The microstructural characterization
reveals that the high strength of Q-P-T steels results from dislocation-type martensite laths and dispersively distributed
fcc NbC or hcp ε-carbides in martensite matrix, while excellent ductility is attributed to the TRIP effect produced by considerable retained
austenite. 相似文献
17.
Step-aging programs, based on principles of particle-dislocation interactions, were developed systematically to obtain increases in the high-temperature strength and ductility properties of Ti-7 at. pct Mo-Al alloys. A triple-step aging program applied to Ti-7 Mo-16 Al produced a yield stress σ 0.2 = 1,500 MN/m 2, elongation to fracture ε F = 4 pct at room temperature, and σ 0.2 = 900 MN/m 2, ε F = 12 pct at 600°C. A two-step aging program resulted in σ 0.2 = 1,350 MN/m 2, ε F = 5 pct at room temperature; σ 0.2 = 800 MN/m 2, ε F = 20 pct at 600°C. 相似文献
18.
The hot workability of Nimonic 115 was studied by means of very high strain rate stress rupture tests in the temperature interval
1323 to 1473 K (1050 to 1200°C) at strain rates of 10 −4 to 10 per s. Hot plasticity, measured as elongation and reduction of area at fracture, increased generally with decreasing
strain rates. Maximum values of about 40 pct elongation and 70 pct reduction of area were obtained between 1398 to 1448 K
(1125 to 1175°C) for strain rates below about 1 per s. For higher rates of strain than about 1 per s, ductility at fracture
fell sharply. Ductility above 1448 K (1175°C) was poor at all strain rates and fell to a minimum at 1473 K (1200°C) regardness
of strain rate. The highest ductility values are associated with intermediate temperatures and intermediate strain rates where
conditions are optimum for significant recovery without encountering grain growth. The presence of excess phases leads to
severe intergranular embrittlement at the highest temperatures and strain rates. 相似文献
19.
The hot workability of Nimonic 115 was studied by means of very high strain rate stress rupture tests in the temperature interval 1323 to 1473 K (1050 to 1200°C) at strain rates of 10 ?4 to 10 per s. Hot plasticity, measured as elongation and reduction of area at fracture, increased generally with decreasing strain rates. Maximum values of about 40 pct elongation and 70 pct reduction of area were obtained between 1398 to 1448 K (1125 to 1175°C) for strain rates below about 1 per s. For higher rates of strain than about 1 per s, ductility at fracture fell sharply. Ductility above 1448 K (1175°C) was poor at all strain rates and fell to a minimum at 1473 K (1200°C) regardness of strain rate. The highest ductility values are associated with intermediate temperatures and intermediate strain rates where conditions are optimum for significant recovery without encountering grain growth. The presence of excess phases leads to severe intergranular embrittlement at the highest temperatures and strain rates. 相似文献
20.
Heat treatments were performed using an isothermal bainitic transformation (IBT) temperature compatible with continuous hot-dip
galvanizing on two high Al–low Si transformation induced plasticity (TRIP)-assisted steels. Both steels had 0.2 wt pct C and
1.5 wt pct Mn; one had 1.5 wt pct Al and the other had 1 wt pct Al and 0.5 wt pct Si. Two different intercritical annealing
(IA) temperatures were used, resulting in intercritical microstructures of 50 pct ferrite ( α)-50 pct austenite ( γ) and 65 pct α-35 pct γ. Using the IBT temperature of 465 °C, five IBT times were tested: 4, 30, 60, 90, and 120 seconds. Increasing the IBT time
resulted in a decrease in the ultimate tensile strength (UTS) and an increase in the uniform elongation, yield strength, and
yield point elongation. The uniform elongation was higher when using the 50 pct α-50 pct γ IA temperature when compared to the 65 pct α-35 pct γ IA temperature. The best combinations of strength and ductility and their corresponding heat treatments were as follows:
a tensile strength of 895 MPa and uniform elongation of 0.26 for the 1.5 pct Al TRIP steel at the 50 pct γ IA temperature and 90-second IBT time; a tensile strength of 880 MPa and uniform elongation of 0.27 for the 1.5 pct Al TRIP
steel at the 50 pct γ IA temperature and 120-second IBT time; and a tensile strength of 1009 MPa and uniform elongation of 0.22 for the 1 pct Al-0.5 pct
Si TRIP steel at the 50 pct γ IA temperature and 120-second IBT time. 相似文献
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