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1.
Shingo Mineta Shigenobu Namba Takashi Yoneda Kyosuke Ueda Takayuki Narushima 《Metallurgical and Materials Transactions A》2010,41(8):2129-2138
The microstructures of as-cast and heat-treated biomedical Co-Cr-Mo (ASTM F75) alloys with four different carbon contents
were investigated. The as-cast alloys were solution treated at 1473 to 1548 K for 0 to 43.2 ks. The precipitates in the matrix
were electrolytically extracted from the as-cast and heat-treated alloys. An M23C6 type carbide and an intermetallic σ phase (Co(Cr,Mo)) were detected as precipitates in the as-cast Co-28Cr-6Mo-0.12C alloy;
an M23C6 type carbide, a σ phase, an η phase (M6C-M12C type carbide), and a π phase (M2T3X type carbide with a β-manganese structure) were detected in the as-cast Co-28Cr-6Mo-0.15C alloy; and an M23C6 type carbide and an η phase were detected in the as-cast Co-28Cr-6Mo-0.25C and Co-28Cr-6Mo-0.35C alloys. After solution treatment, complete precipitate
dissolution occurred in all four alloys. Under incomplete precipitate dissolution conditions, the phase and shape of precipitates
depended on the heat-treatment conditions and the carbon content in the alloys. The π phase was detected in the alloys with carbon contents of 0.15, 0.25, and 0.35 mass pct after heat treatment at high temperature
such as 1548 K for a short holding time of less than 1.8 ks. The presence of the π phase in the Co-Cr-Mo alloys has been revealed in this study for the first time. 相似文献
2.
Alfirano Shingo Mineta Shigenobu Namba Takashi Yoneda Kyosuke Ueda Takayuki Narushima 《Metallurgical and Materials Transactions A》2012,43(6):2125-2132
The microstructures of biomedical ASTM F 75/F 799 Co-28Cr-6Mo-0.25C-0.175N-(0 to 1)Si-(0 to 1)Mo alloys (mass pct) were investigated
before and after heat treatment, with special attention paid to the effect of nitrogen on the phases and the dissolution of
precipitates. The heat treatment temperatures and holding periods employed ranged from 1448 to 1548 K (1175 to 1275 °C) and
0 to 43.2 ks, respectively. A blocky-dense π-phase precipitate and a lamellar cellular colony, which consisted of an M2X type precipitate and a γ phase, were mainly detected in the as-cast alloys with and without added Si, respectively. The addition of nitrogen caused
cellular precipitation, while the addition of Si suppressed it and enhanced the formation of the π phase. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) analyses suggested that a discontinuous
reaction, i.e., γ
1 → γ
2 + M2X, might be a possible formation mechanism for the lamellar cellular colony. Nitrogen was enriched in the M2X type, η-phase, and π-phase precipitates, but was excluded from the M23X6 type precipitate. Complete precipitate dissolution was observed in all of the alloys under varied heat treatment conditions
depending on the alloy composition. The addition of nitrogen decreased the time required for complete precipitate dissolution
at low heat-treatment temperatures. At high temperatures, i.e., 1548 K (1275 °C), complete precipitate dissolution was delayed by the partial melting that accompanied the formation of
the precipitates such as the π phase resulting in the boundary between the complete and incomplete precipitate dissolution regions in having a C-curved
shape. 相似文献
3.
Alfirano Shingo Mineta Shigenobu Namba Takashi Yoneda Kyosuke Ueda Takayuki Narushima 《Metallurgical and Materials Transactions A》2011,42(7):1941-1949
The effect of the addition of Si or Mn to ASTM F75 Co-28Cr-6Mo-0.25C alloys on precipitate formation as well as dissolution
during solution treatment was investigated. Three alloys—Co-28Cr-6Mo-0.25C-1Si (1Si), Co-28Cr-6Mo-0.25C-1Mn (1Mn), and Co-28Cr-6Mo-0.25C-1Si-1Mn
(1Si1Mn)—were heat treated from 1448 K to 1548 K (1175 °C to 1275 °C) for a holding time of up to 43.2 ks. In the case of
the as-cast 1Si and 1Si1Mn alloys, the precipitates were M23C6-type carbide, η phase (M6C-M12C–type carbide), and π phase (M2T3X-type carbide with a β-Mn structure), while in the case of the as-cast 1Mn alloy, M23C6-type carbide and η phase were detected. The 1Si and 1Si1Mn alloys required longer heat-treatment times for complete precipitate
dissolution than did the 1Mn alloys. During the solution treatment, blocky dense M23C6-type carbide was observed in all the alloys over the temperature range of 1448 K to 1498 K (1175 °C to 1225 °C). At the heat-treatment
temperature of 1523 K (1250 °C), starlike precipitates with stripe patterns—comprising M23C6-type carbide and metallic face-centered-cubic (fcc) γ phase—were detected in the 1Si and 1Si1Mn alloys. A π phase was observed
in the 1Si and 1Si1Mn alloys heat treated at 1523 K and 1548 K (1250 °C and 1275 °C) and in the 1Mn alloy heat treated at
1548 K (1275 °C); its morphology was starlike-dense. The addition of Si appeared to promote the formation of the π phase
in Co-28Cr-6Mo-0.25C alloys at 1523 K and 1548 K (1250 °C and 1275 °C). Thus, the addition of Si and Mn affects the phase
and morphology of the carbide precipitates in biomedical Co-Cr-Mo alloys. 相似文献
4.
Bikas C. Maji Madangopal Krishnan M. Sujata Gouthama Ranjit K. Ray 《Metallurgical and Materials Transactions A》2013,44(1):172-185
The effect of Co addition has been studied in Fe-30Mn-6Si-xCo (x = 0 to 9 wt pct) shape memory alloys in terms of their microstructure, martensitic transformation and shape recovery. Microstructural investigations reveal that in Fe-Mn-Si-Co alloys, the microstructure remains single-phase austenite (??) up to 5 pct Co and beyond that becomes two-phase comprising ?? and off-stoichiometric (Fe,Co)5Mn3Si2 intermetallic ??-phases. The forward ??-?? martensite transformation start temperature (M S) decreases with the addition of Co up to 5 pct, and alloys containing more than 5 pct Co, show slightly higher M S possibly on account of two-phase microstructure. Unlike M S, the ??-?? reverse transformation start temperature (A S) has been found to remain almost unaltered by Co addition. In general, addition of Co to Fe-Mn-Si alloys deteriorates shape recovery due to decreasing resistance to plastic yielding concomitant with the formation of stress induced ?? martensite. However, there is an improvement in shape recovery beyond 5 pct Co addition, possibly due to the strengthening effect arising from the presence of (Fe,Co)5Mn3Si2 precipitates within the two-phase microstructure and due to higher amount of stress induced ?? martensite. 相似文献
5.
The microstructure of chromium-tungsten steels 总被引:1,自引:0,他引:1
Chromium-tungsten steels are being developed to replace the Cr-Mo steels for fusion-reactor applications. Eight experimental
steels were produced and examined by optical and electron microscopy. Chromium concentrations of 2.25, 5, 9 and 12 pct were
used. Steels with these chromium compositions and with 2 pct W and 0.25 pct V were produced. To determine the effect of tungsten
and vanadium, three other 2.25Cr steels were produced as follows: an alloy with 2 pct W and 0 pct V and alloys with 0 and
1 pct W and 0.25 pct V. A 9Cr steel containing 2 pct W, 0.25 pct V, and 0.07 pct Ta also was studied. For all alloys, carbon
was maintained at 0.1 pct. Two pct tungsten was required in the 2.25Cr steels to produce 100 pct bainite (no polygonal ferrite).
The 5Cr and 9Cr steels were 100 pct martensite, but the 12Cr steel contained about 25 pct delta-ferrite. Precipitate morphology
and precipitate types varied, depending on the chromium content. For the 2.25Cr steels, M3C and M7C3 were the primary precipitates; for the 9Cr and 12Cr steels, M23C6 was the primary precipitate. The 5Cr steel contained M7C3 and M23C6. All of the steels with vanadium also contained MC. 相似文献
6.
Masaki Taneike Kota Sawada Fujio Abe 《Metallurgical and Materials Transactions A》2004,35(4):1255-1262
The distributions and precipitated amounts of M23C6 carbides and MX-type carbonitrides with decreasing carbon content from 0.16 to 0.002 mass pct in 9Cr-3W steel, which is used
as a heat-resistant steel, has been investigated. The microstructures of the steels are observed to be martensite. Distributions
of precipitates differ greatly among the steels depending on carbon concentration. In the steels containing carbon at levels
above 0.05 pct, M23C6 carbides precipitate along boundaries and fine MX carbonitrides precipitate mainly in the matrix after tempering. In 0.002
pct C steel, there are no M23C6 carbide precipitates, and instead, fine MX with sizes of 2 to 20 nm precipitate densely along boundaries. In 0.02 pct C steel,
a small amount of M23C6 carbides precipitate, but the sizes are quite large and the main precipitates along boundaries are MX, as with 0.002 pct
C steel. A combination of the removal of any carbide whose size is much larger than that of MX-type nitrides, and the fine
distributions of MX-type nitrides along boundaries, is significantly effective for the stabilization of a variety of boundaries
in the martensitic 9Cr steel. 相似文献
7.
R. K. Williams F. W. Wiffen J. Bentley J. O. Stiegler 《Metallurgical and Materials Transactions A》1983,14(3):655-666
Tungsten-base alloys containing 5, 11, and 25 pct Re were irradiated in the EBR-II reactor. Irradiation temperatures ranged from 600 to 1500 °C. All compositions were irradiated to fluences in the range 4.3 to 6.1 X 1025 n/m2 (E > 0.1 MeV), and three 25 pct Re samples were also irradiated to 3.7 X 1026 n/m2 at temperatures 700 to 900 °C. Postirradiation examination included measurement of electrical resistivity at room temperature and lower temperatures, X-ray diffraction, optical metallography, microprobe analysis, and transmission electron microscopy. Irradiation induced resistivitydecreases observed in most of the samples suggested second-phase precipitation. Complete results confirmed the precipitate formation in all samples, in disagreement with existing phase diagrams for the W-Re system. Electron diffraction showed the precipitates to be consistent with the cubic, Re-richX-phase and inconsistent with the σ-phase. Large variations in precipitate morphology and distribution were observed between the different compositions and irradiation conditions. For the 5 and 11 pct Re-alloys, spherically symmetric strain fields surrounded the equiaxed precipitate particles, and were observed even where no particles were visible. These strain fields are believed to arise from local Re enrichment. Thermoelectric data show that the precipitation can lead to decalibration of W/Re thermocouples. 相似文献
8.
Jeonghyeon Do Hyuk-Joong Lee Changwoo Jeon Dae Jin Ha Choongnyun Paul Kim Byeong-Joo Lee Sunghak Lee Yang Su Shin 《Metallurgical and Materials Transactions A》2012,43(7):2237-2250
In the current study, Fe-based alloys were used for powder injection molding (PIM) parts with various qualities and hardness ranges by varying chemical compositions according to thermodynamically calculated phase diagrams. Their microstructure and hardness values were analyzed and compared with those of the PIM specimens made from conventional Fe-based alloy powders or stainless steel powders. The Cr-to-B ratio (X Cr/X B) and the sum of Fe, Cr, and B content (X Fe+X Cr+X B) were varied to design nine Fe-based alloy compositions based on the composition of Armacor ??M?? alloy powders (Liquidmetal Technologies, Lake Forest, CA). According to the microstructural analysis results of the cast and heat-treated Fe-based alloys, large amounts of (Cr,Fe)2B were formed in the tempered martensite matrix. The volume fraction of (Cr,Fe)2B was varied from 42?pct to 91?pct with alloy compositions, and these results were well matched with the thermodynamically calculated volume fractions of (Cr,Fe)2B. The hardness of the fabricated alloys was varied from 300?VHN to 1600?VHN with alloy compositions, and this value increased linearly with the increasing volume fraction of (Cr,Fe)2B. From the correlation data between the volume fraction of (Cr,Fe)2B and hardness, the high-temperature equilibrium phase diagram, which could be used for the design of Fe-based alloys with various fractions and hardness values of (Cr,Fe)2B, was made. 相似文献
9.
The microstructure of 9Cr-2W-0.25V-0.1C (9Cr-2WV), 9Cr-2W-0.25V-0.07Ta-0.1C (9Cr-2WVTa), 7Cr-2W-0.25V-0.07Ta-0.1C (7Cr-2WVTa),
and 5Cr-2W-0.25V-0.07Ta-0.1C (5Cr-2WVTa) steels (all compositions are in wt pct) have been characterized by analytical electron
microscopy (AEM) and atom probe field ion microscopy (APFIM). These alloys have potential applications in fusion reactors
because they exhibit reduced neutron activation in comparison to the conventional Cr-Mo steels. The matrix in all four alloys
was 100 pct martensite. The precipitate type in the steels depended primarily on the chromium level in the alloy. In the two
9Cr steels, the stable phases were blocky M23C6 and small spherical precipitates previously identified as MC. The two lower-chromium steels contained blocky M7C3 and small needle-shaped carbonitrides in addition to M23C6. The AEM and APFIM analyses revealed that, in the steels containing tantalum, the majority of the tantalum was in solid solution.
With the exception of a few of the small spherical precipitates in low-number densities in the 9Cr-2WVTa, none of the other
precipitates contained measurable tantalum. The experimentally observed phases were in agreement with those predicted by phase
equilibria calculations using the ThermoCalc software. However, a similar match between the experimental and predicted values
of the phase compositions did not occur in some instances. Atom probe analyses directly confirmed the crucial role of trace
amounts of nitrogen in the formation of vanadium-rich carbonitrides as predicted by thermodynamic equilibrium calculations. 相似文献
10.
M. B. Winnicka Ph.D. Candidate R. A. Varin 《Metallurgical and Materials Transactions A》1992,23(11):2963-2972
The present article reports and discusses the results of the microstructural characterization of various modifications of
Ll2 trialuminides containing various titanium contents, including the first ever report on their degree of ordering. The Ll2 trialuminide alloys Al3Ti + X, where X = Cu, Fe, Cr, and Mn were studied. The as-cast structure contains a very low level of porosity, and the amount
of second phase depends on the particular alloy. After homogenization, the second phase is reduced in almost all the alloys
to the level less than 0.5 pct, except for the Mn-high Ti alloy in which it remains at about 20 pct and its composition is
67.9 ± 0.6 at. pct Al, 2.2 ± 0.6 at. pct Mn, and 29.9 ± 0.3 at. pct Ti. In almost all the alloys, porosity after homogenization
increases about twofold, except in the Al3Ti + Cr alloy in which it remains at almost the as-cast level. Limited transmission electron microscopic observations have
revealed the existence of very fine (≈10 nm) unidentified precipitates in the homogenized Al3Ti + Cu alloy. The homogenized Al3Ti + Cr and Mn alloys have greater lattice parameters than the Al3Ti + Fe and Cu alloys. It is also found that the long-range order parameterS of the ho- mogenized Ll2 Al3Ti + X alloys dramatically decreases with increasing titanium content. 相似文献
11.
The effect of the precipitation of coherent and incoherent precipitates on the ductility and toughness of high-strength steel 总被引:1,自引:0,他引:1
R. Hamano 《Metallurgical and Materials Transactions A》1993,24(1):127-139
The effect of the coexistence of coherent and incoherent precipitates, such as M2C and NiAl, on the ductility and plane strain fracture toughness of 5 wt pct Ni-2 wt pct Al-based high-strength steels was
studied. In order to disperse coherent and incoherent precipitates, the heat treatments were carried out as follows: (a) austenitizing
at 1373 K, (b) tempering at 1023 or 923 K for dispersing the incoherent precipitates of M2C and NiAl, and then (c) aging at 843 K for 2.4 ks to disperse the coherent precipitate of NiAl into the matrix, which contains
incoherent precipitates, such as M2C and NiAl. The results were obtained as follows: (a) when the strengthening precipitates consist of coherent ones, such as
M2C and/or NiAl, the ductility and toughness are extremely low, and (b) when the strengthening precipitates consist of coherent
and incoherent precipitates, such as M2C and NiAl, the ductility and fracture toughness significantly increase with no loss in strength. It is shown that the coexistence
of coherent and incoherent precipitates increases homogeneous deformation, thus preventing local strain concentration and
early cleavage cracking. Accordingly, the actions of coherent precipitates in strengthening the matrix and of incoherent precipitates
in promoting homogeneous deformation can be expected to increase both the strength and toughness of the material. 相似文献
12.
13.
The effects of tungsten (W) content up to about 20 wt pct on the creep-rupture properties of low-carbon HAYNES 25-(L-605-)
type cobalt-base alloys were investigated at 1089 and 1311 K. An increase in W content of about 5 wt pct resulted in tripling
the rupture life without significant loss of creep ductility at 1311 K. The principal strengthening phases precipitated during
creep at 1311 K were W solid solution and M6C carbide precipitates in the matrix and on the grain boundaries. The amounts of these precipitates, especially precipitates
of W solid solution, increased with increasing W content. The Cr23C6 carbide was also detected in those ruptured specimens of alloys containing more than 17 wt pct W. The creep ductility decreased
a little, and the rupture life did not increase with increasing W content at 1089 K. Two types of carbides (Cr23C6 and M6C), Co2W (Laves phase), and α-Co were confirmed in the specimens ruptured at 1089 K. The amount of Co2W harmful to ductility, as well as the amounts of strengthening phases (carbides), increased with increasing W content. 相似文献
14.
P. N. V. R. S. S. V. Prasada Rao A. K. Patwardhan N. C. Jain 《Metallurgical and Materials Transactions A》1993,24(2):445-457
An experimental study has been made of the effect of heat treatment on the transformation behavior of a 4.8 pct Cr white iron,
alloyed with 6 pct Mn and 1.5 pct Cu, by employing optical metallography, X-ray diffractometry, and differential thermal analysis
(DTA) techniques, with a view to assess the suitability of the different microstructures in resisting aqueous corrosion. The
matrix microstructure in the as-cast condition, comprising pearlite + bainite/martensite, transformed to austenite on heat-treating
at all the temperatures between 900 °C and 1050 °C. Increasing the soaking period at each of the heat-treating temperatures
led to an increase in the volume fraction and stability of austenite. M3C was the dominant carbide present in the as-cast condition. On heat-treating, different carbides formed: M23C6 carbide was present on heat-treating at 900 °C and 950 °C; on heat-treating at 1000 °C, M7C3 formed and persisted even on heattreating at 1050 °C. The possible formation of M5C2 carbide in the as-cast and heat-treated conditions (900 °C and 950 °C) is also indicated. Dispersed carbides (DC), present
in austenite up to 950 °C, mostly comprised M3C and M5C2. On stress relieving of the heat-treated samples, M7C3-type DC also formed. The hardness changes were found to be consistent with the micro-structural changes occurring on heat-treating.
The as-cast state was characterized by a reasonable resistance to corrosion in 5 pct NaCl solution. On heat-treating, the
corrosion resistance improved over that in the as-cast state. After 4 hours soaking, increasing the temperature from 900 °C
to 1050 °C led to an improvement in corrosion resistance. However, after 10 hours soaking, corrosion resistance decreased
on increasing the temperature from 900 °C to 950 °C and improved thereafter on increasing the heat-treating temperature. Deformation
behavior responded to the microstructure on similar lines as the corrosion behavior. Although in an early stage of development,
the composition thus developed betters the performance of 22 pct Ni containing Ni-Resist irons as far as strength and freedom
from pitting and graphitic corrosion are concerned; however, the corrosion resistance is somewhat lower. In conclusion, the
usefulness of the different microstructures in attaining a useful combination of corrosion resistance and deformation behavior
has been assessed. The data thus generated provide definite clues to developing new materials with improved performance for
resisting aqueous corrosion in marine environments.
Formerly Postdoctoral Candidate, University of Roorkee 相似文献
15.
S. M. Skolianos T. Z. Kattamis O. F. Devereux 《Metallurgical and Materials Transactions A》1989,20(11):2499-2516
The microstructure and corrosion behavior of as-cast and heat-treated Al-4.5 pct Cu-2.0 pct Mn alloy specimens solidified
at various cooling rates were investigated. The equilibrium phases Al6Mn and θ-Al2Cu, which are observed in the conventionally solidified alloy in the as-cast condition, were not detected in rapidly solidified
(melt-spun) material. Instead, the ternary compound Al20Cu2Mn3 was present in addition to the α phase, which was present in all cases. The morphological and kinetic nature of corrosion
was investigated metallographically and through potentiostatic techniques in 3.5 wt pct NaCl aqueous solution. Corrosion of
the as-cast material was described by two anodic reactions: corrosion of the intermetallic phases and pitting of the α-Al
solid solution. The corrosion rate increased with cooling rate from that for the furnace-cooled alloy to that for the copper
mold-cast alloy and, subsequently, decreased in the rapidly solidified alloy. In the heat-treated material, corrosion could
be described by two anodic reactions: corrosion of Al20Cu2Mn3 precipitate particles and pitting of the α-Al matrix.
S.M. Skolianos, formerly Graduate Student, Department of Metallurgy, University of Connecticut 相似文献
16.
Specimens of 2 1/4 Cr?1 Mo steel that had been decarburized in sodium or aged in an inert atmosphere for 26,500 h at 839 K were metallurgically examined with optical and transmission electron microscopy. The carbide particle density of the decarburized and aged specimens was considerably less after 26,500 h than after 10,000 h, with the density in the aged specimen being considerably greater than that in the decarburized specimen. For the aged specimen, the density decrease was a result of Ostwald ripening, and the total amount of carbide was not noticeably affected. Decarburization, however, resulted in a gradient in carbide density, with the smallest density near the surface. Carbides were electrolytically extracted and identified by X-ray diffraction. From the before-test material, which was in the annealed condition and had a ferrite-pearlite microstructure, 1.4 wt pct carbide was obtained. This sample contained M3C, M23C6, and M6C, with M23C6 constituting the major portion. In the aged specimen, 2.3 wt pct of precipitate was extracted and contained 60 pct M6C, the balance M23C6. The decarburized specimen contained two regions with different carbide contents. The first 20 pct of the 1.6 mm thick (0.063 in.) specimen yielded 1.3 wt pct precipitate, which contained 94 pct M6C, the balance M23C6. The interior of the specimen contained 1.6 wt pct precipitate, 86 pct M6C. 相似文献
17.
Fusheng Pan Peidao Ding Aitao Tang Mitsuji Hirohashi Yun Lu D. V. Edmonds 《Metallurgical and Materials Transactions A》2004,35(9):2757-2766
The effects of silicon additions up to 3.5 wt pct on the as-cast carbides, as-quenched carbides, and as-tempered carbides
of high-speed steels W3Mo2Cr4V, W6Mo5Cr4V2, and W9Mo3Cr4V were investigated. In order to further understand these effects,
a Fe-16Mo-0.9C alloy was also studied. The results show that a critical content of silicon exists for the effects of silicon
on the types and amount of eutectic carbides in the high-speed steels, which is about 3, 2, and 1 wt pct for W3Mo2Cr4V, W6Mo5Cr4V2,
and W9Mo3Cr4V, respectively. When the silicon content exceeds the critical value, the M2C eutectic carbide almost disappears in the tested high-speed steels. Silicon additions were found to raise the precipitate
temperature of primary MC carbide in the melt of high-speed steels that contained d-ferrite, and hence increased the size of primary MC carbide. The precipitate temperature of primary MC carbide in the high-speed
steels without d-ferrite, however, was almost not affected by the addition of silicon. It is found that silicon additions increase the amount
of undis-solved M6C carbide very obviously. The higher the tungsten content in the high-speed steels, the more apparent is the effect of silicon
additions on the undissolved M6C carbides. The amount of MC and M2C temper precipitates is decreased in the W6Mo5Cr4V and W9Mo3Cr4V steels by the addition of silicon, but in the W3Mo2Cr4V
steel, it rises to about 2.3 wt pct. 相似文献
18.
Y. F. Yang S. D. Luo G. B. Schaffer M. Qian 《Metallurgical and Materials Transactions A》2012,43(12):4896-4906
A systematic study has been conducted of the sintering, sintered microstructure and tensile properties of a range of lower cost Ti-Fe-Si alloys, including Ti-3Fe-(0-4)Si, Ti-(3-6)Fe-0.5Si, and Ti-(3-6)Fe-1Si (in wt pct throughout). Small additions of Si (??1?pct) noticeably improve the as-sintered tensile properties of Ti-3Fe alloy, including the ductility, with fine titanium silicides (Ti5Si3) being dispersed in both the ?? and ?? phases. Conversely, additions of ?>1?pct Si produce coarse and/or networked Ti5Si3 silicides along the grain boundaries leading to predominantly intergranular fracture and, hence, poor ductility, although the tensile strength continues to increase because of the reinforcement by Ti5Si3. Increasing the Fe content in the Ti-xFe-0.5/1.0Si alloys above 3?pct markedly increases the average grain size and changes the morphology of the ??-phase phase to much thinner and more acicular laths. Consequently, the ductility drops to <1?pct. Si reacts exothermically with Fe to form Fe-Si compounds prior to the complete diffusion of the Fe into the Ti matrix during heating. The heat thus released in conjunction with the continuous external heat input melts the silicides leading to transient liquid formation, which improves the densification during heating. No Ti-TiFe eutectoid was observed in the as-sintered Ti-Fe-Si alloys. The optimum PM Ti-Fe-Si compositions are determined to be Ti-3Fe-(0.5-1.0)Si. 相似文献
19.
John A. Wert E. R. Parker V. F. Zackay 《Metallurgical and Materials Transactions A》1979,10(9):1313-1322
Precipitation of the Fe2Nb intermetallic compound has previously been found to cause substantial hardening during aging of Fe rich Fe-Nb alloys. However,
the formation of a wide precipitate free zone adjacent to the grain boundaries caused a degradation of creep resistance. In
an effort to decrease the precipitate free zone width, thereby improving the creep resistance, an extensive study was made
of the precipitation behavior of an Fe-1.7 at. pct Nb(Cb) alloy quenched from the δ-phase field. The quenched alloy was found
to decompose via a two step reaction during aging at temperatures below 550°C. The first step in the decomposition reaction
is thought to occur by clustering of Nb atoms in the ferrite matrix, similar to the clustering of Mo atoms which is known
to occur during aging of Fe-Mo alloys. The second step in the reaction is not well understood. The precipitate free zones
were formed by solute depletion in the vicinity of the grain boundary and the subsequent difficulty of nucleation of the Fe2Nb precipitates in the regions of lowered solute concentration. Using two step aging treatments, an initial low temperature
step to develop the Nb atom clusters followed by a higher temperature step to cause Fe2Nb precipitation, the precipitate free zones were eliminated from the aged alloys. The origin of this effect is thought to
be the heterogeneous nucleation of Fe2Nb precipitates on the clusters developed during the initial aging step. 相似文献
20.
Fatigue crack initiation in extruded X7091 RSP-P/M aluminum type alloys occurs at grain boundaries at both low and high stresses.
By a process of elimination this grain boundary embrittlement was attributed to A12O3 particles formed mainly during atomization and segregated to some grain boundaries. It is not due to the small grain size,
to Co2Al9, to 17 precipitates at grain boundaries, nor to a precipitate free zone. Thermomechanical processing after extrusion of X7091
with 0.8 pct Co was done by Alcoa to produce large recrystallized grains. This resulted in initiation of fatigue cracks at
slip bands, and the resistance to initiation of fatigue cracks at low stresses was much greater. Microcrack growth is, however,
much faster in the thermomechanically treated samples, as well as in ingot alloys, than in extruded and aged X7091. 相似文献