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1.
Containerless processing and rapid solidification of Nb-Si alloys in the niobium-rich eutectic range 总被引:1,自引:0,他引:1
G. A. Bertero W. H. Hofmeister M. B. Robinson R. J. Bayuzick 《Metallurgical and Materials Transactions A》1991,22(11):2713-2721
Containerless processing and rapid solidification techniques were used to process Nb-Si alloys in the Nb-rich eutectic range.
Electromagnetic ally levitated drops were melted and subsequently splat quenched from different temperatures. A variety of
eutectic morphologies was obtained as a function of the degree of superheating or undercooling of the drops prior to splatting.
Metallic glass was observed only in drops quenched from above the melting temperature. Micro-structures of splats deeply undercooled
prior to quenching were very fine and uniform. These results are discussed in terms of classic nucleation theory concepts
and the expected heat evolution at different regions of the splat during the rapid quenching process. The locations of the
coupled-zone boundaries for the α-Nb + Nb3Si eutectic are also suggested.
Formerly Graduate Student, Vanderbilt University. 相似文献
2.
《Acta Metallurgica》1976,24(3):189-196
In iron-cobalt alloys which have been quenched from temperatures above the critical temperature the return to an ordered state occurs by simultaneous growth of ordered regions and growth of the degree of ordering over large distances within these regions. We show that this ordering process does not obey the classical rules of nucleation and growth transformations but rather can be interpreted as a homogeneous ordering process. The transformation is subsequently followed by a period of coalescence of the antiphase domains. The addition of vanadium accelerates the kinetics of the ordering process which can be explained by a large supersaturation of vacancies in the terniary alloy. 相似文献
3.
《Acta Metallurgica》1980,28(10):1375-1383
A very simple experiment is described in which the alteration of the channeling conditions for α particles is observed during room temperature ageing of quenched Al-Cu alloys. The dechanneling increases during the ageing and the kinetics of this phenomenon are measured.With the help of some straightforward hypotheses concerning the elementary mechanisms of dechanneling, kinetical laws are calculated for both nucleation growth and spinodal decomposition. The experimental results support the second process. They also give information on the shape (flat discs) of the Guinier-Preston zones. 相似文献
4.
《钢铁研究学报(英文版)》2011,(Z1):350-354
For a V-Ti-N microalloyed steel with 0.34%C-1.54%Mn,intragranular ferrite (IGF) was obtained in both isothermal austenite decomposition processes and thermomechanical processes simulating the industrial seamless tubing manufacture process.Results show that with decrease of the isothermal temperature in range of 600℃ down to 450℃,not only the morphology of IGF changed from equiaxed to acicular,but also the equiaxed IGF and the acicular IGF were refined.More importantly,it is found that the amount of equiaxed ferrite increased significantly in the thermomechanical process sample water quenched from 550℃ after 800℃ deformation than that in the isothermally treated sample at 550℃ sample without hot deformation.It implies that appropriate controlled deformation with controlled cooling can significantly promote equiaxed IGF formation,and not solely rely on nucleation mechanisms related with inclusions.Hot deformation of austenite without dynamic and complete static recrystallization causes high energy regions,therefore further promotes the nucleation potency of IGF. 相似文献
5.
A. I. Fernández B. López J. M. Rodriguez-Ibabe 《Metallurgical and Materials Transactions A》2002,33(10):3089-3098
Static recrystallization mechanisms have been studied in a coarse-grained Nb microalloyed austenite. An austenite with a coarse
grain size of 800 μm, typical of thin slab casting processes, has been deformed in torsion at a temperature of 1100 °C. After deformation, the
specimens have been held for different times at this high temperature and then water quenched. The microstructural changes
occurring during static recrystallization were characterized by metallographic evaluation. It has been observed that new recrystallized
grains nucleate preferentially on parent austenite grain boundaries and tend to form in clusters. Once all the boundaries
have been consumed, intragranular nucleation is actived at late stages of recrystallization. Clustered nucleation allows impingement
to take place early during the recrystallization process, favoring grain-coarsening phenomena to occur behind the recrystallization
front, which is denoted by the significant reduction in the number of grains per unit volume observed during early stages
of recrystallization. Static recrystallization proceeds heterogeneously, as a result of a nonuniform distribution of stored
energy in the deformed material. A continuous decrease of the average migration rate of the recrystallization front is observed,
which can be ascribed to the reduction of the driving force for migration as recrystallization advances. 相似文献
6.
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
7.
A technique has been developed for the determination of the particle-matrix interface strength in dispersion-strengthened alloys. The procedure involves very rapid resistance heating, which results in the nucleation of voids by the affect of differential thermal expansion between the metal matrix and the dispersed phase particles. The interface stresses during heating are a function of temperature, and thus determination of minimum temperature at which decohesion occurs permits the estimation of interface strength. This determination was accomplished by tensile testing immediately after heating and the findings supported by transmission electron microscopy of quenched samples. The strength of the Ni?ThO2 interface in a TD-Nickel alloy was found to be approximately 30, 100 psi. 相似文献
8.
《Acta Metallurgica Materialia》1991,39(11):2727-2731
Observations of liquid supercooling achieved prior to solidification of rapidly quenched Si thin films as a function of quench rate are examined in terms of a numerical model for transient nucleation, which allows estimation of the kinetics for the transfer of atoms to and from crystalline clusters. The process is found to be thermally activated with an activation energy of 1.09 ± 0.05 eV/atom. This differs appreciably from previous estimates for the activation energy for planar growth of crystalline Si (≈0.3 eV) and possible reasons for this discrepancy are discussed. 相似文献
9.
通过热力学和动力学计算相结合的方法,系统地分析了高磁感取向硅钢中A1N在均热过程中的析出机 制。计算结果表明,在高磁感取向硅钢的均热温度下AIN处在α+γ 两相区,同时具备热力学析出条件。在均匀形核、晶界形核和位错形核3种机制下,A1N的临界形核尺寸处在同一数量级且随温度降低而减小。相同温度下, A1N晶界形核的临界形核功最小,相对形核率最大,即较易发生晶界形核。A1N在α相和γ相中均匀形核、晶界形 核和位错形核的最快析出温度分别为1203 K、1303 K、1243 K和1213 K、1305 K、1233 K。 AIN在均热温度下以晶界形核为主。 相似文献
10.
《Acta Metallurgica Materialia》1994,42(7):2267-2273
The nucleation and kinetics of defects formed by quenching in two AlLi alloys having Li content of 1.7 and 3.74 at.%Li have been studied by positron annihilation spectroscopy. It has been found that the defect formation is sensitive to the aging time at the temperature from which the samples are quenched. This fact has been related to the Li loss experienced by the alloys aged at high temperature. The quenched-in defects have been identified as vacancy-Li clusters and dislocation loops. The latter are formed by the collapse of the Li-rich vacancy complexes and are very sensitive to the Li content; as a consequence, the loops are decorated by Li-rich zones and are revealed as very effective positron traps in comparison to the vacancy-Li complexes, giving rise to an enhanced trapping. 相似文献
11.
12.
《Acta Metallurgica Materialia》1990,38(1):41-54
The fundamental nature of the static restoration processes which result in static softening after a hot deformation has been studied in copper and aluminum. The kinetics of static softening were determined using the double-hit technique applied to hot compression while the microstructural changes were characterized by the quantitative metallography of quenched specimens. A static softening parameter based on the area under the compression flow curve was used to describe the static softening kinetics. The static softening curves exhibited a simple sigmoidal shape showing no inflection. The relative softening occurring prior to the initiation of recrystallization was found to be small when compared with that occurring after the onset of recrystallization, and was dependent on deformation temperature, amount of deformation, purity and stacking fault energy. The static softening was related to the fractional recrystallization in a nonlinear manner; the degree of nonlinearity was dependent on the occurrence of recovery and dynamic recrystallization. The recrystallization process in Al was of the classical type with the nucleation stage being either the boundary bulge or subgrain growth mechanism. In Cu twinning appeared to be the major nucleation mechanism for recrystallization. When the applied prestrain was greater than the critical strain for dynamic recrystallization, recrystallization was observed to be completed before the completion of static softening. In this case, the remaining softening occurred by the operation of multiple recrystallization where high-order twins formed in the already twinned regions. 相似文献
13.
The cooling of pieces being quenched in evapourable fluids is mainly determined by the process of wetting, which is defined as the sequence of the three known cooling stages film boiling, nucleate boiling and convective heat transfer. All these determine the properties of the finished product. The effect of different wetting processes and heat transfer coefficients on the time-dependent distribution of temperature and heat flux in quenched cylinders was examined using numerical methods. Only transformation-free quenching was considered. When the three cooling stages simultaneouly take place on the sample's surface the radial heat fluxes are superimposed by high axial heat fluxes, which could not be pointed out experimentally before. The consequences may be great axial differences of structure and hardness, especially for steels with low hardenability, as well as residual stresses. The axial temperature gradients can be reduced by producing high wetting velocities. An optimum wetting process involves a very fast breakdown of the vapour envelope on the whole surface of the quenched piece. 相似文献
14.
15.
Roy A. Vandermeer 《Metallurgical and Materials Transactions A》1976,7(6):871-878
The uranium +7.5 wt pct niobium +2.5 wt pct zirconium alloy when quenched from 1073 K was found to exist at room temperature
as a metastable phase which was a slight tetragonal distortion of the elevated temperature body-centered-cubic (bcc) phase.
Flat, asquenched specimens have been elastically deformed in four-point bending to maximum outer fiber stresses below the
stress required for plastic deformation to occur but into a range of stress where pseudoelastic behavior has been observed.
Aging of these elastically bent specimens in an oil bath at 423 K, while constrained by the bending jig, resulted in a permanent
deflection and shape change. Further isothermal aging, after removal from the bending apparatus, caused increasing deflection
and continued shape instability in spite of the absence of the applied load. X-ray examination of samples cut from a bent
and aged specimen revealed important preferred orientation and lattice parameter differences between the tension and compression
regions and the high and low stress parts of the specimen. These observations are described and compared to previous findings
on quenched samples of this alloy that had been either deformed separately or aged separately. A rationalization of the shape
instability is presented. Elastic twin nucleation and growth, preferred orientations, solute segregation and the interplay
of all these seem to be involved. 相似文献
16.
Roy A. Vandermeer 《Metallurgical and Materials Transactions A》1976,7(5):871-878
The uranium +7.5 wt pct niobium +2.5 wt pct zirconium alloy when quenched from 1073 K was found to exist at room temperature
as a metastable phase which was a slight tetragonal distortion of the elevated temperature body-centered-cubic (bcc) phase.
Flat, asquenched specimens have been elastically deformed in four-point bending to maximum outer fiber stresses below the
stress required for plastic deformation to occur but into a range of stress where pseudoelastic behavior has been observed.
Aging of these elastically bent specimens in an oil bath at 423 K, while constrained by the bending jig, resulted in a permanent
deflection and shape change. Further isothermal aging, after removal from the bending apparatus, caused increasing deflection
and continued shape instability in spite of the absence of the applied load. X-ray examination of samples cut from a bent
and aged specimen revealed important preferred orientation and lattice parameter differences between the tension and compression
regions and the high and low stress parts of the specimen. These observations are described and compared to previous findings
on quenched samples of this alloy that had been either deformed separately or aged separately. A rationalization of the shape
instability is presented. Elastic twin nucleation and growth, preferred orientations, solute segregation and the interplay
of all these seem to be involved. 相似文献
17.
为了研究热处理工艺对超级13Cr不锈钢组织及拉伸性能的影响,采用了光学显微镜、X射线衍射仪、透射电子显微镜、显微硬度测试及应变速率拉伸等试验方法。结果表明,经过水淬和油淬处理的超级13Cr不锈钢组织及拉伸性能相差不大。但相比于水淬,采用油淬的试样经回火处理后塑性得到更大提升。淬火试样经回火处理后,组织变为回火索氏体。随着回火温度升高,材料的塑性先增加后减小,硬度与强度变化则相反。620 ℃回火试样含有逆变奥氏体,强度塑性组合较好。二次回火能够增加超级13Cr不锈钢中逆变奥氏体含量,但塑性变化不明显,强度下降较大。 相似文献
18.
The mechanical and toughness properties of steam turbine rotors of different production methods and different heat treatments are described and compared. Air and oil quenched rotors have similar properties in respect to creep behaviour while the toughness level of the oil quenched rotors is better. Rotors manufactured since about 1975 have a better level of all properties than older ones which is due to higher degree of purity, lower level of trace elements and a today better controlled temperature process during the whole manufacturing. 相似文献
19.
《Acta Metallurgica》1983,31(5):755-762
A systematic transmission electron microscopy study of the entire carbide precipitate nucleation and growth process in quenched, aged platinum has been made. Five distinct stages have been identified in the growth sequence, four coincide with changes in the Burgers and displacement vectors and the fifth in the configuration of the precipitates. Diffraction contrast analysis has shown that all the precipitate platelets lie in {001} matrix planes, are vacancy in character and are semi-coherent. Precipitates form initially by co-precipitation of vacancies and carbon atoms and the growth sequence follows a ripening type process. At intermediate aging temperatures voids become the preferred growth sites presumably acting as vacancy sources. Various aspects of the precipitation reaction have been explained from the standpoint of the role of vacancies (V), and the system can be treated either as a Pt-C binary or a Pt-C-V ternary alloy. 相似文献
20.
Spray freezing technology has been used in ice building construction in cold regions and artificial snow making. The spray freezing process involves heat and mass transfer and ice nucleation. The freezing temperature of the sprayed water is influenced by many factors, such as droplet size (volume), ambient air temperature, and impurity content of the water. An experimental study was carried out to investigate the influence of the droplet size (volume) and the ambient air temperature on the ice nucleation temperature of the freely suspended droplets of different qualities—piggery wastewater, pulp mill effluent, and oil sands tailings pond water. The time required to initiate freezing in the freely suspended wastewater droplets was measured under various experimental conditions using video-image technology. The ice nucleation temperature of the droplets were predicted based on the required freezing time and the rate of heat and mass transfer. 相似文献