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1.
《Acta Metallurgica Materialia》1990,38(6):889-896
A kinetic model was formulated to account for the influence of the density of orientation, thermal and intersection kinks on the velocity of dislocation ledges during precipitate growth. The relative contributions of each mechanism to the ledge velocity are characterized and the kinetic model is compared with existing experimental data on dislocation ledge motion in the Al-Ag system. 相似文献
2.
Masato Enomoto 《Metallurgical and Materials Transactions A》1991,22(6):1235-1245
A previously developed computer model was modified to simulate the growth of grain boundary precipitates which grow by the ledge mechanism. The ledges were assumed to be nucleated in the grain boundary region at constant, parabolically decreasing, and random rates and to grow under the control of volume diffusion of solute to or from the riser of ledges. At lower under coolings at which the motion of individual ledges is slow, late-nucleated ledges soon catch up with first-nucleated ones, and precipitates tend to extend along the grain boundary: the overall precipitate shape is essentially that of a grain boundary allotriomorph. At larger undercoolings, first-nucleated ledges move fast to form a protuberance similar to Widmanstätten sideplates, while late-nucleated ones stay near the grain boundary region. The transition of precipitate shape from one to the other occurs in a very narrow range of supersaturation. The results are compared with various characteristics of the growth of proeutectoid ferrite allotriomorphs and sideplates in Fe-C alloys documented in the literature. 相似文献
3.
Hubert I. Aaronson R.F. Mehl Medalist 《Metallurgical and Materials Transactions A》1993,24(2):241-276
An integrated overview is presented of a viewpoint on the present understanding of nucleation and growth mechanisms in both
diffusional and shear (martensitic) transformations. Special emphasis is placed on the roles played by the anisotropy of interphase
boundary structure and energy and also upon elastic shear strain energy in both types of transformation. Even though diffusional
nucleation is based on random statistical fluctuations, use of the time reversal principle shows that interfacial energy anisotropy
leads to accurately reproducible orientation relationships and hence to partially or fully coherent boundaries, even when
nucleation at a grain boundary requires an irrational orientation relationship to obtain. Since the fully coherent boundary
areas separating most linear misfit compensating defects are wholly immobile during diffusional growth because of the improbability
of moving substitutional atoms even temporarily into interstitial sites under conditions normally encountered, partially and
fully coherent interphase boundaries should be immovable without the intervention of growth ledges. These ledges, however,
must be heavily kinked and usually irregular in both spacing and path if they, too, are not to be similarly trapped. On the
other hand, the large shear strain energy usually associated with martensite requires that its formation be initiated through
a process which avoids the activation barrier associated with nucleation, perhaps by the Olson-Cohen matrix dislocation rearrangement
mechanism. During growth, certain ledges on martensite plates serve as transformation dislocations and perform the crystal
structure change (Bain strain). However, the terraces between these ledges in martensite (unlike those present during diffusional
growth) are also mobile during non-fcc/hcp transformations; glissile dislocations on these terraces perform the lattice invariant
deformation. Growth ledges operative during both diffusional and shear growth probably migrate by means of kink mechanisms.
However, diffusional kinks appear to be nonconservative and sessile (and therefore resist immediate transmission of elastic
shear strain energy), whereas those associated with martensitic growth must be conservative and glissile (and fully transmit
such strain energy). The broad faces of both diffusionally and martensitically formed plates contain an invariant line, as
emphasized by Dahmen and Weatherly. However, in the diffusional case, minimization of growth ledge formation kinetics seems
to be the main role thereby played, whereas in martensitic growth, the main purpose of such an interface is to minimize elastic
shear strain energy. The latter minimization requires that martensite forms as plates (or perhaps as laths) enclosed by a
pair of invariant line-containing interfaces. During diffusional transformations, on the other hand, other interfaces at which
growth ledge formation kinetics are not too much faster than those at the invariant line interface can also comprise a significant
portion of the interfacial area, thereby leading to the formation of other, quite different morphologies, such as intragranular
idiomorphs and grain boundary allotriomorphs. Critical problems remaining unsolved in diffusional transformations include
calculation of critical nucleus shapes when the crystal structures of the two phases are significantly different, highly accurate
calculation of the energies of the interphase boundaries thus formed, and direct observation of atomic scale kinks on the
risers of growth ledges by means of a yet-to-be-invented three-dimensional (3-D) atomic-resolution form of transmission electron
microscopy. Experimental identification and characterization of transformation dislocations and experimental testing of “nucleation”
mechanisms are now of special importance in fundamental studies of martensitic transformations. 相似文献
4.
《Acta Metallurgica》1987,35(11):2671-2678
A theory of isothermal grain growth in polycrystalline solids, which treats grain growth as a statistical or stochastic process, is presented. In this treatment deterministic equation for the rate of grain growth is made stochastic by the addition of a “noise” term. The noise or fluctuations are used to model the effect of complex topologically connected structure of the specimen on grain boundary motion, in addition to such motion directed by surface tension forces. Such considerations lead to a second order partial differential equation (Fokker-Planck equation) for the grain size distribution. Many of the major attributes of grain growth are shown to be a natural consequence of this equation. The solution obtained for this equation is a modified form of Rayleigh distribution which in many respects is similar of log normal distribution. Grain size distribution is also obtained from independent statistical consideration and is shown to be approximately log normal. Extension of the mathematical analysis to the case of Ostwald ripening is indicated. 相似文献
5.
《Acta Metallurgica Materialia》1990,38(10):2017-2021
A simple model is presented to describe the kinetics of normal grain growth. The final expression incorporates a factor which depends on the grain size distribution but no assumption is made on the distribution function. A similar expression is also derived for particle coarsening. 相似文献
6.
Donald Susan Joseph Michael Richard P. Grant Bonnie McKenzie W. Graham Yelton 《Metallurgical and Materials Transactions A》2013,44(3):1485-1496
Time-lapse SEM studies of Sn whiskers were conducted to estimate growth kinetics and document whisker morphologies. For straight whiskers, growth rates of 3 to 4 microns per day were measured at room temperature. Two types of kinked whiskers were observed. For Type A kinks, the original growth segment spatial orientation remains unchanged, there are no other changes in morphology or diameter, and growth continues. For Type B kinks, the spatial orientation of the original segment changes and it appears that the whisker bends over. Whiskers with Type B kinks show changes in morphology and diameter at the base, indicating grain boundary motion in the film, which eliminates the conditions suitable for long-term whisker growth. To estimate the errors in the whisker growth measurements, a technique is presented to correct for SEM projection effects. With this technique, the actual growth angles and lengths of a large number of whiskers were collected. It was found that most whiskers grow at moderate or shallow angles with respect to the surface; few straight whiskers grow nearly normal to the surface. In addition, there is no simple correlation between growth angles and lengths for whiskers observed over an approximate 2-year period. 相似文献
7.
Min Wang Jianxin Zhou Yajun Yin Hai Nan Dongqiao Zhang Zhixin Tu 《Metallurgical and Materials Transactions B》2017,48(5):2245-2255
A 3D cellular automata model is used to simulate normal austenitic grain growth in this study. The proposed model considers both the curvature- and thermodynamics-driven mechanisms of growth. The 3D grain growth kinetics shows good agreement with the Beck equation. Moreover, the growth exponent and grain size distribution calculated by the proposed model coincides well with experimental and simulation results from other researchers. A linear relationship is found between the average relative grain size and the grain face number. More specifically, for average relative grain sizes exceeding ~0.5, the number of faces increases linearly with relative grain size. For average relative grain sizes <0.5, this relationship is changed. Results simulated by the proposed model are translated to physical meaning by adjusting the actual temperature, space, and time for austenitic grain growth. The calibrated results are found to be in agreement with the simulation results from other research as well as the experimental results. By means of calibration of the proposed model, we can reliably predict the grain size in actual grain growth. 相似文献
8.
《Acta Metallurgica Materialia》1993,41(7):2211-2217
It is shown that defects of rotation type form in polycrystals during the plastic deformation process at junctions and kinks of grain boundaries. Their geometric properties are analysed and it is proved that in general these defects cannot be reduced to disclinations, but represent more complex linear-planar entities. The analysis shows that dipoles of defects of rotation type appear at ledges of boundaries. Such ledges are considered as dipole sources of the partial disclinations. 相似文献
9.
G. Spanos W. T. Reynolds R. A. Vandermeer 《Metallurgical and Materials Transactions A》1991,22(6):1367-1380
The influence of interphase boundary ledges on the growth and morphology of proeutectoid ferrite and proeutectoid cementite
precipitates in steel is examined. After reviewing current theoretical treatments of growth by the ledge mechanism, investigations
that clearly document the presence and motion of ledges with thermionic emission electron microscopy (THEEM) and transmission
electron microscopy (TEM) are reviewed. A fundamental distinction is made between two types of ledges: (1) mobile growth ledges
whose lateral migration displaces the inter-phase boundary and (2) misfit-compensating structural ledges. Both types of ledges
strongly affect the apparent habit plane and aspect ratio of precipitate plates. Agreement between measured growth rates of
proeutectoid ferrite and cementite (plates and allotriomorphs) and predicted growth kinetics assuming volume diffusion-controlled
migration of ledge-free disordered boundaries is shown to be consistently poor. Physically realistic growth models should
incorporate the ledge mechanism. More accurate comparisons of the growth models with experimental data will need to account
for observed ledge heights, interledge spacings, and ledge velocities. In this vein, the sluggish growth kinetics of cementite
allotriomorphs observed in an Fe-C alloy are shown to be quantitatively consistent with a strong increase in interledge spacing
with time.
This paper is based on a presentation made in the symposium “The Role of Ledges in Phase Transformations” presented as part
of the 1989 Fall Meeting of TMS-MSD, October 1–5, 1989, in Indianapolis, IN, under the auspices of the Phase Transformations
Committee of the Materials Science Division, ASM INTERNATIONAL. 相似文献
10.
Numerical simulation of Zn coating solidification 总被引:2,自引:0,他引:2
A. Sémoroz Ph.D L. Strezov M. Rappaz 《Metallurgical and Materials Transactions A》2002,33(8):2685-2694
A numerical model, which simulates nucleation and growth of Zn grains, has been developed in order to describe quantitatively
the solidification of Zn coatings during the hot-dipping process. The inputs of the model are the nucleation distribution,
which has been measured by electron backscattered diffraction (EBSD), and the dendritic growth kinetics, calculated with an
analytical model of a parabolic dendrite tip modified to account for the interactions with the coating interfaces. The model
predicts the shapes of the grain envelopes as a function of the grain orientation and the texture induced by growth. Three
types of grain envelopes have been evidenced, depending on the angle between the c-axis and the normal to the coating plane.
Moreover, it has been shown that growth reinforces the already existing {00.1} nucleation texture, in good agreement with
experimental data. The model also predicts the cooling curve, including recalescence, and the grain size. Thus, it is used
to describe the effects of Pb additions on solidification. In particular, it has been shown that Pb increases the nucleation
undercooling and strongly decreases the density of active nuclei, thus resulting in a much larger grain size. 相似文献
11.
M. Enomoto H. I. Aaronson T. Furuhara 《Metallurgical and Materials Transactions A》1991,22(6):1341-1348
A computer model is developed to simulate the growth of grain-boundary allotriomorphs having more than one set of growth ledges
at their interfaces. The growth is controlled by the volume diffusion of solute to or from the riser of a ledge. The time
dependence of the growth rate of two orthogonal sets of ledges is found to be somewhat different from that of a single set
of ledges. However, the operation of multiple sets of ledges is unlikely to alter significantly the growth kinetics of grain-boundary
allotriomorphs from those predicted from the disordered growth theory, except at small ledge spacings or at short reaction
times. Faster growth kinetics of proeutectoid α allotriomorphs than those of either planar or ellipsoidal disordered boundaries
which have been reported in a Ti-6.6 at. pet Cr alloy are not likely to be accounted for with the heights and spacings of
double sets of ledges actually observed on the interfaces of allotriomorphs. Hence, the grain-and interphase-boundary diffusion-assisted
growth of precipitates, (rejector plate mechanism, RPM) appears to be operative during the growth of a allotriomorphs, as
previously proposed on the basis of growth-rate measurements.
This paper is based on a presentation made in the symposium “The Role of Ledges in Phase Transformations” presented as part
of the 1989 Fall Meeting of TMS-MSD, October 1–5, 1989, in Indianapolis, IN, under the auspices of the Phase Transformations
Committee of the Materials Science Division, ASM INTERNATIONAL. 相似文献
12.
13.
14.
Modeling diffusional growth during austenite decomposition to ferrite in polycrystalline FeC alloys
《Acta Metallurgica Materialia》1990,38(12):2461-2470
A grain-boundary-nucleated, diffusional growth model of austenite decomposition to proeutectoid ferrite is developed for polycrystalline iron-carbon alloys. The diffusion equation is solved under restricted diffusion conditions using the quasi-static method and employing local thermodynamic equilibrium at the disordered austenite:ferrite interface. Decomposition kinetics for a model polycrystalline material consisting of a log-normal distribution of spherical grains are calculated numerically. Effects of temperature, overall carbon concentration, volume change, austenite grain size and carbon buildup in the centers of the austenite grains are included in the treatment. A scaling factor is deduced that enables the effect of austenite grain size on transformation kinetics to be characterized provided kinetic information is available for one grain size. Experiments carried out on a laboratory steel verified the applicability of the scaling factor, Also, partial I-T and C-T diagrams can be computed from the model and sample calculations are presented for an iron + 0.036 wt% carbon steel. 相似文献
15.
T. O. Saetre N. Ryum E. Evangelista 《Metallurgical and Materials Transactions A》1991,22(10):2257-2263
It has been known for many years that subgrain growth can occur by subgrain rotation. However, the kinetics of such a growth
process have not been worked out. In this investigation, a simple model for such a process, namely, a single row of subgrains,
has been studied by computer simulation. The rate of growth of the mean subgrain size and the variation in the distribution
of the subgrain misorientations with time have been established. The differences between subgrain growth by rotation and normal
grain growth are also discussed. 相似文献
16.
采用一种改进的Potts模型Monte Carlo算法,对具有Weibull尺寸分布(参数β=3.47)的晶粒组织进行了3D正常晶粒长大过程的仿真研究.仿真结果表明:整个晶粒长大过程遵循抛物线长大规律,晶粒生长指数为0.501,非常接近理论值0.5.晶粒长大过程可分为过渡阶段与准稳态长大两个阶段.Weibull尺寸分布参数β由过渡阶段的3.47逐渐演变为准稳态阶段的2.76,准稳态阶段晶粒尺寸分布参数保持β=2.76不变.晶粒的平均面数〈f〉随仿真时间的增加而增大,在准稳态阶段后期趋近于稳定数值.晶粒面数分布为Lognormal分布,最高频率面数f为10,个体晶粒面数范围为3~43. 相似文献
17.
C. Devadas I. V. Samarasekera E. B. Hawbolt 《Metallurgical and Materials Transactions A》1991,22(2):335-349
A mathematical model has been developed to compute the changes in the austenite grain size during rolling in a hot-strip mill.
The heat-transfer model described in the first of this series of papers has been employed to calculate the temperature distribution
through the thickness which serves as a basis for the microstructure model. Single-and double-hit compression tests have been
conducted at temperatures of 900 °C, 850°C, 950 °C, and 875 °C on 0.34 and 0.05 pct carbon steels to determine the degree
of recrystallization by metallographic evaluation of quenched samples and by measuring the magnitude of fractional softening.
The Institut de Recherches de la Sidérurgie Francaise, (IRSID) Saint Germain-en-Laye, France equation has been found to yield
the best characterization of the observed recrystallization kinetics. The equations representing static recrystallization
kinetics, recrystallized grain size, and grain growth kinetics have been incorporated in the model. The principle of additivity
has been invoked to permit application of the isothermal recrystallization data to the nonisothermal cooling conditions. The
model has been validated by comparing predicted austenite grain sizes with measurements made on samples quenched after one
to four passes of rolling on the CANMET pilot mill. The austenite grain size evolution during rolling of a 0.34 pct carbon
steel on Stelco’s Lake Erie Works (LEW) hot-strip mill has been computed with the aid of the model. The grain size decreased
from an initial value of 180μm to 35μm in the first pass due to the high reduction of 46 pct. The changes in austenite grain size in subsequent passes were found
to be small in comparison because of the lower per pass reductions. It has been shown that the equation employed to represent
grain growth kinetics in the interstand region has a significant influence on the computed final grain size. Altering the
rolling schedule had a negligible influence on the final grain size for a given finished gage. A 200°C increase in entry temperature
to the mill resulted in a 20μm increase in final grain size, which is significant. This can be attributed to increased grain growth at the higher temperature.
Formerly Graduate Student, The Centre for Metallurgical Process Engineering, The University of British Columbia Metallurgical
transactions a 相似文献
18.
19.
《Acta Metallurgica Materialia》1994,42(3):839-843
The behaviour of an abnormally large grain in a matrix containing stable particles in which normal grain growth is initially possible is examined by means of a theoretical model. It is found that an abnormally large grain will initially decrease its size relative to the critical radius of the distribution. As normal grain growth continues and the grain growth rate decreases due to the increasing action of the particles this situation is reversed and the abnormally large grain starts to increase its size relative to the critical radius. The model satisfactorily explains why the occurrence of severe abnormal grain growth in the presence of particles is often apparently “erratic”. 相似文献
20.
《Acta Metallurgica Materialia》1992,40(6):1159-1166
Normal grain growth in solids is considered in the light of a microstructural path concept. The topological model first presented by Rhines and Craig is corrected and reformulated so as to include the Doherty sweep constant. The reformulated model predicts the experimentally observed linear time dependence for volumetric grain growth only when the average curvature per grain remains constant during growth. A geometrical path function for normal grain growth relating average microstructural properties on a per grain basis is proposed and compared with experimental observations on aluminum and titanium. By combining the geometrical path function with the topological model, further comprehension of normal grain growth kinetics results, specifically the role of the average grain shape. 相似文献