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1.
An On-Heating Dilation Conversional Model for Austenite Formation in Hypoeutectoid Steels 总被引:1,自引:0,他引:1
Seok-Jae Lee Kester D. Clarke Chester J. Van Tyne 《Metallurgical and Materials Transactions A》2010,41(9):2224-2235
Dilatometry is often used to study solid-state phase transformations. While most steel transformation studies focus on the
decomposition of austenite, this article presents an on-heating dilation conversional model to determine phase fraction based
on measured volume changes during the formation of austenite in ferrite-pearlite hypoeutectoid steels. The effect of alloying
elements on the transformation strain is incorporated into the model. Comparison of the conversional model predictions to
measured transformation temperature (A
c3) shows excellent agreement. The pearlite decomposition finish temperature (A
pf
) predicted by the conversional model more closely matches experimental results when compared to standard lever rule calculations.
Results show that including the effects of substitutional alloying elements (in addition to carbon) improves phase fraction
predictions. The conversional model can be used to quantitatively predict intercritical austenite fraction with application
to modeling, induction heating, intercritical annealing, and more complex heat treatments for hypoeutectoid steels. 相似文献
2.
The reaction behavior of shock-consolidated Ti-Si powder mixture compacts, densified at 5 to 7 GPa pressure, was investigated
to determine conditions required for solid-state reaction synthesis leading to the formation of dense Ti5Si3 intermetallic compounds with fine-grained microstructure. It was observed that at temperatures greater than 1000 °C, the
heat released following reaction initiation in the solid state exceeds the rate of heat dissipation causing a self-propagating
combustion-type reaction to take over the synthesis process forming highly porous reaction products. A reaction synthesis
model was developed to allow the prediction of optimum conditions necessary to ensure that the bulk of the reaction in dynamically
densified Ti-Si powder compacts occurs by rapid solid-state diffusion and without being taken over by the combustion process.
The model incorporates mass and heat balance with the kinetics evaluated using experimentally determined apparent activation
energies for solid-state and combustion reactions. Considering the decrease in activation energy (as measure of degree of
shock activation), average particle size, and compact porosity as the main variables, the model plots the fraction reacted
as a function of time for various postshock reaction-synthesis temperatures, illustrating the dominant reaction mechanism
and kinetics. The results show that although changes in average particle size and compact porosity influence the synthesis
temperature above which the reaction may be taken over by the combustion-type process, lowering of the activation energy via shock-compression influences the time for reaction completion in the solid state. 相似文献
3.
4.
Microstructure and Property Evolution for Hot-Rolled and Cold-Rolled Austenitic Stainless Steel 316L
Nitin Kumar Sharma Shashank Shekhar 《Transactions of the Indian Institute of Metals》2017,70(5):1277-1284
Grain boundary character distribution plays an important role in determining the functional and mechanical properties of polycrystalline materials. The aim of this work was to achieve improved coincident site lattice (CSL) fraction without increasing low angle grain boundary (LAGB) proportion. We utilized single-step thermo-mechanical processing route involving rolling followed by short heat treatment and compared the effect of rolling temperature. Our results indicated that rolling at elevated temperature led to significant increase in the fraction of special boundaries while keeping the fraction of LAGB very low, as desired. We conducted thermal stability of our sample-conditions at elevated temperatures for various lengths of time and found the microstructure of the samples to be stable up to 1000 °C. This study showed that even commercially suitable process (single step processing with short heat treatment duration) could lead to microstructure with considerable increase in CSL boundaries fraction, improved hardness values and good thermal stability. 相似文献
5.
Aage Stangeland Asbjørn Mo Dmitry Eskin 《Metallurgical and Materials Transactions A》2006,37(7):2219-2229
The parameters in a recently developed constitutive equation for macroscopic thermal strain in the mushy zone have been determined
for the commercial alloys A356, AA2024, AA6061, and AA7075 in addition to an Al-4 wt pct Cu alloy. The constitutive equation
for macroscopic thermal strain in the mushy zone reflects that there is no thermal strain in the solid part of the mushy zone
at low solid fractions and that the thermal strain in the mushy zone approaches thermal strain in the fully solid material
as the solid fraction increases toward 1. The development of thermal strain in the mushy zone is determined by combining experimentally
measured contraction of a cast sample with thermomechanical stimulations. Experiments were performed at cooling rates in the
range from 2 to 5.5 °C/s. The solid fractions when the tested alloys start to contract,g
s
th, are in the range from 0.63 to 0.94. Grain refinement increasesg
s
th for all the tested alloys. For most of the tested alloys the thermal strain in the mushy zone increases rapidly to the same
level as thermal strain in fully solid material once the solid fraction becomes higher thang
s
th. 相似文献
6.
Sintered materials have significantly higher strength than green compacts. The evolution of that strength during the sintering
cycle involves a combination of annealing, thermal softening, and sintering events. The dynamic interplay between heating
rate, sintering time, and sintering temperature controls the in situ strength and determines the final sintered strength. Although sintered strength is a well-explored subject, the dynamic evolution
of strength requires new models. This research has measured both the sintered and in situ strengths as functions of heating rate, hold times, and temperature for die-compacted prealloyed bronze powder. A core concept
is the use of an integral work of sintering to determine the effective strengthening due to sintering. The model is used to
map strength evolution vs the key processing parameters. It is concluded that, during solid-state sintering of bronze, the key sources of distortion
are the density and thermal gradients. 相似文献
7.
John W. Gibbs Michael J. Kaufman Robert E. Hackenberg Patricio F. Mendez 《Metallurgical and Materials Transactions A》2010,41(9):2216-2223
This work presents a new method of cooling curve analysis to make in situ measurements of the amount of precipitate formed in solid-state phase transformations. The presented technique is based on
a first-principles analysis of thermodynamics and heat flow to develop equations that relate cooling curve data to the amount
transformed. The precipitation of Ag2Al in a binary Al-Ag alloy was examined both as a practical example of this technique and to obtain metallographic measurements
of the amount of precipitation for comparison purposes. 相似文献
8.
T. Koseki T. Matsumiya W. Yamada T. Ogawa 《Metallurgical and Materials Transactions A》1994,25(6):1309-1321
A computational method for the analysis of phase transformation involving solidification was developed with the assumption
of thermodynamic equilibria at interfaces. The region of interest was divided into finite segments, and solute diffusion across
the segments was computed by the use of the direct finite difference method (FDM). Simultaneously, thermodynamic equilibrium
at each interface was updated at every step of the diffusion analysis to determine the location of the interfaces. The temperature
decrease and the increment of fraction solid were calculated based on thermal balance, including a heat extraction condition.
Solid state transformation from δ to γ phase within each FDM segment was modeled by the use of a Clyne-Kurz (C-K) type analysis
with assumptions of complete mixing of solutes in theδ phase and limited back diffusion in theγ phase. The calculation results were compared with welding solidification experiments in the iron-chromium-nickel ternary
system. Good agreement was obtained with respect to solute distribution and residual fraction ofδ phase over different compositions and solidification modes of the alloys used. 相似文献
9.
WCp/Fe—Ni钢基复合材料的抗热疲劳特性 总被引:4,自引:0,他引:4
对WC颗粒体积分数、颗粒尺寸在复合材料热疲劳过程中的影响进行了研究,并探讨了WC颗粒复合材料抗热疲劳特性的形成机理。结果表明,WC颗粒体积分数增加,复合材料抗热疲劳的能力增强,而WC颗粒使复合材料的传热系数降低是影响热疲劳性能的主要原因。 相似文献
10.
A Fundamental Study of Laser Transformation Hardening 总被引:5,自引:0,他引:5
A theoretical and experimental study of heat flow and solid-state phase transformations during the laser surface hardening
of 1018 steel was conducted. In the theoretical part of the study, a three-dimensional heat flow model was developed using
the finite difference method. The surface heat loss, the temperature dependence of the surface absorptivity, and the temperature
dependence of thermal properties were considered. This heat flow model was verified with the analytical solution of Jaeger
and was used to provide general heat flow information, based on the assumptions of no surface heat loss, constant surface
absorptivity, and constant thermal properties. The validity of each of these three assumptions was evaluated with the help
of this heat flow model. In the experimental part of the study, on the other hand, a continuous-wave CO2 laser of 15 kW capacity was used in conjunction with a beam integrator to surface harden 1018 steel plates. The beam power
and the travel speed of the workpiece were varied, and the onset of surface melting was determined. The configurations of
the heat-affected zone observed were compared with those calculated using the heat flow model. The microstructure of the heat-affected
zone was explained with the help of the calculated peak temperature, heating, and cooling rates. 相似文献
11.
D. R. Poirier P. J. Nandapurkar S. Ganesan 《Metallurgical and Materials Transactions B》1991,22(6):889-900
The energy equation for solidifying dendritic alloys that includes the effects of heat of mixing in both the dendritic solid
and the interdendritic liquid is derived. Calculations for Pb-Sn alloys show that this form of the energy equation should
be used when the solidification rate is relatively high and/or the thermal gradients in the solidifying alloy are relatively
low. Accurate predictions of transport phenomena in solidifying dendritic alloys also depend on the form of the solute conservation
equation. Therefore, this conservation equation is derived with particular consideration to an accounting of the diffusion
of solute in the dendritic solid. Calculations for Pb-Sn alloy show that the distribution of the volume fraction of interdendritic
liquid (g
L) in the mushy zone is sensitive to the extent of the diffusion in the solid. Good predictions ofg
L are necessary, especially when convection in the mushy zone is calculated.
Formerly Research Associate, The University of Arizona.
Formerly Graduate Student, The University of Arizona. 相似文献
12.
M. Sherif El-Eskandarany 《Metallurgical and Materials Transactions A》1996,27(10):3267-3278
The rod milling technique using the mechanical alloying (MA) process has been employed for preparing amorphous Al33Ta67 alloy starting from elemental Al and Ta powders. X-ray diffraction (XRD), differential thermal analysis (DTA), differential
scanning calorimetry (DSC), optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM)
are utilized to follow the progress of amorphization. The results show that during the first few kiloseconds of MA time, layered
composite particles of Al and Ta are intermixed and form an amorphous phase upon heating to 685 K by DTA. This process is
called thermally assisted solid-state amorphization (TASSA). During the early stage of milling, the number of layers of the
composite particles increases. This leads to an increase in the heat formation of amorphous Al33Ta67 alloyvia the TASSA process, ΔH
TASSA
a
. After 360 ks (100 h) of the MA time, all Al atoms emigrate to Ta lattices to form a solid solution phase and the powder
particles have no more layered structure. At this stage of milling, the value of ΔH
TASSA
a
becomes zero. This solid solution phase is not stable against the shear forces that are generated by the rods and transforms
completely to an amorphous phase upon milling for 720 ks (200 hours). This phase transformation is attributed to the accumulation
of several lattice imperfections, such as point and lattice defects, which raise the free energy from the more stable phase
(solid solution) to a less stable phase (amorphous). After 1440 ks (400 hours) of MA time, a homogeneous amorphous phase is
formed. The amorphization process in this case is attributed to a mechanical driven solid-state amorphization (MDSSA). The
heat of formation of the amorphous phase formedvia the MDSSA process, ΔH
MDSSA
a
, has been calculated. Moreover, the crystallization characteristics indexed by the crystallization temperature, and the enthalphy
of crystallization, of the amorphous phases formed by TASSA and MDSSA processes are investigated as a function of MA time.
The role of amorphizationvia each process has been discussed.
Formerly lecturer of Materials Science, Department of Mining and Petroleum Engineering, Faculty of Engineering, AI-Azhar University,
Nasr City 11884, Cairo, Egypt 相似文献
13.
对原始状态分别为锻态、固溶态和半时效态的FGH96合金固相扩散连接界面显微组织进行表征,并对连接界面的拉伸性能进行测试,对失效行为进行研究。结果表明,锻态、固溶态和半时效态试样经固相扩散连接后界面均实现了良好的冶金结合,连接界面无孔洞和缝隙等缺陷。锻态试样界面扩散更为充分,组织过渡更为平缓;固溶态和半时效态试样界面存在明显的连接影响区。锻态试样经固相扩散连接和标准热处理后,二次γ?相细小、均匀且呈典型椭球状;固溶态和半时效态试样因固相扩散连接热循环的作用导致γ?相发生长大和分化。二次γ?相尺寸及形貌的不同决定了界面区域性能水平的差异。电子背散射衍射测试结果表明,连接界面处大晶粒的择优取向为{100},距离固相扩散连接界面越近,晶粒的择优取向越明显。拉伸试验结果表明,锻态试样经固相扩散连接和标准热处理后,连接界面处的强度达到基体强度的99%以上。拉伸裂纹主要萌生于连接界面大晶粒及γ?相粗化聚集区域,体现为穿晶的韧窝型断裂。 相似文献
14.
15.
P. W. Voorhees 《Metallurgical and Materials Transactions A》1990,21(1):27-37
A theory of Ostwald ripening has been developed for a solid-liquid mixture consisting of a low volume fraction array of spherical
solid particles in a liquid wherein the coarsening process proceedsvia the transport of both heat and mass. We find that the simultaneous transport of heat and mass during ripening does not alter
the exponents of the temporal power laws governing the ripening process from their classical values but does alter the amplitudes
of these power laws. The growth rate of the cube of the average particle radius, the rate constant, is found to depend both
on the alloy solute concentration and the ratio of the thermal to solutal diffusivities. In most metallic systems, a large
decrease in the rate constant can be expected with small additions of solute to a pure metal. The mean-field temperature and
concentration in the liquid will vary with time during ripening and will approach their equilibrium values along a unique
path which is dependent only on the materials parameters of an alloy. Possible extensions of this theory to the analogous
problem of ripening in isothermal ternary alloys are also discussed. 相似文献
16.
P. J. Jacques F. Delannay J. Ladrière 《Metallurgical and Materials Transactions A》2001,32(11):2759-2768
The mechanical stability of dispersed retained austenite, i.e., the resistance of this austenite to mechanically induced martensitic transformation, was characterized at room temperature
on two steels which differed by their silicon content. The steels had been heat treated in such a way that each specimen presented
the same initial volume fraction of austenite and the same austenite grain size. Nevertheless, depending on the specimen,
the retained austenite contained different amounts of carbon and was surrounded by different phases. Measurements of the variation
of the volume fraction of untransformed austenite as a function of uniaxial plastic strain revealed that, besides the carbon
content of retained austenite, the strength of the other phases surrounding austenite grains also influences the austenite
resistance to martensitic transformation. The presence of thermal martensite together with the silicon solid-solution strengthening
of the intercritical ferrite matrix can “shield” austenite from the externally applied load. As a consequence, the increase
of the mechanical stability of retained austenite is not solely related to the decrease of the M
s temperature induced by carbon enrichment. 相似文献
17.
Heat transfer during Nd: Yag pulsed laser welding and its effect on solidification structure of austenitic stainless steels 总被引:2,自引:0,他引:2
T. Zacharia S. A. David J. M. Vitek T. Debroy 《Metallurgical and Materials Transactions A》1989,20(5):957-967
Theoretical and experimental investigations were carried out to determine the effect of process parameters on weld metal microstructures
of austenitic stainless steels during pulsed laser welding. Laser welds made on four austenitic stainless steels at different
power levels and scanning speeds were considered. A transient heat transfer model that takes into account fluid flow in the
weld pool was employed to simulate thermal cycles and cooling rates experienced by the material under various welding conditions.
The weld metal thermal cycles and cooling rates are related to features of the solidification structure. For the conditions
investigated, the observed fusion zone structure ranged from duplex austenite (γ)+ferrite (δ) to fully austenitic or fully
ferritic. Unlike welding with a continuous wave laser, pulsed laser welding results in thermal cycling from multiple melting
and solidification cycles in the fusion zone, causing significant post-solidification solid-state transformation to occur.
There was microstructural evidence of significant recrystallization in the fusion zone structure that can be explained on
the basis of the thermal cycles. The present investigation clearly demonstrated the potential of the computational model to
provide detailed information regarding the heat transfer conditions experienced during welding. 相似文献
18.
Current analytical thermal models for friction stir welding (FSW) are mainly focused on the steady-state condition. To better
understand the FSW process, we propose a transient thermal model for FSW, which considers all the periods of FSW. A temperature-dependent
apparent friction coefficient solved by the inverse solution method (ISM) is used to estimate the heat generation rate. The
physical reasonableness, self-consistency, and relative achievements of this model are discussed, and the relationships between
the heat generation, friction coefficient, and temperature are established. The negative feedback mechanism and positive feedback mechanism are proposed for the first time and found to be the dominant factors in controlling the friction coefficient, heat generation,
and in turn the temperature. Furthermore, the negative feedback mechanism is found to be the controller of the steady-state
level of FSW. The validity of the proposed model is proved by applying it to FSW of the 6061-T651 and 6063-T5 aluminum alloys. 相似文献
19.
Suli Chang Dongkai Shangguan Doru M. Stefanescu 《Metallurgical and Materials Transactions A》1992,23(4):1333-1346
A model of phase transformations in spheroidal graphite (SG) cast iron has been developed to quantitatively describe the microstructural
evolution during solidification and the subsequent solid-state phase transformations (eutectoid reaction) during continuous
cooling and to predict some of the microstructural characteristics of final phases formed in SG iron castings. Such characteristics
include phase fractions, phase spacings, and grain dimensions. In the model, the nucleation and growth of primary dendrites
and eutectics were described based on existing theories, whereas the mathematical formulation for the eutectoid reaction,i.e., the formation of pearlite and ferrite from the as-cast austenite, was developed based on theories as well as physical evidence
obtained from the experimental work. The Johnson-Mehl equation and the Avrami equation were used to calculate the fraction
of transformed phases under continuous cooling conditions. The role of the grain impingement factor used in these two equations
and the significance of the additivity principle in treating nonisothermal transformations were briefly discussed. The latent
heat method was used for the numerical treatment of the release of latent heat during phase transformations. A two-dimensional
finite element code which can be used in either Cartesian or cylindrical coordinates (ALCAST-2D) was used to solve the time-dependent
temperature distribution throughout the metal/mold system. Numerical predictions were validated against experimental results,
and good agreement was obtained.
DONGKAI SHANGGUAN, Previously Assistant Research Engineer, The University of Alabama, 相似文献