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1.
The thermodynamic approach to the understanding of martensitic transformations can provide insight into the mechanism of transformation, serve as a generalization technique among the alloying systems, and as a framework for the synthesis of data on kinetic, structural, crystallographic, and other aspects of the transformation. The quality and utility of existing thermochemical models and data are examined. The approach requires long, unverifiable extrapolation of the thermodynamic functions and must be used with circumspection. On the other hand, much that is known is derived from a thermodynamic viewpoint. The present authors are not hopeful of great breakthroughs in the quality of the thermodynamic data. More significant, it appears, would be attempts to formally cast the areas of crystallographic analysis (transformation strain energy) and the like into the thermodynamic framework. 相似文献
2.
R. J. Wasilewski 《Metallurgical and Materials Transactions A》1975,6(7):1405-1418
A novel conceptual approach to martensitic transformation is presented. The current concepts are discussed briefly, and a more general qualitative approach to this type of phase change is proposed. It is suggested that the transformation originates in lattice vibration modes, and that once initiated it proceeds adiabatically with no discernible division into nucleation and growth. The transformation does not occur through a dislocation mechanism, and the high dislocation densities observed as associated with martensite formation consist of immobile dislocations. These form to accommodate the transformation strains which are highly localized, but are also self-accommodating and on completion of the transformation are macroscopically nondirectional. All of the anomalies in the mechanical properties reported in the past are ascribed to the effects of an applied stress on the transformation. It is suggested that investigations of such effects on transformation, particularly in single crystal material, should be most fruitful in providing new fundamental insight into the mechanism of the transformation. 相似文献
3.
R. J. Wasilewski 《Metallurgical and Materials Transactions A》1975,6(8):1405-1418
A novel conceptual approach to martensitic transformation is presented. The current concepts are discussed briefly, and a
more general qualitative approach to this type of phase change is proposed. It is suggested that the transformation originates
in lattice vibration modes, and that once initiated it proceeds adiabatically with no discernible division into nucleation
and growth. The transformation does not occur through a dislocation mechanism, and the high dislocation densities observed
as associated with martensite formation consist of immobile dislocations. These form to accommodate the transformation strains
which are highly localized, but are also self-accommodating and on completion of the transformation are macroscopically nondirectional.
All of the anomalies in the mechanical properties reported in the past are ascribed to the effects of an applied stress on
the transformation. It is suggested that investigations of such effects on transformation, particularly in single crystal
material, should be most fruitful in providing new fundamental insight into the mechanism of the transformation. 相似文献
4.
《Acta Metallurgica》1985,33(4):595-601
A formulation is proposed for the factors which have an effect on the start of martensite. They should be subdivided depending on whether they affect the equilibrium temperature To (Ms < To: necessary condition) or the additional undercooling ΔTm = To − Ms (sufficient for the onset of transformation). To depends on chemical composition, degree of order, hydrostatic stress, ΔTm on nucleation and propagation of martensite inside the matrix. This, in turn, is affected by an external shear stress, and the resistance to shear, i.e. hardening mechanisms caused by point defects, dislocations, or particles. The course of Ms as the function of the period of isothermal aging provides an example for a different change of To and ΔTm. 相似文献
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6.
R. J. Wasilewski 《Metallurgical and Materials Transactions B》1971,2(11):2973-2981
The deformation behavior of TiNi at 20°C has been investigated as a function of composition and of the prior heat treatment.
Wide mechanical property variation and significant differences between the effects of tensile and of compressive loading were
observed. Under some conditions anelastic behavior, characterized by a broad hysteresis loop, was reproducibly obtained. The
effects of heating the deformed materials aboveA
f on the subsequent stress-strain behavior indicate the anomalies observed to be directly related to the martensitic transformation.
The effects of stress application on the martensitic transformation are discussed. It is shown that, under some conditions,
the stress-assisted transformation structures may be unstable on the removal of the stress. At test temperatures outside theM
f toA
f range, this can result in anelastic behavior. More complex behavior expected at temperaturesM
f < T < Af, is discussed in some detail. It is shown that both the anelastic behavior and the “shape memory” can be accounted for by
the effects of applied stress. It is also shown that the mechanical properties in this temperature range can vary markedly
with the prior heat treatment, even at temperatures not normally considered of significance. Though based on observations
made on TiNi, the phenomena discussed are inherent in materials undergoing a martensitic transformation over a narrow range
of temperatures. 相似文献
7.
8.
R. J. Wasilewski S. R. Butler J. E. Hanlon D. Worden 《Metallurgical and Materials Transactions B》1971,2(1):229-238
It is shown that the structure homogeneity range of the B2 TiNi compound is very restricted below 500°C. A new phase, containing
58 at. pct Ni and structurally related to Ni3Ti, forms peritectoidally at 625° ± 20°C. The precipitation in nickel-rich TiNi is very slow, and a metastable, supersaturated
B2 structure can be readily retained. At equilibrium the martensitic transformation takes place at ∼60°C, independent of the
overall composition. Quenching from 600°C and above raises the transition temperature in titanium-rich, and lowers it markedly
in nickel-rich compositions. The resulting martensite structure in the latter is also believed to change with increasing the
prior anneal temperature. The observations reported previously on the transformation behavior must be considered as obtained
on nonequilibrium, metastable materials, which may account for the discrepancies in the observed property variation. Close
control of the processing variables is necessary to ensure reproducibility, particularly if nonequilibrium transformation
behavior is desired. 相似文献
9.
Min Jiang Katsunari Oikawa Tamio Ikeshoji 《Metallurgical and Materials Transactions A》2005,36(9):2307-2314
With a potential-energy function of Co described by the embedded-atom method (EAM), molecular-dynamics (MD) simulations were
performed for a series of initial fcc configurations with different types of dislocations or preset hcp embryos. The gliding
process of a Shockley dislocation on a closepacked plane has been observed, which starts from the origin of the dislocation
and proceeds at a high speed of 280 m/s toward a certain direction. An atom which has been swept by the dislocation line was
detected to contribute a displacement close to the Burgers vector of a Shockley dislocation. It is in this way that a new
stacking sequence is produced and an hcp lamella grows in the fcc structure. A similar gliding process has been observed in
the case where an intrinsic stacking fault is preexisting in the fcc structure. The transformation is, again, toward forming
a local hcp region. These results prove that a special dislocation in the fcc structure can act as an embryo of the hcp, as
described in many dislocation mechanisms of the martensitic transformation. The fcc → hcp phase-tranformation process of Co
has been further reproduced by a simulation initiated from an fcc/hcp two-phase configuration. It yields a single hcp crystal
as the final transformed product. 相似文献
10.
11.
《Acta Metallurgica Materialia》1995,43(7):2667-2671
For the analysing of the results from earlier hydrostatic isothermal tests a new athermal test series was conducted. Suitable amounts of martensite were brought into the specimens by quenching. From these specimens the amount of martensite, number of martensitic plates per unit area of random section and hardness were measured. Using the measured data, the effects of the mechanical surroundings around and inside the specimen were taken up for consideration. It seems that the mechanical surroundings inside the specimen have an important effect on martensitic transformation. It also seems that there are temperature dependent components in the transformation. It may be that the temperature dependence of nucleation needed plastic deformation and the temperature dependence of relaxations. 相似文献
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14.
Marc A. Meyers 《Metallurgical and Materials Transactions A》1979,10(11):1723-1727
A method for the determination of nucleation times for martensitic transformation is described. The method utilizes a shock
wave that, upon being reflected at a free surface, generates a tensile wave with a pulse duration that increases as it moves
away from the surface. Once the duration of the reflected pulse is large enough for nucleation to occur, transformation can
take place. The width of the martensite free layer adjoining the surface is measured and compared with wave predictions. A
nucleation time can be obtained. The method requires that the temperature, pulse amplitude, and alloy composition be such
that only the reflected tensile wave induce martensite transformation. For the experimental conditions used by Snell, Shyne,
and Goldberg10 the nucleation time is found to be less than 55 nanoseconds.
MARC A. MEYERS, formerly Assistant Professor, Department of Metallurgical Engineering, South Dakota School of Mines and Technology 相似文献
15.
Martensite formation in ferrous alloys is inextricably connected with local deformation. In order to assess the role of this plastic work in the transformation, the dynamic responses of Fe?Ni?C austenites to imposed changes in deformation variables are investigated. For both stable and metastable compositions direct measurements are made of the separate and combined influences of prestrain and applied stress, and of strain-rate and dynamic strain aging. In addition, new martensite-start temperatures are defined and examined conceptually. A differential form of an energy balance that describes our viewpoint of austenite transformation at stress is then used to interpret the results. Some consequences concerning the importance of plastic work, its structure sensitivity, and the pervasive influence of local stress states are deduced and discussed. 相似文献
16.
17.
T. Xiao Ph.D. Candidate 《Metallurgical and Materials Transactions A》1993,24(5):1067-1071
A physical model and phenomenological treatment are developed to derive an expression for internal friction associated with
the thermoelastic martensitic transformation. Special attention is given to the effect of pseudoelastic hysteresis(h), which has never been previously addressed. The amplitude dependence of the internal friction is discussed in various ranges
of stress amplitude with respect to the critical stresses (σcl, σc2) for stress-induced martensitic transformation. In the case where the applied stress amplitude is much higher than the critical
stresses, the internal friction, due to stress-induced martensitic transformation, is found to be linearly dependent upon
the reciprocal of the stress amplitude, in good agreement with the experimental results. 相似文献
18.
《钢铁冶炼》2013,40(5):404-411
AbstractIn an unidirectional solidification experiment, an 8 kg stainless steel ingot with the composition 0·25%C, 17%Cr, and 1%Mn was solidified under continuous casting conditions. The dwell time of primary cooling was varied, followed by secondary spray cooling. Metallographic investigation, heat transfer, and segregation were carried out to study the solidification mechanism. The partition ratio of the elements present in ferrite and in austenite (martensite) was determined. It was indicated that the solidification follows: L → L + δ → L + δ + γ → δ + γ + carbides. Under high cooling rates γ austenite solidifies as a leading phase. The beginning of spray cooling has the main effect in controlling the obtained microstructures. Carbide thickening is observed in the rapidly cooled zone between the ferrite and the martensitic matrix. Tempered martensite increases by lowering the cooling rate, which gives more time for carbide dissolution and for carbon to diffuse into the ferrite, eventually increasing the austenite (martensite) fraction in the final matrix at the expense of ferrite. 相似文献
19.