Kinetics of phase transformations on isothermal slot channel walls

The determination of the numerical density of molecules flying off from slot channel walls of different geometric shape whose internal surfaces are covered by a sublimable material is reduced to the solution of the Helmholtz equation. Simple formulas are obtained for computation.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 52, No. 1, pp. 73–80, January, 1987.     

Kinetics of isothermal evaporation of a porous or dispersed body

We consider the evaporation of a porous body which consists initially of spheres of identical radii. We find the dependence of the velocity of motion of the boundary of the body on the external parameters.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 43, No. 2, pp. 265–273, August, 1982.     

两种牙科可加工长石瓷的比较研究

目的比较总结两种可加工长石瓷的性能及组织,为牙科可加工长石瓷的制备提供新的选择.方法采用放电等离子烧结技术(简称SPS)在温度1120℃,压力30MPa,升温速度100℃/min制备出可加工陶瓷块,与德国VITA公司的VITA MARKⅡ进行比较;运用直接压痕法测量两种材料的断裂韧性、硬度、弹性模量,分析两种陶瓷的晶相成分及显微组织.结果SPS烧结得到的长石瓷断裂韧性能达1.44～1.53MPa1/2,高于VITA MARK Ⅱ(1.2MPa1/2);硬度为622～644kgf/mm2,弹性模量为60～66kgf/mm2,与VITA MARKⅡ(645kgf/mm2,63 kgf/mm2)相当;经XRD及金相分析表明,两种可加工陶瓷的晶相成分不同,大小也不同.结论SPS技术制备的牙科可加工长石陶瓷材料,硬度适于医用,断裂韧性优于VITAMARK Ⅱ产品,能够达到医用要求.     

Dilatometric study of isothermal phase tranformation in a C-Mn steel

A dilatometric study of the isothermal phase transformations in a low-alloy carbon steel is presented. The results show a TTT diagram consisting of three C shaped curves corresponding respectively to allotriomorphic ferrite-pearlite, Widmanstätten ferrite-pearlite and bainite transformations. These three temperature intervals are even better marked in a plot of total dilatation against temperature. A representation of log (ΔI/I) against logt, at each holding temperature, gives information about the transformations taking place. Hence, it is suggested that the bainitic reaction also occurs in two consecutive stages. Similarly to the other two transformations, ferrite formation in the initial stage is followed by the subsequent precipitation of cementite and further ferrite. It is concluded that the three transformations are of similar nature, controlled by carbon diffusion, the different C-curves being produced by changes either in the growth or nucleation mechanisms.     

Microstructure effect on the properties of a commercial low-fusing dental porcelain

The aim of this study was to investigate the effect of firing cycle on a dental porcelain microstructure in order to correlate microstructure changes with mechanical and thermal properties. A commercial low-fusing dental porcelain powder (Omega 900, Vita) was investigated for this purpose. The powder was treated at different temperatures in the range 750–1000 °C. The fired samples were characterized in terms of their morphology and microstructure, and their mechanical and thermal properties were evaluated. The results showed that firing temperature affects porcelain microstructure influencing significantly in this way both the mechanical properties and the thermal expansion coefficient of the fired objects. Firing at 800 °C led to a homogeneous structure. After treatment at this temperature, the leucite crystals exhibit their maximum concentration and they are well dispersed into the glassy phase. As a consequence the optimum mechanical strength and the maximum thermal expansion coefficient are observed in these samples.     

Ageing reactions in a high carat gold alloy for dental porcelain bonding

The phase transformation during continuous heating of a high carat gold alloy used for porcelain bonding was investigated by electrical resistivity measurements, hardness tests, X-ray diffraction and scanning and transmission electron microscopy. Four reaction stages (I, II, III and IV) were found. Stage I corresponded to the formation of a short-range order. A discontinuous precipitation took place in stage II, which contributed to remarkable hardening. Stages III and IV were reactions to the stable phases at each temperature region, and resulted in softening. The activation energies for stages I, II and III are 27.1, 33.8 and 58.2 kcal/mol, respectively.     

Differential geometry of phase transformations

The foundations have been laid with respect to a generalized theory of phase transformations in solids. In particular, the methods of differential geometry have been employed and such important tensor quantities as distortion, metric, torsion, and anholonomic object have been developed with respect to such transformations. It is further shown that both Riemannian as well as non-Riemannian (dislocation) geometries are needed to describe these transformations properly.     

Crystal phase transformations in nylon 11

Three sample preparations of nylon 11, m-cresol cast, trifluoroacetic acid cast and sodium hydroxide-treated melt-crystallized, are examined by infrared analysis. Digital subtraction spectroscopy is used to distinguish between structural differences in different chemical treatments. M-cresol cast films of nylon 11 appear to be intermediate in conformational structure between melt-crystallized and trifluoroacetic acid cast films. Various bands of the infrared spectrum support this result. The basis for the doublet observed in the amide II band, which results from conformational changes rather than crystal field splitting, is discussed in this paper. Sodium hydroxide treatment promotes the α to γ conversion in melt-crystallized films of nylon 11.     

Crystallographic features of phase transformations in solids

The paper reviews the current knowledge and understanding of the crystallographic features of phase transformations in solid materials – metals, ceramics and alloys. It covers both of the broad classes of phase transformations in crystalline solids – martensitic or ‘displacive’ and ‘diffusional’ or ‘reconstructive’. The factors that govern the crystallographic features of these two classes of transformations are compared and contrasted. This provides an appropriate basis for examining the ‘diffusional–displacive’ transformations that appear to exhibit the characteristics of both classes. After a brief summary of the considerable body of experimental data available on the crystallographic characteristics of these various types of phase transformation, the different models/theories advanced to account for these observations are discussed. The main emphasis is on those models/theories that are capable of predicting, rather than just rationalising or explaining, these crystallographic features. The review purposely adopts a unifying approach and attempts to reconcile the controversy that has on occasions existed between the ‘displacive’ group and the ‘diffusional’ group – particularly in respect of the ‘diffusional–displacive’ transformation. Developing a comprehensive understanding of the crystallographic features of all classes of phase transformations is obviously the ultimate goal. The review concludes by assessing how close we are to this final achievement, identifies the gaps in current knowledge and suggests future work.     

The role of disconnections in phase transformations

The topological model of phase transformations is described in terms of disconnections. Disconnection motion is shown to produce a variety of phase transformations with accompanying plastic strain. These strains, together with elastic strains associated with equilibrium arrays of interface defects, define expected habit planes and orientation relationships. Non-equilbrium defect arrays resulting from kinetic constraints are discussed. Example applications of the topological model are presented for several types of martensitic transformations and several examples of diffusional transformations.     

Some characteristics of first-order phase transformations

The distinction between first-order and second-order phase transformations is brought out. The general characteristics of first-order transformations are described. The salient features of transformations controlled by long-range diffusion, short-range diffusion and by (diffusionless) shear mechanisms are discussed. Martensitic trasnformations, which belong to the shear category, occur in many metallic and non-metallic systems and deserve an intensive study.     

Compatibility and accommodation in displacive phase transformations

The role of disconnections and lattice-invariant deformation in displacive phase transformations is reviewed, particularly the defect structure of equilibrium habit planes, the mechanism of transformation, and the deformation accompanying growth. This is extended to the 3-D topological modeling of a product embedded in its parent phase. Stress concentrations arise where the interface orientation deviates from the equilibrium habit, such as plate tips and edges. Various mechanisms are discussed for the amelioration and accommodation of these stresses. These include the well established mechanism of self-accommodating assemblies of variants. New mechanisms are proposed relating to the defect structure within individual plates and its interaction with crystal dislocations. Some supporting experimental observations are presented.