Competitive growth of stable and metastable Fe- C- X eutectics: Part I. experiments

The effect of small additions of Si, P, Cr, Mn, Ti, Al, and S to pure Fe-C eutectic, upon the transi-tion velocities from grey to white and white to grey cast iron, has been measured by varying the growth rate during directional solidification. As a result, it is found that alloying elements can be classified into three types: graphitizing (Si, Al, P, and S), carburizing (Cr), and “hysteretic effect” (Mn, Ti). The well-known influence of a thermal gradient (or the superheat) has been shown to af-fect only the grey-to-white transition. Growth undercoolings were measured as a function of growth rate, while the average lamellar spacings were determined from transverse sections of directionally solidified samples. A small addition of the element studied can have a marked effect upon these parameters.     

Competitive growth of stable and metastable Fe- C- X eutectics: Part II. mechanisms

The competition between stable and metastable solidification in Fe-C-X alloys has been studied theo-retically, with particular regard to phase stability, nucleation, and growth processes. The effects of small additions of Si, P, Cr, Mn, Ti, Al, and S upon the transition velocities from grey to white and from white to grey in directional solidification are related to their influence on the eutectic tempera-tures, and nucleation and growth undercoolings. It is shown that the consequences of simultaneously adding Si and Cr upon the transition velocities can be deduced from the results of adding of Si and Cr separately only when the detailed effects of these elements upon phase stability, nucleation, and growth are known. The well-known carburizing effect of a high thermal gradient (superheat) has been shown to influence only the nucleation process. A three-phase austenite-graphite-cementite mi-crostructure resulting from the cooperative growth of a double stable/metastable eutectic has been observed for the first time.     

On the kinetics of structural relaxation in metallic glasses

A model is proposed to describe the simultaneous changes in topological (TSRO) and chemical (CSRO) short range order during structural relaxation of metallic glasses. Changes in free volume and order parameter during isothermal and isochronal annealing were calculated. Corresponding changes in internal energy and Curie temperature are—at least qualitatively—in good agreement with experimental observations. The amount of free volume and short range order frozen in during the quench from the liquid state were also calculated and found to be of the correct order of magnitude.     

Nucleation and growth kinetics of inclusions during liquid steel deoxidation

Abstract

The phenomenon of solid inclusion nucleation and growth during the liquid steel deoxidation process is studied numerically. It is shown that the diffusion of atoms, the Ostwald ripening effect, and Brownian movement together control the growth of small size particles. It appears that this stage of initial precipitation lasts for only about 10 s. When the particle diameter exceeds a threshold of ~1 μm, the growth process becomes dominated by particle collisions and agglomeration. The efficiency of these mechanisms depends on the particle size distribution as produced during growth. Comprehensive evaluations are still impeded by the limited capacity of computers at present.     

Plastic deformation in metallic glasses

A theory is presented for the plastic deformation of metallic glasses below their glass transition temperature. The theory is based on two modes of thermally activated shear transformations initiated around free volume regions under an applied shear stress. The regions are typically conceived to be about 5 atom diameters across. At high temperatures (0.6 T_g ≲ T ≲ T_g) the transformation is a diffuse rearrangement producing a relatively small local shear strain in a roughly spherical region. At low temperatures (0 < T ≲ 0.6 T_g) the transformation is in a narrow disk shaped region and resembles closely the nucleation of a dislocation loop. The theory is in good accord with experimental observations.Based on the theory, possible levels of flow dilatation have been computed from which rates of shear localization can be obtained. At low temperatures, very rapid shear localization is predicted which is in good accord with the observations reported in the literature and with recent cinematographic observations.     

Properties of bulk metallic glasses

A relatively small number of revolutionary materials have been discovered in the field of physical metallurgy and metal physics in the last several decades, and bulk metallic glasses are among them. Their strength and hardness are considerably higher and their modulus of normal elasticity is considerably lower than that of crystalline alloys, which leads to large stored elastic strain energy. These materials also have very good corrosion resistance. In this article, we present the properties of bulk metallic glasses, in particular, thermal, mechanical, magnetic, and electrical properties, corrosion resistance, as well as the application fields of these alloys.     

The stable and metastable Ti-Nb phase diagrams

The phase transformations which occur in the Ti-Nb binary alloy system have been discussed in two recent papers. The phase relationships were investigated by varying alloy composition and thermal history. In this paper, these results are summarized in complete and thermodynamically consistent calculations of the stable and metastable phase diagrams. The calculations of the metastable equilibria are relevant to the Ti-V and Ti-Mo systems, as well as to several other titanium and zirconium-based transition metal alloy systems. Formerly with the National Bureau of Standards, Gaithersburg, MD 20899.     

Nb(C,N)沉淀析出动力学计算

对动力学模型进行了优化,对模型中的常数做了修正,引入界面能修正因子为0.38,推导出了动力学计算中的常数B的理论值为1.365×1010;对含Nb贝氏体钢980℃和950℃不同应变速率和应变量计算表明,随着应变速率和应变量的增加,析出时间是减少的,并且最快析出温度在930℃.     

Solidification kinetics and metastable phase formation in binary Ti-Al

Near-equiatomic alloys of Ti-Al were solidified at various bulk undercoolings using electromagnetic levitation. Detailed thermal histories were acquired during experiments using optical pyrometry with sampling rates as fast as 500 KHz. Solidification and other high-temperature transformation pathways were deduced from the thermal data and microstructural analysis. Re- calescence rise times were used to determine semiquantitative primary solidification kinetics for the different phases. Primary β solidification was observed at compositions well into the equi- librium α regime; this is presented as part of a near-equiatomic nucleation domain diagram mat shows the primary solidification phase (β, α, ordered γ, or disordered γ) that results for each combination of nucleation temperature and composition. Solidification kinetics are faster for primary β (V_max ≈ 15 to 18 m s^-1) than they are for primary α (V_max ≈ 10 to 12 m s^-1). For undercoolings less than about 150 K, the primary solidification kinetics are about an order of magnitude slower for γ than for α. However, at an undercooling of about 150 K, the solidi- fication kinetics for γ increase discontinuously. This discontinuity is associated with a change in the primary solidification phase from ordered γ (V_max ≈ 0.5 m s^-1) to disordered γ (V_max ≈ 10 m s^-1). formerly Doctoral Student, Vanderbilt formerly Doctoral Student, Vanderbilt formerly Doctoral Student, Vanderbilt