Microstructure Evolution of Cu-Pb Monotectic Alloys Processed in Drop Tube

Rapid solidification of Cu-Pb monotectic alloys has been accomplished during free fall in a 3 m drop tube. Both macrosegregated and uniformly dispersed structures are observed in Cu-40 wt pct Pb alloy droplets, whereas droplets of composition Cu-64 wt pct Pb exhibit only macrosegregation morphologies. The microstructures are strongly dependent on droplet size. The higher undercooling tends to facilitate liquid phase separation and results in more extensive macrosegregation in smaller droplets. There exists a pronounced tendency for the Pb-rich liquid to occupy the surface of the droplets of both compositions, resulting from the quite lower surface tension of the Pb-rich phase and causing a Pb-rich layer at the surface of the solidified droplet. The nucleation of monotectic cells in the Cu-40 wt pct Pb droplets with dispersed structures preferentially occurs at the droplet surface. A single nucleation event takes place more frequently as droplet size is reduced.

Primary cellular/dendritic spacing selection of Al 4.95%Zn alloy under near rapid directional solidification condition

Al 4.95%Zn alloy is directionally solidified in a modified Bridgman apparatus with higher temperature gradient to investigate response of cellular/dendritic microstructures and primary spacing to the variation of growth velocity under near rapid directional solidification condition. The results show that, with increasing growth rate, there exists a transition from dendrite to fine cell and a wide distribution range in primary cellular/dendritic spacing at the given temperature gradient. The maximum, λ max , minimum, λ min , and average primary spacing, λ , as functions of growth velocity, v , can be given by λ max =12 340 v -0.835 3 , λ min =2 953.7 v -0.771 7 , λ =7 820.3 v -0.833 3 , respectively. , as functions of growth velocity, v , can be given by λ max =12 340 v -0.835 3 , λ min =2 953.7 v -0.771 7 , λ =7 820.3 v -0.833 3 , respectively.

Reliability Analysis for Offshore Platform Structural Systems

A system reliability estimation method for spatial jacket platforms is developed in this paper. The jacket platform is modeled into three-dimensional assembly of spatial beam and plate elements in Finite Element Method (FEM). The limit failure states correspond to collapse of a series of structural members which are identified by engineering design criteria. In this paper the following aspects are taken into account: the punching shear and buckling failures in member failure modes for the tubular joints and tubular columns respectively; incremental loading approach for establishment of the safety margin equations of system failure; the algorithm of enumerating significant failure modes for the structural systems and other concepts, such as the false failure mode and the virtual limit state. The final work is devoted to the reliability analysis for a practical jacket platform presently put into operation on the Bohai Sea. The computed results shows that method suggested in this paper is feasible and effective for the evaluation of the system reliability of offshore platforms.

SOME CHARACTERISTICS OF LIQUID METAL MENISCUS IN ELECTROMAGNETIC FIELD

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Grain Boundary-induced Shielding of Ni-base Single Crystals: Experimental and Analytical Study

Fracture behavior was studied for Ni-base single crystals and Ni-base bicrystals. The single crystals were machined from the bicrystal specimens to ensure that the single crystal samples had the same properties as those of the grains in the bicrystals. Using the three-point bend specimens, the fracture toughness K-c of single crystals acid grain boundaries was measured, respectively. The two toughness values were little dependent on the ratio of the crack length to the height of the specimens. The grain boundary-induced shielding effect of fracture toughness was found in the grain-cracked bicrystals with normal front grain boundaries. The toughness varied with the distance between the crack tip and grain boundary. With the help of SEM analyses and numerical calculations, the mechanisms for the grain boundary-induced shielding are revealed. It is the grain boundary-induced redistribution of stresses and strains near the crack tip that induces the grain boundary-induced shielding. And the strain energy density criterion can be applied to the fracture of bicrystals. Agreement with experiments is good.

Spinodal Decomposition of Ultrahigh Strength Steel 23Co14Ni12Cr3

The depleted and enriched regions of solute atoms formed by spinodal decomposition in the early stage of tempering for ultrahigh strength steel 23Co14Ni12Cr3 were first confirmed by means of transmission electron microscopy (TEM). The segregation of solute atoms provides favorable composition and structure fluctuation for the formation of secondary hardening phase M2C and cementite Fe3C.

A GEOMETRIC THEORY IN INVESTIGATION ON MODE LOCALIZATION AND FREQUENCY LOCI VEERING PHENOMENA

According to the mapping theory in complex plane, the geometric features of eigen frequency loci of systems undergoing free vibrations are investigated. It is concluded that the phenomena of curve coalescence and veering can be described in a unified manner from the singularities of mapping from the complex parameter plane onto the complex frequency plane. The formation of a branch point in the parameter Space is the foundation of explaining localization and veering phenomena. By the use of condensation to reduce the dimension of a system, the scope of application of the geometric theory is widely expanded. The theory is applied to examples to verify the validity of the proposed approach. The present work is an improvement and extension of recent work by M. S. Traintafyllou et al..

Machinable REPM with Excellent Intrinsic Coercive ForcePrepared by DS Technology

To obtain excellent magnetic properties of machinable REPM prepared by directional solidification (DS) technology, Space A, a composition area with theta=0 degrees similar to 5 degrees where easy axis is parallel to solidified direction of alloys, has been established in Zr-Sm-La-Co-Cu-Fe system. In 2/17+1/5+Co, and 2/17+1/5 region near Co phase of Space A, one obtains: V-f = 80.933 - 2.739Zr - 1.828Sm - 0.773Co - 0.147Cu + 0.415Fe + 0.218La Nonequilibrium solidification process of alloys in 2/17+1/5+Co region, is as follows: L-->Co, L-->2/17, L+2/17-->1/5 and L-->1/(5-x)+1/(5+x). The product of partially completed peritectic reaction, and 1/(5-x) and 1/(5+x) phases can all be transformed into a homogeneous 1/5 phase after solution treatment in 1413 K for 5 h. Containing 3 similar to 6% Co phase magnetically hardened by 0.5 similar to 2.5% V, the DS magnets have been achieved with both improved toughness and excellent coercive force of 520 kA/m.

A STATISTICAL INVESTIGATION OF FATIGUE CRACK INITIATION AND GROWTH PROCESS BASED UPON A LARGE SAMPLE SIZE EXPERIMENT

After finishing 102 replicate constant amplitude crack initiation and growth tests on Ly12-CZ aluminum alloy plate, a statistical investigation of the fatigue crack initiation and growth process is conducted in this paper. According to the post-mortem fractographic examination by scanning electron microscopy (SEM), some qualitative observations of the spacial correlation among fatigue striations are developed to reveal the statistical nature of material intrinsic inhomogeneity during the crack growth process. From the test data, an engineering division between crack initiation and growth is defined as the upper limit of small crack. The distributions of crack initiation life N-i, growth life N, and the statistical characteristics of crack growth rate da/dN are also investigated. It is hoped that the work will provide a solid test basis for the study of probabilistic fatigue, probabilistic fracture mechanics, fatigue reliability and its engineering applications.

Establishment of the Constitutive Relationship for MP159 Alloy

The constitutive relationship for MP159 alloy, a kind of high strength cobalt-base superalloy, was studied based on the experimental results of compression tests carried out in the THERMECMASTER-Z hot work simulating test machine. The constitutive relationship model using Zener-Hollomon parameter as primary variable is proposed, and the constitutive equation for MP159 alloy is firstly formulized. This equation well represents the relationship of flow stress with thermomechanical,parameters, i.e. strain, strain rate and temperature, in a large deformation (a true strain of 1.10) and a wide range of deformation temperatures and strain rates, it can be used for the finite element simulation of plastic forming process and the dissipative structure theory and dynamic material modeling.

Thermodynamics of Ordering-Disordering Transformation in Cu-Zn Alloy System

The alpha and beta phases in Cu-Zn alloy system were regarded as regular solutions at elevated temperature, the results calculated from the experimental data have showed that the value of the interaction parameter of components Cu and Zn in the alpha phase E-CuZn(alpha) varied obviously with decreasing component Zn. Substituting the function relation between E-CaZn(alpha) and X-Zn obtained through curve-fitting into expression of free-energy variation of the system, the calculated martensitic transformation temperature M-s was in good agreement with experimental curve, so as to have more comprehensive information for fully making use of binary Cu-base functional materials.

Damping Behaviour and Mechanism of Graphite ParticulateReinforced Metal Matrix Composites

The effect of graphite particulate reinforcement on the resultant damping behaviour of ZA27 metal matrix composites (MMCs) has been investigated in an effort to develop a new functional material. The MMCs were processed by a spray atomization and deposition technique. The damping capacity, as well as the relative dynamic modulus, was measured at frequencies of 1 and 4 Hz in the 30 to 200 degrees C temperature range. There exists a transformation point at 55 degrees C in the internal friction and temperature spectrum of the MMCs. It is shown from microstructural analysis that the damping capacity of ZA27 can be significantly improved by the addition of graphites particulates through spray deposition processing. Finally, the operative damping mechanism is discussed in light of the data obtained from characterization of microstructure and damping capacity.

Nucleation of Bainite around Grain Boundary

The scanning Auger microprobe study on Fe-8Mn-1C alloy shows that the carbon atoms segregate at grain boundary during bainitic transformation and solute atoms depleted regions form near the grain boundary. The observation of optical and TEM indicates that bainite nucleates in solute atoms depleted regions near the grain boundary. (Edited author abstract) 11 Refs.

Self-consistent Constitutive Modeling of the Creep Damage Behaviour of Nickel-base Directionally Solidified Superalloys with Different Grain Orientations

A self-consistent creep damage constitutive model is developed for nickel-base directionally solidified superalloys. Grain degradation and grain boundary voiding are considered. The model parameters are determined from the creep test data of single crystal and directionally, solidified superalloy with a special grain orientation. The numerical analysis shows that the model creep damage behaviours of nickel-base directionally solidified superalloys with different grain orientations are in good agreement with the experimental data.

Influence of Crystal Orientation on the Morphology of a Single Crystal Superalloy during Directional Solidification

The solid-liquid (S/L) interfacial morphology of a single crystal superalloy DD8 has been investigated. The evolutionary behavior of cellular morphology with tilted preferred crystallographic orientation near cell-dendrite transition was dynamically observed, and the effect of crystallographic orientation on primary dendritic arm spacing has been examined. The experimental results show that for planar and cellular morphology, no any S/L interfacial anisotropy exists, but near cell-dendrite transition, the S/L interfacial anisotropy appears and gives rise to the cellular crystal fingers tilted from thermal flow direction to preferred crystallographic orientation. The crystal fingers with their preferred orientation parallel to DS growth direction are more stable than that with tilted orientation. For the tilted fingers, the surface on the side facing DS growth direction is less stable than that on the reverse side,the different stability on the two sides will lead to forming unsymmetrical dendritic microstructure. With this increase of tilted angle of preferred crystallographic orientation, the primary dendrite arm spacing decreases. (Edited author abstract) 7 Refs.

Sub-high Rate Directional Solidification

The main features of morphology evolution of crystal growth during near rapid directional solidification were reviewed and the concept of effective constitutional supercooling was proposed. On the basis of interface stability theory combined with experiments, the behaviour of transition from dendrites to fine cells was quantitatively described, and the transition from cells to plane front of absolute stability was also examined.

Effective Elastic Moduli of Ceramics with Pores

A contribution was given to the theoretical research on the stiffness of ceramic materials, under the approximation of the average stress field, the relation E equals E//0(1 minus p)/(1 plus 2.5p) was derived to describe the dependence of the modulus of elasticity on porosity. The prediction of this model is consistent with the experimental data from several ceramics such as reacting sintered Si//3N//4, MgO and MgAl//2O//3. (Edited author abstract) 12 Refs.

Damping Capacity and Creep Resistance of Spray Deposited High Silicon Alloy ZA27

Alloying and spray deposition technique were adopted to improve the the service temperature limitation of the alloy ZA27 in an effort to develop a new functional material with high damping capacity and good elevated temperature mechanical property. The effects of addition of 5 wt pct of Si on the damping behavior and elevated temperature creep resistance of alloy ZA27 were investigated. The damping capacity, as well as the relative dynamic modulus, were measured at frequency of 1.4 Hz over the 303 to 573 K. No peak phenomenon was observed for the specimens tested in the temperature range of interest. Experimental results indicate that the damping capacity of the deposited material is 2.8 times higher than that of the cast one at 305 K. The ball hardness values were used to evaluate the creep property and the creep activation energy of the deposited material is 3.9-5.7 kJ per mol higher than that of the as-cast one. The creep rate formulation was also derived. Finally, The operative damping and creep mechanisms were discussed in the light of the data obtained from its characterization of microstructure, damping capacity and creep resistance. (Edited author abstract) 12 Refs.

固溶温度对激光立体成形GH4169高温合金组织和性能的影响(英文)

研究激光立体成形GH4169高温合金固溶处理后的组织,分析了固溶过程中合金元素的均匀化,并对材料显微硬度的变化做了讨论。结果表明,随固溶温度的升高,材料的晶界越来越清晰,经1000-1170°C固溶处理1h后材料发生再结晶,当固溶处理温度高于1100°C,有大量孪晶出现,晶粒相比沉积态显著细化,晶粒尺寸约200μm。合金元素的均匀化在1100°C保温1h后基本完成。显微硬度测试结果显示,材料的显微硬度值随固溶温度的升高而降低,在固溶温度低于1100°C时减小趋势较快,高于1100°C时减小趋势减缓。显微硬度的变化与材料中γ′和γ"相及δ相的形态和数量有关。1100°C是激光立体成形GH4169合金比较合适的固溶处理温度下限。

Peritectic solidification under high undercooling conditions

The solidification characteristics of highly undercooled Cu-7.77% Co peritectic alloy has been examined by glass fluxing technique. The obtained undercoolings vary from 93 to 203 K(0.14 T_L). It is found that the a(Co) phase always nucleates and grows preferentially, which is followed by peritectic transformation. This means that the peritectic phase cannot form directly, even though the alloy melt is undercooled to a temperature far below its peritectic point. The maximum recalescence temperature measured experimentally decreases as undercooling increases, which is lower than the thermodynamic calculation result owing to the actual non-adiabatic nature of recalescence process. The dendritic fragmentation of primary α(Co) phase induced by high undercooling is found to enhance the completion of peritectic transformation. In addition, the LKT/BCT dendrite growth model is modified in order to make it applicable to those binary alloy systems with seriously curved liquidus and solidus lines. The dendrite