Paweł Pałka

Ag Powders Consolidated by Reciprocating Extrusion (CEC)

CEC has unique characteristic. These are applicability of very large strain and deformation under high hydrostatic pressure. Due to these abilities of CEC, several unique phenomena have been observed. One of them is the possibility of consolidation of metallic powders in room temperature to the form of bulk material. In the present paper the consolidation of AgSnBi and AgNi to bulk composites was presented. Applying the deformation of  = 0.42 in the single cycle of CEC, under high hydrostatic pressure, the samples without pores and discontinuities were fabricated. The microstructure observations were performed by optical microscopy (MO), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). They show refinement of microstructure at all levels of observation. The nanometric-size subgrains/grains were found inside consolidated granules. The microhardness level of AgSnBi in average achieved level 110 μHV100, and AgNi of about 90 μHV100. The AgSnBi samples consolidated by CEC and additional hydrostatically extruded to wires with 3 mm in diameter average showed 500 MPa yield point.

The Influence of Thermal Sprayed Coats Chemical Composition on the Microstructure and Properties

In the present work, the microstructure, phase composition and microhardness of Cr3C2-NiCr, WCCo, and powdered composite NiCrSiBCr10%,Fe2.5%,Si3.1%,Bi2.1%C; NiCrSiBFe5; NiCrSiBCr5; NiCrSiBFe2.5Cr2.5; HVOF coatings applied on the Al-Si substrate have been compared. The coating cross-sections were examined by optical microscopy (OM), scanning electron microscopy (SEM) and electron transmission microscopy (TEM). For hard Cr3C2-NiCr and WC-Co coatings, a total microhardness level of about 860 and 1240 HV, respectively, was obtained. The microhardness of composite coatings was essentially lower and comprised in the range of values between 420 and 540 HV. The lowest level of microhardness showed the NiCrSiBFe2.5Cr2.5 coating. The most refined microstructure was found in Cr3C2 and WC coatings. The mean size of splat granules obtained in Cr3C2-NiCr had the value of about 2.7 m and of 0.5 m in WC-Co. For comparison, the granules obtained in composite coatings had the dimensions of about 30 μm. An Xray diffraction (XRD) revealed the presence of Cr3C2 and WC carbides in Cr3C2-CrNi and WC-Co coatings. In composite coatings, the phases of Cr3Ni5Si2, Ni3Si, FeSi, Fe2B, Cr3Si, BCr as well as other phases were found. The existence of the analyzed phases was additionally confirmed by the selective etching of coating microstructure. The annealing of coatings at 823K resulted in pore disappearance and increased the coating microhardness.

Numerical Models of Vacancy Diffusion Based on Crystal Structure

The paper discusses influence of main structural factors, as the type of crystal structure and net constant, on macroscopic features of self-diffusion and interdiffusion, in an attempt to provide a consistent description of diffusion that would involve all relevant physical effects, such as Kirkendall effect, dependence of diffusion on vacancy concentration, crystallography, and concentration of individual elements.Basing on the model structure, which is a Simple Regular net along with a corresponding Wigner-Seitz cell, the principal assumptions of the proposed model are presented as well as its implications, in order to determine general relations, which can further be used in numerical calculations involving diffusion streams. Subsequently, example calculations are performed for a simplified case of a two-dimensional Simple Regular net.The conclusions reached through this line of reasoning are then extrapolated to more complicated cases of diffusion – FCC, BCC, and HCP nets – which makes possible to relate particular material structures with diffusion rates. The connection of Wigner-Seitz cells with diffusion constitutes a good demonstration of dependence of the diffusion stream on the crystallographic orientation as well on the corresponding diffusion anisotropy for complex networks.

Microstructure of AgNi and AgSnBi Powders Consolidated by CEC

Powder metallurgy is widely used to the production of AgNi and AgSnBi powders employed for electrical contacts. In the work AgNi and AgSnBi powders were consolidated by the cyclic extrusion compression (CEC) method enabling cyclic unlimited deformation. In the initial stage the AgNi powder contained the two phases Ag and Ni, recognized by the EDX technique using scanning electron microscopy (SEM). The investigations shown that the Ni phase is distributed in the form of small granules around larger Ag granules. In the AgSnBi powder phases Ag + Bi + Ag3Sn (ξ) were distributed uniformly. It was found that after the CEC consolidation phases were excellently joined without cavities and cracks. Detailed observations of microstructure have been performed by the transmission electron microscopy (TEM) and revealed inside the consolidated granules nanometric grains with the nanometric twins inside.