Hardness, Structural Alloys Systems

There is hardly a metal that cannot, or has not, been joined by some welding process. From a practical standpoint, however, the range of alloy systems that may be welded is more restricted. The term weldability specifies the capacity of a metal, or combination of metals, to be welded under fabrication conditions into a suitable structure that provides satisfactory service. It is not a precisely defined concept, but encompasses a range of conditions, eg, base- and filler-metal combinations, type of process, procedures, surface conditions, and joint geometries of the base metals (12). A number of tests have been developed to measure weldability. These tests generally are intended to determine the susceptibility of welds to cracking. 

Recent research on more coercive media with a low noise ratio involved addition of Zn to the Co alloy system [76-79]. Addition of Zn to the cobalt alloy very effectively produces a film with a fine particle structure, which results from codeposition of elements which are hardly soluble in the matrix. Such codeposition causes segregation and hence produces a film microstructure consisting of fine particles. The fine particulate structure lowers the noise ratio and increases the coercivity of the medium. 

Many carbides and silicides of composition AX are formed by transition metals. These carbides and silicides are characterized by very high melting points, extreme hardness, optical opacity and relatively high electrical conductivity. Many of them have the sodium chloride structure but they are not ionic compounds rather do they resemble the corresponding nitrides and phosphides in simulating alloy systems in many of their properties. For this reason they will be discussed later. 

The lead-base babbitts are based upon the lead—antimony—tin system, and, like the tin-base, have a structure of hard crystals in a relatively soft matrix. The lead-base alloys are, however, more prone to segregation, have a lower thermal conductivity than the tin-base babbitts, and are employed generally as an inexpensive substitute for the tin-base alloys. Properly lined, however, they function satisfactorily as bearings under moderate conditions of load and speed. 

The Sm-Fe-Zr system. In the Sm-Fe-Zr system neither the 1 12 nor the A2 phase was observed for mechanically alloyed samples with a high Sm content. Instead, a (Sm,Zr)Fe3 phase forms with the rhombohedral PuNi3 crystal structure. Also this phase proved to be hard magnetic when prepared as a microcrystalline material by mechanical alloying [3.82]. Its coercivity reaches 11.8kA/cm. 

The crystal structures of metallic ruthenium and copper are different, ruthenium having a hexagonal close-packed structure and copper a face-centered cubic structure (7). Although the ruthenium-copper system can hardly be considered one which forms alloys, bimetallic ruthenium-copper aggregates can be prepared that are similar in their catalytic behavior to alloys such as nickel-copper (3,4,8). 

Accurate electronic structure calculations are becoming more and more important because of the increasing need for information about systems which are hard to perform experiments on. Databases compiled from theoretical results are also being used more than ever for applications, and the reliability of the theoretical methods are of utmost importance. In this thesis, the present limits on theoretical alloy calculations are investigated and improvements on the methods are presented. 

Our ignorance concerning even the qualitative nature of catalyst surfaces can be illustrated by reference to alloy catalysis 403a). That a surface alloy can have structure quite different from the normal bulk phases has already been observed by LEED for the Ni-Mo system, in which the surface structures do not correspond at all to the ordinary bulk alloys Ni4Mo and NiaMo 404). In many experiments with alloys an abrupt change of catalytic behavior at a particular alloy composition has been correlated with a change in the electronic band structure of the solid. But what is the nature of the surface Average interior composition of a binary alloy is hardly affected if one kind of alloy atom... 

The wide variety of structures, systems, and components found in DOE nuclear facilities are made from many different types of materials. Many of the materials are alloys with a base metal of iron, nickel, or zirconium. The selection of a material for a specific application is based on many factors including the temperature and pressure that the material will be exposed to, the materials resistance to specific types of corrosion, the materials toughness and hardness, and other material properties. 

The boundary between hardmetals and cermets is not strict because many of these compacts resemble microstructure features of both type of materials [106] faceted WC crystals together with round-shaped titanium carbonitride-based hard particles. Generally, these titaniiun carbonitride hardmetals are comparable with respect to properties and microstructure to WC-based hardmetals. The powders of these materials are liquid phase sintered with Ni or Ni-Co binder metal alloys. The core-and-rim structure of the hard phase usually exhibit a molybdenum- and carbon-rich (Ti,Mo)C rim and a titanium- and nitrogen-rich Ti(C,N) but can also be inverted (compare Fig. 26). The metallurgy of the phase reactions is (because of the complexity of the multicomponent system) not yet fully understood [69]. 

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