Compaction equations: a comparison of the Heckel and Kawakita equations

Two factors could be regarded as primary factors for the compactability of powders: the dominating bond mechanism and the surface area over which these bonds are active. Owing to considerable experimental difficulties, these factors have not been evaluated in any detail for pharmaceutical materials. Instead, more indirect, secondary factors are normally studied and used for correlations with tablet strength. Such secondary factors are particle size, shape and surface texture. Also the importance of volume reduction mechanisms, i.e. elastic deformation, plastic deformation and particle fragmentation have been studied in detail.For the investigation of dominating bond mechanisms and estimation of the magnitude of the surface area of the solids involved in interparticulate attraction in compacts several pharmaceutical excipients representing both plastically deforming materials (sodium chloride, Avicel® PH 101, Sta-Rx 1500®, and sodium bicarbonate) and fragmenting materials (lactose, sucrose, paracet...

Bonding Surface area and Bonding Mechanism-Two Important Factors fir the Understanding of Powder Comparability

The literature dealing with creep of near γ-TiAl alloys is reviewed, including single and multiphase compositions. The characteristics of creep in the primary or transient, minimum strain rate or pseudo-steady state, and tertiary creep regimes are presented. Considerable experimental evidence indicates that a temperature-stress domain exists in which creep is controlled by dislocation climb. For multiphase compositions, the effect of microstructure on creep is discussed, with emphasis on the predominantly lamellar microstructure, which exhibits superior creep resistance. The influence of composition on creep resistance is presented in terms of intrinsic and extrinsic effects, and the associated role of β phase is discussed. Models for the creep of lamellar structures are reviewed. The current state of knowledge regarding creep of near γ-TiAl alloys precludes quantitative conclusions; however preliminary design of creep resistant microstructures is possible.

Current understanding of creep behaviour of near γ-titanium aluminides

The effect of wall friction in the compaction of pharmaceutical tablets with curved faces: a validation study of the Drucker–Prager Cap model

The internal form of compacted ceramic components: a comparison of a finite element modelling with experiment

The use of hardness in the study of compaction behaviour and die loading

Titanium aluminide intermetatlics are of engineering interest due to an attractive combination of mechanical properties and density. This paper discusses the properties and processing of titanium aluminides, concentrating on near γ-TiAl compositions. Inherent structural and deformation characteristics are described. The relationship between microstructure and processing is examined with emphasis placed on powder metallurgical, HIP'ing and hot deformation processes. Mechanical properties attainable with near γ-TiAl alloys are presented and the effect of ternary elements examined.

The technology of titanium aluminides for aerospace applications

The high-temperature deformation behaviour of Mar M200 superalloy powder compacts, pressed below the γ′ solvus, has been examined. Compression tests were carried out at temperatures between 950 and 1200°C and at constant true-strain rates between 10−1 and 10−5 S−1. Superplastic behaviour was found at all the temperatures used and at strain rates below 10−2 S−1. The strain rate sensitivities at low strain rates are in the range 0.4–0.6 while at higher strain rates the exponents fall to between 0.1 and 0.3. In each range the exponent increases with temperature, indicating higher degrees of plasticity. Variations in strain-rate sensitivities are related to the various deformation mechanisms which are thought to contribute to flow. The effects of powder particle mesh size and size distribution on the flow properties of the compacts are also considered. Coarser mesh sizes produce the highest peak flow stresses at all strain rates and temperatures investigated. Compacts made predominantly from fine mesh...

Flow behaviour of Mar M200 powder compacts during isothermal forging

Defects in cold-hydrostatically-extruded aluminium, iron and nickel-base powder compacts

The isothermal forging behavior of a wire reinforced superalloy powder composite has been examined. The material consisted of a Mar-M200 matrix containing 40 pct by volume of tungsten wire and was prepared by hot isostatic pressing. Specimens were deformed by uniaxial compression at constant temperature in the range 1050 °C to 1180 °C, and at constant true strain rates between 10-5 s-1 and 10-1 s-1. Loading was normal to the direction of wire alignment. Microstructural defects existing in the as-pressed composites are compared with defects in the forged materials. An upper bound forming limit occurs when fibers come into contact. However, microstructural damage occurs at lower strains which depends on temperature and strain rate. Observed and calculated values of peak flow stress are used to define practical forming conditions for the material which should avoid the formation of internal damage at low strains.

M.C. de Malherbe

Papers

The use of hardness in the study of compaction behaviour and die loading

The technology of titanium aluminides for aerospace applications

Flow behaviour of Mar M200 powder compacts during isothermal forging

Defects in cold-hydrostatically-extruded aluminium, iron and nickel-base powder compacts

The Fracture Behavior of Tungsten Wire Reinforced Superalloy Composites during Isothermal Forging