Showing papers on "Equiaxed crystals published in 1977"

Deformation characteristics of age hardened Ti-6Al-4V

Abstract: A commercial Ti−6Al−4V alloy with an equiaxed grain shape was investigated after solution annealing at 810°C and after aging at 550 and 350°C. Age hardening at both temperatures produced significant increases in Young's modulus and yield strength. Finely dispersed α2(Ti3Al) precipitates formed within the α phase upon aging at 550°C, but not when aging at 350°C. However, there is evidence of order, probably of oxygen, in the α grains of specimens which were aged at 350°C. The formation of the ordered Ti3Al precipitates at 550°C and the occurrence of oxygen ordering at 350°C can account for the increases in Young's modulus and yield strength.

Method for improving fatigue properties of titanium alloy articles

Abstract: A thermomechanical treatment to improve the fatigue strength of articles made from one of a class of alpha beta titanium alloys. The treatment involves heating the alloy into the beta field, hot deforming the alloy at a temperature within the beta field, rapidly quenching the alloy to room temperature to produce a hexagonal martensite structure and then tempering at an intermediate temperature so as to produce a structure in which discrete equiaxed beta phase particles are presented in an acicular alpha matrix. This structure is particularly resistant to the initiation and propagation of fatigue cracks.

On the columnar to equiaxed transition in small ingots

Abstract: The columnar to equiaxed transition (CET) in small ingots of, aluminum alloys was found to occur more easily for alloys with a larger value of the constitutional supercooling parameter (−mCo(1-k)/k). The CET was found to be completely suppressed by increases in the mold temperature by preheating before casting. These results are discussed in terms of the model proposed by Burden and Hunt that the CET occurs by the effect of the thermal gradient, arising from the slow, solidification of equiaxed dendrites, which increases the undercooling of the columnar dendrites. The application of the model due to Burden and Hunt is shown to require, the use of the ‘big bang’ model for equiaxed nucleation on pouring. A higher density of the nuclei, that grow into equiaxed grains, formed by pouring with lower superheat and into a cold mold, gives a higher thermal gradient immediately in front of the growing columnar grains. Other evidence in favor of the model is briefly discussed.

Endogenous-exogenous freezing characteristics of pure and impure as-cast Zn-Al alloys

Abstract: A study was initially made of the cast structures of pure binary Zn-Al alloys having compositions within the eutectic horizontal. For the fixed casting conditions employed it was observed, in hypoeutectic alloys, that there is a coarsening of the macrostructure with increasing aluminium content accompanied by a change in the mode of freezing from endogenous (equiaxed grain structure predominating) to exogenous (columnar grain structure predominating). On the other hand, hypereutectic alloys were observed to freeze as completely equiaxed structures for all compositions examined. When Pb was added as an impurity to alloys in the range 3–10 wt.-%AI, the general effect was a coarsening of the as-cast macrostructure. However, two major differences were noted between the solidification of hypo- and hypereutectic alloys. In hypoeutectic alloys of neareutectic composition the mode of freezing changed from exogenous to endogenous, whereas the opposite effect occurred in the hypereutectic alloys. Furthermor...

Distribution of non-metallic inclusions in continuously cast carbon steels

Abstract: Metallurgical factors will affect the solidification structure and distribution of inclusion particles in continuously cast carbon steels. The amount of equiaxed structure increases as the temperature of the liquid steel decreases. The number of large sulphide and oxide inclusions is high close to a surface separating the equiaxed and columnar zones or where columnar zones from two different directions meet. A bow-type machine will produce a non-symmetrical distribution of solidification structures and of large inclusions. Small inclusions, however, are formed at a late stage of the freezing process and are not greatly affected by the geometry of the casting machine. A new experimental technique has been applied to examine the composition and size distribution of inclusions. The technique has been valuable in the study of nucleation and growth of oxide and sulphide inclusions.