Papers by Author: Murat Tiryakioğlu

Abstract: The effects of solution treatment time and artificial aging on the work hardening characteristics on Al-7%Si-0.6%Mg (D357) alloy castings were investigated. Four different solution treatment times at 540°C (1, 4, 16 and 64 hours) and six different artificial aging times at 160°C (0, 2.5, 5, 10, 20 and 40 hours) were used. Work hardening characteristics were investigated by Kocks-Mecking plots for each specimen. The effects of Si particle morphology (solution treatment) and matrix strength (aging) on Kocks-Mecking (Stage III) work hardening model parameters are discussed in the paper.

Abstract: The modelling of microstructure, and hence the properties, of cast alloys has so far been attempted assuming the liquid metal is free from defects. The growing appreciation that bifilms, of size measured in millimeters, are usually abundant in liquid metals, and control many features of the casting, has led to this paper in which some of the known and some of the expected effects are reviewed. It is suggested that bifilms cause major effects on microstructure and properties and their presence can vastly overpower the effect of traditionally simulated structural features.

Abstract: We discuss data from a range of heat-treatable aluminum alloys, showing both yield strength and fracture toughness vs time at temperature of interrupted quench. Drop in toughness occurs at much shorter hold time than drop in strength. Concurrently the fracture becomes more intergranular. When later the yield strength falls, fracture becomes more transgranular, and toughness may rise. We attribute this pattern to two mechanisms: 1) Early quench precipitates nucleated on grain and/or subgrain boundaries grow to size sufficient to initiate fracture under tension, long before they withdraw significant solute from subsequent age-hardening. 2) Later quench precipitates nucleated on dispersoids and/or dislocations withdraw solute relatively uniformly, reducing matrix yield strength while increasing matrix ductility. We propose that quantitative modeling of change in strength and toughness with change in quench, requires multiple C-curves for multiple types of quench precipitates, and nonlinear relation of toughness to amount of boundary quench precipitate.