| Products of precipitation strengthened alloys characteristically lose their ability to develop peak properties as the rate of the quench from the solution temperature is reduced. An analytical model was developed which improved on a previous model to predict mechanical properties of such materials from time-temperature-property, TTP, C-curves and a cooling curve. Whereas the previous model was limited to predicting corrosion mode, strength, and hardness, the new model is capable of predicting toughness as well. Moreover, the revised model is inherently more accurate in predicting properties of material that is cooled under conditions that produce low properties. The model was evaluated using plate of an experimental Al-Cu-Li-Zr alloy and was related to a physical model. Effects on structure and properties of time at temperatures between the solvus and aging temperatures were determined using isothermal quenching techniques. Both strength and toughness measured in T8 temper progressively decreased with increasing time at temperature until they asymptotically approached a level which was temperature dependent. Loss in the ability to develop strength with increasing time at temperature was attributed to a loss in copper and lithium by precipitation of T{dollar}sb1{dollar}, T{dollar}sb{lcub}rm B{rcub}{dollar}, and {dollar}thetaspprime{dollar}, while loss in toughness was attributed to precipitation of T{dollar}sb1{dollar} phase in grain and subgrain boundaries. The loss in toughness at particular strength levels depended on the isothermal quench temperature because the morphology of the T{dollar}sb1{dollar} precipitate depended on temperature. The property, time, and temperature data were used to determine constants in a TTP C-curve equation. Properties after continuous cooling were predicted by summing effects which occurred at the isothermal quenching temperatures. In contrast to the previous model, this model recognized that the driving force for precipitation at each temperature was a function of the amount of solute remaining in solution and the amount that can precipitate at that temperature. The results indicate that the new model is an improvement and is useful for technological applications. As part of this investigation, a method was also developed for producing continuous-cooling-property, CCP, curves from TTP C-curves. |
