Cooling precipitation and strengthening study in powder metallurgy superalloy U720LI
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HE excellent high-temperature strength of powder metallurgy (P/M) superalloys is mostly due to the precipitation of ordered and coherent ␥ ⬘ (Ni3Al) phase from the solidsolution matrix, besides some solid-solution strengthening in the ␥ matrix. Control of ␥ ⬘ precipitation through heat treatment is critical to the final mechanical properties, because the nucleation and growth kinetics of the precipitates strongly depend on the cooling rate during quenching. If the cooling rate is fast enough to avoid or limit the formation and growth of extensive cooling ␥ ⬘ precipitates, a high density of fine ␥ ⬘ precipitates can be subsequently developed to achieve desirable properties. Therefore, higher cooling rates result in higher strength.[1] However, the strength gained from fast quenching would be limited by the possibility of quench cracking or severe shape distortion due to the high thermal stresses induced.[2–6] Numerous efforts have been directed at simulating the relationship between the cooling rate and the resultant mechanical properties, as mentioned previously.[7–12] The problem with accurately characterizing this relationship is associated with the description of the cooling rate itself. Traditional cooling-rate studies were mostly conducted by quenching a component in a bath after holding it at the solution temperature. Thus, the cooling profile depended on the heat-transfer coefficient of the quenchant, which varied
as a function of decreasing temperature during quenching. Another method for obtaining different cooling rates was the Jominy bar test, which yielded different cooling profiles at different points along the length of the quenched bar.[13] In this case too, the cooling rates varied as a function of temperature. Therefore, an average cooling rate for the entire process (or for a given temperature range) was adapted as the cooling parameter. In practice, the size of the precipitates depends on the actual cooling profile, which varies not only with quench procedure, but also with the configuration of the component, the heat-transfer coefficient, and the cooling characteristics of the quenchant.[14,15] As a result, using an average cooling rate within a temperature range can only produce an approximate relationship between the size of cooling ␥ ⬘ precipitates and the cooling rate. The main objective of this research is to develop an empirical model that would be capable of predicting the size of the quenched ␥ ⬘ precipitates and their strength, as a function of cooling rate, in a superalloy component. The advantage of this work is the ability to precisely control the linear cooling profile and then to develop a real correlation between the cooling rate and ␥ ⬘ precipitate size. This study will provide valuable input to the thermal process simulation on optimizing thermal process parameters for specific requirements of a given P/M superalloy component.
II. EXPERIMENTAL INVESTIGATION JIAN MAO, Postdoctoral Student, KEH-MINN CHANG, Professor, and WANHONG YANG, former Research Assistant Professor, are with
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