The critical pressure and impeding pressure of Al evaporation during induction skull melting processing of TiAl
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NTRODUCTION
VACUUM induction skull melting (ISM) is usually used to melt reactive metals and their alloys such as titanium and titanium alloys.[1–4] The skull formed in the melting process holds the liquid alloy and prevents the reaction between the crucible material and the alloy melt. However, because of the vacuum condition, the evaporation loss of elements with high equilibrium partial pressure is so serious that the ingot composition may deviate from the original composition. For example, due to the high equilibrium partial pressure of Al that is an indispensable alloying element in almost all of the titanium alloys, there always exists its evaporation loss during the vacuum melting of Al-containing titanium alloys. In some cases, the evaporation loss of Al can be up to 8 wt pct during the cold crucible ISM of gamma TiAl intermetallics. The loss can be up to 10 to 15 wt pct by the electron beam melting process.[5] Undoubtedly, the large quantity of evaporation loss can cause significant influence on gamma TiAl alloy’s strength and ductility, which are very sensitive to composition. Though some researchers have given some principles about the kinetics of induction melting,[6,7,8] unfortunately, the studies on evaporation behavior of the alloying elements are limited. In this study, the author investigated the Pcrit and Pimpe of Al evaporation during the ISM process of Ti-XAl (at pct) (X ⫽ 25 to 50) theoretically, analyzing the effects of Al content and the temperature of the melts. The study aims at being a theoretical guide to the ISM melting practices of Ti-Al–based alloys. II. THE RELATIONSHIP BETWEEN THE EVAPORATION LOSS RATE AND CHAMBER PRESSURE In our previous article,[6] we pointed out the relationship between the evaporation rate of Al in a Ti-35Al (at pct) alloy JINGJIE GUO, JUN JIA, and HENGZHI FU, Professors, GUIZHONG LIU, Postdoctoral Candidate, and YANQING SU and HONGSHENG DING, Associate Professors, are with the School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted October 2, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
melt and the pressure in the vacuum chamber (hereinafter referred to as chamber pressure) based on our theoretical calculation data, as shown in Figure 1. In Figure 1, the curves of Nm-P can be obviously divided into three parts. When the chamber pressure P is less than a certain value, the evaporation rate Nm is the maximum value and almost remains unchanged with the decrease of P. This certain value of pressure can be defined as critical pressure Pcrit of the element evaporation. At the same time, when P is larger than a certain value, the evaporation rate Nm declines to the minimum value with the increase of the chamber pressure. In the same way, this pressure is defined as impeding pressure Pimpe of evaporation. When Pcrit ⬍ P ⬍ Pimpe, with the increase of the chamber pressure, Nm decreases sharply. Some metallurgists have also given the relationship b
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