Thermal conductivity of metallic glassy alloys and its relationship to the glass forming ability and the observed coolin
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Takanobu Saito Institute for Materials Research, Tohoku University, Aoba-Ku, Sendai 980-8577, Japan
Junji Saida Center for Interdisciplinary Research, Tohoku University, Aramaki, Aoba, Sendai 980-8578, Japan
Akihisa Inoue WPI Advanced Institute for Materials Research, Tohoku University, Aoba-Ku, Sendai 980-8577, Japan; and Tohoku University, Aoba-Ku, Sendai 980-8577, Japan (Received 10 December 2007; accepted 19 May 2008)
In this work, we study the cooling behavior of several typical bulk glassy alloys on casing and present the relationship between the thermal conductivity of a glassy alloy and the cooling rate upon mold casting. The cooling rates obtained for Ti-, Zr-, Pd-, and Cu-based bulk glass forming alloys are found to scale with the thermal conductivities of the studied glassy alloys.
I. INTRODUCTION
Bulk glassy alloys [(BGAs), also called bulk metallic glasses]1,2 undergoing glass transition3,4 on casting exhibit good physical and functional properties.5,6 Bulk glassy alloys exhibit high strength, hardness, wear resistance and large elastic deformation,1,2,7 and high corrosion resistance.8,9 Moreover, the fatigue-endurance limits of Zr–Al–Cu–Ni alloys are comparable with those of high-strength structural alloys.10 Glassy alloys being metastable undergo relaxation and devitrification processes on heating.11 The glass-forming ability (GFA) depends on various factors.1,4,12 According to Inoue,1 good metallic glass formers (i) belong to multicomponent systems (analyzed in Ref. 13), (ii) have a significant atomic radius difference above 12%, and (iii) exhibit negative heats of mixing among the constituent elements. Alloys that satisfy these conditions usually exhibit high values of glasstransition temperature (Tg) and low values of liquidus temperature (Tl). The reduced glass-transition temperature (Trg ⳱ Tg/Tl) is one of the good indicators of high GFA.14,15 The width of the supercooled liquid region (⌬Tx ⳱ Tx − Tg) (Tx is
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0286 J. Mater. Res., Vol. 23, No. 8, Aug 2008
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the onset crystallization temperature) as an indicator of the stability of the supercooled liquid against crystallization also correlates reasonably well with the observed GFA.11,16 The ␥ parameter, defined as Tx/(Tg + Tl),17 takes into account both criteria and gives an even better correlation with the experimental GFA data. If the alloy is not a eutectic one the parameter ␣ ⳱ (Ts − Tg)/(Tl − Tg),18 (Ts is the solidus temperature) should be introduced, which takes into account the Tl − Ts difference. Moreover, equilibrium phase diagrams are inapplicable to small castings because of the high cooling rates,11 which cause formation of the metastable phases and shift of the eutectic points. The effect of thermal conductivity of a molten alloy on the cooling rate of Cu- and Ni- based alloys, which influences GFA, was studied recently.19 If the heat-transfer coefficient (h) is constant at the
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