Thermodynamic Characteristic and Phase Evolution in Immiscible Cr-Mo Binary Alloys

doi:10.1007/s40195-015-0297-6

Abstract

Abstract:

This paper systematically reports the thermodynamic characteristic and phase evolution of immiscible Cr-Mo binary alloy during mechanical alloying (MA) process. The Cr-35Mo (in at%) powder mixture was milled at 243 and 258 K, respectively, for different time. For comparative study, Cr-15Mo and Cr-62Mo powder mixtures were milled at 243 K for 18 h. Solid solution Cr(Mo) with body-centered cubic (bcc) crystal structure and amorphous Cr(Mo) alloy was obtained during MA process caused by high-energy ball milling. Based on the Miedema’s model, the free-energy change for forming either a solid solution or an amorphous in Cr-Mo alloy system is positive but small at a temperature range between 200 and 300 K. The thermodynamical barrier for forming alloy in Cr-Mo system can be overcome when MA occurs at 243 K, and the supersaturated solid solution crystal nuclei with bcc structure form continually, and three supersaturated solid solutions of Cr-62Mo, Cr-35Mo and Cr-15Mo formed. Milling the Cr-35Mo powder mixture at 258 K, the solid solution Cr(Mo) forms firstly, and then the solid solution Cr(Mo) transforms into the amorphous Cr(Mo) alloy with a few of nanocrystallines when milling is prolonged. At higher milling temperature, it is favorable for the formation of the amorphous phase, as indicated by the thermodynamical calculation for immiscible Cr-Mo alloy system.

Key words: Thermodynamic characteristic, Immiscible alloy system, Phase evolution, Solid solution, Amorphous, Mechanical alloying

Chong-Feng Sun, Sheng-Qi Xi, Yue Zhang, Xiao-Xue Zheng, Jing-En Zhou. Thermodynamic Characteristic and Phase Evolution in Immiscible Cr-Mo Binary Alloys[J]. Acta Metallurgica Sinica (English Letters), 2015, 28(8): 1074-1081.

Figures/Tables 12

Table 1 Parameters used for free-energy change calculation [14-16]

Elements	T _m (K)	V ^2/3 (cm²)	φ * (V)	n _ws ^1/3 [(d.u.)^1/3]	K (10¹⁰ Pa)	μ (10¹⁰ Pa)	P (kJ V^-2 cm^-1)	Q (kJ V^-1)	R/P
Mo	2890	4.4	4.65	1.73	26.12	12.56	14.1	132.54	0
Cr	2130	3.7	4.65	1.77	16.02	11.53	14.1	132.54	0

Fig. 1 Free-energy change of Cr-Mo alloy system at different milling temperatures

Fig. 2 XRD patterns of the Cr-35Mo powder milled at 243 K for different time

Fig. 3 SEM images of Cr-35Mo powder milled at 243 K for 10 h a, 18 h b

Fig. 4 XRD patterns of the Cr-62Mo, Cr-35Mo and Cr-15Mo mixed powders milled at 243 K for 18 h Angles of the peaks in Fig. 5 are used to determine the interplanar spacing by the Bragg equation [20]: λ=2dhkl sinθ, (2)

Fig. 5 Changes in the interplanar spacing, d, with Mo content in Cr(Mo) solid solution

Table 2 The lattice parameters of the three supersaturated solid solutions

Solid solutions	a _m	a _ss	μ
Cr-62Mo	0.3096	0.3056	1.013
Cr-35Mo	0.3032	0.2993	1.013
Cr-15Mo	0.2968	0.2946	1.007

Fig. 6 a XRD patterns of Cr-35Mo powder milled at 258 K for different time; b analysis of the diffraction peak at ~42.45° for Cr-65Mo powder milled at 258 K for 18 h

Fig. 7 TEM image and SAEDP of Cr-35Mo powder milled at 258 K for 18 h

Fig. 8 TEM image and SAEDP of Cr-35Mo powder milled at 258 K for 22 h

Fig. 9 HR-TEM image of amorphous Cr-35Mo powder after milled 18 h at 258 K

Fig. 10 XRD patterns of Cr-35Mo powder milled for 18 h under 243 and 258 K

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