Effect of Phase Transformation During Long-Term Solution Treatment on Microstructure, Mechanical Properties, and Bio-corrosion Behavior of Mg-5Zn-1.5Y Cast Alloy

doi:10.1007/s40195-018-0707-7

Abstract

Abstract:

The influence of long-term solution treatment for various intervals on the microstructure, mechanical properties, and corrosion resistance of the as-cast Mg-5Zn-1.5Y alloy was investigated. Variation of secondary phases was studied during solution treatment through thermal analysis test and thermodynamic calculations. Tensile and hardness tests, as well as polarization and immersion tests, were performed to evaluate the mechanical properties and corrosion behavior of the as-cast and heat-treated alloy, respectively. Results show that solution treatment transforms I-phase (Mg₃Zn₆Y) into W-phase (Mg₃Zn₃Y₂), as well as dissolves it into the α-Mg matrix to some extent; therefore, the amount of W-phase increases. Moreover, prolonged solution treatment decreases the volume fraction of the phases. In the first stage of solution treatment for 14 h, the tensile properties significantly increase due to the incomplete phase transformation. Although long-term solution treatment sharply decreases the tensile and hardness properties of the alloy, it improves the corrosion resistance due to the transformation of I-phase into W-phase. In fact, it decreases corrosion potential and simultaneously dissolves intermetallic compounds into the α-Mg matrix, resulting in the reduction in galvanic microcells between the matrix and compounds. It is found that the optimum time for long-term solution treatment is 14 h, which improves both corrosion behavior and mechanical properties.

Key words: Magnesium alloy, Long-term solution treatment, Microstructure, Mechanical properties, Bio-corrosion

H. Jafari, E. Heidari, A. Barabi, M. Dashti Kheirabadi. Effect of Phase Transformation During Long-Term Solution Treatment on Microstructure, Mechanical Properties, and Bio-corrosion Behavior of Mg-5Zn-1.5Y Cast Alloy[J]. Acta Metallurgica Sinica (English Letters), 2018, 31(6): 561-574.

Figures/Tables 17

Fig. 1 Schematic illustration of steel mold (A: cavity, B: pouring basin; unit: mm)

Fig. 2 Cooling and corresponding first derivative curves of Mg-5Zn-1.5Y alloy (T temperature, t time)

Fig. 3 Thermodynamical predictions of weight percentage of phases in alloys as a function of temperature a, at 25 °C b, 415 °C c

Fig. 4 OM a, SEM b images of as-cast alloys and EDS analysis of points denoted as A c, B d in Fig. 4b

Fig. 5 OM images of alloys after heat treatment for 6 h a, 14 h b, 24 c, 36 h d, 50 h e

Fig. 6 Volume fraction of intermetallic compounds as a function of solution time

Fig. 7 SEM micrographs of alloys after heat treatment for 14 h a, 36 h b

Fig. 8 XRD patterns of as-cast alloy a, specimens after heat treatment for 14 h b, 36 h c, JCPDS cards corresponding to α-Mg, I- and W-phases d

Fig. 9 Morphologies of as-cast alloy a, specimen after heat treatment for 14 h b, 36 h c used for calculating I- and W- phases (Red regions: α-Mg, green regions: W-phase, blue regions: I-phase)

Fig. 10 YS, UTS, and elongation (EL) of experimental specimens

Fig. 11 Fracture surfaces of as-cast alloy a, specimens solution treated for 14 h b, 36 h c, higher magnification image of area in c, d

Fig. 12 Brinell hardness of as-cast and solution-treated specimens

Fig. 13 a pH alteration, b weight loss curves of as-cast and solution-treated specimens after immersion in SBF for 144 h

Table 1 Weight loss results of as-cast and solution-treated specimens after immersion test in SBF for 144 h

Specimen	As-cast	Solution treatment time (h)
Specimen	As-cast	6	14	24	36	50
Weight loss (%)	5.5	5.0	4.1	3.5	3.1	2.2

Fig. 14 SEM images of surface of as-cast a, 36 h solution-treated b specimens after immersion test and EDS analysis of points A c, B d in b

Fig. 15 Polarization curve of as-cast and solution-treated Mg-5Zn-1.5Y alloy

Table 2 Electrochemical parameters extracted from polarization curves of as-cast and solution-treated Mg-5Zn-1.5Y alloy (βc: cathodic Tafel slope, βa: anodic Tafel slope)

Specimen	I_corr (μA cm^-2)	E_corr (V)	β_a (V dec^-1)	β_c (V dec^-1)	CR (mm y^-1)
As-cast	223 ± 8	- 1.71	0.066	- 0.061	5.10 ± 0.18
6 h	208 ± 7	- 1.69	0.06	- 0.061	4.82 ± 0.16
14 h	184 ± 7	- 1.67	0.059	- 0.062	3.93 ± 0.15
24 h	149 ± 7	- 1.66	0.06	- 0.059	3.52 ± 0.15
36 h	127 ± 6	- 1.63	0.059	- 0.062	2.92 ± 0.11
50 h	73 ± 5	- 1.58	0.050	- 0.062	1.87 ± 0.08

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