- Email: [email protected]
Surface microstructure and property modifications in a Mg-8Gd-3Y-0.5Zr magnesium alloy treated by high current pulsed electron beam
Journal of Alloys and Compounds 788 (2019) 231e239
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Surface microstructure and property modifications in a Mg-8Gd-3Y-0.5Zr magnesium alloy treated by high current pulsed electron beam T.C. Zhang a, K.M. Zhang a, *, J.X. Zou b, P. Yan b, H.Y. Yang b, L.X. Song a, X. Zhang a a
School of Materials Engineering, Shanghai University of Engineering Science, Shanghai 201620, China National Engineering Research Center of Light Alloys Net Forming & Shanghai Engineering Research Center of Mg Materials and Applications, Shanghai Jiao Tong University, Shanghai 200240, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 October 2018 Received in revised form 2 February 2019 Accepted 10 February 2019 Available online 12 February 2019
In the present study, the effect of high current pulsed electron beam (HCPEB) surface treatments on the microstructure, composition, stress states and properties of a Mg-8Gd-3Y-0.5Zr alloy has been investigated. The surface layers showed different features in microstructure and residual stress states under different number of pulses. Such a variation is essentially due to the different level of the dissolution of bMg5(Gd,Y) particles in remelted layers and the impact from thermal stresses generated by HCPEB treatments. The results have shown that, after 5 pulses of HCPEB treatment, maximum hardness in the surface layer of the sample was increased by 35% and the corrosion rate in 3.5 wt% NaCl water solution was reduced by 30% as compared with those of the untreated sample. The formation of nano structured cells of Mg-RE solid solution and b-Mg5(Gd,Y) nano precipitates, together with the residual compressive stress in the surface layer of the 5 pulsed sample account for the increased microhardness and improved corrosion resistance of the HCPEB treated Mg-8Gd-3Y-0.5Zr Mg alloy. © 2019 Elsevier B.V. All rights reserved.
Keywords: High current pulsed electron beam (HCPEB) Mg-RE alloys Surface treatment Corrosion Hardness
1. Introduction Mg based alloys are promising light weight structural materials due to their distinct advantages, such as high specific strength, high stiffness, good machinability and low density over traditional structural materials (steels, aluminum alloys, etc.) [1]. These properties render the opportunities for Mg based alloys to be used in automobile, aeronautic and electronic industries for weight saving purpose. In particular, when rare earth elements (REs) are introduced into Mg or its alloys, strength, corrosion resistance and high temperature properties can be significantly improved [2e5]. Recent years, Mg-Gd-Y-Zr (GWK) Mg based alloys are attracting more and more attentions in the fields of scientific research and industrial applications owing to their excellent mechanical properties at both room and elevated temperatures as well as good creep resistance. It has been established that the addition of Gd and Y can strengthen Mg due to the grain refinement and the formation of Mg5(Gd,Y) precipitates [6,7]. Besides, Y is also found to improve
* Corresponding author. E-mail address: [email protected] (K.M. Zhang). https://doi.org/10.1016/j.jallcom.2019.02.130 0925-8388/© 2019 Elsevier B.V. All rights reserved.
the corrosion resistance of Mg alloys by reducing the corrosion rate and increasing corrosion potentials [1,2]. Nevertheless, the corrosion resistance of GWK Mg alloys cannot yet meet the requirements for industrial applications. Usually, it is hard to achieve both improvement on strength and on corrosion resistance as the introduction of precipitates into Mg alloys has two sides: The precipitates can pin grain boundaries and dislocations, thus strengthen Mg alloys; On the other hand, they also provide the galvanic corrosion sites in corrosive environments due to their different potentials from Mg matrix. Considering that corrosion is closely related to the surface states of a material, surface modifications would be an efficient way to improve the corrosion resistance of Mg based alloys without changing the bulk structures and compositions [2]. Some surface modification methods, such as laser cladding, physical vapor deposition, chemical vapor deposition, electroless plating, etc. have been applied on to Mg alloys with the aim to improve their surface properties [8e10]. As compared to these surface treatment methods, high current pulsed electron beam (HCPEB) technology, which was initially invented in Tomsk in Russia [11,12], has been developed in recent years and adopted for surface modifications of metallic materials [13e19] with some
232
T.C. Zhang et al. / Journal of Alloys and Compounds 788 (2019) 231e239
advantages, such as low energy consuming, less pollution and high efficiency. With the extremely intensive energy input into the surface layer of the target material within a short pulse of microseconds, the processes include rapid heating (~108 Ks1) [20], melting and even evaporation [21], together with thermal stress (~GPa) and rapid self-quenching (~107 Ks1) [20] occurred drastically, giving rise to the non-equilibrium surface structure modifications accompanied by the improved hardness, corrosion resistance and wear resistance. In addition, the compositions in the surface layer of materials can be changed through HCPEB alloying while the substrate materials remain unchanged [19]. Thus the surface wear or corrosion resistances can be improved while the mechanical properties can be maintained. Zhang et al. [22]studied the surface modifications of stainless steels and observed the improved corrosion resistance after HCPEB treatments due to the selective surface purification effect and composition homogenizations. Liu et al. [23]have investigated the surface modification of the Mg-4Sm alloy by HCPEB treatments and found the formation of nanocrystalline Mg41Sm5 in the modified surface layers. As a result, corrosion resistance of the Mg-4Sm alloy is significantly improved with the increasing number of pulse processing. Gao et al. [24]have applied the HCPEB treatment on the surface of Mg-Al-Zn and MgZn-Y alloys, and the results showed that the composition in the surface layer was homogenized and hardened after treatments. Lee et al. [25] have studied the effect of Large Pulsed Electron Beam (LPEB) irradiation treatments on the magnesium alloy AZ31B and found the formation of an Al-enriched re-melted layer owing to a selective evaporation effect, with nano-grained structure containing Mg3.1Al0.9 metastable phase. The results showed that the formation of such a layer after the LPEB irradiation improved the corrosion resistance of the AZ31B Mg alloy in the 3.5% NaCl water solution. In the present work, the effect of HCPEB treatments on the surface microstructure, hardness and corrosion resistance of a Mg8Gd-3Y-0.5Zr alloy was investigated. It is found that, with proper pulsed electron beam treatments, the corrosion resistance and surface hardness of the Mg-8Gd-3Y-0.5Zr alloy can be both improved, which are advantageous for its industrial applications.
2. Experimental procedures 2.1. Materials The chemical composition (in wt.%) of the Mg-8Gd-3Y-0.5Zr magnesium alloy used in the present research is as follows: ~8 wt % of Gd, ~3 wt% of Y and ~0.5 wt% of Zr (Mg in balance). The ingot sample was heat treated by holding it at 773 K for 6 h (T4 state) [26]. The as received bulk material was cut into small cylinders with the dimension of Ф 15 mm 2 mm.
2.2. HCPEB treatments Prior to the HCPEB treatment, the sample surfaces were mechanically polished from 600-grit sand-papers down to 0.5 mm diamond paste, and were ultrasonically cleaned in acetone. The electron beam system used in this work is a “Hope-1” type HCPEB source [19,22,27], which can produce an electron beam with the following parameters: accelerating voltage of 10e30 kV, peak current of ~10 kA, pulse duration of 0.5e5 s, beam diameter of Ф 60 mm, and energy density of 1e30 J/cm2. In order to avoid the formation of cracks on the treated surface, the parameters set for HCPEB irradiations in the present work were: accelerating voltage 18 kV, energy density 4 J/cm2, pulse duration 1.5 s, number of pulses 5 and 15 and 10s dwelling time between each pulse.
2.3. Microstructure characterizations The phase components of untreated and HCPEB treated samples were examined using X-ray diffraction (XRD) of a Rigaku D/max2000 X-ray diffractometer equipped with a Cu Ka radiation source. The scanning speed for XRD analyses was set at 5 /min in a 2q range from 10 to 90 . The surface and cross-section microstructures were observed using a VHX-600 optical microscope (OM) and a S-3400 N scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS). Before microstructure observations, crosssection of samples was polished and then etched in a 4 vol% nitric acid alcohol solution. To obtain more details about microstructure modifications in the treated surface layers, TEM (FEI Talos-F200X type transmission electron microscope) observation was carried out on the untreated and 5 pulses treated samples. The disc samples for TEM observation were prepared by mechanical grounding from untreated side to 30e50 mm in thickness, followed by dimpling and ion beam (Arþ) thinning from the untreated side to obtain proper thin area for TEM observations. 2.4. Hardness and electrochemical tests Vickers hardness of the samples were measured on the crosssection of the treated samples under 50 g load using a HXD-1000 TMSC/LCD type hardness tester. The loading time is 15s for each measurement. The microhardness at each depth is obtained by doing 3 parallel measurements to get an average value. The electrochemical measurements of untreated and HCPEB treated samples were carried out by a traditional standard threeelectrode cell connected with a PARSTAT 4000 type electrochemical workstation. Platinum and saturated calomel electrode were used as the counter electrode and reference electrode, respectively, and 3.5 wt% NaCl water solution was selected as the electrolyte. All samples were soaked in the 3.5 wt% NaCl water solution for 0.5 h to get a stable open circuit potential. The area exposed to the NaCl water solution was 0.28 cm2. During the measurements, the temperature of the solution was maintained at room temperature. Potentiodynamic polarization curves were measured with a scanning rate of 2 mVS1 ranging from -2 V (vs.SCE) to 1 V. For better reproducibility, all above electrochemical measurements were repeated three times. The polarization curves were used to evaluate the corrosion current density (Icorr) and the corrosion potential (Ecorr) by Tafel extrapolation method. 3. Results and discussions 3.1. Microstructures Fig. 1 shows the OM topography and XRD pattern of the untreated sample. As XRD pattern shown in Fig. 1b, the main phases of the untreated sample are a-Mg and compound b-Mg5(Gd,Y), which is consistent with previously published works [6,26]. As shown in Fig. 1a, the average size of the a-Mg grains are about 75 mm, and the b-Mg5(Gd,Y) particles (arrowed in Fig. 1a) are distributed fairly homogeneous without segregating along grain boundaries. Fig. 2 shows OM images taken at different magnifications of the two treated samples. After 5 pulses and 15 pulses HCPEB treatment, the morphology of the treated surfaces becomes undulated and lots of craters were observed, as shown in Fig. 2a and d. The mechanisms of the crater formation on metal surfaces irradiated by pulsed electron beam have been reported in many previous works [22]. Melting starts at sublayer and micro-irregularities, leading to the eruption of liquid matters from sublayer at the site of inclusions
T.C. Zhang et al. / Journal of Alloys and Compounds 788 (2019) 231e239
233
Fig. 1. Typical OM image (a) and XRD pattern (b) of the untreated Mg-8Gd-3Y-0.5Zr magnesium alloy sample.
Fig. 2. Typical OM images with different magnifications for the HCPEB treated Mg-8Gd-3Y-0.5Zr magnesium alloy samples. (a) 5 pulses, 100; (b) 5 pulses, 300 (a); (c) 5 pulses, 500; (d) 15 pulses, 100; (e) 15 pulses, 300, (f) 15 pulses, 500.
or second phase particles, which creates the volcano-like morphology. Such a morphology is kept at room temperature due to the rapid cooling. It is worth noting that the surface of 15 pulses treated sample (shown in Fig. 2d) is even flatter and the density of
large craters were reduced when compared with that on the 5 pulses treated surface (shown in Fig. 2a). With increasing the pulse number, more and more b-Mg5(Gd,Y) particles were dissolved in the melted surface layer and results in the reduced nucleation sites
234
T.C. Zhang et al. / Journal of Alloys and Compounds 788 (2019) 231e239
for the crater formation. Besides, numerous small shrinkage pinholes emerged on the modified surface, as denoted in Fig. 2c and f. This is due to the rapid solidification in the melted layer after the HCPEB treatment, for which the liquid state duration is short so that the shrinkage pinholes cannot be filled during solidification. Such phenomenon was also observed on HCPEB treated steels and NiTi alloy [27,28]. Fig. 3 shows the XRD patterns of untreated and treated samples. As discussed above, the untreated sample is mainly composed of a-Mg and b-Mg5(Gd,Y) phases. It can be clearly seen that a trend that the peaks of Mg5(Gd,Y) tend to disappear after the HCPEB treatments. There is no new phase formed after the HCPEB treatment. Meanwhile, as shown in Fig. 3b, diffraction peaks of a-Mg shifted after HCPEB treatments, which reflects the variation of the interplanar spacing for Mg lattice. The relationship between the scattering angle q and the interplanar spacing d can be described based on Bragg's law:
2d sinq ¼ nl
(1)
where n is a positive integer and l is the wavelength of the incident wave. After 5 pulses of HCPEB treatment, the diffraction peaks of Mg shifted to higher angles, reflecting a decreased interplanar spacing. On the contrary, for 15 pulses treated sample, the peaks of Mg shifted to lower angles, indicating the increase in the interplanar spacing. Usually, two major factors may strongly affect the lattice parameters, i.e., residual stresses and variations of solute content. If only considered the modification in the solute content, the dissolution of Gd/Y atoms into a-Mg lattice would increase the lattice parameters as the diameter of Gd/Y atoms(rGd ¼ 0.180 nm,rY ¼ 0.162 nm) is larger than that of the Mg atom(rMg ¼ 0.136 nm). Since the amount of Gd/Y dissolved into the layer increased with the pulses number, the lattice parameter of the a-Mg in the unstressed state should increase with pulses number. The c value of hcp a-Mg increases slightly from 0.52171 nm for the untreated sample to 0.52174 nm for sample treated by 15 pulses but the c value decreases to 0.52119 nm for sample treated by 5 pulses and the a value of the samples shows the same variation trend, as listed in Table 1. This means that the surface layer undergoes a deformation process during the HCPEB treatment and a residual compressive stress forms after the treatment. As is reported in previous works [29], a temperature gradient induced nonstationary thermal stress filed is generated in the surface layers during the HCPEB treatment. This nonstationary thermal stress filed consists mainly of quasistatic compressive stress and thermal stress waves [30]. The thermal stress wave is a typical nonlinear wave with small amplitudes of about 0.1 MPa while the maximum value of quasistatic compressive stress in the near surface layer can reach hundreds of MPa and is sufficiently high to induce severe plastic deformation, both in dislocation and twinning modes in the target metallic material. After 5 pulses treatment, the influence of compressive stress on lattice parameter exceeds that of Gd/Y atoms dissolution, thus the crystal lattice of a-Mg was compressed and resulted in the decrease of interplanar spacing. However, when the pulse numbers increase to 15, with the dissolution of b-Mg5(Gd,Y) precipitates and selective evaporation of Mg [25], the amount of Gd/Y atoms dissolved into the layer increased and the compressive stress would release because the dwelling time of treated layer in liquid state increases. So the influence of Gd/Y atoms dissolution on lattice parameter exceeds that of residual compressive stress. Besides, it is also worth noting that the orientation of grains in the surface layer changes after HCPEB treatments with different numbers. According to XRD patterns, the relative peak intensity of a-Mg (002) increases for the 5 pulses treated sample while the peak intensity of a-Mg (101) increases after 15 pulses treatment when compared to those of untreated sample. It is reported that a fiber texture with (002)//surface can be generated in the Mg alloy after compression [31]. The texture of (101)//surface is a common solidification texture with [101]//thermal gradient direction for hcp metals [23]. These confirm that the compressive stress plays also a major role for the texture state in the surface layer of the 5 pulsed sample while the rapid solidification induced grain growth determines the texture state after 15 pulses treatment. It is also worth noting in Fig. 3b that the half width of the a-Mg peaks increases, indicating that grain refinement occurs on the surface of the sample after the HCPEB treatment.
Table 1 The lattice constants of the Mg-8Gd-3Y-0.5Zr magnesium alloy samples before and after HCPEB treatments.
Fig. 3. XRD patterns (a) and local enlarged patterns (b) of the Mg-8Gd-3Y-0.5Zr magnesium alloy samples before and after the HCPEB treatments.
Samples
a(nm)
c(nm)
Untreated 5 pulses 15 pulses
0.32254 0.32172 0.32291
0.52171 0.52119 0.52174
T.C. Zhang et al. / Journal of Alloys and Compounds 788 (2019) 231e239
235
Fig. 4. Typical SEM images taken on the surface of the Mg-8Gd-3Y-0.5Zr magnesium alloy samples after 5 pulses (a) and15 pulses of HCPEB treatments.
Fig. 4 shows the typical SEM images taken on the two treated samples. It can be clearly seen that b-Mg5(Gd,Y) particles disappeared and grain boundaries became blurred after HCPEB irradiations (as shown in Fig. 4a and b). This is due to the reduction in the scale second phase particles and formation of undulated morphology induced by the surface melting, evaporation and rapidly solidification process [21,32]. It also can be seen that there are some domains in white color having blurred aspect formed around shrinkage pinholes. These domains are the remnant of the melted b-Mg5(Gd,Y) phase and their formation can be explained by the repeated surface melting and rapid solidification of the treated layer that induces liquid mixing and thereby the size of segregates is reduced. Besides, the white domains in the surface layer treated by 15 pulses are smaller than those in the surface layer treated by 5 pulses and the color distribution of the surface layer treated by 15 pulses time is more homogeneous than that of 5 pulses treated one, indicating that the surface chemical composition after 15 pulses treatment is more homogeneous than that after 5 pulses treatment. As is mentioned above, this is due to the increased dwelling time of the melted layer at liquid state when the electron beam pulse number increases. As is seen in Fig. 4b, there are some small white droplets (as arrowed in Fig. 4b) distributed on the treated surface. This is a result of the eruption of subsurface matter such as second phase particles (b-Mg5(Gd,Y)) during the formation of craters. Besides, some parallel deformation marks were observed on the surface, as indicated in Fig. 4a and b. These marks are slip bands formed due to the high thermal stress generated deformations in the surface layer during the HCPEB treatment. Similar phenomenon is also observed in other materials treated by HCPEB [23,24,32]. Fig. 5 shows the SEM images and results of EDS analyses on the Mg-8Gd-3Y-0.5Zr alloy samples after 15 pulses of HCPEB treatment. Some micro-voids and spherical droplets can be observed. EDS analysis taken on the surface showed that the Gd and Y contents of the droplet (marked as A with the white arrow in Fig. 5a, Gd: 5.61 at %, Y: 4.35 at%) were both higher than those of the matrix (marked as B with the white arrow in Fig. 5a, Gd: 2.39 at%, Y: 1.55 at%). This means that the droplet was actually the erupted b-Mg5(Gd,Y) particle. Fig. 6a and b show the typical SEM images of the cross-section of the 5 pulses and 15 pulses treated samples, respectively. It can be clearly seen that a layer having less or even no b-Mg5(Gd,Y) particles is formed on the top surface. With the increase of pulses number, the thickness of this HCPEB modified layer increases from about 10 mm for 5 pulses treated sample to about 12 mm for the 15 pulsed sample. Such a phenomenon is also observed in other metal and alloys, such as AZ91 Mg alloy, Mg-4Sm alloy and 2024 Al alloy [13,21,23], treated by HCPEB and is attributed to the accumulated energy in the surface layers and temperature field induced epitaxial
grain growth with increasing number of pulses [19,21]. Compared to the 15 pulses treated sample (shown in Fig. 6b), there are still some white domains visible inside the HCPEB modified layer of the 5 pulses treated sample (shown in Fig. 6a). This confirms a more homogeneous composition in the surface layer of the 15 pulses treated sample, which is consistent with above analysis (observation in Fig. 4). Fig. 7 gives typical TEM images showing the microstructures of the untreated sample and 5 pulses treated sample. It can be observed that there are some square-like particles in the untreated sample (shown in Fig. 7a) and these particles have been identified as Mg5(Gd,Y) according to the selected area diffraction (SAED) pattern shown inset of Fig. 7a. Fig. 7b shows that many fine equiaxed cells/domains with an average size of 200e300 nm form after 5 pulses treatment. Under higher magnification, it is seen in Fig. 7c that there are lots of fine black particles located at cell/domain boundaries, triple junctions and inside cells and the dimension of these fine black particles is less than 10 nm. These nano-particles were nanosized b-Mg5(Gd,Y) precipitates formed during the HCPEB treatment. Large amount of these nano-particles distributed along boundaries can impede the growth of the a-Mg cells/domains during repeatedly electron beam irradiation induced heating process. The formation of such nanostructured cells/domains with fine precipitates were also observed in the HCPEB treated D2 steels and was attributed to the rapid solidification induced cellular growth mode at the liquid-solid interface [15]. 3.2. Microhardness analysis Fig. 8 shows the cross-sectional microhardness profile along depth for the two HCPEB treated samples. The variation tendencies of microhardness value of the two treated samples are similar and unanimous. That is, the hardness increases firstly from top surface to subsurface, and then reaches a maximum value (108 HV for 5 pulses treated sample and 93 HV for 15 pulses treated sample) at a depth about 70 mm from the top surface, then decreases, and achieves a constant about 80 HV when the depth is more than 400 mm from the top. This type of deep-hardening effect is also observed in the cases of AZ31 Mg alloy [14] and 316 L stainless steel [33] treated by HCPEB and can be attributed to the action of thermal stress and shock stress wave on the sub-surface layers of the treated material [29,33]. When the depth is more than 400 mm, the affection of the HCPEB treatment is fairly weak and the microhardness drops to the value for the substrate. Besides, it is visible that the peak hardness value of 5 pulses treated sample is higher than that of 15 pulses treated sample in the depth from 5 mm to 15 mm. This means that the strengthening effect produced by the quasistatic compressive stress is much stronger
236
T.C. Zhang et al. / Journal of Alloys and Compounds 788 (2019) 231e239
Fig. 5. Surface SEM image (a) of the Mg-8Gd-3Y-0.5Zr magnesium alloy samples after 5 pulses of the HCPEB treatment, and EDS results of positions A(b), B (c) and C (d).
Fig. 6. The Typical cross-section SEM images of the Mg-8Gd-3Y-0.5Zr magnesium alloy samples after the HCPEB treatments with 5 pulses (a) and 15 pulses (b).
after 5 pulses treatment than that after 15 pulses treatment. The repeated heat input after high number of pulses may cause the grain growth at subsurface layer and induce softening instead of hardening in the heat effected layer. 3.3. Potentio-dynamic polarization tests Fig. 9 shows the potentiodynamic polarization curves of the untreated and HCPEB treated samples and the corresponding corrosion data are collected in Table 2. The Ecorr and the Icorr of the
untreated sample are 1.52 V and 2.28 105 A/cm2, respectively. After 5 pulses treatment, the Icorr decreased to 1.58 105 A/cm2 and the Ecorr is reduced to 1.59 V, while the Icorr of 15 pulses treated sample increased to 2.43 105 A/cm2 and the Ecorr also increased to 1.46 V. After HCPEB treatments, b-Mg5(Gd,Y) particles were dissolved into the matrix and the melted layer were rich in Gd and Y solutes. The presence of Gd/Y could help to form a protective oxide films, increasing the stability of the alloy surface [34]. On the contrary, the formation of the defects such as craters and shrinkage pinholes
T.C. Zhang et al. / Journal of Alloys and Compounds 788 (2019) 231e239
237
Fig. 7. Typical TEM images of the untreated sample with SAED pattern (a), 5 pulses HCPEB treated sample under low magnification (b) and high magnification (c).
Fig. 8. Cross-sectional microhardness profiles of the Mg-8Gd-3Y-0.5Zr magnesium alloy samples after HCPEB treatments.
accelerates the pitting corrosion rates. After 15 pulses treatment, the melted surface is richer in Gd and Y solutes content than that for the 5 pulsed sample. This layer is
Fig. 9. Potentiodynamic polarization curves of the Mg-8Gd-3Y-0.5Zr magnesium alloy samples before and after the HCPEB treatments.
thus more stable with the higher Ecorr. Meanwhile, the defects in the melted layer also increases with number of HCPEB pulses, accelerating the corrosion rates and resulting in the increased Icorr.
238
T.C. Zhang et al. / Journal of Alloys and Compounds 788 (2019) 231e239
Table 2 Corrosion data of the Mg-8Gd-3Y-0.5Zr magnesium alloy samples before and after the HCPEB treatments. Sample
Ecorr/vs.SCE
icorr/A$cm2
Untreated 5 pulses 15 pulses
1.52 1.59 1.46
2.28 105 1.58 105 2.43 105
Acknowledgement This work was supported by the National Natural Science Foundations of China (No. 51471109, 51271121); Prof. Zhang would like to thank the support from the “Graduate innovation” Project of Shanghai University of Engineering and Science (16KY0505). References
According to XRD analyses and TEM observations, a surface layer with residual compressive stress and refined microstructure formed after 5 pulses treatment. As mentioned above, although Gd and Y solutes were dissolved into the layer, severe plastic deformation took place in the layer and twin formed during the electron beam irradiation. This led to the decrease of Ecorr for the treated samples. The microstructure of 5 pulsed sample is more compact as compared with untreated and 15 pulsed sample due to the compressive stress and Liu's work has shown that the existence of compressive stress can prevent the protective oxide film from rupture in the corrosive environment [35]. Both of these will retard the destroy of the protective film and the diffusion of the Cl into the bulk, resulting in the decreased corrosion rates.
4. Conclusions In this work, the HCPEB surface treatment has been applied onto the Mg-8Gd-3Y-0.5Zr alloy to investigate the effect of different pulse numbers on microstructure, residual stress state, hardness and corrosion resistance of the alloy. It is demonstrated that the corrosion resistance and hardness of the Mg-8Gd-3Y-0.5Zr alloy can be improved simultaneously when treated by HCPEB under proper parameters. The main results are summarized as follows: 1. After the HCPEB treatment, the surface of the alloy was melted with a selective evaporation of Mg taking place at different magnitudes depending on the number of pulses, and the bMg5(Gd,Y) particles were dissolved into the matrix. Therefore, the melted layers are rich in Gd and Y solutes content. The content of Gd and Y solutes and the thickness of the HCPEB modified layers increases with the pulses number. 2. The different pulse number results in different residual stress states in the treated surface layers. A compressive residual stress forms in the surface of 5 pulsed sample, which causes the lattice shrinkage of the Mg even Gd and Y are dissolved into Mg lattice. Meanwhile, the compressive stress is released due to the repeated melting after 15 pulses and the lattice parameter for Mg of the 15 pulsed sample increases. 3. The microhardness of the treated layers increased compared with the substrate due to the formation of nanostructures on the surface and residual stresses in the treated layers. The microhardness in the surface layers of the 5 pulses treated sample is higher than that of the 15 pulses treated sample. 4. The potentio-dynamic polarization measurements taken in the 3.5 wt% NaCl water solution shows that the Icorr of the 5 pulsed sample is the lowest. This is mainly attributed to the dissolution of Gd and Y into the melted surface layer, the selective evaporation of Mg and the residual compressive stress state in the treated layer, all of which help the formation of a protective oxide film on the HCPEB treated surface, leading to the improved the corrosion resistance. The results gathered in this study and the mechanisms proposed for the surface modifications may guide the applications of HCPEB treatments on Mg rare earth alloys.
[1] W. Michels, Magnesium alloys and their applications, Mater. Technol. 13 (2016) 121e122. [2] S. Shrestha, Magnesium and surface engineering, Surf. Eng. 26 (2013) 313e316. , V. O sek, Creep behaviour of the [3] F. Hnilica, V. Janík, B. Smola, I. Stulíkova cena creep resistant MgY3Nd2Zn1Mn1 alloy, Mater. Sci. Eng. A 489 (2008) 93e98. [4] J. Wang, J. Meng, D. Zhang, D. Tang, Effect of Y for enhanced age hardening response and mechanical properties of MgeGdeYeZr alloys, Mater. Sci. Eng. A 456 (2007) 78e84. [5] X.B. Liu, R.S. Chen, E.H. Han, Effects of ageing treatment on microstructures and properties of MgeGdeYeZr alloys with and without Zn additions, J. Alloy. Comp. 465 (2008) 232e238. [6] S.M. He, X.Q. Zeng, L.M. Peng, X. Gao, J.F. Nie, W.J. Ding, Microstructure and strengthening mechanism of high strength Mge10Gde2Ye0.5Zr alloy, J. Alloy. Comp. 427 (2007) 316e323. [7] H.R. Jafari Nodooshan, G. Wu, W. Liu, G. Wei, Y. Li, S. Zhang, Effect of Gd content on high temperature mechanical properties of MgeGdeYeZr alloy, Mater. Sci. Eng. A 651 (2016) 840e847. [8] X.J. Wang, D.K. Xu, R.Z. Wu, X.B. Chen, Q.M. Peng, L. Jin, Y.C. Xin, Z.Q. Zhang, Y. Liu, X.H. Chen, G. Chen, K.K. Deng, H.Y. Wang, What is going on in magnesium alloys? J. Mater. Sci. Technol. 34 (2018) 245e247. [9] F. Pan, M. Yang, X. Chen, A review on casting magnesium alloys: modification of commercial alloys and development of new alloys, J. Mater. Sci. Technol. 32 (2016) 1211e1221. [10] K. Fritzsch, A. Buchwalder, R. Zenker, A. Jung, Influence of electron beam liquid phase surface treatments on the corrosion resistance of AZ91D using different additive deposition techniques, Corros. Sci. 88 (2014) 109e117. [11] D.I. Proskurovsky, V.P. Rotshtein, G.E. Ozur, et al., Pulsed electron-beam technology for surface modification of metallic materials, J. Vac. Sci. Technol. Vacuum Surfaces Films 16 (4) (1998) 2480e2488. [12] D.I. Proskurovsky, V.P. Rotshtein, G.E. Ozur, et al., Physical foundations for surface treatment of materials with low energy, high current electron beams, Surf. Coating. Technol. 125 (1e3) (2000) 49e56. [13] M.C. Li, S.Z. Hao, H. Wen, R.F. Huang, Surface composite nanostructures of AZ91 magnesium alloy induced by high current pulsed electron beam treatment, Appl. Surf. Sci. 303 (2014) 350e353. [14] B. Gao, S. Hao, J. Zou, T. Grosdidier, L. Jiang, J. Zhou, C. Dong, High current pulsed electron beam treatment of AZ31 Mg alloy, J. Vac. Sci. Technol. Vacuum, Surfaces, and Films 23 (2005) 1548e1553. [15] J.X. Zou, T. Grosdidier, K.M. Zhang, et al., Mechanisms of nanostructure and metastable phase formations in the surface melted layers of a HCPEB-treated D2 steel, Acta Mater. 54 (20) (2006) 5409e5419. [16] T. Grosdidier, J.X. Zou, N. Stein, et al., Texture modification, grain refinement and improved hardness/corrosion balance of a FeAl alloy by pulsed electron beam surface treatment in the "heating mode, Scripta Mater. 58 (12) (2008) 1058e1061. [17] J. Kim, H.W. Park, Influence of a large pulsed electron beam (LPEB) on the corrosion resistance of Ti6Al7Nb alloys, Corros. Sci. 90 (6) (2015) 153e160. [18] J.W. Murray, P.K. Kinnell, A.H. Cannon, et al., Surface finishing of intricate metal mould structures by large-area electron beam irradiation, Precis. Eng. 37 (2) (2013) 443e450. [19] J.X. Zou, T. Grosdidier, K.M. Zhang, C. Dong, Cross-sectional analysis of the graded microstructure in an AISI D2-steel treated with low energy highcurrent pulsed electron beam, Appl. Surf. Sci. 255 (2009) 4758e4764. [20] A.B. Markov, V.P. Rotshtein, Calculation and experimental determination of dimensions of hardening and tempering zones in quenched U7A steel irradiated with a pulsed electron beam, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 132 (1) (1997) 79e86. [21] J.X. Zou, K.M. Zhang, T. Grosdidier, C. Dong, Analysis of the evaporation and re-condensation processes induced by pulsed beam treatments, Int. J. Heat Mass Transf. 64 (2013) 1172e1182. [22] K.M. Zhang, J.X. Zou, T. Grosdidier, C. Dong, D. Yang, Improved pitting corrosion resistance of AISI 316L stainless steel treated by high current pulsed electron beam, Surf. Coating. Technol. 201 (2006) 1393e1400. [23] Y.R. Liu, K.M. Zhang, J.X. Zou, D.K. Liu, T.C. Zhang, Effect of the high current pulsed electron beam treatment on the surface microstructure and corrosion resistance of a Mg-4Sm alloy, J. Alloy. Comp. 741 (2018) 65e75. [24] B. Gao, Y. Hao, G.F. Tu, S.W. Li, F.X. Yu, L. Zuo, L. Hu, Surface modification of Mg67eZn30eY3 quasicrystal alloy by high current pulsed electron beam, Surf. Coating. Technol. 229 (2013) 42e45. [25] W.J. Lee, J. Kim, H.W. Park, Improved corrosion resistance of Mg alloy AZ31B induced by selective evaporation of Mg using large pulsed electron beam irradiation, J. Mater. Sci. Technol. (2018). https://doi.org/10.1016/j.jmst.2018.
T.C. Zhang et al. / Journal of Alloys and Compounds 788 (2019) 231e239 12.004. [26] Z.Q. Wang, B. Zhang, D.J. Li, R. Fritzsch, X.Q. Zeng, H.J. Roven, W.J. Ding, Effect of heat treatment on microstructures and mechanical properties of high vacuum die casting Mge8Gde3Ye0.4Zr magnesium alloy, Trans. Nonferrous Metals Soc. China 24 (2014) 3762e3768. [27] T. Grosdidier, J.X. Zou, B. Bolle, S.Z. Hao, C. Dong, Grain refinement, hardening and metastable phase formation by high current pulsed electron beam (HCPEB) treatment under heating and melting modes, J. Alloy. Comp. 504 (2010) S508eS511. [28] K.M. Zhang, J.X. Zou, T. Grosdidier, et al., Surface modification of Ni (50.6at.%) Ti by high current pulsed electron beam treatment, J. Alloy. Comp. 434e435 (2007) 682e685. [29] Y. Qin, C. Dong, Z. Song, S. Hao, X. Me, J. Li, X. Wang, J. Zou, T. Grosdidier, Deep Modification of materials by thermal stress wave generated by irradiation of high-current pulsed electron beams, J. Vac. Sci. Technol.: Vacuum, Surfaces, and Films 27 (2009) 430e435. [30] Y. Qin, J.X. Zou, C. Dong, et al., Temperature-stress fields and related phenomena induced by a high current pulsed electron beam, Nucl. Instrum.
[31]
[32]
[33]
[34]
[35]
239
Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 225 (4) (2004) 544e554. M. Jahedi, B.A. McWilliams, P. Moy, M. Knezevic, Deformation twinning in rolled WE43-T5 rare earth magnesium alloy: influence on strain hardening and texture evolution, Acta Mater. 131 (2017) 221e232. P. Yan, T. Grosdidier, X. Zhang, J.X. Zou, Formation of large grains by epitaxial and abnormal growth at the surface of pulsed electron beam treated metallic samples, Mater. Des. 159 (2018) 1e10. J.X. Zou, K.M. Zhang, S.Z. Hao, et al., Mechanisms of hardening, wear and corrosion improvement of 316 L stainless steel by low energy high current pulsed electron beam surface treatment, Thin Solid Films 519 (4) (2010) 1404e1415. A.E. Coy, F. Viejo, P. Skeldon, G.E. Thompson, Susceptibility of rare-earthmagnesium alloys to micro-galvanic corrosion, Corros. Sci. 52 (12) (2010) 3896e3906. X. Liu, G.S. Frankel, Effects of compressive stress on localized corrosion in AA2024-T3, Corros. Sci. 48 (2006) 3309e3329.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Surface microstructure and property modifications in a Mg-8Gd-3Y-0.5Zr magnesium alloy treated by high current pulsed electron beam T.C. Zhang a, K.M. Zhang a, *, J.X. Zou b, P. Yan b, H.Y. Yang b, L.X. Song a, X. Zhang a a
School of Materials Engineering, Shanghai University of Engineering Science, Shanghai 201620, China National Engineering Research Center of Light Alloys Net Forming & Shanghai Engineering Research Center of Mg Materials and Applications, Shanghai Jiao Tong University, Shanghai 200240, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 October 2018 Received in revised form 2 February 2019 Accepted 10 February 2019 Available online 12 February 2019
In the present study, the effect of high current pulsed electron beam (HCPEB) surface treatments on the microstructure, composition, stress states and properties of a Mg-8Gd-3Y-0.5Zr alloy has been investigated. The surface layers showed different features in microstructure and residual stress states under different number of pulses. Such a variation is essentially due to the different level of the dissolution of bMg5(Gd,Y) particles in remelted layers and the impact from thermal stresses generated by HCPEB treatments. The results have shown that, after 5 pulses of HCPEB treatment, maximum hardness in the surface layer of the sample was increased by 35% and the corrosion rate in 3.5 wt% NaCl water solution was reduced by 30% as compared with those of the untreated sample. The formation of nano structured cells of Mg-RE solid solution and b-Mg5(Gd,Y) nano precipitates, together with the residual compressive stress in the surface layer of the 5 pulsed sample account for the increased microhardness and improved corrosion resistance of the HCPEB treated Mg-8Gd-3Y-0.5Zr Mg alloy. © 2019 Elsevier B.V. All rights reserved.
Keywords: High current pulsed electron beam (HCPEB) Mg-RE alloys Surface treatment Corrosion Hardness
1. Introduction Mg based alloys are promising light weight structural materials due to their distinct advantages, such as high specific strength, high stiffness, good machinability and low density over traditional structural materials (steels, aluminum alloys, etc.) [1]. These properties render the opportunities for Mg based alloys to be used in automobile, aeronautic and electronic industries for weight saving purpose. In particular, when rare earth elements (REs) are introduced into Mg or its alloys, strength, corrosion resistance and high temperature properties can be significantly improved [2e5]. Recent years, Mg-Gd-Y-Zr (GWK) Mg based alloys are attracting more and more attentions in the fields of scientific research and industrial applications owing to their excellent mechanical properties at both room and elevated temperatures as well as good creep resistance. It has been established that the addition of Gd and Y can strengthen Mg due to the grain refinement and the formation of Mg5(Gd,Y) precipitates [6,7]. Besides, Y is also found to improve
* Corresponding author. E-mail address: [email protected] (K.M. Zhang). https://doi.org/10.1016/j.jallcom.2019.02.130 0925-8388/© 2019 Elsevier B.V. All rights reserved.
the corrosion resistance of Mg alloys by reducing the corrosion rate and increasing corrosion potentials [1,2]. Nevertheless, the corrosion resistance of GWK Mg alloys cannot yet meet the requirements for industrial applications. Usually, it is hard to achieve both improvement on strength and on corrosion resistance as the introduction of precipitates into Mg alloys has two sides: The precipitates can pin grain boundaries and dislocations, thus strengthen Mg alloys; On the other hand, they also provide the galvanic corrosion sites in corrosive environments due to their different potentials from Mg matrix. Considering that corrosion is closely related to the surface states of a material, surface modifications would be an efficient way to improve the corrosion resistance of Mg based alloys without changing the bulk structures and compositions [2]. Some surface modification methods, such as laser cladding, physical vapor deposition, chemical vapor deposition, electroless plating, etc. have been applied on to Mg alloys with the aim to improve their surface properties [8e10]. As compared to these surface treatment methods, high current pulsed electron beam (HCPEB) technology, which was initially invented in Tomsk in Russia [11,12], has been developed in recent years and adopted for surface modifications of metallic materials [13e19] with some
232
T.C. Zhang et al. / Journal of Alloys and Compounds 788 (2019) 231e239
advantages, such as low energy consuming, less pollution and high efficiency. With the extremely intensive energy input into the surface layer of the target material within a short pulse of microseconds, the processes include rapid heating (~108 Ks1) [20], melting and even evaporation [21], together with thermal stress (~GPa) and rapid self-quenching (~107 Ks1) [20] occurred drastically, giving rise to the non-equilibrium surface structure modifications accompanied by the improved hardness, corrosion resistance and wear resistance. In addition, the compositions in the surface layer of materials can be changed through HCPEB alloying while the substrate materials remain unchanged [19]. Thus the surface wear or corrosion resistances can be improved while the mechanical properties can be maintained. Zhang et al. [22]studied the surface modifications of stainless steels and observed the improved corrosion resistance after HCPEB treatments due to the selective surface purification effect and composition homogenizations. Liu et al. [23]have investigated the surface modification of the Mg-4Sm alloy by HCPEB treatments and found the formation of nanocrystalline Mg41Sm5 in the modified surface layers. As a result, corrosion resistance of the Mg-4Sm alloy is significantly improved with the increasing number of pulse processing. Gao et al. [24]have applied the HCPEB treatment on the surface of Mg-Al-Zn and MgZn-Y alloys, and the results showed that the composition in the surface layer was homogenized and hardened after treatments. Lee et al. [25] have studied the effect of Large Pulsed Electron Beam (LPEB) irradiation treatments on the magnesium alloy AZ31B and found the formation of an Al-enriched re-melted layer owing to a selective evaporation effect, with nano-grained structure containing Mg3.1Al0.9 metastable phase. The results showed that the formation of such a layer after the LPEB irradiation improved the corrosion resistance of the AZ31B Mg alloy in the 3.5% NaCl water solution. In the present work, the effect of HCPEB treatments on the surface microstructure, hardness and corrosion resistance of a Mg8Gd-3Y-0.5Zr alloy was investigated. It is found that, with proper pulsed electron beam treatments, the corrosion resistance and surface hardness of the Mg-8Gd-3Y-0.5Zr alloy can be both improved, which are advantageous for its industrial applications.
2. Experimental procedures 2.1. Materials The chemical composition (in wt.%) of the Mg-8Gd-3Y-0.5Zr magnesium alloy used in the present research is as follows: ~8 wt % of Gd, ~3 wt% of Y and ~0.5 wt% of Zr (Mg in balance). The ingot sample was heat treated by holding it at 773 K for 6 h (T4 state) [26]. The as received bulk material was cut into small cylinders with the dimension of Ф 15 mm 2 mm.
2.2. HCPEB treatments Prior to the HCPEB treatment, the sample surfaces were mechanically polished from 600-grit sand-papers down to 0.5 mm diamond paste, and were ultrasonically cleaned in acetone. The electron beam system used in this work is a “Hope-1” type HCPEB source [19,22,27], which can produce an electron beam with the following parameters: accelerating voltage of 10e30 kV, peak current of ~10 kA, pulse duration of 0.5e5 s, beam diameter of Ф 60 mm, and energy density of 1e30 J/cm2. In order to avoid the formation of cracks on the treated surface, the parameters set for HCPEB irradiations in the present work were: accelerating voltage 18 kV, energy density 4 J/cm2, pulse duration 1.5 s, number of pulses 5 and 15 and 10s dwelling time between each pulse.
2.3. Microstructure characterizations The phase components of untreated and HCPEB treated samples were examined using X-ray diffraction (XRD) of a Rigaku D/max2000 X-ray diffractometer equipped with a Cu Ka radiation source. The scanning speed for XRD analyses was set at 5 /min in a 2q range from 10 to 90 . The surface and cross-section microstructures were observed using a VHX-600 optical microscope (OM) and a S-3400 N scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS). Before microstructure observations, crosssection of samples was polished and then etched in a 4 vol% nitric acid alcohol solution. To obtain more details about microstructure modifications in the treated surface layers, TEM (FEI Talos-F200X type transmission electron microscope) observation was carried out on the untreated and 5 pulses treated samples. The disc samples for TEM observation were prepared by mechanical grounding from untreated side to 30e50 mm in thickness, followed by dimpling and ion beam (Arþ) thinning from the untreated side to obtain proper thin area for TEM observations. 2.4. Hardness and electrochemical tests Vickers hardness of the samples were measured on the crosssection of the treated samples under 50 g load using a HXD-1000 TMSC/LCD type hardness tester. The loading time is 15s for each measurement. The microhardness at each depth is obtained by doing 3 parallel measurements to get an average value. The electrochemical measurements of untreated and HCPEB treated samples were carried out by a traditional standard threeelectrode cell connected with a PARSTAT 4000 type electrochemical workstation. Platinum and saturated calomel electrode were used as the counter electrode and reference electrode, respectively, and 3.5 wt% NaCl water solution was selected as the electrolyte. All samples were soaked in the 3.5 wt% NaCl water solution for 0.5 h to get a stable open circuit potential. The area exposed to the NaCl water solution was 0.28 cm2. During the measurements, the temperature of the solution was maintained at room temperature. Potentiodynamic polarization curves were measured with a scanning rate of 2 mVS1 ranging from -2 V (vs.SCE) to 1 V. For better reproducibility, all above electrochemical measurements were repeated three times. The polarization curves were used to evaluate the corrosion current density (Icorr) and the corrosion potential (Ecorr) by Tafel extrapolation method. 3. Results and discussions 3.1. Microstructures Fig. 1 shows the OM topography and XRD pattern of the untreated sample. As XRD pattern shown in Fig. 1b, the main phases of the untreated sample are a-Mg and compound b-Mg5(Gd,Y), which is consistent with previously published works [6,26]. As shown in Fig. 1a, the average size of the a-Mg grains are about 75 mm, and the b-Mg5(Gd,Y) particles (arrowed in Fig. 1a) are distributed fairly homogeneous without segregating along grain boundaries. Fig. 2 shows OM images taken at different magnifications of the two treated samples. After 5 pulses and 15 pulses HCPEB treatment, the morphology of the treated surfaces becomes undulated and lots of craters were observed, as shown in Fig. 2a and d. The mechanisms of the crater formation on metal surfaces irradiated by pulsed electron beam have been reported in many previous works [22]. Melting starts at sublayer and micro-irregularities, leading to the eruption of liquid matters from sublayer at the site of inclusions
T.C. Zhang et al. / Journal of Alloys and Compounds 788 (2019) 231e239
233
Fig. 1. Typical OM image (a) and XRD pattern (b) of the untreated Mg-8Gd-3Y-0.5Zr magnesium alloy sample.
Fig. 2. Typical OM images with different magnifications for the HCPEB treated Mg-8Gd-3Y-0.5Zr magnesium alloy samples. (a) 5 pulses, 100; (b) 5 pulses, 300 (a); (c) 5 pulses, 500; (d) 15 pulses, 100; (e) 15 pulses, 300, (f) 15 pulses, 500.
or second phase particles, which creates the volcano-like morphology. Such a morphology is kept at room temperature due to the rapid cooling. It is worth noting that the surface of 15 pulses treated sample (shown in Fig. 2d) is even flatter and the density of
large craters were reduced when compared with that on the 5 pulses treated surface (shown in Fig. 2a). With increasing the pulse number, more and more b-Mg5(Gd,Y) particles were dissolved in the melted surface layer and results in the reduced nucleation sites
234
T.C. Zhang et al. / Journal of Alloys and Compounds 788 (2019) 231e239
for the crater formation. Besides, numerous small shrinkage pinholes emerged on the modified surface, as denoted in Fig. 2c and f. This is due to the rapid solidification in the melted layer after the HCPEB treatment, for which the liquid state duration is short so that the shrinkage pinholes cannot be filled during solidification. Such phenomenon was also observed on HCPEB treated steels and NiTi alloy [27,28]. Fig. 3 shows the XRD patterns of untreated and treated samples. As discussed above, the untreated sample is mainly composed of a-Mg and b-Mg5(Gd,Y) phases. It can be clearly seen that a trend that the peaks of Mg5(Gd,Y) tend to disappear after the HCPEB treatments. There is no new phase formed after the HCPEB treatment. Meanwhile, as shown in Fig. 3b, diffraction peaks of a-Mg shifted after HCPEB treatments, which reflects the variation of the interplanar spacing for Mg lattice. The relationship between the scattering angle q and the interplanar spacing d can be described based on Bragg's law:
2d sinq ¼ nl
(1)
where n is a positive integer and l is the wavelength of the incident wave. After 5 pulses of HCPEB treatment, the diffraction peaks of Mg shifted to higher angles, reflecting a decreased interplanar spacing. On the contrary, for 15 pulses treated sample, the peaks of Mg shifted to lower angles, indicating the increase in the interplanar spacing. Usually, two major factors may strongly affect the lattice parameters, i.e., residual stresses and variations of solute content. If only considered the modification in the solute content, the dissolution of Gd/Y atoms into a-Mg lattice would increase the lattice parameters as the diameter of Gd/Y atoms(rGd ¼ 0.180 nm,rY ¼ 0.162 nm) is larger than that of the Mg atom(rMg ¼ 0.136 nm). Since the amount of Gd/Y dissolved into the layer increased with the pulses number, the lattice parameter of the a-Mg in the unstressed state should increase with pulses number. The c value of hcp a-Mg increases slightly from 0.52171 nm for the untreated sample to 0.52174 nm for sample treated by 15 pulses but the c value decreases to 0.52119 nm for sample treated by 5 pulses and the a value of the samples shows the same variation trend, as listed in Table 1. This means that the surface layer undergoes a deformation process during the HCPEB treatment and a residual compressive stress forms after the treatment. As is reported in previous works [29], a temperature gradient induced nonstationary thermal stress filed is generated in the surface layers during the HCPEB treatment. This nonstationary thermal stress filed consists mainly of quasistatic compressive stress and thermal stress waves [30]. The thermal stress wave is a typical nonlinear wave with small amplitudes of about 0.1 MPa while the maximum value of quasistatic compressive stress in the near surface layer can reach hundreds of MPa and is sufficiently high to induce severe plastic deformation, both in dislocation and twinning modes in the target metallic material. After 5 pulses treatment, the influence of compressive stress on lattice parameter exceeds that of Gd/Y atoms dissolution, thus the crystal lattice of a-Mg was compressed and resulted in the decrease of interplanar spacing. However, when the pulse numbers increase to 15, with the dissolution of b-Mg5(Gd,Y) precipitates and selective evaporation of Mg [25], the amount of Gd/Y atoms dissolved into the layer increased and the compressive stress would release because the dwelling time of treated layer in liquid state increases. So the influence of Gd/Y atoms dissolution on lattice parameter exceeds that of residual compressive stress. Besides, it is also worth noting that the orientation of grains in the surface layer changes after HCPEB treatments with different numbers. According to XRD patterns, the relative peak intensity of a-Mg (002) increases for the 5 pulses treated sample while the peak intensity of a-Mg (101) increases after 15 pulses treatment when compared to those of untreated sample. It is reported that a fiber texture with (002)//surface can be generated in the Mg alloy after compression [31]. The texture of (101)//surface is a common solidification texture with [101]//thermal gradient direction for hcp metals [23]. These confirm that the compressive stress plays also a major role for the texture state in the surface layer of the 5 pulsed sample while the rapid solidification induced grain growth determines the texture state after 15 pulses treatment. It is also worth noting in Fig. 3b that the half width of the a-Mg peaks increases, indicating that grain refinement occurs on the surface of the sample after the HCPEB treatment.
Table 1 The lattice constants of the Mg-8Gd-3Y-0.5Zr magnesium alloy samples before and after HCPEB treatments.
Fig. 3. XRD patterns (a) and local enlarged patterns (b) of the Mg-8Gd-3Y-0.5Zr magnesium alloy samples before and after the HCPEB treatments.
Samples
a(nm)
c(nm)
Untreated 5 pulses 15 pulses
0.32254 0.32172 0.32291
0.52171 0.52119 0.52174
T.C. Zhang et al. / Journal of Alloys and Compounds 788 (2019) 231e239
235
Fig. 4. Typical SEM images taken on the surface of the Mg-8Gd-3Y-0.5Zr magnesium alloy samples after 5 pulses (a) and15 pulses of HCPEB treatments.
Fig. 4 shows the typical SEM images taken on the two treated samples. It can be clearly seen that b-Mg5(Gd,Y) particles disappeared and grain boundaries became blurred after HCPEB irradiations (as shown in Fig. 4a and b). This is due to the reduction in the scale second phase particles and formation of undulated morphology induced by the surface melting, evaporation and rapidly solidification process [21,32]. It also can be seen that there are some domains in white color having blurred aspect formed around shrinkage pinholes. These domains are the remnant of the melted b-Mg5(Gd,Y) phase and their formation can be explained by the repeated surface melting and rapid solidification of the treated layer that induces liquid mixing and thereby the size of segregates is reduced. Besides, the white domains in the surface layer treated by 15 pulses are smaller than those in the surface layer treated by 5 pulses and the color distribution of the surface layer treated by 15 pulses time is more homogeneous than that of 5 pulses treated one, indicating that the surface chemical composition after 15 pulses treatment is more homogeneous than that after 5 pulses treatment. As is mentioned above, this is due to the increased dwelling time of the melted layer at liquid state when the electron beam pulse number increases. As is seen in Fig. 4b, there are some small white droplets (as arrowed in Fig. 4b) distributed on the treated surface. This is a result of the eruption of subsurface matter such as second phase particles (b-Mg5(Gd,Y)) during the formation of craters. Besides, some parallel deformation marks were observed on the surface, as indicated in Fig. 4a and b. These marks are slip bands formed due to the high thermal stress generated deformations in the surface layer during the HCPEB treatment. Similar phenomenon is also observed in other materials treated by HCPEB [23,24,32]. Fig. 5 shows the SEM images and results of EDS analyses on the Mg-8Gd-3Y-0.5Zr alloy samples after 15 pulses of HCPEB treatment. Some micro-voids and spherical droplets can be observed. EDS analysis taken on the surface showed that the Gd and Y contents of the droplet (marked as A with the white arrow in Fig. 5a, Gd: 5.61 at %, Y: 4.35 at%) were both higher than those of the matrix (marked as B with the white arrow in Fig. 5a, Gd: 2.39 at%, Y: 1.55 at%). This means that the droplet was actually the erupted b-Mg5(Gd,Y) particle. Fig. 6a and b show the typical SEM images of the cross-section of the 5 pulses and 15 pulses treated samples, respectively. It can be clearly seen that a layer having less or even no b-Mg5(Gd,Y) particles is formed on the top surface. With the increase of pulses number, the thickness of this HCPEB modified layer increases from about 10 mm for 5 pulses treated sample to about 12 mm for the 15 pulsed sample. Such a phenomenon is also observed in other metal and alloys, such as AZ91 Mg alloy, Mg-4Sm alloy and 2024 Al alloy [13,21,23], treated by HCPEB and is attributed to the accumulated energy in the surface layers and temperature field induced epitaxial
grain growth with increasing number of pulses [19,21]. Compared to the 15 pulses treated sample (shown in Fig. 6b), there are still some white domains visible inside the HCPEB modified layer of the 5 pulses treated sample (shown in Fig. 6a). This confirms a more homogeneous composition in the surface layer of the 15 pulses treated sample, which is consistent with above analysis (observation in Fig. 4). Fig. 7 gives typical TEM images showing the microstructures of the untreated sample and 5 pulses treated sample. It can be observed that there are some square-like particles in the untreated sample (shown in Fig. 7a) and these particles have been identified as Mg5(Gd,Y) according to the selected area diffraction (SAED) pattern shown inset of Fig. 7a. Fig. 7b shows that many fine equiaxed cells/domains with an average size of 200e300 nm form after 5 pulses treatment. Under higher magnification, it is seen in Fig. 7c that there are lots of fine black particles located at cell/domain boundaries, triple junctions and inside cells and the dimension of these fine black particles is less than 10 nm. These nano-particles were nanosized b-Mg5(Gd,Y) precipitates formed during the HCPEB treatment. Large amount of these nano-particles distributed along boundaries can impede the growth of the a-Mg cells/domains during repeatedly electron beam irradiation induced heating process. The formation of such nanostructured cells/domains with fine precipitates were also observed in the HCPEB treated D2 steels and was attributed to the rapid solidification induced cellular growth mode at the liquid-solid interface [15]. 3.2. Microhardness analysis Fig. 8 shows the cross-sectional microhardness profile along depth for the two HCPEB treated samples. The variation tendencies of microhardness value of the two treated samples are similar and unanimous. That is, the hardness increases firstly from top surface to subsurface, and then reaches a maximum value (108 HV for 5 pulses treated sample and 93 HV for 15 pulses treated sample) at a depth about 70 mm from the top surface, then decreases, and achieves a constant about 80 HV when the depth is more than 400 mm from the top. This type of deep-hardening effect is also observed in the cases of AZ31 Mg alloy [14] and 316 L stainless steel [33] treated by HCPEB and can be attributed to the action of thermal stress and shock stress wave on the sub-surface layers of the treated material [29,33]. When the depth is more than 400 mm, the affection of the HCPEB treatment is fairly weak and the microhardness drops to the value for the substrate. Besides, it is visible that the peak hardness value of 5 pulses treated sample is higher than that of 15 pulses treated sample in the depth from 5 mm to 15 mm. This means that the strengthening effect produced by the quasistatic compressive stress is much stronger
236
T.C. Zhang et al. / Journal of Alloys and Compounds 788 (2019) 231e239
Fig. 5. Surface SEM image (a) of the Mg-8Gd-3Y-0.5Zr magnesium alloy samples after 5 pulses of the HCPEB treatment, and EDS results of positions A(b), B (c) and C (d).
Fig. 6. The Typical cross-section SEM images of the Mg-8Gd-3Y-0.5Zr magnesium alloy samples after the HCPEB treatments with 5 pulses (a) and 15 pulses (b).
after 5 pulses treatment than that after 15 pulses treatment. The repeated heat input after high number of pulses may cause the grain growth at subsurface layer and induce softening instead of hardening in the heat effected layer. 3.3. Potentio-dynamic polarization tests Fig. 9 shows the potentiodynamic polarization curves of the untreated and HCPEB treated samples and the corresponding corrosion data are collected in Table 2. The Ecorr and the Icorr of the
untreated sample are 1.52 V and 2.28 105 A/cm2, respectively. After 5 pulses treatment, the Icorr decreased to 1.58 105 A/cm2 and the Ecorr is reduced to 1.59 V, while the Icorr of 15 pulses treated sample increased to 2.43 105 A/cm2 and the Ecorr also increased to 1.46 V. After HCPEB treatments, b-Mg5(Gd,Y) particles were dissolved into the matrix and the melted layer were rich in Gd and Y solutes. The presence of Gd/Y could help to form a protective oxide films, increasing the stability of the alloy surface [34]. On the contrary, the formation of the defects such as craters and shrinkage pinholes
T.C. Zhang et al. / Journal of Alloys and Compounds 788 (2019) 231e239
237
Fig. 7. Typical TEM images of the untreated sample with SAED pattern (a), 5 pulses HCPEB treated sample under low magnification (b) and high magnification (c).
Fig. 8. Cross-sectional microhardness profiles of the Mg-8Gd-3Y-0.5Zr magnesium alloy samples after HCPEB treatments.
accelerates the pitting corrosion rates. After 15 pulses treatment, the melted surface is richer in Gd and Y solutes content than that for the 5 pulsed sample. This layer is
Fig. 9. Potentiodynamic polarization curves of the Mg-8Gd-3Y-0.5Zr magnesium alloy samples before and after the HCPEB treatments.
thus more stable with the higher Ecorr. Meanwhile, the defects in the melted layer also increases with number of HCPEB pulses, accelerating the corrosion rates and resulting in the increased Icorr.
238
T.C. Zhang et al. / Journal of Alloys and Compounds 788 (2019) 231e239
Table 2 Corrosion data of the Mg-8Gd-3Y-0.5Zr magnesium alloy samples before and after the HCPEB treatments. Sample
Ecorr/vs.SCE
icorr/A$cm2
Untreated 5 pulses 15 pulses
1.52 1.59 1.46
2.28 105 1.58 105 2.43 105
Acknowledgement This work was supported by the National Natural Science Foundations of China (No. 51471109, 51271121); Prof. Zhang would like to thank the support from the “Graduate innovation” Project of Shanghai University of Engineering and Science (16KY0505). References
According to XRD analyses and TEM observations, a surface layer with residual compressive stress and refined microstructure formed after 5 pulses treatment. As mentioned above, although Gd and Y solutes were dissolved into the layer, severe plastic deformation took place in the layer and twin formed during the electron beam irradiation. This led to the decrease of Ecorr for the treated samples. The microstructure of 5 pulsed sample is more compact as compared with untreated and 15 pulsed sample due to the compressive stress and Liu's work has shown that the existence of compressive stress can prevent the protective oxide film from rupture in the corrosive environment [35]. Both of these will retard the destroy of the protective film and the diffusion of the Cl into the bulk, resulting in the decreased corrosion rates.
4. Conclusions In this work, the HCPEB surface treatment has been applied onto the Mg-8Gd-3Y-0.5Zr alloy to investigate the effect of different pulse numbers on microstructure, residual stress state, hardness and corrosion resistance of the alloy. It is demonstrated that the corrosion resistance and hardness of the Mg-8Gd-3Y-0.5Zr alloy can be improved simultaneously when treated by HCPEB under proper parameters. The main results are summarized as follows: 1. After the HCPEB treatment, the surface of the alloy was melted with a selective evaporation of Mg taking place at different magnitudes depending on the number of pulses, and the bMg5(Gd,Y) particles were dissolved into the matrix. Therefore, the melted layers are rich in Gd and Y solutes content. The content of Gd and Y solutes and the thickness of the HCPEB modified layers increases with the pulses number. 2. The different pulse number results in different residual stress states in the treated surface layers. A compressive residual stress forms in the surface of 5 pulsed sample, which causes the lattice shrinkage of the Mg even Gd and Y are dissolved into Mg lattice. Meanwhile, the compressive stress is released due to the repeated melting after 15 pulses and the lattice parameter for Mg of the 15 pulsed sample increases. 3. The microhardness of the treated layers increased compared with the substrate due to the formation of nanostructures on the surface and residual stresses in the treated layers. The microhardness in the surface layers of the 5 pulses treated sample is higher than that of the 15 pulses treated sample. 4. The potentio-dynamic polarization measurements taken in the 3.5 wt% NaCl water solution shows that the Icorr of the 5 pulsed sample is the lowest. This is mainly attributed to the dissolution of Gd and Y into the melted surface layer, the selective evaporation of Mg and the residual compressive stress state in the treated layer, all of which help the formation of a protective oxide film on the HCPEB treated surface, leading to the improved the corrosion resistance. The results gathered in this study and the mechanisms proposed for the surface modifications may guide the applications of HCPEB treatments on Mg rare earth alloys.
[1] W. Michels, Magnesium alloys and their applications, Mater. Technol. 13 (2016) 121e122. [2] S. Shrestha, Magnesium and surface engineering, Surf. Eng. 26 (2013) 313e316. , V. O sek, Creep behaviour of the [3] F. Hnilica, V. Janík, B. Smola, I. Stulíkova cena creep resistant MgY3Nd2Zn1Mn1 alloy, Mater. Sci. Eng. A 489 (2008) 93e98. [4] J. Wang, J. Meng, D. Zhang, D. Tang, Effect of Y for enhanced age hardening response and mechanical properties of MgeGdeYeZr alloys, Mater. Sci. Eng. A 456 (2007) 78e84. [5] X.B. Liu, R.S. Chen, E.H. Han, Effects of ageing treatment on microstructures and properties of MgeGdeYeZr alloys with and without Zn additions, J. Alloy. Comp. 465 (2008) 232e238. [6] S.M. He, X.Q. Zeng, L.M. Peng, X. Gao, J.F. Nie, W.J. Ding, Microstructure and strengthening mechanism of high strength Mge10Gde2Ye0.5Zr alloy, J. Alloy. Comp. 427 (2007) 316e323. [7] H.R. Jafari Nodooshan, G. Wu, W. Liu, G. Wei, Y. Li, S. Zhang, Effect of Gd content on high temperature mechanical properties of MgeGdeYeZr alloy, Mater. Sci. Eng. A 651 (2016) 840e847. [8] X.J. Wang, D.K. Xu, R.Z. Wu, X.B. Chen, Q.M. Peng, L. Jin, Y.C. Xin, Z.Q. Zhang, Y. Liu, X.H. Chen, G. Chen, K.K. Deng, H.Y. Wang, What is going on in magnesium alloys? J. Mater. Sci. Technol. 34 (2018) 245e247. [9] F. Pan, M. Yang, X. Chen, A review on casting magnesium alloys: modification of commercial alloys and development of new alloys, J. Mater. Sci. Technol. 32 (2016) 1211e1221. [10] K. Fritzsch, A. Buchwalder, R. Zenker, A. Jung, Influence of electron beam liquid phase surface treatments on the corrosion resistance of AZ91D using different additive deposition techniques, Corros. Sci. 88 (2014) 109e117. [11] D.I. Proskurovsky, V.P. Rotshtein, G.E. Ozur, et al., Pulsed electron-beam technology for surface modification of metallic materials, J. Vac. Sci. Technol. Vacuum Surfaces Films 16 (4) (1998) 2480e2488. [12] D.I. Proskurovsky, V.P. Rotshtein, G.E. Ozur, et al., Physical foundations for surface treatment of materials with low energy, high current electron beams, Surf. Coating. Technol. 125 (1e3) (2000) 49e56. [13] M.C. Li, S.Z. Hao, H. Wen, R.F. Huang, Surface composite nanostructures of AZ91 magnesium alloy induced by high current pulsed electron beam treatment, Appl. Surf. Sci. 303 (2014) 350e353. [14] B. Gao, S. Hao, J. Zou, T. Grosdidier, L. Jiang, J. Zhou, C. Dong, High current pulsed electron beam treatment of AZ31 Mg alloy, J. Vac. Sci. Technol. Vacuum, Surfaces, and Films 23 (2005) 1548e1553. [15] J.X. Zou, T. Grosdidier, K.M. Zhang, et al., Mechanisms of nanostructure and metastable phase formations in the surface melted layers of a HCPEB-treated D2 steel, Acta Mater. 54 (20) (2006) 5409e5419. [16] T. Grosdidier, J.X. Zou, N. Stein, et al., Texture modification, grain refinement and improved hardness/corrosion balance of a FeAl alloy by pulsed electron beam surface treatment in the "heating mode, Scripta Mater. 58 (12) (2008) 1058e1061. [17] J. Kim, H.W. Park, Influence of a large pulsed electron beam (LPEB) on the corrosion resistance of Ti6Al7Nb alloys, Corros. Sci. 90 (6) (2015) 153e160. [18] J.W. Murray, P.K. Kinnell, A.H. Cannon, et al., Surface finishing of intricate metal mould structures by large-area electron beam irradiation, Precis. Eng. 37 (2) (2013) 443e450. [19] J.X. Zou, T. Grosdidier, K.M. Zhang, C. Dong, Cross-sectional analysis of the graded microstructure in an AISI D2-steel treated with low energy highcurrent pulsed electron beam, Appl. Surf. Sci. 255 (2009) 4758e4764. [20] A.B. Markov, V.P. Rotshtein, Calculation and experimental determination of dimensions of hardening and tempering zones in quenched U7A steel irradiated with a pulsed electron beam, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 132 (1) (1997) 79e86. [21] J.X. Zou, K.M. Zhang, T. Grosdidier, C. Dong, Analysis of the evaporation and re-condensation processes induced by pulsed beam treatments, Int. J. Heat Mass Transf. 64 (2013) 1172e1182. [22] K.M. Zhang, J.X. Zou, T. Grosdidier, C. Dong, D. Yang, Improved pitting corrosion resistance of AISI 316L stainless steel treated by high current pulsed electron beam, Surf. Coating. Technol. 201 (2006) 1393e1400. [23] Y.R. Liu, K.M. Zhang, J.X. Zou, D.K. Liu, T.C. Zhang, Effect of the high current pulsed electron beam treatment on the surface microstructure and corrosion resistance of a Mg-4Sm alloy, J. Alloy. Comp. 741 (2018) 65e75. [24] B. Gao, Y. Hao, G.F. Tu, S.W. Li, F.X. Yu, L. Zuo, L. Hu, Surface modification of Mg67eZn30eY3 quasicrystal alloy by high current pulsed electron beam, Surf. Coating. Technol. 229 (2013) 42e45. [25] W.J. Lee, J. Kim, H.W. Park, Improved corrosion resistance of Mg alloy AZ31B induced by selective evaporation of Mg using large pulsed electron beam irradiation, J. Mater. Sci. Technol. (2018). https://doi.org/10.1016/j.jmst.2018.
T.C. Zhang et al. / Journal of Alloys and Compounds 788 (2019) 231e239 12.004. [26] Z.Q. Wang, B. Zhang, D.J. Li, R. Fritzsch, X.Q. Zeng, H.J. Roven, W.J. Ding, Effect of heat treatment on microstructures and mechanical properties of high vacuum die casting Mge8Gde3Ye0.4Zr magnesium alloy, Trans. Nonferrous Metals Soc. China 24 (2014) 3762e3768. [27] T. Grosdidier, J.X. Zou, B. Bolle, S.Z. Hao, C. Dong, Grain refinement, hardening and metastable phase formation by high current pulsed electron beam (HCPEB) treatment under heating and melting modes, J. Alloy. Comp. 504 (2010) S508eS511. [28] K.M. Zhang, J.X. Zou, T. Grosdidier, et al., Surface modification of Ni (50.6at.%) Ti by high current pulsed electron beam treatment, J. Alloy. Comp. 434e435 (2007) 682e685. [29] Y. Qin, C. Dong, Z. Song, S. Hao, X. Me, J. Li, X. Wang, J. Zou, T. Grosdidier, Deep Modification of materials by thermal stress wave generated by irradiation of high-current pulsed electron beams, J. Vac. Sci. Technol.: Vacuum, Surfaces, and Films 27 (2009) 430e435. [30] Y. Qin, J.X. Zou, C. Dong, et al., Temperature-stress fields and related phenomena induced by a high current pulsed electron beam, Nucl. Instrum.
[31]
[32]
[33]
[34]
[35]
239
Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 225 (4) (2004) 544e554. M. Jahedi, B.A. McWilliams, P. Moy, M. Knezevic, Deformation twinning in rolled WE43-T5 rare earth magnesium alloy: influence on strain hardening and texture evolution, Acta Mater. 131 (2017) 221e232. P. Yan, T. Grosdidier, X. Zhang, J.X. Zou, Formation of large grains by epitaxial and abnormal growth at the surface of pulsed electron beam treated metallic samples, Mater. Des. 159 (2018) 1e10. J.X. Zou, K.M. Zhang, S.Z. Hao, et al., Mechanisms of hardening, wear and corrosion improvement of 316 L stainless steel by low energy high current pulsed electron beam surface treatment, Thin Solid Films 519 (4) (2010) 1404e1415. A.E. Coy, F. Viejo, P. Skeldon, G.E. Thompson, Susceptibility of rare-earthmagnesium alloys to micro-galvanic corrosion, Corros. Sci. 52 (12) (2010) 3896e3906. X. Liu, G.S. Frankel, Effects of compressive stress on localized corrosion in AA2024-T3, Corros. Sci. 48 (2006) 3309e3329.