Bulk and Surface Low Temperature Phase Transitions in the Mg-Alloy EZ33A

: Low-temperature phase transitions in the EZ33A Mg-cast alloy have been investigated. Based on the structure assessment of the alloy after annealing at 150 ◦ C (1826 h) and at 200 ◦ C (2371 h) a grain boundary wetting transition by a second solid phase was documented. Within a 50 ◦ C temperature range, substantial di ﬀ erences in the α (Mg) grain boundary fraction wetted by the (Mg,Zn) 12 RE intermetallic were observed. In contrast to what was reported in the literature, two di ﬀ erent types of precipitates were found within α (Mg) grains. With increasing annealing temperatures, both types of precipitates dissolved.


Introduction
In modern materials technology, different Mg-based alloys are used in various applications.This class of materials exhibits low density and good strength.Mg-Zn cast alloys with the addition of different alloying elements represent an important part of this materials class.Quite frequently, in addition to being light weight and strong, these alloys have other beneficial features, like low microporosity and weldability.Unfortunately, they possess low ductility due to brittle intermetallic precipitates which nucleate and grow on grain boundaries (GBs).Many recent investigations [1][2][3] aim at increasing the ductility of Mg-Zn alloys, by exploring different thermomechanical treatments to reduce cracking.
The scientific objective of the present work is to contribute to these efforts by focusing on the role of wetting surface phase transitions [4].Special emphasis is placed on the analysis of the solid phase wetting transition and the associated low temperature precipitation processes in the Mg cast alloy EZ33A.

Background
The as-cast EZ33A alloy (Figure 1a) has a polycrystalline dendritic structure with a second phase occupying 75% of the GBs and regions in the α(Mg) grain interiors with nanoprecipitates.According to literature, this second phase represents a (Mg,Zn) 12 RE intermetallic compound [1][2][3], which has a high hardness and strength.It increases the overall strength of the alloy, especially its creep resistance, by slowing down GB mobility [5,6].The precipitation in the α(Mg) grain interior results from an increased Zr concentration (saturation) in this region [2,3].The Zr-saturated areas appear medium-grey in Figure 1a.It is difficult to quantify the chemical composition of the precipitates (highlighted by white arrows with question marks in Figure 1a), as the interaction volume for the energy-dispersive X-ray spectroscopy (EDX) method is larger than the particles.The micrograph in Figure 1b suggests, that 550 • C annealing results in a dissolution of the nanoparticles and a chemical homogenization across the grain [4].The as-cast EZ33A alloy (Figure 1a) has a polycrystalline dendritic structure with a second phase occupying 75% of the GBs and regions in the α(Mg) grain interiors with nanoprecipitates.According to literature, this second phase represents a (Mg,Zn)12RE intermetallic compound [1][2][3], which has a high hardness and strength.It increases the overall strength of the alloy, especially its creep resistance, by slowing down GB mobility [5,6].The precipitation in the α(Mg) grain interior results from an increased Zr concentration (saturation) in this region [2,3].The Zr-saturated areas appear medium-grey in Figure 1a.It is difficult to quantify the chemical composition of the precipitates (highlighted by white arrows with question marks in Figure 1a), as the interaction volume for the energy-dispersive X-ray spectroscopy (EDX) method is larger than the particles.The micrograph in Figure 1b suggests, that 550 °C annealing results in a dissolution of the nanoparticles and a chemical homogenization across the grain [4].The sample in this figure was annealed at 550 • C for only 2 h, which is a much shorter annealing time than in the experiments of the present work described below.This sample was a part of the previous experimental studies that confirmed the GB wetting phase transition by the second liquid phase in the present alloy in the temperature range from 530 to 590 • C [4].The short annealing time is explained by the high annealing temperature which results in the partial melting of the structure.The presence of a second liquid phase boosts the diffusion processes significantly and results in shorter transformation times for structure stabilisation.It is clear from Figure 1 that the initial as-cast structure is very similar to the sample structure after a high temperature annealing and subsequent quenching.The similarities of these structures are in detail discussed in [4].However, the as-cast structure can also be a result of a solid state wetting (spreading) surface phase transition.To examine this possibility long low temperature annealings were performed in this work.

Material
The investigated EZ33A alloy was provided by the CanMet Materials (Hamilton, ON, Canada).The chemical composition of the material in weight percentage is: 2.5% Zn, 0.4% Zr, 3% rare earth elements (RE), and Mg balance.The RE metals were added to the Mg-Zn-Zr alloy in the form of a 'Mischmetal' addition, consisting mainly of Ce, La, and Nd.

Methods
Slices that were 2-mm-thick were spark-eroded from the as-cast EZ33A ingot.The ingot and the samples were chemically etched in a 75% HCl + 25% H 2 O etching solution.After cutting the samples were encapsulated into evacuated quartz ampoules (residual pressure: 4 × 10 −4 Pa) to prevent oxidation.The samples were then subjected to two annealing treatments, 150 • C/1826 h and 200 • C/2371 h.The annealing temperatures and times were chosen specifically to investigate the solid state surface transformations based on the previous experience of the author with such experiments.In both samples, it was expected either to find small numbers of completely wetted grain boundaries to confirm the assumptions or to find a structure similar to the as-cast structure (Figure 1a) which would disprove the existence of the solid state wetting transition.After quenching, the samples were sealed in an electrically conductive resin and mechanically ground and polished.
Polished samples were examined using a LEO 1530 VP scanning electron microscope (SEM) equipped with an Energy-dispersive X-ray (EDX) detector and a backscatter detector (BSD) (LEO Electron Microscopy Inc., Thornwood, NY, USA).The measurements were performed at a working distance of 10 mm and an acceleration voltage of 25 kV.All micrographs of the sample structure were quantified by the Image Expert Pro software (20/20 Software, Inc., Stamford, CT, USA).Values like grain size, number of wetted (covered) by the second phase grain boundaries and surface fractions of the phases were extracted and analyzed.The X-ray diffraction (XRD) method (Siemens D−500 diffractometer, Siemens AG, Munich, Germany) was used to determine the phase composition for all samples.

Results and Discussion
XRD patterns demonstrated that all samples contain only two phases, namely the matrix α-(Mg) solid solution phase with a hexagonal crystal structure (P63/mmc) and the second Mg 12 RE phase with a tetragonal crystal structure (I4/mmm).Figure 2 shows the XRD pattern for the as-cast EZ33A structure.It witnesses the absence of any other phases.
The microstructure after annealing at 150 • C (Figure 3a) reveals significant difference from the initial state.The intermetallic (Mg,Zn) 12 RE phase has undergone a shape change.Moreover, the volume fraction of the continuous GB second phase layers decreased to only about ~7%.Most of the (Mg,Zn) 12 RE phase regions, covering 6.2% of the investigated cross section, are situated on GB triple junctions with almost equiaxed and even round particle shapes, and an average particle size of 16 µm.
junctions with almost equiaxed and even round particle shapes, and an average particle size of 16 μm.The precipitates found in the as-cast state have grown in size (Figure 3b).The particle shapes have evolved.One can find lath like, round, and in some cases hexagonal particles of sizes up to 2 μm.These particles do not seem to have a fixed orientation relationship with the matrix.The EDX mappings document a high Zn concentration, moderate concentrations of Zr and Nd and almost no Mg in the (Mg,Zn)12RE particles (Figure 3c-e).Similar precipitates were reported in the literature, after different annealing temperatures and times [5] with higher concentrations of RE [3].A second type of needle-like precipitates was found in the grain interior (Figure 3b).Unlike the first particle family described so far, these are needle-shaped and do seem to have specific orientations.The size of the needles reached 1-2 μm in length and 120 nm in thickness.The EDX mapping (Figure 3c) allows us to conclude that the small precipitates have a very high Zr  The precipitates found in the as-cast state have grown in size (Figure 3b).The particle shapes have evolved.One can find lath like, round, and in some cases hexagonal particles of sizes up to 2 μm.These particles do not seem to have a fixed orientation relationship with the matrix.The EDX mappings document a high Zn concentration, moderate concentrations of Zr and Nd and almost no Mg in the (Mg,Zn)12RE particles (Figure 3c-e).Similar precipitates were reported in the literature, after different annealing temperatures and times [5] with higher concentrations of RE [3].A second type of needle-like precipitates was found in the grain interior (Figure 3b).Unlike the first particle family described so far, these are needle-shaped and do seem to have specific orientations.The size of the needles reached 1-2 μm in length and 120 nm in thickness.The EDX mapping (Figure 3c) allows us to conclude that the small precipitates have a very high Zr The precipitates found in the as-cast state have grown in size (Figure 3b).The particle shapes have evolved.One can find lath like, round, and in some cases hexagonal particles of sizes up to 2 µm.These particles do not seem to have a fixed orientation relationship with the matrix.The EDX mappings document a high Zn concentration, moderate concentrations of Zr and Nd and almost no Mg in the (Mg,Zn) 12 RE particles (Figure 3c-e).Similar precipitates were reported in the literature, after different annealing temperatures and times [5] with higher concentrations of RE [3].
A second type of needle-like precipitates was found in the grain interior (Figure 3b).Unlike the first particle family described so far, these are needle-shaped and do seem to have specific orientations.The size of the needles reached 1-2 µm in length and 120 nm in thickness.The EDX mapping (Figure 3c) allows us to conclude that the small precipitates have a very high Zr concentration.According to the literature these particles might be oriented parallel to the [0001] orientation in the hexagonal Mg lattice.They are the main source of precipitation hardening and creep resistance [7], as the main Metals 2020, 10, 1127 5 of 7 dislocation slip systems in hexagonal Mg are microscopic crystallographic slip systems of type {0001} <1,1,−2,0> (basal slip).Some authors suggest [3] that in EZ33A the needle shaped particles represent an intermediate β' 1 phase, which precedes the formation of the lath-shaped β' 2 phase, which was detected in super saturated Mg-Zn and Mg-Zn-RE alloys [8,9].Based on our EDX mapping results, we suggest that segregation of Zr to the (0001) planes in the α(Mg) hampers the formation of the β' 1 phase in EZ33A and instead promotes the direct formation of the β' 2 lathes.
A new research on the microstructure and mechanical characterization of a ZE41 alloy after equal channel angular pressing (ECAP) deformation, which is an Mg-based alloy very similar to the EZ33A, is reporting a formation of nano-sized Zr 22 Zn precipitates in the Mg grain interior [10].The needle-like precipitates could be related to the nano-sized Zr 22 Zn phase but as the alloys differ in composition, heat treatment and the shape of the particles further certain conclusions can be made only after extensive transmission electron microscopy (TEM) of the EZ33A samples from the present research.
The microstructure after annealing at 200 • C (Figure 4a) shows more similarities to the initial as-cast structure (Figure 1a).The intermetallic (Mg,Zn) 12 RE phase covers about 81% of the GBs, where it forms continuous layers with a mean thickness of 7 µm.As the microstructure after such long annealing times is close to equilibrium, the difference to the fraction of covered GBs in the first sample confirms the existence of the solid phase GB wetting transition in EZ33A.Similar phenomenon of GBs completely and/or incompletely covered with layers of a second solid phase have been observed also in Al- [11,12], Cu- [13,14], Co- [15], Fe- [16], Zr- [17], and Ti-based alloys [18,19].Thermodynamically, this phenomenon is very similar to the GB wetting by the melt [20,21].
concentration.According to the literature these particles might be oriented parallel to the [0001] orientation in the hexagonal Mg lattice.They are the main source of precipitation hardening and creep resistance [7], as the main dislocation slip systems in hexagonal Mg are microscopic crystallographic slip systems of type {0001} <1,1,−2,0> (basal slip).Some authors suggest [3] that in EZ33A the needle shaped particles represent an intermediate β'1 phase, which precedes the formation of the lath-shaped β'2 phase, which was detected in super saturated Mg-Zn and Mg-Zn-RE alloys [8,9].Based on our EDX mapping results, we suggest that segregation of Zr to the (0001) planes in the α(Mg) hampers the formation of the β'1 phase in EZ33A and instead promotes the direct formation of the β'2 lathes.
A new research on the microstructure and mechanical characterization of a ZE41 alloy after equal channel angular pressing (ECAP) deformation, which is an Mg-based alloy very similar to the EZ33A, is reporting a formation of nano-sized Zr22Zn precipitates in the Mg grain interior [10].The needle-like precipitates could be related to the nano-sized Zr22Zn phase but as the alloys differ in composition, heat treatment and the shape of the particles further certain conclusions can be made only after extensive transmission electron microscopy (TEM) of the EZ33A samples from the present research.
The microstructure after annealing at 200 °C (Figure 4a) shows more similarities to the initial as-cast structure (Figure 1a).The intermetallic (Mg,Zn)12RE phase covers about 81% of the GBs, where it forms continuous layers with a mean thickness of 7 μm.As the microstructure after such long annealing times is close to equilibrium, the difference to the fraction of covered GBs in the first sample confirms the existence of the solid phase GB wetting transition in EZ33A.Similar phenomenon of GBs completely and/or incompletely covered with layers of a second solid phase have been observed also in Al- [11,12], Cu- [13,14], Co- [15], Fe- [16], Zr- [17], and Ti-based alloys [18,19].Thermodynamically, this phenomenon is very similar to the GB wetting by the melt [20,21].It is important to mention that such small temperature interval (50 °C) of the solid phase wetting (spreading) from 7% to 81% of covered GBs in the EZ33A alloy was rather unexpected.In other similar systems the solid state wetting takes place in a much broader temperature range.This can be the result of either a steep surface energy temperature dependence of the solid/solid interface that crosses the different GB energy temperature dependences very fast in the present temperature interval or a very small difference in GB energy of all those GBs that were wetted in the present temperature interval.It is important to mention that such small temperature interval (50 • C) of the solid phase wetting (spreading) from 7% to 81% of covered GBs in the EZ33A alloy was rather unexpected.In other similar systems the solid state wetting takes place in a much broader temperature range.This can be the result of either a steep surface energy temperature dependence of the solid/solid interface that crosses the different GB energy temperature dependences very fast in the present temperature interval or a very small difference in GB energy of all those GBs that were wetted in the present temperature interval.
Similar to the initial state of the alloy, only the lath-shaped precipitates can be found in the structure (Figure 4b,e).Their size is comparable to the precipitate size in the as-cast state.They are situated in the bright Zr rich area of the α(Mg).Although there appear to be no Zr-precipitates, it is possible that they do not dissolve in α(Mg), but rather are too small to be resolved by a SEM [3].Previous research results [22] suggest that higher temperatures of the order of 250 • C are required for Zr-precipitates to dissolve and/or significantly shrink.

Conclusions
The results obtained in the present work have shown, that grain boundary precipitates redistribution associated with the wetting of GBs occurs in EZ33A.In the 50 • C temperature interval, between 150 • C and 200 • C, the area fraction of wetted GBs rises from 7% to 81%.Two different types of precipitates were found in the grain interior.Needle-shaped Zr-rich particles of 1-2 µm length were all oriented in one direction, presumably [0001].Lath-shaped hexagonal particles of up to 2 µm diameter were also found, which are enriched in Zn and RE.The existence of the precipitates up to the dissolution at 250 • C provides the high creep resistance.Funding: This research was funded by Russian Science Foundation, grant number 18-72-00243.

Figure 1 .
Figure 1.Scanning electron microscope (SEM) micrographs of Mg-cast alloy EZ33A.(a) As-cast structure of ingot.White question marks indicate presence of nanoparticles in grain interiors.(b) Microstructure after annealing at 550 °C for 2 h [4].

Figure 1 .
Figure 1.Scanning electron microscope (SEM) micrographs of Mg-cast alloy EZ33A.(a) As-cast structure of ingot.White question marks indicate presence of nanoparticles in grain interiors.(b) Microstructure after annealing at 550 • C for 2 h [4].

Figure 2 .
Figure 2. The X-ray diffraction (XRD) pattern for the as-cast EZ33A sample.According to the computer analysis the XRD resolves only two different phases, the matrix α-(Mg) solid solution phase and the second Mg12RE phase.

Figure 3 .
Figure 3. SEM results obtained for Mg-alloy EZ33A after annealing at 150 °C for 1826 h.(a) Overall SEM back scatter micrograph.(b) SEM back scatter micrograph of the grain interior taken at a higher magnification.Sizes, shapes, and distances between precipitates can be appreciated.Energy-dispersive X-ray spectroscopy (EDX) element distribution maps from microstructural region shown in Figure 3b: (c) Zr, (d) Zn, and (e) Nd.

Figure 2 .
Figure 2. The X-ray diffraction (XRD) pattern for the as-cast EZ33A sample.According to the computer analysis the XRD resolves only two different phases, the matrix α-(Mg) solid solution phase and the second Mg 12 RE phase.

Figure 2 .
Figure 2. The X-ray diffraction (XRD) pattern for the as-cast EZ33A sample.According to the computer analysis the XRD resolves only two different phases, the matrix α-(Mg) solid solution phase and the second Mg12RE phase.

Figure 3 .
Figure 3. SEM results obtained for Mg-alloy EZ33A after annealing at 150 °C for 1826 h.(a) Overall SEM back scatter micrograph.(b) SEM back scatter micrograph of the grain interior taken at a higher magnification.Sizes, shapes, and distances between precipitates can be appreciated.Energy-dispersive X-ray spectroscopy (EDX) element distribution maps from microstructural region shown in Figure 3b: (c) Zr, (d) Zn, and (e) Nd.

Figure 3 .
Figure 3. SEM results obtained for Mg-alloy EZ33A after annealing at 150 • C for 1826 h.(a) Overall SEM back scatter micrograph.(b) SEM back scatter micrograph of the grain interior taken at a higher magnification.Sizes, shapes, and distances between precipitates can be appreciated.Energy-dispersive X-ray spectroscopy (EDX) element distribution maps from microstructural region shown in Figure 3b: (c) Zr, (d) Zn, and (e) Nd.

Figure 4 .
Figure 4. SEM results obtained for Mg-alloy EZ33A after annealing at 200 °C for 2371 h.(a) Overall SEM back scatter micrograph.(b) SEM back scatter micrograph of the grain interior taken at a higher magnification.Sizes, shapes, and distances between precipitates can be appreciated.EDX element distribution maps from microstructural region shown in Figure 4b: (c) Zr, (d) Zn, and (e) Nd.

Figure 4 .
Figure 4. SEM results obtained for Mg-alloy EZ33A after annealing at 200 • C for 2371 h.(a) Overall SEM back scatter micrograph.(b) SEM back scatter micrograph of the grain interior taken at a higher magnification.Sizes, shapes, and distances between precipitates can be appreciated.EDX element distribution maps from microstructural region shown in Figure 4b: (c) Zr, (d) Zn, and (e) Nd.