Effect of Phase Transformations on Hardness in Zn – Al – Cu Alloys

Over recent years Zinc–rich aluminum alloys with iron or copper additions have been highly interesting for engineering since because the addition of these alloying elements enhances the wear resistance, elastic modulus, yield strength and corrosion resistance under the service conditions of stress and temperature without having a significant detrimental effect on the superplastic behavior during production of components1-4. The properties of these alloys change with the increase in copper content; however, this alloying element also causes the formation of intermetallic compounds which can affect the phase reactions in the solid-state. Additionally, the presence of copper enhances the resistance to the creep and corrosion. Nevertheless, the Cu addition, in certain proportions, diminishes certain physical properties such as, ductility5. Most of the research works6-8 in the Zn–Al–Cu alloys are related to alloy compositions close to the eutectoid composition, see Figure 1. However, it is also important to know the phase transformations and mechanical properties of Al-rich and Zn-rich Zn-Al-Cu alloys. The α, η and τ ́ phases are expected to be formed in alloy compositions close to the eutectoid one after slow cooling. The α phase is Al-rich with an fcc crystalline structure, while the η phase is Zn-rich with a cph crystalline structure. The (Al4Cu3Zn) τ ́ phase is stable at low temperatures and it has an ordered rhombohedral structure. It is formed by the four-phases reaction α + ε → η + τ ́, which also involves the α and η phases. This reaction takes place at aging temperatures lower than 268 °C[6] and it is also related with the dimensional instability of this type of alloys4. The copper addition also favors the appearance of metaestable (Cu4Zn) ε phase with a hexagonal crystalline structure. A high content of copper may also promote the formation of the stable (CuAl2) θ phase. In general, the Cu-containing phases cause an increase in hardness in this type of alloys5. The β phase with an fcc structure is stable at high temperatures and it may decompose into a lamellar product composed of the α and η phases according to the following eutectoid reaction β → α + η[5]. In the Al-Zn rich side, it has been reported6 to occur the spinodal decomposition of the supersaturated α phase into a mixture of the α and η phases at the early stages of aging at low temperatures. In general, the occurrence of these phase transformations is expected to be affected by the alloy composition. The Zn-Al-Cu alloys are usually used in the as-cast condition; however, a homogenization heat treatment can be performed in these alloys for improving their mechanical properties since the dendritic structure and microsegregation are eliminated as a result of the atomic diffusion process during heating above 350 °C for prolonged times4. Thus, the purpose of this work is to investigate the effect of the chemical composition on the microstructure and mechanical properties, hardness, in order to establish relationships among them to determine the hardness in both the as-cast and homogenized Zn-Al-Cu alloys.


Introduction
Over recent years Zinc-rich aluminum alloys with iron or copper additions have been highly interesting for engineering since because the addition of these alloying elements enhances the wear resistance, elastic modulus, yield strength and corrosion resistance under the service conditions of stress and temperature without having a significant detrimental effect on the superplastic behavior during production of components [1][2][3][4] .
The properties of these alloys change with the increase in copper content; however, this alloying element also causes the formation of intermetallic compounds which can affect the phase reactions in the solid-state.Additionally, the presence of copper enhances the resistance to the creep and corrosion.Nevertheless, the Cu addition, in certain proportions, diminishes certain physical properties such as, ductility 5 .
Most of the research works [6][7][8] in the Zn-Al-Cu alloys are related to alloy compositions close to the eutectoid composition, see Figure 1.However, it is also important to know the phase transformations and mechanical properties of Al-rich and Zn-rich Zn-Al-Cu alloys.The α, η and τ´ phases are expected to be formed in alloy compositions close to the eutectoid one after slow cooling.The α phase is Al-rich with an fcc crystalline structure, while the η phase is Zn-rich with a cph crystalline structure.The (Al 4 Cu 3 Zn) τ´ phase is stable at low temperatures and it has an ordered rhombohedral structure.It is formed by the four-phases reaction α + ε → η + τ´, which also involves the α and η phases.This reaction takes place at aging temperatures lower than 268 °C [6] and it is also related with the dimensional instability of this type of alloys 4 .The copper addition also favors the appearance of metaestable (Cu 4 Zn) ε phase with a hexagonal crystalline structure.A high content of copper may also promote the formation of the stable (CuAl 2 ) θ phase.In general, the Cu-containing phases cause an increase in hardness in this type of alloys 5 .The β phase with an fcc structure is stable at high temperatures and it may decompose into a lamellar product composed of the α and η phases according to the following eutectoid reaction β → α + η [5] .In the Al-Zn rich side, it has been reported 6 to occur the spinodal decomposition of the supersaturated α phase into a mixture of the α and η phases at the early stages of aging at low temperatures.In general, the occurrence of these phase transformations is expected to be affected by the alloy composition.
The Zn-Al-Cu alloys are usually used in the as-cast condition; however, a homogenization heat treatment can be performed in these alloys for improving their mechanical properties since the dendritic structure and microsegregation are eliminated as a result of the atomic diffusion process during heating above 350 °C for prolonged times 4 .
Thus, the purpose of this work is to investigate the effect of the chemical composition on the microstructure and mechanical properties, hardness, in order to establish relationships among them to determine the hardness in both the as-cast and homogenized Zn-Al-Cu alloys.

Experimental Procedure
Sixteen Zn-Al-Cu alloy compositions were prepared by melting pure elements at 750 °C in an alumina crucible with an electric furnace under an argon atmosphere and subsequently poured into a steel mold.These specimens were designated as M1 to M8 and M9 to M16, corresponding to line 1 and 2, respectively, as shown in Figure 1.Their compositions are also shown in Tables 1 and 2, respectively.The alloy compositions were chosen from the two isopletic lines shown in Figure 1 and they were defined by the following equations:

Effect of Phase Transformations on Hardness in Zn-Al-Cu Alloys
Where X Zn líne 1 and X Zn líne 2 are the Zn atomic fraction for each line.X Cu is the Cu atomic fraction.The alloy compositions were selected to be around the eutectoid Zn-22%Al-2%Cu alloy.All the as-cast ingots of 2 × 2 × 20 cm were homogenized at 350 °C for 180 h in an electric tubular furnace to eliminate the dendritic structure followed by slow cooling.The as-cast and homogenized alloy specimens were prepared metallographically using the standard metallographic procedure and subsequently etched in a solution composed of 0.5 ml hydrofluoric acid and 99.5 ethylic alcohol.Metallographic specimens were examined using both optical (OM) and scanning electron microscopy (SEM) at 15 kV equipped with an EDX spectrometer and X-ray diffraction (XRD) analysis with monochromated copper K α radiation at a scan rate of 2°/min from an angle 2θ of 35 to 50°.Backscattered electron images (BEI) were also used to distinguish the microconstituents in these alloys.The Rockwell "B" hardness of specimens was determined with a load of 100 kg f and 1/16 in diameter steel ball according to the standard procedure 9 .A multiple regression analysis was conducted between the hardness and chemical composition of alloys using a commercial software.

Structural and microstructural characterizations of alloys
The X-ray diffraction patterns of the as-cast M1-M8 and M9-M16 alloys are shown in Figures 2 and 3, respectively.It can be seen that the α, η, ε and τ' phases are present in almost all the as-cast alloys.The α, η and ε phases are present in the case of the as-cast M8 alloy.Besides, the θ and β phases are present in almost all the specimens except in the as-cast M10-M13 alloys.According to the   equilibrium Al-Cu-Zn diagram 8 , shown in Figure 1, the α, η and τ' phases are the equilibrium ones for these alloy compositions at low temperature.The cooling rate during the casting process does not follow the equilibrium conditions, slow cooling, thus the phases formed in the as-cast alloys may not correspond to the equilibrium ones.Conversely, the homogenizing treatment is expected to cause the formation of the equilibrium phases in these alloys.The increase in Cu content was observed to be related to the increase in the intensity of X-ray diffraction peaks corresponding to the θ phase.This phase may be formed by the eutectic reaction L → α + θ located in the Al-Cu rich side 6 .In contrast, the peak intensity of the Zn-rich η phase also increases with the increase in Zn content.This fact suggests the increase in the volume fraction of this phase.The amount of the α and ε phases showed no clear tendency for increasing or decreasing the contents of either Al or Zn.A low volume fraction of the β phase is present in the as-cast M1 to M7 and M9, M14 to M16 alloys.This presence seems to indicate that the alloy chemical compositions on the line 1 are closer to the β phase field at high temperatures than the alloy compositions on line 2. Likewise, the τ´ phase is, in general, more stable as the Cu content increases.This suggests that the four-phase reaction, α + ε → η + τ´, took place during the cooling of these alloy compositions.Figures 4 and 5 show the X-ray diffraction patterns of the homogenized M1-M8 and M9-M16 alloys, respectively.These patterns indicate that the α, η and τ' phases are present in all samples.These three phases are in agreement with the equilibrium ones as shown in Figure 1.The increase in the intensity of X-ray diffraction peaks corresponding to the Al-rich α phase is related to the increase in the Al content, whereas the presence of Zn-rich η phase increases with the increase in the Zn content.The presence of τ'  The former phase can be formed by its precipitation from the supersaturated αphase, α sss → α + θ [6] .The only presence of the expected equilibrium α, η and τ' phase mixture was not observed in any composition.This fact suggests that the homogenizing time should be longer than 180 h in order to obtain only the equilibrium phases.
The β phase is presented after homogenizing only in the alloys with high contents of Cu and Al, the M6 to M8 and M14 to M16 alloys.This means that the Cu and Al alloying  elements may retain the β phase at room temperature.In the case of the other alloys, the β phase is transformed into a mixture of α and η phases according to the following phase reaction: β → α + η [4] .
The optical observation of the as-cast alloys shows the presence of dendritic structure.SEM micrographs of the ascast M1, M3, M5 and M7 alloys are shown in Figures 6a-d, respectively, for line 1, In contrast, Figures 7a-d show the SEM micrographs corresponding to the as-cast M10, M12, M14 and M16 alloys, respectively.There are several microconstituents in these SEM micrographs.They correspond mainly to the phases shown in the XRD pattern shown in Figures 2 and 3.The identified microconstituents are indicated in these figures.The increase in volume fraction of the θ and the α phases is more evident with the increase in the content of Cu and Al, respectively, for alloys of both lines 1 and 2. Figure 8 shows the secondary electron SEM micrograph, for instance, for the as-cast M4 alloy and their corresponding Zn, Al and Cu SEM-EDS elemental mapping images.These SEM images verify the identification of microconstituents shown in Figures 6 and 7.
On the other hand, Figures 9 and 10a-d show the SEM micrographs corresponding to the homogenized M1, M3, M5 and M7, and M10, M12, M14 and M16 alloys, respectively.The presence of five microconstituents can be observed in these micrographs.The α, η, ε and τ' phases are observed in the homogenized M1, M3, M10 alloys and M12.While the α, η, θ and τ' phases are observed in the M5, M7, M14 and M16 alloys.These microconstituents correspond to those detected in the XRD pattern of these alloys, shown in Figures 4 and 5.The volume fraction of the θ and ε phases, and the α phase increases with the increase in the content of Cu and Al, respectively.

Hardness of the as-cast and homogenized alloys
Figures 11 and 12 show the average Rockwell "B" hardness, HRB, of the as-cast and homogenized M1-M5 alloys as a function of the copper content.The hardness of  the as-cast alloys is higher than that corresponding to the homogenized alloys.This behavior is mainly associated with the disappearance of dendritic structure and the ε phase.In general, there is an increase in hardness with the increase in the Cu and Al contents.This can be attributed to the presence of θ phase which is the phase with the highest hardness 4 .
The linear regression analyses were conducted in order to determine the correlation between the hardness and chemical composition of the as-cast and homogenized  alloys.The regression equations for the as-cast alloys were determined to be as follows: HRB as-cast line 1 = 93.28+ 5.32ln(X Cu ) -4.74ln(X Zn ) HRB as-cast line 2 = 98.22 + 11.01ln(X Cu ) -13.92ln(X Zn ) (4) In contrast, the equations corresponding to the homogenized alloys were the following:   HRB Homogenized line 1 = 103.55+ 13.7ln(X Cu ) -4.88ln(X Zn ) (5) HRB Homogenized line 2 = 136.84+ 27.81ln(X Cu ) -2.84ln(X Zn ) (6)   It is interesting to notice that the regression coefficients are positive for the Cu composition and negative for the Zn composition.These coefficients suggest that the increase in volume fraction of the Cu-containing phases such as, the ε, θ and τ' phases, are mainly responsible for the increase in hardness in both alloys.Conversely, the presence of the Znrich η phase causes the decrease in hardness for both alloys.This type of equations might be useful for the alloy design.For instance, these can be used to estimate the hardness of either the as-cast or homogenized Zn-Al-Cu alloys based on its chemical composition.Additionally, it can also be used to determine the chemical composition of a new alloy with a given hardness value.

Conclusions
The conclusions of the present work can be summarized as follows: (1) The hardness for both the as-cast and homogenized alloys increased with the increase in volume fraction of the Cu and Al due to the increase in volume fraction of the Cu-containing θ and τ´ phases.

Figure 11 .
Figure 11.Plot of hardness vs. composition for the as-cast and homogenized alloys of line 1.

Figure 12 .
Figure 12.Plot of hardness vs. composition for the as-cast and homogenized alloys of line 2.

( 2 )
The elimination of the dendritic structure and the ε phase during homogenizing treatment promoted the decrease in hardness for the homogenized alloys.The homogenizing treatment caused the formation of most of the equilibrium phases.(3)A multiple linear regression analysis permitted to obtain four equations to estimate the hardness in the as-cast and homogenized Zn-Al-Cu alloys which can be useful for designing alloys with specific hardness values.

Table 1 .
Chemical composition of the M1 to M8 alloys.

Table 2 .
Chemical composition of the M9 to M16 alloys.