Microstructural, Mechanical, and Corrosion Properties of AZXX Magnesium Alloy: A Review of Processing Methods

Abstract

Magnesium (Mg) and its alloys are considered an ideal material for aerospace, medical, energy, and automotive purposes, because of their low density and high specific strength. Researchers are interested in AZ alloys because of their superior flow characteristics. This review makes an effort to summarise the numerous processing methods that have been adapted for use with AZXX alloy. One of the main obstacles to Mg alloys being used in their intended context is the difficulty of processing Mg and its alloys. Curiously, the homogenization process is often used in tandem with extrusion and rolling. It also gives an insight into the microstructure, mechanical (hardness, tensile, impact, fatigue, and creep), and electrochemical corrosion properties of AZXX alloys. The improvement of AZXX alloy can be attributed to the grain boundary strengthening and the second phase strengthening mechanisms. The effects of Al content and phases on properties are extensively discussed. This article summarises what has recently happened with AZXX wrought Mg alloy and offers some predictions for its future.

1. Introduction

Demand for lighter products has been reported in numerous sectors, including aviation, healthcare, energy, transportation, construction, chemicals, and the 3C (Computer, Communication, and Consumer Electronics) sector. There is a lot of interest in magnesium (Mg) alloys because of their high strength-to-weight ratio and low weight [1,2]. At 1.74–2.0 g/cm³, magnesium is 33% lighter than aluminium and 77% lighter than steel [3]. Despite its low weight, magnesium is not widely used in aerospace and automotive applications for a number of reasons. These include magnesium’s poor cold workability and strength, toughness, and creep resistance at elevated temperature; its high chemical activity at room and elevated temperature; and its relatively low ductility [4]. Magnesium is essential for the body’s proper functioning, serving essential functions like muscle support and energy production. However, due to a low Pilling-Bedworth ratio of 0.81 and the extremely negative standard potential of −2.37 V, Mg-based alloys have a high corrosion rate and degrade completely in the bio-fluid. To combat these drawbacks while still maintaining their mechanical strength and biocompatibility, Mg-based bio-composites and alloys were created [5]. Magnesium alloys have a high corrosion rate, leading to premature loss of mechanical integrity, which limits their clinical use in load-bearing sections [6]. Natural bone has a density between 1.8 and 2.1 g/cm³, which is also similar to Mg’s density. Pure magnesium has an elastic modulus of 45 GPa, which is nearly equal to the modulus of human bone (40 to 57 GPa). Magnesium, with its similar elastic modulus to water, could be used to prevent stress shielding and bone resorption in hard tissue engineering applications. Therefore, it has the potential to be a material for the development of biodegradable orthopaedic implants [7]. Hydrogen is safely stored in metal hydrides due to its exothermic oxidation in the atmosphere, and scientists have been working for years to adapt Mg for this purpose.

Studies show that at 400 °C, pure Mg can store up to 7.8% hydrogen [8,9,10]. Magnesium can also be used as an anode in a metal-air battery, with oxygen from the air serving as the cathode. However, Mg’s self-corrosive properties limit the metal’s applications. Mg (anode) performance is limited because Mg(OH)₂ readily forms on the surface when dissolved in an electrolyte with a pH between 0 and 10.5. Because of this, Mg is always used in conjunction with an alloying element to enhance its properties [11]. Aluminium is by far the most important metal, and it was one of the first alloying components for magnesium. Almost all structural uses involve the Mg-Al alloy system (mainly AZ91 and AM60). In practice, Al-based Mg alloys are often alloyed with trace amounts of zinc to increase their strength and corrosion resistance. Conventional primary processing techniques (casting and forming) rely heavily on the AZXX alloying system [12], which features Al concentrations from 3 to 9% wt and Zn content from less than 1% wt. Corrosion rates in aqueous chloride solutions were decreased in Mg alloys from the AZ-series and AM-series when the alloys were treated with rare earth elements (REE). According to the findings, the effectiveness against atmospheric corrosion can also be improved through alloying with Zinc (Zn) and Calcium (Ca) dissolved in a Mg-matrix. Zn is dispersed throughout the Mg matrix, forming a compact surface layer that improves the alloy’s resistance to degradation. Addition of Ca to an alloy improves Mg’s resistance to corrosion. Because of this, it is clear that the properties and behaviour of Magnesium are modified due to the presence of the alloying elements [13]. There is room to improve the application-specific properties of Mg and its alloys, despite their identification as potential materials for a wide range of engineering and biomedical applications [14]. The primary barrier to Mg’s use in its final applications is the difficulty of processing Mg and its alloys [1]. According to the literature [14,15], Mg alloys in the AZ series are known to have desirable properties and are used for a variety of purposes. The purpose of this article is to investigate the function/importance of AZ series Mg alloy through a systematic review of the literature that establishes the relationship between mechanical characteristics and microstructure. It also summarizes the relationship between the microstructure and phases of the alloy with corrosion behaviour. Additionally, it emphasizes the importance of the Al alloying with Mg. This article summarizes what has recently happened with AZXX wrought Mg alloy and offers some predictions for its future.

2. Processing of AZXX Mg Alloy

Casting and extruding are typically the first steps in the production process when making Mg alloy. There have been successful squeeze castings of AZ91, as well as rheo and thixocastings of AZ91, AZ71, and AZ80. Table 1 [16] displays the results of solutionizing at 400 °C for 24 h to improve the characteristics of the examined alloy. AZ61 alloys were extruded at a speed of 4 m/min and a temperature of 300 °C [17]. Forward extrusion involves separating the top and bottom halves and homogenising them at 380 °C for 10 h [18]. Thermomechanical processing, in the form of two or three passes of hot rolling, was used to achieve a fine-grained microstructure that is resistant to superplastic deformation [18]. Direct hydraulic extrusion at 300 and 400 °C with an extrusion ratio of 25 has been used to create AZ91 after solid solutionizing (homogenization) at 420 °C [19]. The melting point of the alloying element is largely determined by the temperature at which the material is extruded or deformed. The phase diagram of Mg–1Zn–xAl (x = 1–20 wt%) (shown in Figure 1) demonstrates the temperature-dependent decrease in solubility.

Rolling mills are designed so that the friction between the rolls and the workpiece drives the material forward as it is rolled [17]. Compressive stress results in a thinner, finer grain structure in the work material, which is the direct result of the stress. Based on the recrystallization temperature (TR), bulk forming can be divided into two distinct categories. Hot working (above TR) and cold working (below TR) describe the two processes, respectively [20]. The initial press speed of 10 mm/s was used to facilitate the multidirectional forging, which was controlled along two orthogonal directions to ensure workability. Quickly dipping the samples into water helps freeze the microstructure. This proved the deformation-caused lengthening along the z axis. The efficiency of the forging process can be determined from the freezing microstructure of the Mg alloy.

Graphite powder was used as a high-temperature lubricant, which tends to prevent sticking of working material at high temperatures, to achieve reasonable homogenous deformation. Before forging, a resistive furnace was used to heat the homogenised samples and two anvils to 360 °C for half an hour. During forging, the samples were not annealed. After the final pass, these forgings were cooled in water. The AZ80 was made stronger through age hardening at 175 °C and subsequent forging [21]. High pressure die casting (HPDC) has recently been applied to the production of Mg alloy systems of Mg, Al, Zn, and Mn in an effort to achieve superior mechanical properties without compromising the cost of processing. Because the mechanical properties of various processed materials are listed in Table 1, it is easy to see how the physical phenomenon or processing parameters of the alloy affects the microstructure and the phases in the alloy [22]. The depositing process is called wire additive manufacturing, and it employs extruded AZ80M alloy with a diameter of 1.6 mm. There is micro-cracking in the inter-transition layer and inhomogeneity in the grain structure in AZ80M that was produced using additive manufacturing. The horizontal direction also has greater tensile strength than the vertical [23]. Different properties along the horizontal and vertical directions, as well as their significance depending on the product’s intended use, are present in additively made parts. Rolling and extrusion, two common bulk deformation processes, are frequently used in the processing of AZXX Mg alloys [24]. The refined grains were observed on the outer surface, but they became coarser towards the centre. On other hand, the casting process leads to uniform grain size throughout the specimen with the coarser grains. Refined grains, homogeneity, and hardness of AZXX Mg alloy cannot be achieved by a single processing technique. However, it is possible through the combination of processing techniques, along with a heat treatment process (solid solutionizing, and age hardening) [17]. Table 1

Processing of AZXX Mg alloy and its corresponding mechanical properties.

S. No.	Material	Process	Solutionizing—Temp. (C); Time (Hours); Quenching Medium	T6 Temp.(C); Time (Hours)	Hardness (HV)	YS(MPa)	UTS(MPa)	Ductility(%)	Ref.
1	AZ80	Casting	-	-	-	100	190	6	[25]
			420; 4; water		-	110	270	17
			420; 8; water		-	100	260	18
			420; 10; water		-	90	265	16
			420; 16; water		-	85	270	15
			420; 4; water	200; 10	-	150	280	7
			420; 8; water	200; 10	-	140	260	12
			420; 10; water	200; 10	-	150	290	9
			420; 16; water	200; 10	-	130	280	7
2	AZ80	Extrusion	-	-	69.1	201	312	10.1	[26]
			400; 10; water		66.6	185	305	10.3
			400; 10; water	200	92	227	334	4
4	AZ80	Casting	-	-	61.4	-	-	-	[27]
			360; 1; water	170; 2	62.5	-	-	-
			360; 1; water	170; 4	61.7	-	-	-
			360; 1; water	170; 6	60.8	-	-	-
			360; 1; water	170; 8	53	-	-	-
5	AZ80	Casting	-	-	61.2	-	-	-	[28]
			420; 10; water		64	-	-	-
			420; 10; water	200; 10	83.5	-	-	-
			380; 10; air cooling +420 C; 10; water	-	63.7	-	-	-
			380; 10; air cooling +420 C; 10; water	200; 10	86	-	-	-
6	AZ80	casting	-	-	68	-	-	-	[29]
			250; 6	-	69	-	-	-
			250; 12	-	73	-	-	-
			350; 6	-	67.5	-	-	-
			350; 12	-	70	-	-	-
			450; 6	-	66	-	-	-
			450; 12	-	66	-	-	-
7	AZ80	wire arcadditivemanufacturing	-	-	-	146	308.7		[23]
8	AZ80	Multitemperature Multidirectional Forging + Extrusion	400; 24	-	-	292	420	-	[30]
9	AZ80	Casting			-	73.1–84.4	138–153.7	-	[31]
10	AZ80	Horizontal Flat Die Extrusion			64–72	287	162	-	[32]
	AZ80	Thixocasting	-	-	-	102	187	3.5	[16]
			400; 24; water	-	-	92	224	7.7
	AZ91	Squeeze casting	-	-	-	104	183	4.5
		Rheocasting	-	-	-	105	171	3.4
			400; 24; water	-	-	94	241	8.3
	AZ71		-	-	-	98	185	4.7
			400; 24; water	-	-	88	262	11.2
11	AZ21	Gravity Die Casting			53.9 ± 2.1	89	113	3.4	[33]
12	AZ31	Hot rolled			147	-	260	22.5	[34]
	AZ61	Hot rolled			-	-	260	22.5	[34]
13	AZ31	Extrusion			-	191.8	268.1	23.9	[35]
14	AZ31	Extrusion	450; 3; water	-		162	-	-	[36]
	AZ61		420; 6; water	-	55.4	198	-	-
	AZ91		400; 10; water	-	62.7	227	-	-
	AZ91		400; 10; water	210; 15	81.6	271	-	-
15	AZ91D	HPDC				188	255	3.0	[37]
16	AZ91	HPDC				150	240	3.0	[38]

3. Microstructural Analysis

Casting, extruding, rolling, homogenising, solutionizing, age hardening, and other processing methods all have a significant impact on the microstructure of an alloy. In Figure 2a, a secondary electron micrograph of the as-cast AZ61 alloy is displayed. This micrograph reveals the presence of second phases of α-Mg, which are located in the grain boundaries. The inverse pole figure of the corresponding micrograph is shown in Figure 2b [18,39].

It is the α-Mg phase and the secondary β-Mg₁₇Al₁₂ phase that AZ31 is designed to house. The twin-roll cast (TRC) has lower accumulation of the second phase on grain boundaries than conventionally cast (CC) AZ31 [40]. When comparing CC AZ31 and TRC AZ31, you can see that the grain size of the former is 300 µm while the latter is 200 µm. There is also more deformation near the surface during the TRC than in the middle of the strip [41]. The surface grain size is reduced to 50 µm as a result. After 10 h of ageing at 450 °C, the grains of cast AZ31 were seen to have become slightly coarser. The secondary β-Al₁₂Mg₁₇ phase in the cast AZ31 partially dissolves with age and almost no β-phase particles were observed within the grains. Morphology reveals that alloy microstructures are significantly impacted by partial remelting temperatures [42]. After rolling, grain sizes were reduced for all materials, and it was noticed that this was also correlated with a decrease in rolling temperature. In particular, they found that the material’s yield strength had increased during the treatment [43]. Dendritic structure is drastically reduced after homogenization [44].

Figure 3 shows the effect of heat treatment (T4, T5, and T6) on microstructure of the extruded AZ80 [45]. After 12 h of ageing, precipitates formed along grain boundaries and grew into grains in AZ80, where the typical grain size of the as-cast alloy is 80–100 m [46]. At 36 h, DP’s characteristic behaviour was nearly over. As compared to its age-hardened for 12 h counterpart, the T6–12 h sample demonstrates superior sedimentation behaviour in the fine grain region. After this, β-Mg₁₇Al₁₂ phase precipitation is more evident in both large and small grains. As the ageing time was increased to 36 h, the area proportion of precipitated regions steadily increased in both aged alloys. As a result of this, aged T6 samples have relatively low hardness values [47]. The AZXX alloys are composed primarily of a matrix of α-Mg with β-Mg₁₇Al₁₂ along the grain boundaries. The AZXX non-equilibrium solidification process produces the intermetallic β-Mg₁₇Al₁₂ phase. The second β-Mg₁₇Al₁₂, which is separated from the α-Mg solid solution up on cooling, forms a lamellar microstructure around the eutectic phase. Mg₁₇Al₁₂ particles tend to cluster near grain boundaries, but most of those particles were dispersed [48]. The inverse pole figure of Mg, AZ31, and AZ91 at various deformation stages is depicted in Figure 4.

Studies have shown that as extrusion temperature is lowered, fewer dynamic recrystallized (DRX) grains are produced [50,51]. Strain-induced precipitates are affected by the high effective strain at the interface between the extruder and along the sample, which decreases gradually toward the centre [52,53]. As strain increases, the lamellar Mg₁₇Al₁₂ phases surrounding grain boundaries become thinner and narrower, and the percentage of dynamically recrystallized grains in the sample’s volume rises. DRX typically initiates at gradients near grain boundaries because these regions are rich in potential nucleation sites [54]. As strain increases, the defect density and significant distortion of the lattice increase, leading to the formation of tiny crystal particles at the nucleation sites. Hot deformation causes a reduction in the amount of lamellar β-Mg₁₇Al₁₂ phases and an increase in the number of fine grains [48] by breaking the coarse and dendritic microstructures of the Mg and β-Mg₁₇Al₁₂ phases, elongating the grains, and dissolving the majority of the β-Mg₁₇Al₁₂ phases into the Mg matrix. The coarse grain boundaries were clearly seen, and the average grain size was measured to be 40 m. Figure 5 shows an XRD pattern for the extruded alloy, which reveals the presence of three intermetallic phases with distinct shapes: Mg₁₇Al₁₂ (lamellar), Al₄Ce (block), and Al₁₁Mn₄. The microstructures of the extruded magnesium alloy showed non-uniform grain structure with fine and coarse grains after being in solution at 370 °C for 12 h [55]. The benefits of using finer grains are many times greater than the costs associated with using coarser grains [56]. Alloying elements, such as rare earth elements (REE), are frequently used in AZXX alloy to increase its strength. Since Al has a higher electronegativity than Mg, the REE readily forms intermetallic phases with Al (for example, Al₂Nd) instead of Mg [57,58,59] In Mg alloys like AZ31, AZ61, AZ80, and AZ91, the presence of the lamellar-shaped Mg₁₇Al₁₂ phase has been observed [59].

4. Mechanical Properties

The Vickers microhardness test revealed that the hardness of Mg AZ80 alloy was constant at its core, but ranged from 64 HV to 66 HV over its surface. Microhardness in the surface layer is highly heterogeneous in the width direction. It is hardest at the plate’s outermost edge and gradually softens inward [32]. The findings also corroborate the hypothesis that hardness improves with smaller grain size [32,60,61]. The tensile properties of AZ31 were found to be significantly impacted by the material’s microstructure. The homogenization process did not significantly alter or improve the mechanical properties. Specifically, if the temperature at which the material ages is raised, the yield strength will fall, and vice versa [62]. In general, the yield strength and tensile strength of AZXX increase with the weight percentage of Al content, while the castability and weldability rapidly decrease [63]. The ductility improves dramatically across the board for twin rolling deformation temperatures. The dislocation substructure is removed, and homogeneous fine-grained structures form as a result, leading to more consistent deformation. Due to this, the homogenised material’s yield strength decreases over time [44]. The mechanical properties of AZXX are modified by the presence of brittle secondary phases [64]. Figure 6 shows the effect of processing technique on tensile properties of the AZ80 alloy.

As the AZ61 alloy is heat treated, the intermetallic particles dissolve, which leads to the formation of Al in higher concentrations on the Mg matrix, that in return induces solid solution strengthening [65]. In AZ91 alloy, it is observed that the alloys with increasing content of Sc have enhanced tensile properties as a result where Al is not segregated along the grain boundaries in the Mg matrix. It also pins down motion of dislocations causing dispersion hardening [66]. In most cases, the cracks are initiated at the intermetallic phases. These intermetallic phases are the main cause of failure, where the distribution and shape of the intermetallic phases play a significant role in altering the strength of the alloy [45].

The LCF (Low-Cycle Fatigue) lifetime of the AZ91 alloy at a given strain amplitude grows longer as the extrusion temperature rises. Since LCF failure results from the loss of fracture ductility due to cyclic deformation, the elongation of a metallic material is directly related to its LCF resistance. According to the research, the tensile elongation of AZ91 extruded at 400 °C is significantly lower than that of AZ91 extruded at 300 °C. The former has longer LCFs than the latter do.

Regardless of the material, a flat fracture surface with small fatigue striations is produced when fatigue cracks initiate on the surface of a specimen and propagate into its interior. Fatigue fracture occurs when the load-bearing area of a material is gradually reduced [19]. There are secondary cracks and features that look like fatigue striations in the area where the fatigue crack is propagating [67]. It has been estimated using a Charpy impact testing machine how tough hot-extruded AZ31B alloy is. The results show that the impact toughness can be increased by nearly half (10.7 J/cm²) when processed with four passes at an extrusion temperature of 260 °C. However, the products are anisotropic because of the directionality of the processing method [68]. There is an intermetallic phase called Mg₁₇Al₁₂ in AZXX’s microstructure. Due to its low thermal stability, this phase has a lower creep resistance at higher temperatures [69]. With a decrease in creep strain and a higher creep rate throughout the 120 °C and 70–90 MPa creep tests, the material’s resistance to creep decreases with increasing stress [70]. By vying with lattice diffusion, dislocation creep is found to be the dominant creep mechanism. Microcracks are seen to form and begin propagating during the acceleration creep stage [71]. Processing methods have a significant impact on the creep behaviour of magnesium alloys made via extrusion or die-casting. The strain-hardening coefficient increases as the fraction of Al and its intermetallic phases in the total metal weight rises [72]. In comparison to AZ82, AZ84, AZ124, AZ86, and AZ126, AZ122 has weaker creep properties because of its higher Zn content, and alloys with a high Al content are susceptible to softening at high temperatures due to the presence of intermetallic phase [73]. This is the stage where cracks start to form in Mg₁₇Al₁₂ because of the material’s brittleness. The ductility or percentage elongation of an AZXX alloy is determined by the distribution of the phase and the volume fraction of the phase [16,74]. Overall, the grain size and the presence of intermetallic phases alter the mechanical properties. It is observed that hardness increases with grain refinement; tensile strength, strain-hardening coefficient, and the yield strength improve with more saturation of Al in Mg matrix whereas creep resistance decreases.

5. Corrosion Behaviour

There are a number of electrochemical methods that can be used to forecast the corrosion behaviour of any given material (potentiodynamic polarisation is a commonly used technique). In potentiodynamic polarisation, the chemical reaction is kicked off by applying an electric potential difference between a reference and working electrode (E_Corr), and the rate of corrosion is then calculated from the corresponding current, referred to as the corrosion current (i_Corr), which is measured in amperes per square centimetre. A greater resistance to corrosion would be indicated by a larger E_Corr and a smaller i_Corr. In a 3.5% NaCl solution, the i_Corr values for AZ01, AZ21, AZ41, AZ61, and AZ91 are found to be 65.86, 5.74, 3.16, 9.80, and 36.0 A/cm², respectively. AZ41 has been found to have excellent corrosion resistance [75]. Corrosion resistance improves with increasing Al content up to about 4%, but then decreases as a result of coarsening intermetallic phases [75,76]. Corrosion resistance was improved in other works, regardless of Al content (≥4%) [77]. When it comes to corrosion, the distribution of the phase is more important than the weight percentage of Al [78], as demonstrated by AZ80, which has a corrosion rate of 0.03 mm/year after the T6 process. Grain refinement and α-Mg protection under the continuously distributed particles that act as a coverage make plastic deformation an effective method for reducing AZXX’s grain size and increasing the material’s resistance to corrosion. Corrosion in AZ80 increases with increasing deformation at 250 °C. Corrosion resistance at 400 °C is independent of the degree of deformation [79]. The AZXX alloys usually contain three regions such as α-Mg, β-Phase which is due to the increased percentage of Aluminum (>1%) and a eutectic region with a mixture of α and β phases. Heat treatment reduces the proportion of the second phase, which reduces corrosion in AZ31 and AZ91Mg alloys [80]. According to reports, the morphology of the secondary phase in AZ91 alloy makes it more susceptible to corrosion in 3.5% NaCl than other AZ alloys like AZ21, AZ41, and AZ61. The intermetallic phase forms AlMg_x(OH)_y which interrupts the normal growth of the oxide film [75]. Diagrammatically depicted in Figure 7 is the corrosion behaviour of AZXX alloys. When it comes to the corrosion behaviour of AZXX Mg alloy, the volume fraction of the intermetallic or secondary or β phase is crucial. Galvanic or localised corrosion is facilitated by the Al-rich phase acting as a cathode [81]. This limits the anodic nature of the α-Mg. Secondary phases were dissolved in the Mg matrix up to the application of heat, which may cause widespread corrosion [82]. Corrosion resistance in AZXX alloy is improved with age due to the formation of a continuous phase along the grain boundaries [78]. Corrosion rate is affected by the shape, type, and distribution of the phase due to the potential difference between the α and β phases [83]. In particular, AZ80 has a lower corrosion resistance because of the fine and discontinuous distribution of Mg₁₇Al₁₂ [84].

Additionally, the propensity for hydrogen embrittlement is increased by the increased transportation of hydrogen in the alloy. When NaCl medium is used instead of air, ductile fractures transform into trans-granular stress corrosion cracking [85]. The threshold stresses for AZ91 and AZ31 during Stress Corrosion Cracking (SCC) in pure water were 55–75 MPa and 105–170 MPa, respectively [86]. The corrosion rate of an alloy is affected not only by the concentration of the corrosion medium, but also by its flow rate. Both static and moving corrosion rates were reported by the studies. As the flow rate of the electrolyte (corrosive medium) is increased, the corrosion rate of the AZ80 alloy is decreased, according to the study [87]. However, in a biological setting, the fluid’s dynamic nature mitigates inflammation while hastening degradation [88]. As the flow rate of the electrolyte rises, the corrosion rate rises as well because the corrosion-protective corrosion products or passive layer on the alloy’s surface have been washed away and the new surface is in contact with the electrolyte [89,90,91]. Furthermore, hydrogen evaluation and passivation rate are affected by the pH of the electrolyte, which in turn affects corrosion behaviour [87,92]. Anodizing, heat treating, or coating with composites are all examples of surface modifications that can be used to boost corrosion resistance [93]. While many studies have focused on improving Mg alloys’ resistance to corrosion, there is still room to learn more about the material’s behaviour under the conditions encountered in practical settings. Table 2 shows the corrosion rate and electrical parameters corresponding to it. It is clearly understood that the increase in Al content leads to an increase in I_Corr, which increases the anodic nature of the Mg alloy. The T6 heat treatment also shows a more promising corrosion resistance than as-fabricated and T4 alloys, due to the formation of the continuous intermetallic phases. This acts as a cathodic barrier and enhances the corrosion rate of α-Mg, as a result of galvanic corrosion between α-Mg and Mg₁₇Al₁₂. The corrosion of the alloy is initiated at the α-Mg phase.

6. Summary and Outlook

Because of the correlation between microstructure, mechanical properties, and corrosion behaviour, this review sheds light on the significance of the magnesium AZ series alloys. Many types of AZXX alloys are processed by extrusion and homogenization. However, there is a need for the homogenization process because the extrusion method exhibits surface-to-core grain refinement variation that must be addressed extensively. To ensure that the AZXX alloys are processed with consistent bulk properties, modern casting techniques are adapted for use with these materials, and heat treatment is often combined with casting to further improve properties. When processing coarse and dendritic Mg AZXX alloys, second phase precipitates of Mg can be seen along the grain boundaries. In the case of AZXX, the mechanical and corrosion properties are influenced by the size, shape, and distribution of the secondary phases, which are primarily a brittle Mg₁₇Al₁₂. For engineering applications, the presence of phases is undesirable because they improve corrosion resistance at the expense of mechanical properties. Therefore, research efforts should be redirected toward achieving an optimum volume fraction of secondary phases through possible processing techniques, which may aid in broadening the application of AZXX Mg alloys. Alloy properties are modified because of how the processing techniques modify the microstructure of the alloy. Grain size and morphology can be predicted by modelling static and dynamic recrystallization phenomena, even though there is work reported on various processing techniques of AZ31, AZ41, AZ61, AZ80, AZ91, and other AZXX alloys. Despite decades of study, processing techniques for AZ series Mg alloys can be refined further to increase their tensile, creep, and corrosion resistance.

Author Contributions

Conceptualization, A.M. and A.R.A.; methodology, S.P.B., T.U.D., S.D. and M.S.; formal analysis M.S. and A.R.A.; investigation, S.P.B., T.U.D., S.D. and M.S.; resources, S.P.B., T.U.D. and S.D.; data curation, S.P.B., T.U.D. and S.D.; writing—original draft preparation, S.P.B. and M.S.; writing—review and editing, S.P.B., M.S., A.M. and A.R.A.; supervision, A.M. and A.R.A. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Tables

Figure 1. Phase diagram of Mg–1Zn–xAl (PANDAT Software) [19].

Figure 2. As-cast AZ61 Magnesium alloy (a) SE micrograph (b) inverse pole figure [39].

Figure 3. Microstructure of extruded AZ80 Mg alloy after heat treatment (a,b) T4, (c,d) T5, and (e,f) T6. CP, continuous precipitate; DP, discontinuous precipitate [45].

Figure 4. Electron Back Scattered Diffraction’s Inverse pole figure maps of rolled Mg, AZ31 and AZ91 alloys at different deformation stages: (a–c) Region A—initial (d–f) Region B—medium, (g–i) Region C—final [49].

Figure 5. XRD results of rare elements based AZXX alloy [55].

Figure 6. Stress vs strain graph of AZ80 alloy with different processing techniques [45].

Figure 7. Schematic illustration corrosion mechanism on (a) AZ21 and AZ41 and (b) AZ61 and AZ91 [75].

Table 2

Corrosion behaviour of AZXX Mg alloys.

Materials	Electrolyte	Method	ECorr (V)	ICorr (mA/cm2)	C.R. (mm/Year)	Ref.
AZ80—T4 390 °C 24 h	3.5% NaCl	Immersion test	-	-	18.2	[78]
AZ80—T6 250 °C 16 h			-	-	3
AZ80—T6 250 °C 30 h			-	-	5
AZ80—T6 250 °C 48 h			-	-	3
AZ80—T6 330 °C 16 h			-	-	12
AZ80—T6 330 °C 30 h			-	-	11
AZ80—T6 330 °C 48 h			-	-	9
AZ80	3.5% NaCl	Immersion test	-	-	8	[84]
		Potentiodynamic	−1.446	6.037 × 10−2	-
AZ80—T4 420 °C 12 h		Immersion test	-	-	14
		Potentiodynamic	−1.424	5.574 × 10−2	-
AZ80—T6 175 °C 28 h		Immersion test	-	-	5
		Potentiodynamic	−1.436	5.713 × 10−2	-
AZ80	SBF—pH: 4	Potentiodynamic	−1.7016	8.5627 × 10−2	-	[87]
	SBF—pH: 7.4		−1.5957	3.7453 × 10−3	-
	SBF—pH: 9		−1.4905	6.8479 × 10−4	-
AZ31	3.5% NaCl	Potentiodynamic	−1.535	44.25	-	[94]
AZ61			−1.529	26.74	-
AZ91			−1.489	24.02	-

References

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