Effect of Isothermal Forging on the Microstructure and Mechanical Properties of Mg-8Gd-3Y-0.5Zr Alloy

HOU Xiangwu，WANG Yanbo，ZHOU Haitao，SUN Xin，GONG Zhengxuan，JIANG Shanyao，XIAO Lü

（Shanghai Spaceflight Precision Machinery Institute，Shanghai 201600，China）

Abstract:Aiming at the problems of poor plastic forming ability，narrow forging temperature range，and strain rate sensitivity of rare earth magnesium alloys，a study on the microstructure and mechanical properties of Mg-8Gd-3Y-0.5Zr alloy with different isothermal forging processes is carried out.The microstructure and properties of the alloy in the as-cast，isothermal forged，and post-aging states after forging are studied with optical microscope（OM），scanning electron microscope（SEM），and tensile testing.The results show that significant dynamic recrystallization occurs during the isothermal forging process，a fine equiaxed grain structure is formed，and the mechanical properties of the alloy are greatly improved.When the isothermal forging temperature is 460 °C and the strain rate is 0.02 s-1，the alloy structure performance is the best，the room temperature tensile yield strength（TYS）is 218 MPa，the ultimate tensile strength（UTS）is 299 MPa，and the fracture elongation（FE）is 19.2%.When the alloy is post-forging artificial aged，the α-Mg matrix is dispersed，the Mg5（Gd，Y）phase is precipitated，the UTS of the alloy is increased to 392 MPa，and the FE is reduced to 12.0%.

Key words:Mg-8Gd-3Y-0.5Zr alloy；isothermal forging；microstructure；mechanical property

0 Introduction

In recent years，as the demands for energy sav‑ing，weight reduction，and environmental protection in the fields such as aerospace and automotive indus‑tries have become increasingly urgent，lightweight components have become an important development trend，and the demands for lightweight and highstrength materials are increasing.Magnesium alloy is currently one of the lightest metal structural materi‑als in industrial applications.It has won a widespread favor owing to its excellent performance，and is con‑sidered to be the most promising metal materials for the development and application in the 21st century non-ferrous metal materials.However，the mechan‑ical properties of magnesium alloys are low，which limits their applications.Therefore，how to improve the mechanical properties of magnesium alloys has become the focus of research on magnesium alloy ma‑terials and their preparation processes.

The isothermal forging technology，as a near net shape technology，is a new forging process developed on the basis of the traditional forging technology.The technology mainly heats the dies and the blank to the optimal temperature required for material defor‑mation，so that the material can deform at a lower strain rate while ensuring that the temperature of the blank remains basically unchanged during the defor‑mation process.The isothermal forging technology is generally used for metals with a narrow forging tem‑perature range，e.g.，magnesium alloys and titanium alloys.This kind of metal is very sensitive to the de‑formation temperature.Especially，when forging thin webs，high ribs，and frame components，the tempera‑ture drops sharply，the deformation resistance increas‑es rapidly，and the plasticity decreases sharply.Therefore，it is difficult to form by conventional forg‑ing methods.The isothermal forging technology can significantly improve the plasticity and fluidity of this type of alloy.At present，the maximum aspect ratio of the ribs formed by isothermal precision forging can reach 23∶1，and the thickness of the web can reach 0.5 mm-2 mm.Therefore，it has been widely used in the manufacture of hard-to-deform alloy parts in the aviation and aerospace fields.

RAO et al.studied the isothermal forging pro‑cess of AZ31B magnesium alloy webs.The results showed that the basal structure of the plate and the slip of the prismatic crystals at lower temperatures led to the anisotropy of the flow during rolling，and the pyramidal slip and cross slip in the dynamic re‑crystallization destroyed the texture and restored the metal flow symmetry.HE et al.studied the isother‑mal forging technology of AZ80 magnesium alloy support beam.The results showed that the average grain size of the part with a sufficient deformation amount is small，a fine and uniform recrystallized structure was obtained，and the mechanical proper‑ties were excellent.The technology has been success‑fully applied to a certain type of helicopter.SALE‑VATI et al.studied the relationship between the microstructure and mechanical properties of Mg-6Al-0.35Mn alloy formed under multi-directional forging（MDF）isothermal conditions，and showed that the MDF process significantly increased the dislocation density，thereby improving the mechanical proper‑ties.SHAN et al.studied the isothermal forging process of Mg-10Gd-2Y-0.5Zn-0.3Zr alloy rib-web bracket，and showed that the isothermal forged micro‑structure of the alloy refined obviously and amounts of secondary phases precipitated on the matrix during the isothermal forging process.WU et al.studied the isothermal forging process of the Mg-Gd-Y-Zn-Zr Mg alloy complex shape part.Isothermal precision forging experiments of magnesium parts with high performance and complex shape were successfully carried out.

In this paper，the microstructure evolutions of Mg-8Gd-3Y-0.5Zr alloy during isothermal forging are investigated.The mechanical properties of the alloy after isothermal forging are also studied to illustrate the effects of microstructure characteristics on the me‑chanical properties.In addition，the post-aging treat‑ment is also conducted to illuminate the precipitation characteristic and precipitation hardening behavior.

1 Material and methods

In this paper，the as-cast Mg-8Gd-3Y-0.5Zr al‑loy is selected as the material，and is solid-solution treated.The solid solution treatment parameter is 480 °C×15 h.The nominal composition of the alloy is illustrated in Tab.1.

Tab.1 Nominal composition of Mg-8Gd-3Y-0.5Zr alloy

A 6 300 kN pressure testing machine is used to perform the isothermal forging experiment of the sol‑id solution alloy at the forging temperature of 400 °C，430 °C，460 °C，490 °C，and the strain rate of 0.01 s，0.02 s，0.05 s，0.1 s.The blank size before forging is80 mm×45 mm.The defor‑mation is 80%，and the blank size after forging is180 mm× 9 mm.The macroscopic morphology of the isothermal forged Mg-8Gd-3Y-0.5Zr alloy is shown in Fig.1.In order to improve the tensile strength of the isothermal forged alloy，post artificial aging treatment is carried out on the alloy.The aging treatment process parameter is 215 °C×30 h.

Fig.1 Macromorphology of isothermal forged Mg-8Gd-3Y-0.5Zr alloy

In this paper，the GX60-DS metallographic anal‑ysis system is used to observe the alloy microstruc‑ture，and the FEIQUANTA450 scanning electron microscope and its equipped energy spectrometer are used to observe the alloy microstructure.The CMT5305 electronic universal material testing ma‑chine is used for the tensile test，and the tensile rate is 2 mm/min.The locations for optical observation and tensile test samples are shown in Fig.2.

Fig.2 Locations for optical observation and tensile test samples

2 Results and discussion

The microstructure of the as-cast Mg-8Gd-3Y-0.5Zr alloy obtained with optical microscope（OM）and scanning electron microscope（SEM）is shown in Fig.3.It can be seen that the as-cast structure of the alloy is mainly composed of the primary α-Mg sol‑id solution and a skeleton-like eutectic phase distribut‑ed at the grain boundary，and spherical cores are found inside the grains.According to Ref.［15］，it can be seen that the as-cast alloy structure is mainly composed of the α-Mg matrix and the Mg（Gd，Y）eutectic second phase at the grain boundary.Due to the low solid solubility of Gd and Y elements in mag‑nesium alloy，these two elements are prone to enrich‑ment and segregation during solidification，and the enrichment of the solute elements increases the sub‑cooling of the composition，resulting in the increase in the secondary dendrites.A skeleton-like eutectic phase is formed，in which the concentration of rare earth（RE）elements is higher than that inside the crystal grains.

Fig.3 Microstructure of the as-cast Mg-8Gd-3Y-0.5Zr alloy

The microstructure of the as-homogenized Mg-8Gd-3Y-0.5Zr alloy is shown in Fig.4.After the as-cast Mg-8Gd-3Y-0.5Zr alloy is homogenized at 480 °C for 15 hours，the eutectic phase at the grain boundary is fully dissolved into the α -Mg matrix，and the square-shaped phase forms in the grain and near the grain boundary.The grain size is still sta‑ble，and an excellent solid solution effect is obtained consequently.After the solution treatment，with the dissolution of the eutectic phase，the RE elements originally segregated at the grain boundaries are uni‑formly distributed in the α-Mg matrix.

Fig.4 Microstructure of the as-homogenized Mg-8Gd-3Y-0.5Zr alloy

The microstructures of the isothermal forged Mg-8Gd-3Y-0.5Zr alloy at different forging tempera‑tures（the strain rate is 0.02 s）are shown in Fig.5.Under the condition of a certain strain rate，there is no significant difference between the grains of the al‑loy with an isothermal forging temperature of 400 °C and the as-homogenized structure.The structure of the Mg-8Gd-3Y-0.5Zr alloy is not refined.The struc‑ture is loose and scattered，while the grains are coarse.The SEM results show the presence of Zr particles and square-shaped RE-rich phases.When the isothermal forging temperature reaches 430 °C，dynamic recrystallized grains are found at the grain boundary.This is because the cross-slip and non-in‑terface slip system of the magnesium alloy can be started after the temperature rises，and a small amount of dynamic recrystallization takes place at the grain boundary of the magnesium alloy.When the iso‑thermal forging temperature rises to 460 °C，the tem‑perature is sufficient to support the full dynamic re‑crystallization of the Mg-8Gd-3Y-0.5Zr alloy.The deformation is sufficient，and the grains are fully re‑fined.The grains become dense，and the structure state is the best.It can be seen that with the increase in the forging temperature，the diffusion capacity of RE elements such as Gd and Y is enhanced，and the supersaturated solid solution is formed in the α-Mg matrix.The square-shaped RE-rich phase is dis‑persed and distributed in the alloy.The recrystalliza‑tion area gradually expands，the dynamic recrystalli‑zation becomes an equiaxed grain structure of differ‑ent sizes，and the size of the crystal grains becomes smaller and smaller.However，too high forging tem‑perature is not conducive to the refinement of alloy grains.When the isothermal forging temperature reaches 490 °C，the internal grains of the alloy are coarse.It can be seen that the forging temperature has a significant effect on the isothermal forging struc‑ture of the Mg-8Gd-3Y-0.5Zr alloy.

Fig.5 Microstructures of the isothermal forged Mg-8Gd-3Y-0.5Zr alloy at different forging temperatures

The mechanical properties of the isothermal forged Mg-8Gd-3Y-0.5Zr alloy at different forging temperatures（the strain rate is 0.02 s）are shown in Fig.6.It can be seen that the tensile yield strength，ultimate tensile strength（UTS），and fracture elonga‑tion（FE）of the Mg-8Gd-3Y-0.5Zr alloy gradually in‑crease to the highest points and then slowly decrease with the increase in the forging temperature.When the isothermal forging temperature is 460 °C，the al‑loy structure performance is the best，the room tem‑perature tensile yield strength（TYS）is 218 MPa，the UTS is 299 MPa，and the FE is 19.2%.This re‑sult is due to the continuous increase in the amount of deformation during the alloy deformation，and the work hardening caused by the dislocation entanglement in the alloy increases the deformation resistance of the alloy.When the deformation contin‑ues，the alloy produces fine dynamic recrystallized grains.When the isothermal forging temperature in‑creases，the number of recrystallized grains increas‑es，which causes softening and decrease in the tensile strength of the alloy.When the forging temperature increases to 490 °C，the mechanical properties of the alloy decrease slightly.This is because the high tem‑perature makes the internal grains of the alloy coarse.In addition，the higher the forging temperature is，the easier it is for the alloy to deform.Moreover，the internal stress generated cannot be eliminated，caus‑ing the material to fracture prematurely.

Fig.6 Mechanical properties of the isothermal forged Mg-8Gd-3Y-0.5Zr alloy at different forging temperatures

The isothermal forged microstructure of the Mg‑8Gd‑3Y‑0.5Zr alloy at different strain rates（the forg‑ing temperature is 460 °C）is shown in Fig.7.Under the certain conditions of isothermal forging tempera‑ture，the alloy grain structure with a strain rate of 0.01s，owing to the lower isothermal forging strain rate，has a higher degree of dynamic recrystallization，Zr particles and RE‑rich phases are streamlined.However，since the deformation time is long，the re‑crystallized grains grow slightly.When the strain rate is 0.02 s，（see Fig.5（e）and Fig.5（f）），the dyn‑amic recrystallization degree of the alloy reaches the maximum，and the strengthening effect of the grain boundary is strengthened，so that the deformation is concentrated in the area where recrystallization does not take place，which leads to an increase in the twin density.The recrystallization process is accelerated，and the uniform complete recrystallized structure is obtained.When the strain rate is 0.05 s，the effect of twinning is obviously weakened，and there are no more twins in the new‑born recrystallized grains.This is because when the alloy structure is refined to a certain extent，the effect of the grain boundary slip is enhanced，twins no longer dominate the alloy deform‑ation process，and thus the alloy can no longer be refined through twin recrystallization.When the strain rate is 0.1 s，the heat generated by the severe defor‑mation of isothermal forging is difficult to dissipate in a short time，which causes the temperature of the alloy to rise significantly.Under the strong thermal activation，the surface energy of the grain boundary decreases spontaneously through the growth of crystal grains，which results in the growth of recrystallized grains.

Fig.7 Microstructure of the isothermal forged Mg-8Gd-3Y-0.5Zr alloy at different strain rates

The mechanical properties of the isothermal forged Mg-8Gd-3Y-0.5Zr alloy at different strain rates（the forging temperature is 460 °C）are shown in Fig.8.It can be seen that the TYS and UTS of the Mg-8Gd-3Y-0.5Zr alloy gradually increase to the highest points and then slowly decrease with the increase in the strain rate.The FE gradually decreases with the increase in the strain rate.When the strain rate is 0.02 s，the alloy mechanical properties are better，the room temperature TYS is 218 MPa，the UTS is 299 MPa，and the FE is 19.2%.Appropriately reducing the strain rate of isothermal forging，the internal plastic deformation of the alloy could be slow enough to cause sufficient dynamic recrystallization inside the alloy.Then，the second phase of the alloy will be broken，gradually disperse and streamlined，the grains will be fully refined，and the structure will become densified and uniform.The refined grains and the distribution of the second phase are also effective to prevent the generation and propagation of cracks to a certain extent.At the same time，the residual stress inside the alloy disappears，and the mechanical properties，especially the FE，are greatly improved.

Fig.8 Mechanical properties of the isothermal forged Mg-8Gd-3Y-0.5Zr alloy at different strain rates

According to the influence law of isothermal forging temperature and strain rate on the microstructure and mechanical properties of the Mg-8Gd-3Y-0.5Zr alloy，the optimal isothermal forging process is determined.The forging temperature is 460 °C，the strain rate is 0.02 s.The energy dispersive spectroscopy（EDS）analysis is performed on the Mg-8Gd-3Y-0.5Zr alloy with the isothermal forging process parameters，as shown in Fig.9.According to the EDS result in Fig.9（b），the square-shaped phase is the RE-rich phase，and the particle phase is Zr.From the element mappings in Fig.9（c），Fig.9（d），Fig.9（e），and Fig.9（f），it can be seen that the contents of Mg element in the square-shaped phase and the particle phase are less，while the content in the α-Mg matrix is higher.The distribution of Gd element is relatively uniform，and the content of Gd element in the square-shaped phase is slightly higher.The Y element mainly distributes in the square-shaped phase.The Zr element has the highest content in the granular phase，and a slightly higher content in the square-shaped phase.RE elements and Zr element accumulate in the RE-rich square-shaped phase.Zr atoms aggregate to form a granular phase.The streamlined dispersion distribution of the RE-rich phase and Zr particles can effectively prevent the generation and propagation of cracks，and greatly improve the mechanical properties of the alloy.

Fig.9 EDS results of the isothermal forged Mg-8Gd-3Y-0.5Zr alloy

The Mg-8Gd-3Y-0.5Zr alloy under the best iso‑thermal forging process is post-forging artificial aged.The aging treatment process parameter is 215 °C×30 h.The microstructure of the post-forging aged Mg-8Gd-3Y-0.5Zr alloy is shown in Fig.10.It can be seen that the second phase Mg（Gd，Y）is dis‑persed and distributed during the post-forging artifi‑cial aging process.The precipitation method of alloy aging is based on discontinuous precipitation and then continuous precipitation.Since the lamellar new phase has the smallest relative capacity strain energy when the non-coherent new phase is formed，it is in an advantageous position in terms of energy and is easy to form.Therefore，in the early stage of the ag‑ing process，the second phase preferentially precipi‑tates into the grain at the grain boundary with higher contents of Gd and Y in the discontinuous precipita‑tion mode，and the distribution of the precipitated phases is in the form of bands.When the discontinu‑ous precipitation reaches a certain stage，the second phase will begin to precipitate in the form of fine dots and needles.During the precipitation，the Gd and Y contents of the surrounding matrix continue to decrease，and the lattice constant changes continu‑ously，i.e.，it precipitates in a continuous precipita‑tion mode.With the massive precipitation and shape transformation of Mg（Gd，Y），the microstructure and mechanical properties of the alloy change.

Fig.10 Microstructure of the post-forging aged Mg-8Gd-3Y-0.5Zr alloy

The mechanical properties of the Mg-8Gd-3Y-0.5Zr alloy in different states are shown in Fig.11.For the as-cast alloy，the room temperature TYS is 131 MPa，the UTS is 218 MPa，and the FE is 9.3%.For the isothermal forged alloy，the room temperature TYS is 218 MPa，the UTS is 299 MPa，and the FE is 19.2%，which are 66.4%，37.2%，and 106% higher than those of the as-cast alloy，re‑spectively.The alloy undergoes dynamic recrystalli‑zation during the isothermal forging process，which makes the grain refined，so that the grain boundary area per unit volume becomes larger.The resistance of the small continuous grain boundary to the move‑ment of the dislocation also becomes greater，thus making its strength and plasticity better.For the postforging aged alloy，the room temperature TYS is 309 MPa，the UTS is 392 MPa，and the FE is 12.0%，which are 136%，79.8%，and 29.0% high‑er than those of the as-cast alloy，respectively.This is because the post-forging artificial aged alloy precip‑itates the Mg（Gd，Y）phase，which hinders the movement of dislocations.Since dislocations accumu‑late in a large amount at the grain boundaries，the stress is large，and microcracks are induced，which leads to the TYS and UTS of the alloy increase and the plastic toughness decrease.

Fig.11 Mechanical properties of the Mg-8Gd-3Y-0.5Zr alloy in different states

The tensile fracture morphology of the Mg-8Gd-3Y-0.5Zr alloy is shown in Fig.12.The results of Fig.12（a）show that the RE-rich phase of the isother‑mal forged Mg-8Gd-3Y-0.5Zr alloy is further broken or integrated into the grains.Under stress conditions，cavities are formed at the second phase particles.Under the action of slip，the cavity gradually grows up and connects with other cavities to form a dimple fracture.Therefore，it appears as a ductile fracture.After isothermal forging，the alloy is completely re‑crystallized，the grains are refined，and the second phase distribution is further improved.Therefore，the isothermal forged Mg-8Gd-3Y-0.5Zr alloy has better strength and toughness.The results of Fig.12（b）show that after artificial aging treatment，the ten‑sile fracture morphology of the alloy has changed significantly，which is manifested as the brittle frac‑ture dominated by cleavage，along with an increase in the intergranular fracture and tearing edges.This is because the Mg（Gd，Y）phase precipitates in the grain or grain boundary，grows into the grain after aging treatment，which plays a pinning role，greatly enhances the bonding force between the grains，and makes the tensile fracture present the characteristic of the brittle fracture dominated by cleavage.The cleavage fractures increase，and the dimples decrease.

Fig.12 Tensile fracture morphology of the Mg-8Gd-3Y-0.5Zr alloy

3 Conclusions

The effects of isothermal forging and artificial aging after forging on Mg-8Gd-3Y-0.5Zr alloy are studied.The detailed analysis of the microstructure characterization and mechanical properties leads to the following conclusions.

1）The mechanical properties of the Mg-8Gd-3Y-0.5Zr alloy gradually increase and then decrease with the increase in the isothermal forging tempera‑ture.The forging temperature at the highest point of performance is 460 °C.At 460 °C，the alloy under‑goes sufficient dynamic recrystallization，the crystal grains are sufficiently refined，and the microstructure is the best.

2）The tensile strength of the Mg-8Gd-3Y-0.5Zr alloy increases gradually and then decreases with the increase in the strain rate.The strain rate at the highest point of strength is 0.02 s.The FE of the alloy gradually decreases with the increase in the strain rate.At 0.02 s，the dynamic recrystallization degree of the alloy reaches the maximum.

3）The optimal isothermal forging process of the Mg-8Gd-3Y-0.5Zr alloy is determined.The forging temperature is 460 °C，and the strain rate is 0.02 s.Under this condition，the alloy has a room tempera‑ture TYS of 218 MPa，a UTS of 299 MPa，and an FE of 19.2%.The EDS results show that the block phase is RE-rich phase and the particle phase is Zr.

4）For the post-forging aged Mg-8Gd-3Y-0.5Zr alloy，the room temperature TYS is 309 MPa，the UTS is 392 MPa，and the FE is 12.0%.The postforging artificial aged alloy precipitates Mg（Gd，Y）phase，which greatly enhances the bonding force between the grains，resulting in an increase in the tensile strength of the alloy and a decrease in the plastic toughness.