Enhanced microstructural stability and mechanical properties of the Ag-containin

S.Najafi,R.Mahmudi

School of Metallurgical and Materials Engineering,College of Engineering,University of Tehran,Tehran,Iran

Received 23 March 2020;received in revised form 19 July 2020;accepted 12 August 2020 Available online 29 September 2020

Abstract The effect of 0.5,1 and 1.5 wt% Ag addition on the microstructural evolution,thermal stability and mechanical properties of an Mg–5 wt% Gd–1 wt% Y(GW51)alloy was investigated.The as-cast microstructure of the base alloy consisted of the Mg5(Gd,Y)phase in the α-Mg matrix.The obtained results revealed that Ag addition refines the dendritic microstructure of the base alloy,promotes the formation of the new Mg16Gd2YAg phase,and increases the volume fraction of the Mg5(Gd,Y)particles.These events resulted in improved hardness,strength,and microstructural stability of the Ag-containing alloys in the as-cast condition and after prolonged exposure to high temperature.The superior mechanical properties of the quaternary alloys over those of the tertiary alloy at low and high temperatures stems from the solid solution hardening effect of Ag,presence of the thermally stable Mg16Gd2YAg particles,and higher volume fraction of the Mg5(Gd,Y)particles.These particles can slow down the grain growth during exposure to high temperature,enhancing the stability and strength of the alloys at both room and high temperatures.

Keywords:Mg–Gd–Y alloy;Ag addition;Thermal stability;Mechanical properties.

1.Introduction

Nowadays,it is vital to reduce fuel consumption by weight reduction in automotive industries in order to improve the energy conversion efficiency and decrease environmental issues.Magnesium alloys are promising choices for various industries due to their availability,low density and high specific strength.However,weak high-temperature strength,poor microstructural stability and extensive grain growth at elevated temperatures limit their widespread application[1,2].To deal with these drawbacks,several approaches such as alloying with different alloying elements have been proposed[3].Alloying can improve the high-temperature mechanical properties and creep resistance of Mg through various mechanisms of;(i)solid solution hardening,(ii)grain boundary strengthening,and(iii)precipitation hardening[4,5].

Al and Zn are the most common additives for Mg alloys and the AZ and AM series alloys have flourished due to their suitable balanced elongation and strength[6,7].Nevertheless,the applications of these alloys are limited to temperatures below 130°C,above which microstructural softening happens and mechanical properties degrade.This can be attributed to the presence of low melting point precipitates such as Mg17Al12,which is dissolved into theα-Mg matrix at high temperatures,leading to grain coarsening[8,9].Accordingly,other Mg alloy series containing rare-earth(RE)elements Gd and Y have been developed for high-temperature applications[10,11].It has been found that Gd can improve the thermal stability of Mg alloys by forming stable and metastable precipitates at relatively high temperatures[12].Although the Mg–Gd alloys with less than 10 wt% Gd show a slight increase in hardness after precipitation hardening,increasing Gd alone causes a price rise and a drop in ductility[13].As a result,the use other elements is needed to address these shortcomings.

Among various rare-earth elements,addition of Y has shown the most remarkable results.Solid solution strengthening is a consequence of the high solubility of Gd and Y elements in the Mg matrix[14].Since solubility of these elements decreases rapidly with a decrease in temperature,they can provide significant hardening[15].On the other hand,accumulation of these elements at grain boundaries and the subsequent arrested grain growth result in better thermal stability and mechanical properties at ambient and high temperatures,through grain boundary strengthening[16–18].Accordingly,several studies have been conducted on the development as well as microstructure and mechanical properties of the Mg–Gd–Y alloys[19–22].In a study,the effect of adding different amounts of Gd on the microstructure and mechanical properties of an Mg–3Y–0.5Zr alloy was investigated.It was reported that the volume fraction and size of precipitates increased by increasing Gd content up to 12 wt%.Also,after age hardening,the 12 wt% Gd-containing alloy showed finer microstructure than the rest of the alloys and had the highest hardness of 124Hv among all tested materials[10].The high thermal stability of these alloys is generally related to the stability of the Mg24Y5,Mg5Gd and Mg3Gd precipitates,which have been observed in this group of alloys[23,24].The thermally stable precipitates can effectively prohibit grain growth of the Mg–Gd–Y alloys at high temperatures[12].

Although Mg–Gd–Y alloys have shown an exceptional age hardening response,the significant cost of long solutionizing and aging heat treatments has encouraged researchers to look for a method to improve the high-temperature mechanical properties of the Mg alloys without utilizing such long heat treatments.This goal can be reached through the addition of alloying elements,such as Ag,which can encourage the formation of thermally stable precipitates.The addition of Ag has been reported to increase the strength of the Gdcontaining Mg alloys by modifying the microstructure,increasing the volume fraction of the particles and causing segregations at grain boundaries and twin boundaries[25–29].It has also been reported that adding Ag to Mg–Gd alloys increases the thermal stability by increasing the volume fraction of precipitates and locking of the grain boundaries[30,31].

Since previous studies have been mostly focused on the room temperature mechanical properties of the Ag-containing Mg–Gd alloys[32,33],the aim of the present study is to examine the effects of Ag on the microstructure and hightemperature shear strength of an Mg–Gd–Y alloy in the ascast condition and after annealing at high temperature for different times.The strength was assessed by the localized shear punch test(SPT),which has been employed as an efficient method to evaluate the mechanical properties of various as-cast[3,8]and wrought[34]Mg alloys.The use of this miniature testing method has been justified by the ease of sample preparation,the use of small sized samples,and the correlation of the measured strength data with those of the tensile test[35,36].

Table 1.Measured chemical composition of the tested materials.

2.Experimental procedures

2.1.Materials and processing

Four alloys with the nominal compositions of Mg–5 wt%Gd–1 wt% Y−x wt% Ag(x=0,0.5,1.0,1.5)were considered.Appropriate amounts of high purity Mg(99.8 wt%)and Ag(99.99 wt%)together with two Mg–20 wt% Gd and Mg–20 wt% Y master alloys were melted in a graphite crucible under the protection of the Foseco MAGREX 36 covering flux in an electrical furnace.To obtain a homogeneous composition,the melt was held at 780 °C for 20min and mechanically stirred for 2min,before being poured into a steel mold preheated to 200 °C.A tilt-casting technique was employed to minimize the casting defects caused by the turbulent flow of the melt.From the cast bars,1mm×3mm×30mm slices were cut for shear punch testing and microstructural characterization,using electro discharge machining.The actual chemical compositions of the studied alloys,obtained by inductively coupled plasma spectroscopy(ICP),are listed in Table 1.Thermal stability of the studied alloys was evaluated by annealing some of the cast specimens at 450 °C for 4,24 and 96h,followed by cooling in air.

2.2.Microstructural characterization

Microstructural characterization was accomplished by optical microscopy(OM),scanning electron microscopy(SEM)and energy dispersive X-ray spectroscopy(EDS).The specimens for microstructural examination were polished with 0.3-μm alumina powder and were etched using an acetic picric solution(10ml acetic acid,70ml ethanol,10ml distilled H2O and 4.2g picric acid)for 5 to 10s.OM images taken at a given magnification were used to measure the grain size of different alloys,where at least five images were used for each condition.A pictorial analysis program of Digimizer was utilized to measure the average grain size according to the ASTM-112E standard.The same image analysis software was used to measure the volume fraction of particles on at least 5 random SEM images taken at a given magnification.Phase identification was performed using EDS analysis and X-ray diffraction(XRD)with Cu-Kαradiation(k=1.5405˚A)at a scanning speed of 2° min–1.

2.3.Evaluation of shear strength

The effects of Ag as well as annealing process on mechanical properties were evaluated using SPT.From alloys annealed for 0,4,24 and 96h,a number of 10 mm×10 mm slices were cut and thinned to 0.7mm.The SPT was performed in the temperature range of 25–400 °C using a 3.125mm diameter flat cylindrical punch and a shear punch die with a 3.225mm diameter hole.For this purpose,a STM-20 SANTAM universal testing machine with a crosshead speed of 0.25mm/min was used.Using the load–displacement curves,the shear stress can be determined from Eq.(1)[37]:

Fig.1.Optical micrographs of the as-cast and annealed alloys containing different amounts of Ag.

wherePis the load,tis the specimen thickness,andDis the average of the punch and die diameters.For each condition,three different specimens were tested and the average was reported.

3.Results and discussion

3.1.Microstructural evolution and stability

Fig.1 shows the optical microstructures of the alloys in the as-cast condition and after annealing at 450°C for 4,24 and 96h.Comparison of the microstructures of the base alloy with those of the Ag-containing alloys reveals that in all specimens the as-cast conditions possess a dendritic microstructure,which is significantly refined after addition of Ag.In fact,addition of Ag in the as-cast materials leads to the breakage of the dendrites and reduction of dendrite arm spacing(DAS)in the microstructure.However,the dendritic structure disappears with annealing;so that after annealing for 4h,the dendritic microstructures of all of the studied alloys are completely transformed into equiaxed grain structures.It is worth noting that after annealing for different times,the alloys containing Ag have smaller grain sizes than the base alloy,so that increasing the Ag content causes a further decrease in the grain size.

In general,the effect of different alloying elements on grain size can be explained by the growth restriction factor(GRF).This factor can be calculated from the binary phase diagrams of the constituent elements and Eq.(2)[38]:

wheremis the slope of the liquidus line of the phase diagram,C0is the concentration of the solute element andkis its distribution coefficient.GRF values for Gd,Y and Ag in Mg are 1.03C0,1.71C0and 2.56C0,respectively[39].It can be seen that GRF of Ag is much higher than those of Gd and Y,indicating the stronger grain refining effect of this element.The higher value of GRF for Ag indicates the higher concentration of the solute Ag atoms in the liquid in front of the solid–liquid interface during solidification,which results in constitutional undercooling,and thus,reduced dendrite size[40].This argument is consistent with the microstructural evolution observations,in which the finest as-cast microstructure belongs to the GW51–1.5Ag alloy.This alloy exhibits the highest degree of thermal stability after annealing,mainly due to its finer initial dendritic structure and the presence of intermetallic compounds that impede grain growth by blocking grain boundary migration.

Fig.2.Average grain sizes of the materials after annealing at 450 °C for different times.

Fig.2 summarizes the grain sizes of all alloys after 4,24 and 96h annealing.Heat treatment destroys the dendritic structure by breaking the dendritic arms and removing the segregation in the microstructure.According to Fig.1,after 4h of annealing,the dendritic structure is still observed within the grains.It can be argued that it is not rational to discuss the grain size in as-cast condition,as it is practically after annealing that the grain structure is observed.For this reason,grain size has not been reported for the alloys in as-cast condition.On the other hand,the as-cast alloys are annealed to evaluate their grain growth,so their microstructures contain distinct grains with different sizes.Therefore,several OM images were considered and the average grain size for each sample was calculated after annealing.Fig.1 provides a qualitative comparison of microstructural changes occurring after annealing for different times.

Fig.3.SEM micrographs and EDS analysis of different phases of:(a)GW51 and(b)GW51−1.5Ag alloys in the as-cast condition.

In accordance with Fig.2,the average grain size of GW51 increases from 170μm to 289μm by prolonging the annealing time from 4 to 96h.Unlike the distinct changes observed in the grain size of the base alloy,the Ag-containing alloys exhibit higher stability and less grain growth after exposure to high temperature.For example,after annealing for 96h,the grain size of alloys containing 0.5,1 and 1.5 wt% Ag reaches 148,117 and 87μm,respectively.It is also observed that the GW51–1.5Ag alloy exhibits the least grain size variations,confirming the effective role of Ag in enhancing the thermal stability of the base GW51 alloy.

The SEM micrographs and EDS analysis of the base GW51 and GW51–1.5Ag alloys are demonstrated in Fig.3.The dark area(point A in both alloys)in the background representsα-Mg and the white particles that are sparsely dispersed in the matrix represent the secondary phases.EDS analysis was performed on points A–C to identify the corresponding phases.According to this analysis,the chemical compositions of the particles(points B and C)in the GW51 alloy,shown in Fig.3a,can be approximated by Mg5(Gd,Y).In the GW51–1.5Ag alloy,however,the second phase particles have a denser distribution,as depicted in Fig.3b.The EDS analysis of these particles shows the average composition of Mg79.9Gd9.8Y5.2Ag5.1,which corresponds to the Mg16Gd2YAg compound,as reported for other similar alloys earlier[34].

SEM images of the GW51 and GW51–1.5Ag alloys after 96h annealing are shown in Fig.4a and b,respectively.It can be observed that the base alloy containsα-Mg,along with undissolved Mg5(Gd,Y)intermetallic particles.The microstructure of the GW51–1.5Ag alloy,however,is composed of the same dispersed Mg5(Gd,Y)particles together with the more frequently occurring Ag-containing particles on both grain boundaries and inside the grains.It can also be inferred that the microstructure consists of fine and coarse particles with a cuboid morphology.EDS analysis indicated that the cubelike particles are Mg16Gd2YAg and the others are Mg5(Gd,Y)compounds.It can also be noticed that after annealing the morphology of the precipitates does not change significantly.In sum,particles with different morphologies were found in all of the microstructural observations.

Fig.4.SEM micrographs of the 96h annealed specimens:(a)GW51 and(b)GW51−1.5Ag.

The XRD patterns of the as-cast and 96 h-annealed base and 1.5 wt% Ag-containing alloys are depicted in Fig.5.These patterns confirm the presence ofα-Mg and Mg5(Gd,Y)phases in both conditions of these alloys.This is in contrast to most studies on Mg–Gd–Y alloys that have reported the presence of Mg24(Gd,Y)5phase.The patterns of the Ag-containing alloys show peaks corresponding to both Mg5(Gd,Y)and Mg16Gd2YAg intermetallics.According to Fig.5a,annealing process has no particular effects on the XRD results of the base alloy.However,in the GW51–1.5Ag alloy,peak intensities have been changed and the volume fraction of the Mg5(Gd,Y)phase is increased after annealing(Fig.5b).

Since both microstructural stability and mechanical properties are affected by the alloying elements and their capability in solid solution strengthening as well as second-phase particle hardening,it is important to elucidate these effects caused by the Gd,Y and Ag elements.In accordance with the Mg–Gd phase diagram[41],Gd has a limited solubility in Mg at ambient temperature.Based on the results of EDS analysis,exhibited in Fig.3,the matrix of the GW51 and GW51–1.5Ag alloys contain 0.6 and 0.4 at% Gd,respectively.This trend is somehow in agreement with the concentration of the dissolved Y in the matrix of the tested alloys,which shows a slight drop from 0.3 to 0.1 at%after Ag addition.This implies that the microstructure of both alloys in the as-cast condition consists of supersaturated solid solutions of Gd and Y in the Mg matrix.It can be inferred from Fig.3 that the volume fraction of the second phase particles increases in the alloy with 1.5 wt% Ag.By calculating the volume fraction of particles from SEM images,it was found that addition of 0.5,1 and 1.5 wt% Ag will increase the volume fraction of particles by 6,10 and 13%,respectively.The higher number density of the particles in the Ag-containing alloy can be partly due to the formation of the quaternary Mg16Gd2YAg intermetallic compound,which has been accomplished by the reaction of the free Ag with Gd and Y.Dissolution of Ag into the matrix can result in the depletion of Gd and Y atoms in solid solution,making them more available to the undissolved Ag to form Mg16Gd2YAg.Another reason for the higher volume fraction of the particles in the GW51–1.5Ag alloy is that the available Gd and Y atoms also promote the formation of the ternary Mg5(Gd,Y)particles.This argument is in agreement with the higher XRD peak intensities of the Mg5(Gd,Y)phase in the Ag-containing alloy(Fig.5b),compared to the Ag-free base alloy(Fig.5a).As can be seen in Fig.5a and b,not only 96h annealing could not diminish the peak intensity of the intermetallic compounds in the Ag-free or Ag-containing alloys,but also it is increased in some cases.Also,the change in the intensity of the peaks and the increase in the number of peaks indicate a change in the particles volume fraction.Increasing the volume fraction of particles in the Ag-containing alloys can be due to the dissolution of Ag in the matrix.Ag with an atomic radius lower than that of Mg,occupies interstitial spaces,reduces the solubility of Gd and Y in the matrix and facilitates forming new precipitates.On the other hand,due to the high electronegativity difference between Ag and Mg,Ag can act as a site for inhomogeneous nucleation and cause the formation of new precipitates and increase the volume fraction of Mg5(Gd,Y)particles.This implies that both types of the intermetallic compounds possess high thermal stability,which can help the alloys retaining their strength at relatively high temperatures.

Fig.5.XRD patterns of the as-cast and 96h annealed specimens:(a)GW51 and(b)GW51−1.5Ag.

Fig.6.High magnification SEM micrographs of(a)GW51 and(b)GW51–1.5Ag after 96h annealing treatment,showing the shape and size of the second phase particles.

To further examine the morphology of the particles,highmagnification SEM images of the GW51 and the GW51–1.5Ag alloys after annealing for 96 h are shown in Fig.6.As discussed in the previous section,EDS analysis was used to determine the chemical composition of different particles(at least 10 particles were selected randomly for this purpose).

Fig.7.Vickers hardness data for the alloys in the as-cast and annealed conditions.

According to Fig.6a,the Mg5(Gd,Y)particles have a cuboid morphology.The size of these particles is less than 2μm,and they are distributed both inside the grains and on the grain boundaries according to Fig.4.The microstructure of the GW51–1.5Ag alloy(Fig.6b)contains the same coarser Mg5(Gd,Y)particles together with some finer cuboid particles which are identified as Mg16Gd2YAg.These finer particles are mostly formed adjacent to the coarser Mg5(Gd,Y)particles,and their size is about 0.5μm.The presence of both Ag-containing and Ag-free particles have been also reported in a recent study on Mg–Gd–Y–Ag–Zr alloys[42].Due to the shape and distribution of these particles throughout the microstructure,it is difficult to calculate their volume fraction separately,as it is impractical to differentiate the Mg16Gd2YAg particles from the Mg5(Gd,Y)ones.

3.2.Mechanical properties

Fig.8.Typical SPT curves of:(a)GW51,(b)GW51−0.5Ag,(c)GW51−1.0Ag and(d)GW51−1.5Ag alloys in the as-cast condition.

Vickers hardness results of all alloys after annealing for different times are given in Fig.7.It can be observed that the addition of Ag increases the hardness in both as-cast and annealed specimens.Regardless of the annealing duration,the highest hardness is obtained in the GW51–1.5Ag alloy.In addition,a steeper decline in hardness with the annealing time is observed for the base alloy,which causes a significant difference in the hardness of the Ag-free and Ag-containing alloys.According to Fig.7,hardness of the base alloy drops by about 22% that is in contrast to the Ag-containing alloys,which exhibit less pronounced reductions in their hardness values after 96h of annealing.The hardness decrements for alloys containing 0.5,1 and 1.5 wt% Ag are 13,12 and 10%,respectively.This indicates that the rate of hardness drop in the base alloy is greater than those of the Ag-containing alloys.

Fig.9.Effect of Ag addition on USS of the GW51,GW51−0.5Ag,GW51−1.0Ag and GW51−1.5Ag alloys in the temperature range of 25–400°C for:(a)as-cast,(b)4h annealed,(c)24h annealed and(d)96h annealed specimens.

With respect to the Hall–Patch relationship,strength and hardness are inversely related to grain size.So the larger the grain size,the lower the hardness of the sample.According to Fig.2,the grain grows with increasing annealing time in all alloys.As a result,although the precipitates are not dissolved in the matrix,the hardness of the samples after annealing is reduced.It also shows that at high annealing times,the grain boundary movement increases and grain growth exceeds the pinning effect,resulting in a hardness drop.

Fig.8 exhibits the shear stress versus normalized displacement for the as-cast GW51,GW51–0.5Ag,GW51–1.0Ag and GW51–1.5Ag alloys,obtained by SPT.Similar to the conventional tensile tests,after a linear elastic behavior,the curves deviate from linearity,yield,and then pass through a maximum point before failure.The stress at the maximum point is known as the ultimate shear strength(USS).In all of the alloys,an increase in the test temperature from 25 to 400°C leads to a reduction in the USS values,the drop being more pronounced for the base GW51 alloy.

The USS values obtained from Fig.8 are plotted against the test temperature to elucidate the softening behavior of the alloys,as depicted in Fig.9.This figure also reveals the effects of Ag addition on thermal stability of the materials under different annealing conditions.According to the variations of USS with temperature in the as-cast condition,shown in Fig.9a,the strength of all alloys drops with increasing temperature,where the alloy with the highest Ag content has the highest strength level at all temperatures.The same type of behavior can be observed for the annealed conditions,depicted in Fig.9b–d.For example,in the GW51 alloy at 400°C,USS decreases from 70MPa in the as-cast condition to 52MPa in the 96h annealed condition(25% reduction),while the USS of the GW51–1.5Ag alloy decreases from 88 to 81MPa(only 8% reduction)at the same test temperature.The sudden strength drop after 350°C can be ascribed to the high-temperature recovery processes,structural instability and grain growth at high temperature.It can be inferred that with increasing annealing time,the difference between the strength of the base and Ag-containing alloys becomes more pronounced.The superiority of the Ag-containing alloys over the base alloy stems from the co-existence of the thermally stable Mg16Gd2YAg and Mg5(Gd,Y)particles.These particles enhance the softening resistance of the alloys,while the base alloy with Mg5(Gd,Y)particles,as the only second phase present in its matrix,can soften more readily at high temperatures.Strengthening by particle hardening in the present alloys can be manifested by grain boundary pinning and retardation of recovery and recrystallization during deformation at ambient and high temperatures.The thermal stability of the Ag-containing second phase particles is partly,in addition to their high melting points,due to the low diffusion coefficients of Gd and Y in Mg.The shear strength of the as-cast alloys obtained in this study shows better mechanical properties,as compared to other commercial Mg alloys such as Mg–Zn–Sb[3],Mg–Al–Mn[7],Mg–Al–Zn–RE[8],Mg–Sn–Ca[43],and Mg–Zn–Y[44]at all temperatures.

Fig.10.SEM microstructures of the shear deformation area for:(a,b)GW51 and(c,d)GW51–1.5Ag after SPT at 300°C.

Microstructural evolution during shear deformation of the GW51 and GW51–1.5Ag alloys subjected to SPT at 300°C is shown in the SEM images of Fig.10.Comparison of Fig.10a and c,indicates that the volume fraction of particles in the Ag-containing alloy is higher than that of the Ag-free alloy,where the particles are oriented in the flow direction.By comparing the microstructure in the enlarged shear deformation area,shown in Fig.10b and d,it can be inferred that the particles have remained almost undeformed.Fracture and morphological changes do not occur in these particles and only the Mg matrix is deformed within the deformation area.Similar observations have also been reported in an article examining Mg–Gd particles after deformation[45].The observed behavior is indicative of the high thermal stability of Mg5(Gd,Y)and Mg16Gd2YAg cuboid particles and their good compatibility with Mg matrix during deformation.No cracks or cavities were found around the particles,resulting in significant high-temperature strength of the alloys

4.Conclusions

The effect of 0.5,1.0 and 1.5 wt% Ag addition on the microstructure,thermal stability and mechanical properties of GW51 alloy in the as-cast and annealed conditions was investigated.The results are briefly summarized as follows:

1)Microstructure of the as-cast Mg–5 wt% Gd–1 wt% Y(GW51)alloy contained Mg5(Gd,Y)precipitates in theα-Mg matrix.Adding Ag to this alloy resulted in microstructural refinement and formation of the quaternary Mg16Gd2YAg particles.

2)Significant grain coarsening occurred in the base alloy after long time exposure to high temperature.In the Ag-containing alloys,however,the coexistence of the Mg5(Gd,Y)and Mg16Gd2YAg particles enhanced microstructural stability with trivial grain growth.

3)Addition of Ag to the base alloy increased strength and hardness in all conditions.Microstructural refinement,solid solution strengthening and hardening by the thermally stable Mg5(Gd,Y)and Mg16Gd2YAg particles were the main mechanisms responsible for the observed improvement of mechanical properties.

Funding

This research did not receive any specific grant from funding agencies in the public,commercial,or not-for-profit sectors.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.