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Influence of heat treatment on the microstructure and wearbehavior of end-chill cast Zn–27Al alloys with different copper

content

R. Arabi Jeshvaghani1 • H. Ghahvechian1 • H. Pirnajmeddin1 • H. R. Shahverdi1

Received: 19 September 2015 / Accepted: 29 October 2015 / Published online: 15 March 2016

Springer-Verlag Berlin Heidelberg 2016

Abstract The aim of this paper was to study the effect of

heat treatment on the microstructure and wear behavior of Zn–27Al alloys with different copper content. In order to

study the relationship between microstructure features and

wear behavior, the alloys prepared by an end-chill cast

apparatus and then heat treated. Heat treatment procedure

involved solutionizing at temperature of 350 C for 72 h

followed by cooling within the furnace to room tempera-

ture. Microstructural characteristics of as-cast and heat-

treated alloys at different distances from the chill were

investigated by scanning electron microscopy (SEM),

energy dispersive spectroscopy (EDS) and X-ray diffrac-

tion. Wear tests were performed using a pin-on-disk test

machine. To determine the wear mechanisms, the worn

surfaces of the samples were also examined by SEM and

EDS. Results showed that heat treatment led to the com-

plete dissolution of as-cast dendritic microstructure and

formation of a fine lamellar structure with well-distributed

microconstituents. Moreover, addition of copper up to

1 wt% had no significant change in the microstructure,

while addition of 2 and 4 wt% copper resulted in formation

of e (CuZn4) particle in the interdendritic regions. The

influence of copper content on the wear behavior of the

alloys was explained in terms of microstructural charac-

teristics. Delamination was proposed as the dominant wear

mechanism.

1 Introduction

During the last three decades, zinc–aluminum alloys (ZA)

have occupied attention of both researchers and industries

for their unique combination of properties. Some of the

attractive properties of these alloys are lower density,

excellent castability, good machinability, excellent corro-

sion resistance in a variety of environments and superior

wear properties [1–5]. Desirable characteristics and low

manufacturing cost of these alloys enable them to be

considered as competing materials for cast metals such as

high strength aluminum alloys, cast iron, copper alloys and

brass. Typical uses of these alloys include machine tools,

internal combustion engines, bearing bushings and flanges,

fuel-handling components, pulleys, electrical fittings and

hardware components [6, 7].

It is known that mechanical and tribological properties

of ZA alloys strongly depend on their microstructures and

can be further improved by controlling the as-cast

microstructure. The final microstructure of casting is a

complex function of the composition and cooling rate [8].

In this regard, several studies have been done on the

relationships between cooling rate, composition and prop-

erties of ZA alloys.

Savaskan and Turhal [9] have studied the effect of

cooling rate and copper content on the structure and

mechanical properties of monotectoid Zn–40Al–Cu alloys,

which were cast in a permanent mold. They reported that

with increasing cooling rate the hardness, tensile strength,

elongation and impact energy increased in a linear manner.

Furthermore, copper content was found to be more effec-

tive in optimizing the mechanical properties than cooling

rate. Savaskan et al. [10] have also investigated the effect

of copper content on the sliding wear behavior of mono-

tectoid-based Zn–Al–Cu alloys. They found that the

H. Ghahvechian and H. Pirnajmeddin have contributed equally to this

work.

& H. R. Shahverdi

[email protected]

1Department of Materials Engineering, Tarbiat Modares

University, P.O. Box 14115-143, Tehran, Iran

1 3

Appl. Phys. A (2016) 122:397

DOI 10.1007/s00339-016-9820-5

http://crossmark.crossref.org/dialog/?doi=10.1007/s00339-016-9820-5&domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1007/s00339-016-9820-5&domain=pdf


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highest wear resistance obtained with the monotectoid-

based alloy containing 2 wt% copper and the wear resis-

tance deteriorated at higher copper contents. Mojaver and

Shahverdi [11] observed that decrease in cooling rate of

Zn–27Al during end-chill casting resulted in increase in

dendrite arm spacing, percentage porosity and the inter-

dendritic region.

It is well known that addition of copper improvesmechanical and tribological properties of these alloys, but

the existence of copper more than 1 wt% results in the

formation of metastable e (CuZn4) phase during solidifi-

cation [12]. The tendency of the e phase to transform to the

stable T 0 (Al4Cu3Zn) phase leads to the dimensional

instability, which limits the applications of ZA alloys at

low operating temperatures [13]. One of the possible

measures for overcoming this problem is applying a suit-

able heat treatment. Accordingly, several heat treatment

procedures have been suggested by authors, which mainly

involve solutionizing with subsequent water quenching and

artificial aging [2, 14].Babic et al. [1, 2] studied the effects of heat treatment,

involving solution treatment at temperature of 370 C for 3

and 5 h followed by quenching in water, on tribological

behavior of ZA27 alloys. They reported that higher wear

resistance of the heat-treated alloys over as-cast ones is

attributed to the finer and more uniform distributed

microconstituents. Savaskan and Bican [14] investigated

the wear behavior of Al–25Zn–3Cu–3Si alloy subjected to

T7 heat treatment, which includes solutionizing at a tem-

perature of 375 C for 36 h followed by rapid water

quenching and aging at 180 C for 8 h. They found that

heat treatment reduced the wear resistance of the Al–25Zn–

3Cu–3Si alloy, but greatly increased its ductility.

Another interesting procedure is the furnace cooling

process, which includes solutionizing at a high temperature

for a period of time to obtain one or more phases at that

temperature following by slow cooling to room tempera-

ture within the furnace. Slow cooling is common in cast

ingot production. It is interesting to note that phase trans-

formation and microstructural changing in the slow cooling

process are much more complicated than in solution-trea-

ted quenched alloy. In this process the phase or phases

obtained by solution treatment decompose to various pha-

ses during cooling, which is similar to what happens in

some of the advanced metallurgical processes, e.g. con-

tinuous casting. Thus, investigation of the furnace-cooled

eutectoid Zn–Al based alloy is of practical importance for

the advanced metallurgical processes [15, 16].

To the best of our knowledge, most of published papers

have focused on the aging characteristics of conventional

zinc–aluminum alloys [1, 2]. However, little effort has

been made to investigate the microstructural changes and

phase transformation during slow cooling process.

Furthermore, up to now, very limited information exists on

the influence of heat treatment on the response of copper

modified zinc–aluminum alloys. In view of the above, the

aim of this paper is to study the effect of slow cooling

process on the microstructure and wear behavior of Zn–

27Al alloys containing different copper content. In this

regard, an end-chill cast was conducted on Zn–27Al alloys

with different copper content to produce a variety of microstructural features. Then heat treatment, involving

solutionizing at temperature of 350 C for 72 h followed

by cooling within the furnace, was carried out.

2 Experimental procedures

2.1 Alloy preparation

In the present work, Zn–27Al alloys containing different

copper content were produced from pure zinc (99.99 wt%),

pure commercial aluminum (99.9 wt%) and an Al–Cumaster alloy. The alloys were melted in a resistance fur-

nace and degassed with zinc chloride and poured into the

end-chill sand mold in the form of cylindrical castings

(size: 40 mm in diameter, 160 mm height). The details of

the end-chill apparatus, used to promote downward heat

extraction, have been described in our previous works [11,

17]. The chemical compositions of the produced alloys

which determined by atomic absorption spectroscopy are

listed in Table 1.

2.2 Heat treatment procedure and microstructural

examination

In order to study the effect of heat treatment, the as-cast

alloys were solutionized at 350 C for 72 h and then cooled

to room temperature in the furnace. For complete disso-

lution of as-cast dendritic microstructure and interdendritic

phases, solutionizing time was chosen so long. Due to the

shrinkage pipe at the top of the casting and the limited

effective amplitude of the chill, only the first 110 mm of

the castings was considered for study. After cutting the

cylinder longitudinally, one of the halves was cut at 10-mm

intervals for microstructural examinations and the other

Table 1 Chemical composition of prepared alloys

Alloys Elements (wt%)

Zn Fe Al Cu Mg

Zn–27Al Bal. 0.1 25.5 – 0.01

Zn–27Al–1Cu Bal. 0.1 26.2 1.1 0.01

Zn–27Al–2Cu Bal. 0.1 26.9 2.2 0.01

Zn–27Al–4Cu Bal. 0.1 27.6 4.2 0.01

397 Page 2 of 9 R. Arabi Jeshvaghani et al.

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half was machined into two pins for wear tests. Figure 1 is

a schematic of the preparation of samples. To study the

microstructural features, selected surfaces were polished

and etched using Palmerton’s reagent. After etching, the

samples were rinsed with a solution of CrO3 (20 mg) in

H2O (100 ml) [18]. Finally, the microstructures were

examined by scanning electron microscope equipped with

energy dispersive spectroscopy. The phase identificationwas also conducted by means of X-ray diffraction analysis.

2.3 Wear testing

Dry sliding wear tests were performed using a pin-on-disk

machine. The test materials in the form of pins of 9 mm

diameter were made to slide against a rotating steel disk

with hardness of 58 HRC. Wear tests were carried out in an

applied normal pressure of 1 MPa, a sliding speed of

0.5 m/s and total sliding distance of 3000 m. Three sec-

tions of each alloy, taken at distances of 20, 50 and

110 mm from the chill, as shown in Fig. 1, were tested.These sections were selected based on the data obtained

from prior tests to show enough difference in microstruc-

tural features. Each sample was ultrasonically cleaned and

then accurately weighed before the wear test using a bal-

ance with an accuracy of ±1 mg. After 100, 300, 600,

1000, 1500, 2000 and 3000 m of sliding, the test sample

was removed, cleaned with solvents and weighed to

determine the weight loss. The weight loss of each section

reported as average of two observations. To determine the

wear mechanisms, the worn surfaces of the samples were

examined by SEM and EDS.

3 Results and discussion

3.1 Microstructures of as-cast alloys

Figure 2a shows SEM micrographs of Zn–27Al alloy. As

can be seen in this figure, the as-cast microstructure indi-

cated dendritic structure comprising primary a-dendrites

surrounded by eutectoid a ? g and residual g phase in the

interdendritic regions. This obviously inhomogeneous

structure is as a consequence of solidification under non-equilibrium conditions. A magnified view of the

microstructure and results of EDS analysis are shown in

Fig. 2b. The EDS results indicated that aluminum had the

Fig. 1 Positions of

metallography and wear

samples in the end-chill casting

Fig. 2 a SEM micrographs of

as-cast Zn–27Al alloy and b

higher magnification image of

the microstructure and the EDS

results of the corresponding

compounds

Influence of heat treatment on the microstructure and wear behavior of end-chill cast Zn–27Al… Page 3 of 9 397

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highest concentration in the dendrite core (EDS 1), then

decreased on the periphery of the dendrite (EDS 2) and was

lowest in the interdendritic region (EDS 3). Furthermore,

the highest concentration of zinc in the interdendritic

regions revealed that zinc-rich g phase precipitated in these

areas.

According to the aluminum–zinc phase diagram [19] in

Fig. 3, solidification of Zn–27Al alloy begins with pre-cipitation of the Al-rich a phase with a dendritic structure

at 493 C. With decreasing the melt temperature to 443 C,

the peritectic reaction ( L ? a ? b) occurs and due to the

rapid solidification condition only a thin layer of the b

phase forms at the edges of the a phase. The residual liquid

becomes enriched with zinc, and the solidification is fre-

quently completed by a divorced eutectic reaction,

L ? b ? g. The eutectic b phase attaches to the peritectic

b phase, and g phase distributes in the form of interden-

dritic layers. The b phase is unstable and decomposes intoa and g at eutectoid temperature [20].

The X-ray diffraction pattern of this alloy is shown in

Fig. 4. It is clear that a and g were predominant phases in

the microstructure and there was no evidence of the pres-

ence of the b phase. This confirms that the b phase has

entirely decomposed into a and g phases.

Figure 5 shows the microstructures of Zn–27Al alloys

with different copper content. The composition of labeled

regions in Fig. 5 determined by EDS is given in Table 2.

The microstructure of the ternary Zn–27Al–1Cu alloy is

shown in Fig. 5a. As can be seen in this figure, addition of

copper up to 1 wt% made no obvious change in thestructure. The results of EDS of areas marked by A and B in

Fig. 5a verified that the area A with a lamellar structure

was eutectoid a ? g and the area marked by B was inter-

dendritic g phase. According to the aluminum–zinc phase

diagram and the lever rule, the majority of dendritic

structure of Zn–27Al alloy transforms into eutectoid a ? g

at 275 C. This was confirmed by the presented

microstructures in Fig. 5, which showed extensive lamellar

eutectoid a ? g. The EDS results also revealed the

Fig. 3 Binary Al–Zn equilibrium phase diagram [19]

Fig. 4 X-ray diffraction pattern of as-cast Zn–27Al alloy

Fig. 5 SEM micrographs of as-cast Zn–27Al alloys with different copper (wt%): a 1 %, b 2 % and c 4 %

Table 2 Chemical composi-

tions of selected areas shown inFig. 5

Area Elements (wt%)Zn Al Cu

A 57.7 38.7 3.6

A0 49.0 42.4 8.6

B 65.5 31.6 2.9

C 85.1 0.1 14.8


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existence of copper in the matrix, which led to solid

solution strengthening effect. As reported by Costa et al.

[21], added copper in low concentration (\1 wt%) dis-

tributes almost uniformly in a macro- and microscopic

scale, presenting complete solubility in the matrix. Since

the solubility of copper in a-Al is generally higher than that

in g- Zn, the copper content in primary dendrites was higher

than that of interdendritic regions.Figure 5b and c shows the microstructures of Zn–27Al

alloys containing 2 and 4 wt% copper, respectively. It is

obvious that the addition of copper more than 1 wt% led to

the formation of a new phase in the interdendritic areas.

The composition of area marked with C in Fig. 5b con-

firmed formation of copper-rich phase in the interdendritic

regions. The X-ray diffraction patterns of these alloys in

Fig. 6 revealed the existence of a, g and copper-rich e

phase in the microstructure.

As mentioned earlier, copper has low solubility in zinc.

Therefore, when the copper content exceeds 1 wt%, the

surplus copper becomes concentrated in the eutectic liquidduring the final stage of solidification and as a result a

metastable copper-rich e phase rejects from the liquid in theFig. 6 X-ray diffraction patterns of as-cast Zn–27Al alloys withdifferent copper (wt%): a 2 % and b 4 %

Fig. 7 SEM micrographs of as-cast Zn–27Al alloys with different copper (wt%) at two different positions from the chill: a, b 1 %, c, d 2 % and

e, f 4 %


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form of discrete irregular particles [21, 22]. Another fea-

ture in Fig. 5b, c is size dependency and distribution of e

phase on the copper content. It is obvious that increasing

copper content increased the size and density of the e

phase.

Figure 7 shows the microstructures of Zn–27Al alloys

with different copper content at two different positions (i.e.

20 and 110 mm) from the chill. It can be seen that withincreasing distance from the chill, the interdendritic

regions became more extensive and size as well as density

of the e phase increased. It is clear that with increasing

distance from the chill, cooling rate decreases. Thus, there

is enough time for melting of small dendrite arms, which

have high surface area to volume ratio. Dissolution of

small dendrite arms increases dendrite arm spacing (DAS),

which leads to significant change in size and density of

interdendritic phases [4].

3.2 Microstructures of heat-treated alloys

Figure 8 shows SEM micrographs of heat-treated Zn–27Al

alloy. Comparing with the as-cast microstructure, it can

clearly be seen that applied heat treatment refined the

microstructure and reduced the segregation effects.

According to the aluminum–zinc phase diagram, solu-

tionizing of this alloy at 350 C results in dissolution of

interdendritic phases into the matrix and formation of b

phase. This phase is unstable below the eutectoid temper-

ature. Thus, during furnace cooling decomposes into the

equilibrium a and g phases through a eutectoid reaction and

forms a lamellar structure [12, 23].

It can be seen in Fig. 8 that during prolonged solution-

izing, the dendrite cores and interdendritic phases com-

pletely dissolved. Therefore, the final microstructure

consisted of a fine lamellar structure of eutectoid a ? g

with a distinct contrast; the dark a and light g phases are

also visible along the grain boundaries which is one of the

characteristics of the b phase decomposition through the

cellular reaction: b ? a ? g. The EDS results of the areas

labeled with D and E in Fig. 8b reveal the presence of a

and eutectoid a ? g phases, respectively. The results of

EDS are listed in Table 3.

Figure 9 illustrates SEM micrographs of heat-treated

Zn–27Al alloys with different copper content in the dis-

tance of 20 mm from the chill. According to the Fig. 9a,

the microstructure of alloy containing 1 wt% copper was

quite similar to the microstructure of copper-free alloy. As

mentioned before, copper dissolves in the matrix and

results in solid solution strengthening [4].

Figure 9b, c shows the microstructures of Zn–27Al con-

taining 2 and 4 wt% copper, respectively. The microstruc-

tures comprised the lamellar a ? g and e phases. By

comparing Fig. 9b, c, it is possible to observe an increase in

the relative quantity of the e phase in the alloy with higher

copper content. Liu et al. [23] also reported that the addition

of copper more than 1 wt% resulted in formation of the e

phase as well as with increasing the copper content the size,

density and distribution of this phase increased.

Figure 10 shows SEM micrographs of heat-treated Zn–

27Al–4Cu alloy at three different positions from the chill. It

is obvious that with increasing distance from the chill, heat-

Fig. 8 a SEM micrographs of heat-treated Zn–27Al alloy in the

distance of 20 mm from the chill and b higher magnification image

Table 3 Chemical composi-

tions of selected areas shown in

Figs. 8 and 10

Area Elements (wt%)

Zn Al Cu

D 18.1 81.9 –

E 44.7 55.3 –

F 10.5 38.8 50.7


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treated structures became coarser. These microstructural

features are due to the use of chill that establishes different

cooling rates in the various distances from the chill and

creates different microstructures along the cast. For this

reason, heat-treated sections nearer the chill showed a finer

structure. Meanwhile, with increasing the size and density of

the e phase at higher position from the chill, the gray pre-

cipitates (area marked by F in Fig. 10c) observed inside the e

phase, which was recognized as T 0 (Al4Cu3Zn). It is reported

that this phase forms during decomposition of the e phase

through the four-phase transformation, a ? e?

T 0

? g[24]. The composition of this phase was determined using

EDS and is listed in Table 3.

3.3 Wear test results

The weight losses obtained for three different sections for

each alloy are plotted versus distance from the chill in

Fig. 11. As can be seen, addition of copper reduced the

weight loss of Zn–27Al alloy. However, addition of more

than 1 wt% copper increased the weight loss. As cited

before, Savaskan et al. [10] also reported that addition of

copper more than 2 wt% deteriorated the wear resistance

of the monotectoid-based Zn–Al alloy. This observation

can be explained in terms of microstructural changes that

occur during solidification and heat treatment.

As mentioned earlier, copper up to 1 wt% completely

dissolves in the matrix and leads to the solid solution

strengthening effect, which improves wear resistance [21].

When the copper content exceeds 1 wt%, the e phase forms

in the interdendritic regions. Seemingly, microstructureswith two phases, one hard and the other soft, are ideal for

bearing materials [25], but formation of the e phase results

in a reduction in the copper content of the a phase which is

the matrix of the alloys and hence reduces the effect of

solid solution strengthening. In addition, formation of the e

phase can increase the cracking susceptibility of the alloys

[10]. Therefore, decrease in wear resistance of the alloy

with 2 wt% copper can be related to the above-mentioned

reasons. Moreover, the lower wear resistance of the alloy

Fig. 9 SEM micrographs of heat-treated Zn–27Al alloys with different copper (wt%) in the distance of 20 mm from the chill: a 1 %, b 2 % and

c 4 %

Fig. 10 SEM micrographs of heat-treated Zn–27Al–4Cu alloy at three different positions from the chill: a 1 mm, b 20 mm and c 110 mm. The

high-magnification images of each position also are shown in top right-hand corner of each micrograph


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with 4 wt% copper may be attributed to the formation of

the T 0 phase, which reduces hardness [13].

According to Fig. 11, the weight losses of alloys without

and with 1 wt% copper increased with increasing distance

from the chill, while for alloys containing 2 and 4 wt%

copper the trend reversed. In the case of Zn–27Al and Zn–

27Al–1Cu alloys, deterioration of wear resistance with

increasing distance from the chill is attributed to thecoarsening of structure. Applying heat treatment breaks the

dendrite structure, decreases the fraction of interdendritic

regions and forms a fine lamellar structure. However, the

as-cast structure affects the characteristics of the heat-

treated structure. With increasing distance from the chill

the grain size increased greatly. It is reported that with

decreasing grain size the strain hardening occurs because

of the dislocation pileup. Moreover, a reduction in grain

size increases the shear strength. Therefore, the structureswith smaller grain size show better wear resistance. This

can be ascribed to the increase in work hardening and shear

strength of the alloy [26, 27]. Concerning alloys with 2 and

4 wt% copper, the inverse trend was due to the increase in

the size and density of the e phase. As reported by Mojaver

and Shahverdi [17], the morphology of the e phase is also

effective on the wear resistance of Zn–27Al alloys.

Comparing the wear results of heat-treated and as-cast

alloys, reported in our previous papers [11, 17], showed

that heat treatment had a positive effect on the wear

resistance of Zn–27Al alloys with different copper content,

because heat treatment dissolved the dendriticmicrostructure of as-cast alloys and created a fine lamellar

structure with well-distributed microconstituents. Further-

more, Prasad [13] reported that the presence of various

phases with different thermal characteristics and mechan-

ical properties in the structure of non-homogeneous as-cast

alloys results in creation of residual stresses on a micro-

scale. Based on this fact, Miroslav et al. [2] concluded thatFig. 11 Weight losses obtained from all the tested sections versusdistance from the chill

Fig. 12 Worn surfaces of heat-

treated Zn–27Al alloys withdifferent copper (wt%) in the

distances of 20 and 110 mm

from the chill: a, b 1 % and c, d

2 %


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obtained uniformity after heat treatment relieves the

residual stresses and enhances the wear characteristics.

3.4 Morphology of the worn surfaces

Since the worn surfaces of all the sections were similar to

each other, typical micrographs of the worn surfaces are

shown in Fig. 12. It is clear that the worn surfaces arerough and have patches with a fractured appearance. From

these patches, it can be deduced that a layer of a material

has been removed as debris. These features propose the

delamination wear as the main wear mechanism.

The delamination is based on the shear plastic defor-

mation, crack initiation and crack propagation [28]. During

the sliding process because of the applied stresses, micro-

cracks nucleate at the phase/matrix interfacial regions.

When these cracks grow and become interconnected,

delamination occurs. It is worthwhile to be mentioned that

the presence of second-phase microconstituents can pro-

mote crack initiation at the phase/matrix interfacial regionsbecause of the inferior coherency between the phase and

the surrounding matrix, which makes the interfacial regions

weak and more susceptible to the cracking [29].

Except the patches, shallow grooves are also visible on

the worn surfaces. These grooves are the consequence of

plowing action of hard debris particles emanated from the

sample surface and the steel disk, which activates three-

body abrasion [13].

4 Conclusions

The influence of heat treatment on the microstructure and

wear behavior of Zn–27Al alloys with different copper

content has been studied. Based on the results and dis-

cussion, the following conclusions can be drawn:

• End-chill casting established different cooling rates in

the various distances from the chill and created

different microstructures along the cast.

• Addition of copper more than 1 wt% resulted in the

formation of copper-rich e phase. The number, size and

distribution of this phase increased with the increase in

copper content.• Solutionizing followed by furnace cooling completely

dissolved the as-cast dendritic structure. The

microstructure of heat-treated alloys is refined with

uniform distribution of microconstituents.

• Heat-treated microstructure of the sections solidified at

higher cooling rates showed a fine lamellar structure

compared with those solidified at lower cooling rates.

The lamellar structure mainly consisted of a and g

phases.

• Copper addition improved the wear resistance of Zn–

27Al alloy. The best wear resistance obtained with

adding copper up to 1 wt%. Wear surfaces of the alloys

were characterized by delamination and shallow

grooves.

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