UDC 669.721715857:621.762.224

STRUCTURAL CHANGES IN MAGNESIUM ALLOY MA14

UNDER THE ACTION OF PROCESS FACTORS

E. F. Volkova,1 I. V. Iskhodzhanova,1 and L. V. Tarasenko2

Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 12, pp. 19 23, December, 2010.

Results of a study of recrystallization processes in commercial high-strength magnesium alloy AM14, which

occur under the action process deformation factors, are presented. The possibilities of attaining a stable level

of the main mechanical characteristics and effective lowering of their anisotropy by forming an equiaxed

fine-grained structure due to optimization of the process parameters for the case of isothermal die forging are

considered.

Key words: high-strength magnesium alloy, recrystallization processes, isothermal deformation,

geometric parameters of grains, lowering of the anisotropy of properties.

INTRODUCTION

Commercial high-strength magnesium alloy MA14 of

the Mg Zn Zr system (a counterpart of alloy AZ31A,

USA) is used for making pressed, stamped, and forged

semiproducts. According to the results of earlier studies, the

range of potential applications of the alloy from the stand-

point of growth in its mechanical characteristics by advanc-

ing the process of production of deformed semiproducts can

be widened considerably [1, 2].

Magnesium-base high-strength alloys present special in-

terest where the development of efficient processes elevating

their deformability at the forming temperature due to the use

of the effect of superplasticity is concerned. In particular, this

makes it possible to produce precision pressed preforms

without additional mechanical treatment, elevates the coeffi-

cient of utilization of metal (CUM), provides leveling of the

properties over the volume of the part, and lowers the aniso-

tropy of the main mechanical characteristics of alloys. The

general requirements on process parameters providing the

occurrence of plastic deformation of alloys in the state of

superelasticity have been formulated [3, 4].

The aim of the present work consisted in studying the ef-

fect of process deformation parameters on the occurrence of

recrystallization and formation of structure in alloy MA14

and on the main mechanical characteristics and their aniso-

tropy for subsequent optimization of the process cycle in the

production of bulk forged semiproducts with complex geo-

metry.

METHODS OF STUDY

We studied forged and stamped preforms from alloy

MA14 fabricated at the pilot production of FGUP VIAM.

Ingots with a size of 110 185 mm were melted with VI-2

flux. The composition of the alloy matched the requirements

of the GOST 14957 Standard for alloy MA14, i.e., Mg

5.18 wt.% Zn + 0.52 wt.% Zr. The content of impurities

matched the range specified by GOST 14957.

After mechanical treatment and homogenizing annealing

the ingots were subjected to the first stage of deformation

(upsetting) in different temperature-rate regimes in a vertical

1600 tonf die press. In the second stage of deformation a

batch of pressed model ribbed preforms was produced from

the upset preforms (Fig. 1). The developed surface of a

pressed model preform allowed us to imitate complex-con-

figuration actual parts and to study the structure and proper-

ties in four directions.

Pressed model preforms were deformed under isothermal

conditions in one operation using low rates and different

temperature regimes in a 630 tonf hydraulic press equipped

with a UIDIN isothermal die block with induction heating.

The mechanical properties of the pressed model preforms

from alloy MA14 were determined under uniaxial tension

according to GOST 1497 in an Instron device.

The microstructure of the alloy was studied under a

Leica DM IRM light inverted microscope. The images were

Metal Science and Heat Treatment, Vol. 52, Nos. 11 12, March, 2011 (Russian Original Nos. 11 12, November December, 2010)

592

0026-0673/11/1112-0592 2011 Springer Science + Business Media, Inc.

1All-Russia Institute for Aircraft Materials (FGUP VIAM),

Moscow, Russia.2

N. . Bauman Moscow State Technical University, Moscow, Russia.

obtained with the help of a VEC video camera connected to a

computer (3 megapixel resolution). The images were ana-

lyzed and processed with the help of domestic Image Expert

Pro 3x software. In addition to the microscopic study of the

structural features of the die-pressed preforms from alloy

MA14 we performed quantitative analysis of the grain struc-

ture and processed the results statistically. We determined the

mean grain size, the oblongness of the grains, the shape pa-

rameters and the volume fraction of recrystallized and de-

formed grains.

RESULTS AND DISCUSSION

The first stage of deformation of alloy AM14 consisted

of upsetting the initial homogenized preforms at varied tem-

perature, rate, and degree of the deformation.3 Upsetting was

performed after preliminary heating of the preforms to 250

450C with a hold of 1 5 h upon a change in the deforma-

tion rate from 0.5 to 100 mmmin and in the degree of the

deformation from 10 to 80%.

In the process of upsetting we determined the flow

stresses (ups

). The results of the tests were used to plot the

dependences ups

= f(, t ) (Fig. 2). Analysis of these curves

shows that 40 50% deformation during upsetting in the

whole of the studied temperature range is accompanied by

minimum flow stresses (Fig. 2a ). An additional factor affect-

ing this parameter is the rate of the deformation. At 40 50%

deformation and deformation rate of 0.5 5.0 mmmin thespecific forces decrease to 20 35 MPa accordingly, i.e., to

the values typical for superplastic flow of the metal

(Fig. 2b ).

Figure 3 presents the microstructure of the alloy after the

first deformation stage at a temperature of 400C and a rate

of upsetting of 0.5 5.0 mmmin. The typical cast structureof alloy MA14 becomes much finer after two-three reduction

operations (Fig. 3a ) The size of the deformed grains at a

constant temperature of upsetting depends on the number of

reduction operations. After five such operations in the upset-

ting process the structure of the alloy is refined still more

(Fig. 3b ).

It has been shown that the degree of deformation in each

reduction should not exceed 45 50% and the number of re-

duction operations in the first stage of the deformation

should be at most 5 in order to avoid the appearance of

cracks.

Since the deformation occurs at a quite high temperature,

the formation of a strained structure is accompanied by

recrystallization processes (dynamic and, partially, static

ones) of different intensity depending on the chosen mode of

upsetting. The results of a comparative study of the micro-

structure of upset preforms, including the use of quantitative

Structural Changes in Magnesium Alloy MA14 under the Action of Process Factors 593

X

YV

Z

Fig. 1. Appearance of a pressed model ribbed preform from alloy

MA14.

220

180

140

100

60

20

180

140

100

60

20

10 30 50 70 90

, %

ups , P

ups , P

tups ,

b

250

300

350

400

450

200 250 300 350 400 450

10

100

5

0,5

Fig. 2. Dependence of the specific force of upsetting of alloy MA14

on: a) the degree of deformation () at vdef

= 10 mmmin at differenttemperatures (given at the curves in C); b ) the temperature at

= 50% at different deformation rates (given at the curves in

mmmin).

3With participation of N. V. Moiseev from FGUP VIAM.

diffraction analysis, allow us to judge on the efficiency of the

chosen modes of the first stage of deformation (see Fig. 4).

The effect of the use of the Image Expert Pro 3x soft-

ware becomes obvious when we compare the microstructure

in the initial form (Fig. 4b ) and after processing the same im-

age (Fig. 4c ).

Other process parameters of the first deformation stage

being equal, we studied in greater detail the effect of the tem-

perature on the special features of structure formation in the

alloy.

After upsetting at 370C the volume fraction of de-

formed grains in the structure of preforms is the highest

(47.3%) of the three types of structure formed at 370, 400,

and 450C. At 370C many geometric parameters of de-

formed grains are the highest, namely, the mean grain diame-

ter (77.3 m), the mean maximum Feret diameter (116.7 m),

the oblongness of the grains (3.21), and the scattering with

respect to the mean diameter of deformed grains (Table 1).

The scattering with respect to the mean diameter of grains

characterized the differences in the grain sizes. Thus, the

scattering of grain sizes in the structure at 370C remains

considerable, which reflects insufficient efficiency of the dif-

fusion processes.

When the temperature of upsetting is increased to 450C,

the geometric parameters of the deformed grains decrease

progressively; their volume fraction falls from 47.3 to 36.7%,

and the mean maximum and minimum Feret diameters de-

crease, which may be a result of intense growth of the fine

recrystallized grains (Fig. 4, Table 2).

Note that the picture for recrystallized grains is inverse.

Intensification of diffusion processes upon growth in the

temperature of upsetting activates the process of nucleation

of new grains and accelerates the growth of the already

formed recrystallized grains (Fig. 4b e). This is confirmed

by changes in the geometric characteristics of the grains, i.e.,

the mean diameter of the fine recrystallized grains increases

from 5.1 m (at tups

= 370C) to 7.2 m (at tups

= 450C).

The diameter of the circle of the equivalent area of

recrystallized grains increases by a factor of 1.4 and the dif-

ference between the minimum and maximum Feret diame-

ters increases (Table 2).

Comparative analysis of these results shows that after

upsetting at 400C the structure of alloy MA14 is character-

ized by the most balanced proportion between the

recrystallized and deformed grains and higher roundedness

of the grains. At this temperature the oblongness of the de-

formed grains is the lowest (2.41) and the scattering with re-

spect to the mean diameters of the deformed grains is the

lowest ( 1.83) too. For the recrystallized grains the differ-

ence between the values of the mean diameters of fine (6.7)

and coarse (24.3) grains is the lowest.

Thus, at 400C the structure of the alloy is more prepared

for the second stage of deformation (Fig. 4c, Tables 1 and 2).

In order to raise the adaptability of the alloy to manufac-

ture and stabilize the formed structure, we performed heat

treatment (low-temperature annealing) between the first and

second stages of deformation.

The second stage of deformation of preforms from alloy

MA14 (after the heat treatment) was performed under iso-

thermal conditions at a low rate. We produced a test batch of

594 E. F. Volkova et al.

TABLE 1. Geometrical Characteristics of Deformed Grains in Specimens of Alloy MA14 (after the First Stage of Defor-

mation)

tups

, C Vd.g

, %, scattering Dg

, m Dmax

Dmin

Dferet(max)

, m DFeret(min)

, m

370 47.3 (44.9 49.6) 77.3 4.13 3.21 0.14 116.7 6.8 37.9 2.00

400 45.5 (40.4 54.4) 66.3 1.83 2.41 0.05 92.3 2.72 38.2 1.17

450 36.7 (32.8 42.0) 57.6 3.0 3.08 0.10 84.9 4.36 30.2 1.77

Notations: tups

) temperature of upsetting, Vd.g

) volume fraction of deformed grains, Dg

) mean grain diameter;

Dmax

Dmin

) oblongness of grains; DFeret(max)

) mean maximum Feret diameter, Dferet(min)

) mean minimum Feret diameter.

10 m

10 m

b

Fig. 3. Microstructure of an intermediate preform from alloy MA14

after the first stage of deformation at 400C ( 1000): a) 2 reduction

operations in upsetting; b ) 5 reduction operations.

model pressed preformed with deep ribbing, the geometric

shape of which allowed us to study the structure and the level

of mechanical characteristics in four directions, i.e., X, Y, V, Z

(see Fig. 1 and Table 3). The highest strength properties were

determined in directions Z and X and the lowest one were de-

termined in direction V. The strength properties of the model

pressed preforms were stable; their scattering for each stud-

ied direction did not exceed 5%. The anisotropy of the ulti-

mate rupture strength and, what is especially important, of

the yield strength was also not high and did not exceed

7 12%, including the case of comparison of properties in

directions Z and V, X and V.

It should be noted that the anisotropy of the yield

strength (the most structurally sensitive characteristic) in

pressed semiproducts from alloy MA14 fabricated by the

conventional process is usually 40 50%. Thus, in commer-

cial pressed preforms from alloy MA14 the yield strength in

the direction transverse to the axis of deformation is

40 50% lower than that in the longitudinal direction.

The ductility margin in the model pressed preforms is

preserved at a stably good level; the elongation is 10 13%

for all the studied specimens in whatever direction they have

been cut (Table 3).

The decrease in the anisotropy and the stabilization of

mechanical properties of the model pressed preforms in our

Structural Changes in Magnesium Alloy MA14 under the Action of Process Factors 595

TABLE 2. Geometric Characteristics of Recrystallized Grains in Specimens of Alloy MA14 (after the First Stage of De-

formation)

tups

, C

Grain category

according

to averaged size*

Dg

, m Dc

, m Dmax

Dmin

DFeret(max)

, m Dferet(min)

, m

370 Fine 5.1 0.18 4.5 0.16 1.53 0.03 6.1 0.22 4.1 0.15

Coarse 21.2 0.79 18.2 0.70 1.58 0.041 25.3 0.92 17.0 0.72

400 Fine 6.7 0.14 5.9 0.12 1.55 0.02 8.0 0.17 5.3 0.11

Coarse 24.3 1.09 20.2 0.88 1.73 0.06 30.4 1.45 18.2 0.92

450 Fine 7.2 0.12 6.4 0.10 1.61 0.02 8.8 0.16 5.6 0.10

Coarse 30.6 0.99 26.0 0.81 1.71 0.04 37.9 1.25 23.3 0.83

*We agree that the fine grains have a mean diameter 10 m and the coarse grains have a mean diameter > 10 m.

Notations: tups

) temperature of upsetting; Dg

) mean grain diameter; Dc

) diameter of the circle of the equivalent area;

Dmax

Dmin

) oblongness of grains; DFeret(max)

) mean maximum Feret diameter; DFeret(min)

) mean minimum Feret diameter.

10 m

10 m

10 m

10 m

a

c

b

d

Fig. 4. Microstructure of MA14 al-

loy after upsetting at 370 (a, b ),

400 (c) and 450C (d ). Recrystal-

lized grains at different growth

stages are colored with yellow and

blue, the other area is attributed to

non-recrystallized structure ( 600).

case are explainable by the structural changes that have oc-

curred in alloy MA14 due to isothermal deformation in a

state close to that of superplastic flow. We have established

that the microstructure formed at specific deformation pa-

rameters is primarily recrystallized and fine-grained and

does not contain streak segregations commonly observed in

the alloy subjected to deformation by the standard technol-

ogy. The volume fraction of the recrystallized grains is close

to 90%, the mean grain diameter is about 7.5 m, the mini-

mum and maximum Feret diameters of the recrystallized

grains have decreased to 5 and 15 m respectively (see

Fig. 5).

This kind of microstructure is observed in specimens cut

from pressed preforms of alloy MA14 in all the studied di-

rections including directions Y and V characterized by the

highest difference in the yield strength and in the ultimate

rupture strength but not, however, exceeding 10 12% (Ta-

ble 3).

The results obtained agree well with the data of our pre-

vious studies [2, 5].

CONCLUSIONS

1. Alloy MA14 deformed at a temperature 370C with

deformation degree 50% undergoes active recrystallization

as a result of which the volume fraction of recrystallized

grains may exceed 50%.

2. We have developed a process cycle including two-

stage deformation and intermediate heat treatment, which

yields complex-configuration pressed preforms from alloy

MA14. The process provides stabilization of the mechanical

properties of the alloy and simultaneously lowers the aniso-

tropy of the properties to 7 12%.

3. The optimum combination of properties in model

pressed preforms from alloy MA14 produced by the sug-

gested process cycle is a result of the formed structure cha-

racterized by equiaxed and fine grains (the mean grain size is

about 7.5 m) in the directions along and across the fibers.

REFERENCES

1. E. F. Volkova and G. I. Morozova, Structure-phase state and

properties of zirconium-bearing magnesium alloy MA14, Me-

talloved. Term. Obrab. Met., No. 1, 24 28 (2006).

2. E. F. Volkova, Effect of deformation and heat treatment on the

structure and properties of magnesium alloys of the Mg Zn Zr

system, Metalloved. Term. Obrab. Met., No. 11, 38 42 (2006).

3. R. S. Jefkins, Mechanisms of superplastic strain, in: Super-

plastic Forming of Structural Alloys [in Russian translation],

Metallurgiya, Moscow (1985), pp. 11 36.

4. N. G. Zaripov and R. O. Kaibyshev, Dynamic recrystallization

and superplasticity of magnesium alloys. Superplasticity and

superplastic forming, TMS, 91 95 (1988).

5. E. F. Volkova and N. V. Moiseev, Special features of deforma-

tion of high-strength magnesium alloys in the mode of super-

plasticity, in: E. N. Kablov (ed.), Aircraft Materials and Tech-

nologies, Issue Promising Magnesium and Titanium Alloys [in

Russian], VIAM, Moscow (2002), pp. 136 142.

596 E. F. Volkova et al.

TABLE 3. Mechanical Properties of Model Pressed Preforms from

Alloy MA14 (Optimum Deformation Mode)

Direction of cutting of

the specimen (in accor-

dance with Fig. 1)

r, MPa

0.2, MPa

, %

X 290 298 235 238 11 13

Y 282 286 230 236 11 14

Z 298 300 230 234 10 14

V 276 289 229 230 11 14

Note. We present minimum and maximum values of each charac-

teristic after testing 5 specimens.

10 m

10 m

a

b

Fig. 5. Microstructure of a model die forging directed (according to

Fig. 1) in Y (a) and V (b ) ( 600).

AbstractKey wordsINTRODUCTIONMETHODS OF STUDYRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES

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