Development and comparison of processing maps of Mg-3Sn-1Ca alloy from data obtained intension versus compression

Rao, K. P.; Suresh, K.; Prasad, Y.V.R.K.; Hort, N.

Published in:IOP Conference Series: Materials Science and Engineering

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Publication details:Rao, K. P., Suresh, K., Prasad, Y. V. R. K., & Hort, N. (2018). Development and comparison of processing mapsof Mg-3Sn-1Ca alloy from data obtained in tension versus compression. IOP Conference Series: MaterialsScience and Engineering, 297, [012014]. https://doi.org/10.1088/1757-899X/297/1/012014

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Development and comparison of processing maps of Mg-3Sn-1Ca alloyfrom data obtained in tension versus compressionTo cite this article: K.P. Rao et al 2018 IOP Conf. Ser.: Mater. Sci. Eng. 297 012014

 

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8th TSME-International Conference on Mechanical Engineering (TSME-ICoME 2017) IOP Publishing

IOP Conf. Series: Materials Science and Engineering 297 (2018) 012014 doi:10.1088/1757-899X/297/1/012014

Development and comparison of processing maps of Mg-3Sn-1Ca

alloy from data obtained in tension versus compression

K.P. Rao1,*, K. Suresh2, Y.V.R.K. Prasad3 and N. Hort4

1 Department of Mechanical and Biomedical Engineering, City University of Hong

Kong, Tat Chee Avenue, Kowloon, Hong Kong

2 Department of Physics, Bharathiar University, Coimbatore 641046, Tamilnadu,

India (formerly with City University of Hong Kong)

3 processingmaps.com, No. 2/B, Vinayaka Nagar, Hebbal, Bengaluru 560024, India

(formerly with City University of Hong Kong)

4 Helmholtz-Zentrum Geesthacht, Institute of Materials Research, Magnesium

Innovation Centre, Max-Planck-Straße 1, Geesthacht 21502, Germany

* Corresponding Author: mekprao@cityu.edu.hk, +852 3442 8409, +852 3442 0172

Abstract. The hot workability of extruded Mg-3Sn-1Ca alloy has been evaluated by developing

processing maps with flow stress data from compression and tensile tests with a view to find the

effect of the applied state-of-stress. The processing maps developed at a strain of 0.2 are

essentially similar irrespective of the mode of deformation – compression or tension, and exhibit

three domains in the temperature ranges: (1) 350 – 425 oC, and (2) 450 – 550 oC and (3) 400 –

500 oC, the first two occurring at lower strain rates and the third occurring at higher strain rates.

In all the three domains, dynamic recrystallization occurs and is caused by non-basal slip and

controlled by lattice self-diffusion in the first and second domains and grain boundary self-

diffusion in the third domain. The state-of-stress imposed on the specimen (compression or

tension) does not have any significant effect on the processing maps.

1. Introduction

Magnesium alloys have assumed technological importance in recent years due to their light weight and are

being used for several applications in automobiles and aerospace. Two important limitations for use of these

alloys are their lower corrosion resistance and creep strength. In recent years, newer alloys are being

developed to combat these limitations and Mg-Sn-Ca system has been explored for this purpose. It has

been observed that addition of Sn in magnesium improves the latter’s corrosion resistance [1] and tensile

properties [2]. Ternary addition of Ca improves the higher temperature creep strength by forming stable

intermetallic CaMgSn particles in Mg-Sn matrix [3, 4]. It is observed that the ratio of Sn to Ca is important

for microstructural control to optimize the strength and corrosion properties [5]. A ratio of 3:1 binds entire

Ca in forming CaMgSn particles while the remaining Sn stays in solid solution. On the other hand, higher

calcium content promotes the formation of Mg2Ca particles [2], the presence of which is detrimental to the

corrosion properties. Thus, Mg-3Sn-1Ca composition gives acceptable combination of corrosion resistance

and creep strength. The hot workability of the as-cast alloy has been evaluated using processing map [6],

which revealed that the alloy may be extruded at a much higher temperature (500 oC) than other

conventional Mg alloys (300 oC). This processing map, the principles of which were discussed earlier [7-

9], was developed on the basis of the temperature and strain rate dependence of flow stress obtained in

constant strain rate isothermal compression tests. Uniaxial compression testing is preferred for this purpose

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8th TSME-International Conference on Mechanical Engineering (TSME-ICoME 2017) IOP Publishing

IOP Conf. Series: Materials Science and Engineering 297 (2018) 012014 doi:10.1088/1757-899X/297/1/012014

since large strains may be imposed on the sample without the onset of complications like necking as in

tension or a large non-uniformity of strain from center to surface as in a torsion bar.

It is useful to examine whether the state-of-stress in these testing techniques has any effect on the

characterization of hot deformation by processing maps. In this direction, the processing maps developed

on AISI 403L stainless steel on the basis of compression versus torsion data [10], have been compared and

the results indicated that either techniques may be used to predict the workability domain but the instability

regime is wider in the map made with torsion data than that made with compression data. This is

understandable since shear stresses accentuate flow instability by adiabatic shear bands or flow localization.

The aim of the present investigation is to compare the characteristics of hot deformation of high temperature

extruded TX31 alloy as evaluated by processing maps developed from uniaxial tensile versus compression

test data.

A processing map depicts the conditions of temperature and strain rate or speed that are best suited for

performing industrial metal working operations, without resorting to trial and error attempts on shop-floor.

The relevance of such a map is particularly important for successful conversion of cast alloys into their

wrought form so to improve their mechanical properties through appropriate control of the evolution of

microstructure during thermomechanical or hot working processes. The principles and procedures for

developing processing maps [7-9] are briefly summarized here. Briefly, the work-piece undergoing hot

deformation is considered to be a dissipator of power and the strain rate sensitivity (m) of flow stress is the

factor that partitions power between deformation heat and microstructural changes. The efficiency of power

dissipation compared to an ideal linear dissipator, occurring through microstructural changes during

deformation as a function of temperature and strain rate is given by:

η = 2m/(m+1) (1)

The variation of efficiency of power dissipation with temperature and strain rate represents a power

dissipation map which is generally viewed as an iso-efficiency contour map. Further, the extremum

principles of irreversible thermodynamics as applied to continuum mechanics of large plastic flow [11] are

explored to define a criterion for the onset of flow instability given by the equation for the instability

parameter ξ:

0

ln

)]1/(ln[)(

m

mm

(2)

The variation of the instability parameter as a function of temperature and strain rate represents an

instability map which delineates regimes of instability where ξ is negative. A superimposition of the

instability map on the power dissipation map gives a processing map which reveals domains (efficiency

contours converging towards a peak efficiency) where individual microstructural processes occur and the

limiting conditions for the regimes (bounded by a contour for = 0) of flow instability. Processing maps

help in identifying temperature – strain rate windows for hot working where the intrinsic workability of the

material is maximum (e.g. dynamic recrystallization (DRX) or superplasticity) and also in avoiding the

regimes of flow instabilities (e.g. adiabatic shear bands or flow localization).

2. Experimental

The nominal composition (wt.%) of the starting material Mg-3Sn-1Ca was extruded at 500 oC at a ram

speed of 3 mm s-1 using a round shear die of 12 mm diameter with an extrusion ratio of 17:1. The average

grain diameter was about 9 µm. For compression testing, cylindrical specimens with 10 mm diameter and

15 mm height were machined with the compression axis parallel to the extrusion direction. Uniaxial

compression tests were conducted in the temperature range of 300 – 500 oC and true strain rate range of

0.0003 – 1 s-1. Constant true strain rates were achieved by an exponential decay of actuator speed in the

servo-hydraulic machine. Details of the test set-up and experimental procedure are given earlier [12]. For

high temperature tensile tests, cylindrical specimens with a gage length of 30 mm and gage diameter of 6

mm were used and constant true strain rate had been achieved by ramping up the speed of the actuator

exponentially. The load – stroke data were converted into true stress – true strain curves using standard

equations. Processing maps were developed using the procedure described earlier using the flow stress

data and different temperatures and strain rates at a constant strain. The deformed specimens were sectioned

parallel to the compression or tensile axis as the case may be, polished and etched with an aqueous solution

3

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8th TSME-International Conference on Mechanical Engineering (TSME-ICoME 2017) IOP Publishing

IOP Conf. Series: Materials Science and Engineering 297 (2018) 012014 doi:10.1088/1757-899X/297/1/012014

containing 10% picric acid. The microstructures were recorded close to the fracture end in the case of tensile

specimens while in compression specimens they were examined in the middle of the specimen.

3. Results and Discussion

3.1. Stress-strain behavior

The true stress – true strain curves obtained on TX31 deformed in compression and tension at a temperature

of 300 oC and at different strain rates are shown in figure 1. The compression curves were showing flow

softening at higher strain rates and lower temperatures and a near steady state flow at lower strain rates and

higher temperatures. The tensile curves on the other hand showed steady state flow only for the strain rate

of 0.0003 s-1 and exhibited normal shapes for higher strain rates, namely reaching an ultimate tensile stress

before dropping towards fracture when necking occurs.

The curves at higher temperatures are shown in figure 2 with 500 oC as an example, for compression

and tension. The compression curves are of flow softening type at higher strain rates and steady-state type

at lower strain rates similar to those obtained at 300 oC (figure 1). The tensile curves (figure 2) exhibited

nearly steady state flow for all the strain rates except that for a strain rate of 1 s-1, which started to neck at

strains higher than about 0.2. From the stress – strain curves given in figures 1 and 2, it is clear that for the

purpose of comparison, the flow stress data up to a strain of 0.2 at different temperature and strain rates

may be valid for developing processing maps for compression versus tension. At higher strains, the data

will acquire complications due to the three-dimensional state-of-stress in necked region in tension rendering

it unacceptable for comparison with compression.

Figure 1. Stress-strain curves of TX31 alloy

obtained at a temperature of 300 oC in

compression versus tension at different strain

rates.

Figure 2. Stress-strain curves of TX31 alloy

obtained at a temperature of 500 oC in

compression versus tension at different strain

rates.

3.2. Processing maps

The processing maps obtained at a strain of 0.2 from flow stress obtained from compression and tension

tests are shown in figure 3 and figure 4, respectively. The numbers on the contours represent efficiency of

power dissipation in percent. The maps exhibit three domains in each case – two in the lower strain rate

range and the third at higher strain rate range. For compression map (figure 3), the lower strain rate domain

(#1) occurs over a wide range of temperature of 340 – 420 oC and extend over the strain rate range of 0.0003

– 0.002 s-1 with a high peak efficiency of power dissipation at 28%. The higher temperature domain (#2),

however, occurs in temperature and strain rate ranges given by 450 – 500 oC and 0.0003 – 0.01 s-1 with a

peak efficiency of about 29%. In comparison, the map for tension shown in figure 4 also exhibited three domains

in a similar temperature range although the upper strain rates have moved to higher values in the case of Domain #1

and Domain #2. The peak efficiency values were also higher in these two domains. It is possible that the true strain

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8th TSME-International Conference on Mechanical Engineering (TSME-ICoME 2017) IOP Publishing

IOP Conf. Series: Materials Science and Engineering 297 (2018) 012014 doi:10.1088/1757-899X/297/1/012014

rates in tension will be lower than the nominal values, which may be the reason for the higher values exhibited by the

processing maps for tension.

Figure 3. Processing maps of TX31 alloy at a

strain of 0.2 for compression.

Figure 4. Processing maps of TX31 alloy at a

strain of 0.2 for tension.

The microstructures obtained in Domain #1 at 500 oC/0.0003 s-1 on the compression and tension

specimens, shown in figure 5(a) and 5(b) are similar. The microstructures obtained on specimens deformed

under compression and tension in Domain #2 are shown in figure 6(a) and 6(b) and those in Domain #3 are

shown in figure 7(a) and 7(b) and these are also similar. The microstructures in figure 5 to figure 7 clearly

show that dynamic recrystallization has occurred in these domains. It is likely that Domain #1 and Domain

#2 will merge into a single domain at large strain since the temperature range for these domains is favorable

for the occurrence of non-basal slip including prismatic and second order pyramidal slip [13].

(a)

(b)

Figure 5. Microstructures obtained on TX31 alloy processed at 400 oC/0.0003 s-1 (Domain #1):

(a) Compression, and (b) Tension.

The higher strain rate domain is not fully developed at the strain selected for the comparison, i.e., 0.2,

but it generally represents DRX by basal+prismatic slip and recovery by climb controlled by grain boundary

self-diffusion [13]. However, all of the three domains are essentially similar for both tension and

compression and represent DRX process. Thus, the results show that the hot working mechanisms have not

changed irrespective of deformation imposed either by compression or tension.

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8th TSME-International Conference on Mechanical Engineering (TSME-ICoME 2017) IOP Publishing

IOP Conf. Series: Materials Science and Engineering 297 (2018) 012014 doi:10.1088/1757-899X/297/1/012014

(a)

(b)

Figure 6. Microstructures obtained on TX31 alloy processed at 500 oC/0.0003 s-1 (Domain #2):

(a) Compression, and (b) Tension.

(a)

(b)

Figure 7. Microstructures obtained on TX31_HTX alloy processed at 450 oC/1.0 s-1 (Domain #3):

(a) Compression, and (b) Tension.

4. Conclusions

The hot working characteristics of high temperature extruded TX31 alloy deformed in compression has

been compared with those evaluated on the basis of tensile data. The following conclusions are drawn:

(1) The shapes of true stress – true strain curves are similar and exhibited a steady state flow in both modes

of deformation until a strain of about 0.2, beyond which the occurrence of necking in tension has restricted

the comparison.

(2) The processing maps developed at a strain of 0.2 with flow stress data at different temperatures and

strain rates, are essentially similar irrespective of the mode of deformation – compression or tension.

(3) The processing maps exhibit three domains in the temperature ranges: (1) 350 – 425 oC, and (2) 450 –

550 oC and (3) 400 – 500 oC, the first two occurring at lower strain rates and the third occurring at higher

strain rates.

(4) All the three domains represent dynamic recrystallization process caused by non-basal slip and

controlled by lattice self-diffusion in the first and second domains and grain boundary self-diffusion in the

third domain (higher strain rates).

(5) The state-of-stress imposed on the specimen (compression or tension) does not have any significant

effect on the processing maps.

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8th TSME-International Conference on Mechanical Engineering (TSME-ICoME 2017) IOP Publishing

IOP Conf. Series: Materials Science and Engineering 297 (2018) 012014 doi:10.1088/1757-899X/297/1/012014

Acknowledgement

The work presented in this paper has been supported by a strategic research grant from the City University

of Hong Kong (Project #7002013).

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