Microstructure and High Temperature Impression ... - Springer Link

Trans Indian Inst Met (2017) 70(3):649–654 DOI 10.1007/s12666-017-1068-z

TECHNICAL PAPER

Microstructure and High Temperature Impression Creep Properties of Mg–3Ca–xZr (x 5 0.3, 0.6, 0.9 wt%) Alloys Widyani Darham1 • Ahmad Lutfi Anis1 • Izzul Adli Mohd Arif1 • Nagamothu Kishore Babu2 • Mohamad Kamal Harun1 • Mahesh Kumar Talari1

Received: 30 November 2016 / Accepted: 6 February 2017 / Published online: 21 February 2017 Ó The Indian Institute of Metals - IIM 2017

Abstract The current study has investigated the influence of zirconium (Zr) addition to Mg–3Ca–xZr (x = 0.3, 0.6, 0.9 wt%) alloys prepared using argon arc melting on the microstructure and impression properties at 448–498 K under constant stress of 380 MPa. Microstructural analysis of as-cast Mg–3Ca–xZr alloys showed grain refinement with Zr addition. The observed grain refinement was attributed to the growth restriction effect of Zr in hypoperitectic Mg–3Ca–0.3 wt% Zr alloys. Heterogeneous nucleation of a-Mg in properitectic Zr during solidification resulted in grain refinement of hyperperitectic Mg–3Ca– 0.6 wt% Zr and Mg–3Ca–0.9 wt% Zr alloys. The hardness of Mg–3Ca–xZr alloys increased as the amount of Zr increased due to grain refinement and solid solution strengthening of a-Mg by Zr. Creep resistance of Mg–3Ca– xZr alloys increased with the addition of Zr due to solid solution strengthening of a-Mg by Zr. The calculated activation energy (Qa) for Mg–3Ca samples (131.49 kJ/mol) was the highest among all alloy compositions. The Qa values for 0.3, 0.6 and 0.9 wt% Zr containing Mg–3Ca alloys were 107.22, 118.18 and 115.24 kJ/mol, respectively. Keywords Mg–3Ca Zirconium Grain refinement Impression creep

& Mahesh Kumar Talari [email protected] 1

Faculty of Applied Sciences, Universiti Teknologi MARA, 40450 Shah Alam, Malaysia

2

Empa, Swiss Federal Laboratories for Material Science and Technology, Laboratory for Advanced Materials Processing, Feuerwerkerstrasse 39, 3602 Thun, Switzerland

1 Introduction Magnesium (Mg) alloys are widely used for structural applications due to their specific characteristics of low density, high specific strength and lightweight [1]. However, usage of Mg alloys is restricted due to their inability to withstand creep deformation and possess low strength at high temperatures. The most common Mg alloys are those based on the Mg–Al system (AZ and AM series). However, formation of b-Mg17Al12 precipitates causes these alloys to exhibit relatively poor high temperature mechanical properties. This is because, the precipitates have a low melting point (Tm = 732 K) and tend to be formed at the grain boundaries and inter-dendritic regions [1, 2]. The low melting point precipitates tend to soften and dissolve at temperatures higher than 393 K, thus resulting in low creep resistance. Mg–Ca alloys containing Mg2Ca phase have been developed for high temperature applications. The higher melting point (Tm = 988 K) of Mg2Ca phase resists softening at elevated temperature, thereby increasing the creep resistance. Calcium (Ca) is a relatively inexpensive alloying element that can assist in grain refinement. Addition of Ca C 1% into Mg is reported to enhance tensile strength, ductility and corrosion resistance of Mg alloys [3]. Yang et al. [4] have reported that the addition of 1.5% Ca into Mg–5Zn–5Sn alloys improves the yield strength and creep properties. Grain refinement is considered an effective approach to achieve better quality and improved strength, toughness and ductility of Mg alloys. Zirconium (Zr) refined Mg alloys usually provide a good combination of mechanical properties at ambient and elevated temperatures. Zr is generally added into Mg alloys as an effective grain refiner due to its high growth restriction factor (GRF = 38.29) and also Zr has almost identical crystal

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Trans Indian Inst Met (2017) 70(3):649–654

structure with Mg [5, 6]. However, Zr cannot be added into aluminium (Al), silicon (Si), manganese (Mn), nickel (Ni) or antimony (Sb) containing Mg alloys. This is due to the formation of intermetallic phases with Al, Si, Mn, Ni or Sb which will reduce the grain refining effect of Zr [2, 7]. Impression creep test has attracted researchers as it is a relatively new tool to study the creep deformation behaviour of the material. The impression creep technique uses cylindrical shaped punch with a flat end, at constant load, forced against flat surfaced samples. Therefore, only small amount of material is sufficient for the test. Conventional tensile creep tests include measurement of plastic deformation at constant stress or constant strain rate with respect to time, whereas impression creep measures indentation depth with respect to indentation time [8]. Creep deformation usually occurs at temperature approximately 0.5Tm, where Tm is the absolute melting temperature. Investigation of creep rate, e_ has been related to temperature, T, and stress, r, by the following equation e_ ¼ k1 expðQa =RT Þðr=EÞn

ð1Þ

where k1 is a function of structure, E is Young’s modulus, n is stress exponent, Qa is activation energy and R is universal constant [9]. The minimum impression velocity, Vi, can be obtained from the slope of the secondary steady state region of the indentation depth versus time plot [10]. Steady state creep velocity, e,_ can be obtained by dividing Vi with the punch diameter [10]. The activation energy, Qa, for the creep deformation of the alloys can be estimated from the slopes of ln e_ versus the reciprocal of temperature (1/T) plot. Although creep deformation can be usually explained by the diffusion-controlled creep equation, other mechanisms such as grain boundary sliding (GBS) and diffusional flow can also influence creep deformation, particularly in fine grain materials. In this work, Mg–3Ca–xZr (x = 0.3, 0.6, 0.9 wt%) alloys were prepared by argon arc melting technique. Hardness and impression creep tests were performed on the arc melted specimens. Microstructural analysis and relation to creep resistance were discussed.

2 Materials and Methods Four different alloys of nominal composition of Mg–3 wt% Ca–x wt% Zr alloys (x = 0, 0.3, 0.6 and 0.9) were prepared as listed in Table 1. Mg–3 wt% Ca–x wt% Zr alloys with x = 0, 0.3, 0.6 and 0.9 shall be here after referred to as Mg–3Ca, Mg–3Ca–0.3Zr, Mg–3Ca–0.6Zr and Mg–3Ca– 0.9Zr, respectively. The alloys were prepared using arc melting furnace on a water-cooled copper (Cu) hearth in argon (Ar) atmosphere. High purity Mg and Zr (99.9%) and 70 wt% Mg–30 wt% Ca master alloy were used for

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Table 1 Composition of alloys after melting obtained from ICP-OES analysis Alloy

Wt% Zr

Ca

Mg

Mg–3Ca

0

2.9

97.1

Mg–3Ca–0.3Zr

0.32

2.78

96.9

Mg–3Ca–0.6Zr Mg–3Ca–0.9Zr

0.63 0.97

2.97 2.83

96.4 96.2

melting. The melting of each sample was performed 5 times to ensure homogeneity of the samples. Compositions of prepared samples after melting were analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES, Perkin Elmer Optima 4300DV). Table 1 show the ICP-OES chemical analysis results of arc melted samples. The samples were metallographically prepared prior to etching using a solution of 2% nitric acid and 98% ethyl alcohol at room temperature. Microstructural examination of samples was conducted using a field emission scanning electron microscope (FESEM) equipped with attachment for energy-dispersive spectroscopy (EDS) (Zeiss Supra 40VP). The phase identification was performed using Panalytical X-ray diffractometer (XRD) with Cu Ka radiation. The measurements were conducted with step scan (2h) from 20° to 90° with increment of 0.02°. A Vickers hardness tester (Shimadzu HMV-2) was employed to measure the hardness with a load of 500 g and a holding time of 15 s. Impression creep testing of the samples were carried out using Wear and Friction Tech (Chennai, India) impression creep equipment at temperatures of 423–498 K. The creep load of 380 MPa was selected based on indenter size and preliminary laboratory studies. Furnace temperature control and impression load accuracy were within ±0.5 K and ±0.1 N respectively. Cylindrical samples of 5 mm height and 15 mm diameter were machined and their surface was metallographically polished. A flat end cylindrical tungsten carbide indenter with 1 mm diameter was used during this investigation. The data of indentation depth and time was obtained by displacement sensor (1 lm resolution) connected to a computer by data acquisition hardware.

3 Results and Discussion 3.1 Microstructure The FESEM micrographs of Mg–3Ca–xZr alloys in Fig. 1 shows dendritic grain structure for all the samples. The gradual transformation from coarse grained dendritic


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Fig. 1 SEM micrograph of; a Mg–3Ca, b Mg–3Ca–0.3Zr, c Mg–3Ca–0.6Zr, d Mg–3Ca–0.9Zr, and e lamellar eutectic mixture in interdendrite region of Mg–3Ca–0.9Zr sample

structure to fine grained dendritic structure can be clearly seen from the micrographs with increasing Zr addition. The dendritic arm spacing (DAS) values are shown in Table 2. The value of DAS has been measured using linear intercept method and calculated using the Eq. [11] DAS ¼ LT =PM

ð2Þ

where LT is the total length of measuring line, M is magnification and P is crossing point between measuring line and secondary dendrite. The DAS values for Mg–3Ca also get refined after the addition of Zr. It can be observed that the microstructure of Mg–3Ca consists primarily of a-Mg matrix and lamellar eutectic mixture of a-Mg and Mg2Ca in the intergrain and interdendritic arm regions (Fig. 1e) [3].

The average grain size in the Mg–3Ca–xZr alloys reduces as Zr is increased. EDS analysis reveals that the lamellar eutectic structure consists of higher amounts of Ca compared to grain interiors. Figure 2 shows the indexed XRD patterns of Mg–3Ca–xZr alloys showing the presence of a-Mg and Mg2Ca phases. Mg–Ca binary phase diagram shows the formation of high temperature Mg2Ca intermetallic phase at 715 °C while Mg rich corner shows eutectic reaction of liquid into a-Mg and Mg2Ca phases during solidification [12, 13]. The solidification of Mg-3Ca alloys starts with the formation of proeutectic a-Mg with hcp crystal structure and subsequent eutectic reaction results in the formation of lamellar eutectic mixture of a-Mg and Mg2Ca. Mg rich corner of

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Table 2 The Vickers hardness value and DAS of Mg–3Ca–xZr samples Zr content (wt%)

Vickers hardness (Hv)

Grain size (lm)

DAS (lm)

0

57 ± 2

66 ± 8

0.072 ± 0.003

0.3

59 ± 2

53 ± 4

0.064 ± 0.004

0.6 0.9

60 ± 4 64 ± 2

50 ± 2 40 ± 3

0.054 ± 0.005 0.048 ± 0.001

Fig. 2 XRD patterns of the Mg–3Ca–xZr alloys

Mg–Zr binary phase diagram shows peritectic reaction with a peritectic composition of 0.58 wt% Zr [5]. According to Wang et al. [14], addition of peritectic forming elements in an alloy system results in higher driving force for the solidification and subsequent higher nucleation rate. Another mechanism that has been discussed is the growth restriction factor of Zr in Mg. In Zr enriched-Mg alloys, it promotes constitutional undercooling during solidification at the solid/liquid interface that may retard the grain growth. Reduction in grain size and DAS with 0.3 wt% Zr addition to Mg–3Ca alloy may be attributed to the higher nucleation rate and growth restriction by Zr during solidification. Addition of 0.6 and 0.9 wt% Zr in Mg exceeds the value of 0.58 wt% and falls in hyper-peritectic region, where formation of Zr precedes the solidification of a-Mg [5]. Heterogeneous nucleation caused by properitectic Zr when Zr content is more than 0.58 wt% can be another factor for grain refinement. StJohn et al. [6] reiterated that undissolved Zr actually helps in Mg grain refinement. According to Mg–Zr binary phase diagram, properitectic Zr dissolves back in a-Mg after complete solidification. Rapid cooling to room temperature after solidification can arrest the Zr precipitation from super saturated a-Mg solid solution. XRD patterns of

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Zr added Mg–3Ca alloys also does not show any distinguishable Zr peaks (Fig. 2). Furthermore, FESEM analysis of the Mg–3Ca–xZr alloys does not show any Zr precipitates within the grain or at grain boundaries. 3.2 Mechanical Properties Table 2 shows the average value of Vickers hardness of Mg–3Ca–xZr alloys. The result reveals that Zr-free alloy has the lowest hardness value (57 ± 2 Hv) compared to Zrcontaining alloys and the hardness increases with increase in Zr content in Mg–3Ca–xZr alloys. Mg–3Ca–0.9Zr alloy displays highest hardness value of 64 ± 2 Hv. Increase in hardness with the addition of Zr can be attributed to the combined effect of grain size reduction and solid solution strengthening by Zr. Figure 4 shows the typical impression creep curves expressed as depth of impression (mm) versus time (s). The impression curves exhibit primary and secondary creep stage. During the primary stage of creep, the creep resistance of the material increases by virtue of its deformation as the material undergoes strain hardening. During the secondary stage, the creep rate is nearly constant as a result of a balance between competing hardening and softening mechanisms [15, 16]. The impression creep deformation plots show that Mg–3Ca–0.9Zr sample exhibit highest creep resistance compared to Mg–3Ca–0.3Zr and Mg–3Ca–0.6Zr samples. The increase in creep resistance with the increase in Zr content can be due to solid solution strengthening of a-Mg by Zr. According to the Mg–Zr phase diagram, there is a possibility of fine Zr precipitation in the a-Mg during cooling below eutectic temperature. However, SEM micrographs does not reveal the presence of any precipitates in a-Mg matrix. Furthermore, reduction in grain size reduces the thickness and amount of low melting point eutectic Mg and Mg2Ca mixture at the grain boundaries, which can also contribute to the improvement of the creep resistance. Impression creep plots of Mg–3Ca–xZr alloys are shown in Fig. 3. Mg–3Ca displays lower creep rate (8.67 9 10-6 mm/s) compared to pure Mg (0.19 9 10-6 mm/s) at 423 K. High creep resistance of the Mg–3Ca alloy can be attributed to the presence of a-Mg and Mg2Ca eutectic mixture at the grain boundaries. Thermally stable Mg2Ca (Tm = 988 K) within grain boundaries can enhance creep resistance by slowing down the dislocation movement and grain boundary sliding at elevated temperatures [16, 17]. It can be seen from Fig. 3 that Zr-added alloys show lower creep rate at all temperatures compared to Mg–3Ca sample. This implies that addition of Zr improves creep resistance of Mg–3Ca alloy and can be attributed to the solid solution strengthening of a-Mg by Zr. Zr, when dissolved in a-Mg, results in higher lattice distortions due to bigger atomic radius (206 pm) compared to solvent Mg atoms (145 pm)


(a)

(c)

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(b)

(d)

Fig. 3 Impression creep deformation plots of Mg–3Ca–xZr at various temperatures; a 423 K, b 448 K, c 473 K, and d 498 K

and increases resistance to dislocation motion during deformation [18]. Figure 4 shows ln e_ versus the reciprocal of temperature (1/T) plots and Qa values. The Qa value for Mg–3Ca samples (131.49 kJ/mol) is the highest among all alloy compositions. The Qa values for 0.3, 0.6 and 0.9 wt% Zr containing Mg–3Ca alloys are 107.22, 118.18 and 115.24 kJ/mol, respectively. The values are lower than that for self-diffusion of Mg (Qa = 135 kJ/mol) [18]. In Zr-free alloy, Qa is closer to lattice self-diffusion of Mg, suggesting that dislocation climb is assisted by lattice self-diffusion of Mg atoms. Mg–3Ca–xZr alloys, in general, displays lower activation energies compared to Mg–3Ca alloy. As discussed earlier, Zr added Mg–3Ca–xZr alloys displays smaller grain size values compared to Mg–3Ca alloy. Thus, increase in grain boundary area for Mg–3Ca–xZr alloys results in increased grain-boundary diffusion (Qa = 92 kJ/mol) which also contribute to the creep deformation along with the lattice self-diffusion [18, 19]. However, the

activation energies of Mg–3Ca–0.6Zr and Mg–3Ca–0.9Zr alloys are slightly higher than the Mg–3Ca–0.3Zr alloy, in spite of smaller grain size for 0.6 and 0.9 wt% Zr alloys, which may be attributed to the decrease in diffusivities with the increase in solute content (Zr) in the a-Mg matrix.

4 Conclusions Mg–3Ca–xZr (x = 0.3, 0.6, 0.9 wt%) alloys were successfully prepared by argon arc melting technique. Impression creep tests were performed on the samples at temperatures 423–498 K. From the analysis of the results obtained it can be concluded that 1.

Microstructural analysis showed that the coarse dendritic structure of Mg–3Ca was generally refined with the addition of 0.3, 0.6 and 0.9 wt% Zr. The microstructures consisted primarily of a-Mg matrix

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References

Fig. 4 ln e_ versus 1/T plots of Mg–3Ca–xZr samples at a constant load of 380 MPa

2.

3.

4.

and lamellar a-Mg and Mg2Ca eutectic structure in intergrain and interdendritic arm regions. Growth restriction effect due to Zr in Mg–3Ca–0.3Zr alloys and inoculation effect due to properitectic Zr in Mg–3Ca–0.6Zr and Mg–3Ca–0.9Zr alloys resulted in grain refinement. Grain refinement and solid solution strengthening of aMg by Zr resulted in increased hardness of Zr added Mg–3Ca alloys. The creep rates of Mg–3Ca alloys gradually decreased as higher amount of Zr was added. 0.9Zr added alloy had the lowest creep rates at all temperatures among all the alloys.

Acknowledgements Authors acknowledge the financial support provided by Universiti Teknologi MARA, Malaysia during the implementation of this project.

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