Effect of Ti on the Mechanical Properties and Corrosion of Cast Az91 Magnesium Alloy

Ti addition to AZ91 alloy was been investigated with conventional casting. The microstructure and mechanical property were examined. The Corrosion resistance of all the different Ti addition content alloy was studied in 3.5% NaCl solution through weight loss measurement in constant immersion conditions and potentiodynamic polarization experiments. The results show that addition of Ti with an amount of 0.1~0.5%wt resulted in a refinement of the as-cast microstructure. The morphology of phases is changed from coarse, uneven, semi-continuous skeletal network to small, uniform, short rod-like or granular. When the content of Ti is 0.4wt%, the tensile strength and elongation go up to maximum value of 197 MPa and 6.9% respectively. The corrosion resistant improved through addition Ti element is related to the morphology and distribute of phases. The mechanism of mechanical properties and corrosion resistant improvement caused by Ti addition is discussed.


INTRODUCTION
As the most promising lightweight materials, magnesium alloys have high specific strength and specific stiffness, good yield strength, excellent machinability and good damping capacity.So, more and more magnesium alloy products have been used in automobile, aerospace and communication industries.AZ and AM series of alloys constitute about 90% of all structural applications of magnesium.AZ91D alloy has been used to fabricate a variety of automobile parts, such as cam covers, baffles, oil adapters, clutch housings, steering wheels, and so on; AM60 and AM50 alloys are frequently employed to manufacture instrument panels, steering wheel armatures, and seat risers.However, complex hard and brittle phases mostly formed at grain boundaries [1,2], they deteriorated the mechanical properties of magnesium alloys.At same time, corrosion limits the application of magnesium alloys [3,4].Several studies [5,6] have shown that the corrosion behavior is significantly influenced by microstructure, particularly the amount and distribution of the intermetallic phases -Mg 17 Al 12 (the following are called phases).
Ti element have excellent properties such as high corrosion resistance, good mechanical properties [7].Our previous work showed Ti element could refine the as-cast microstructure and changed the morphology of phases.The present investigation is concentrated on studying the effect of Ti element on microstructure mechanical properties and corrosion behavior of as-cast AZ91 alloys.

Materials
The compositions of four experimental alloys are designed as Table 1.The raw materials for the alloys are magnesium (99.8wt %) ingots, aluminum (99.7wt %) ingots, Zinc (99.8wt%) ingots and Al-6Ti alloy ingots.Smelting processes were carried out in an 12 kg well crucible electric resistance furnace under condition of shielding RJ2.The ingots were melt completely, up to a temperature of 670 the Al-6Ti alloy was added.When the temperature of the molten metal was raised to 750 , the melt was held for 20 min to make sure that Ti was completely dissolved.Afterwards the melt was poured into a preheated steel mould at approximately 720 .In this process the melt was stirred several times to ensure homogeneous distribution of Ti and other alloying elements.The base nominal composition of the alloys studied was Mg-9Al-0.8Zn-0.2Mnconformed to that of AZ91.The dimension of sample is 20 mm 20 mm 20 mm.

Corrosive Medium
The 3.5% NaCl solution test medium was made with AR grade NaCl and distilled water.The pH value was controlled at 7-7.5.All corrosion tests were carried out at room temperature.

Constant Immersion Testing
The specimens were ground on progressively finer emery papers up to 1000 grade and then polished using 1μm diamond paste.The polished and preweighed specimens were exposed to 400 ml of 3.5% NaCl solution for a constant time.The corroded specimens were immersed in a solution of 20wt% CrO 3 + 1wt% AgNO 3 in 200 ml of boiling water for 5 minutes at ambient temperature to remove the surface corrosion products and then were washed by flowing water, cleaned with acetone and absolute alcohol respectively for 2 min and cold dried again.The weight loss was subsequently measured by means of an electronic balance with a precision of ±0.1mg.The corrosion rate was calculated according to the following Eq. 1 where V denotes the corrosion rate of the specimen, W1the weight before the corrosion, W2 the weight after the corrosion (wipping off the corrosion products), and A the surface area and t the corrosion time.The experimental results were the mean value of three samples.

Electrochemical Testing
The specimens for electrochemical measurements, of dimensions 20 mm 20 mm 5 mm, were polished to1500 grit SiC paper and a copper wire was welded on the backsurface for electrical contact.The specimens were embedded with epoxy resin so that about 1 cm 2 was exposed to the solution.The specimens were given a metallographic polish prior to each experiment, followed by washing with distilled water and acetone.Polarization measurements were carried out in a corrosion cell containing 400 ml of 3.5% NaCl solution using a CHI660C electrochemical measurement system: saturated colomel as a reference with a platinum electrode as counter and the sample as the working electrode.Specimens were immersed in the 3.5% NaCl solution.A polarisation scan was carried out from cathodic potential to anodic potential at a rate of 1 mV s -1 , after allowing a steady state potential to develop.

Microstructure and Surface Morphology
The microstructure and surface morphology were observed by an Olympus-BX51M optical microscopy.Determination of crystalline structure of different phases in the alloys studied was carried out by energy dispersive X-ray diffraction (XRD).The morphology of phases was investigated by a JSM-6360LV scanning electron microscope (SEM) coupled with energy dispersed spectroscopy (EDS) system.

Mechanical Properties Testing
Tensile testing was performed using Instron5569 electron universal testing machine at room temperature (RT) with strain rate of 1.7 10 3 s 1 .Tensile specimens with gauge section of 35mm 15mm 3mm were cut from the ingots prepared by electric spark machining.

Fig. (1a
) is an optical micrograph taken from the as-cast specimen of the alloy AZ91 showing coarse dendrites.SEM observations revealed that the microstructure of as-cast AZ91 was composed of -Mg matrix and two types of phases segregated on the boundaries of the -Mg dendrite structures.The first one was coarse with an irregular morphology.The second one was tiny laminar shaped surrounding the first one, as shown in Fig. (1b).According to the Mg-Al phase diagram and the previous investigations [8][9][10][11], these particles should be the -Mg 17 Al 12 phase.Small amount of Ti elements addition to the AZ91 alloy resulted in significant refinement of the as-cast microstructure, as shown in Figs.(2)(3)(4) and Table 2.When 0.8wt% Ti was added, it can be found that there are some bright, rod particles lying in grain and grain boundaries.According to the XRD results (Fig. 5), it can be confirmed that rod-like precipitates are TiAl 3 .However, compared to the AZ91 alloy, the other three alloys with a small amount of Ti added have finer grain sizes.These changes about the microstructure of AZ91 magnesium alloy can be analyzed from the following two aspects: On the one hand, Ti element is surface-active element, so during the course of alloy solidification, it can significantly reduce the alloy solid -liquid interfacial tension.According to nucleation formula, see Eq. 2.
where r* is the critical nucleation radius, LS solid-liquid interfacial tension and G m Gibbs free energy for the solidification reaction.Ti element reduces the critical nucleation radius by reducing the liquid-solid interfacial tension, and the ultimately nuclear volume of the -Mg in the liquid phase is improved.Compared with the AZ91 magnesium alloy, under the same conditions of cooling rate, AZ91-Ti melt, during the initial solidification, has more -Mg nuclei which lead -Mg dendrite smaller.Furthermore, Ti adsorbed on the tip of growth phases, thus prevents them from growing up, and reduces their size.As results, the amount of phases on the grain boundaries become short and the dispersion degree of phases improves.
On the other hand, because the chemical action does not occur between Ti and Mg elements [12], Al elements and Ti elements combine to form intermetallic compounds rather than the combinations of Mg and Ti on the same condition.From the thermodynamic point of view, in the Mg-Al-Ti system, Al elements firstly combine with Ti elements to intermetallic compounds, while, the remaining Al elements combine with Mg elements to form the compounds, or solve in the Mg matrix [13,14].Mostly, Ti elements and Al elements have the response to form AlTi or TiAl 3 two kinds of intermetallic compounds.But from the perspective of thermodynamics, the reaction of producing intermetallic compounds TiAl 3 is easier.In this alloy, TiAl 3 distributes in the matrix and the second phases by cosh shape (Fig. 4b).Although Al-Ti compounds are not considered as the core of crystal formations, it can impede the growth of -Mg branch crystal.The reason is that with the growth of -Mg grains, Al-Ti compounds concentrate along the grain boundaries to prevent the grain growth, suppress Al elements spreading into the melt, to prevent phases formation and growth, to   reduce the number of phases, Thus this provides a driving force to other nucleation particle in liquid in front of solidification for shaping core.

Tensile Properties
The mechanical properties including ultimate tensile strength (UTS) and elongation ( ) of the as-cast alloys at RT are shown in Fig. (6).It can be seen that addition Ti element improves the mechanical properties of alloys.When Ti additions are 0.4 (wt%), the AZ91-0.4Tialloy exhibits highest UTS and in the system alloys, the values are 197MPa, 6.9% respectively.However, if the Ti contents are above 0.4 (wt%), the ultimate tensile strength and the elongation reduce.The tensile results demonstrate that the influence of Ti element addition in metal on tensile properties was significant.After Ti elements addition in alloy, the size of grain becomes small, grain boundaries increasing, and the quantity of phases is few or disperses.For these reasons, the crack tendency is reduced, fracture growth is slowed down, and thus the tensile strength has the enhancement.The addition of Ti element improves the mechanical properties by refining and diapering phases.Diffuse distribution of phases can reduce the fragmentation action on the matrix and the weakening of the role of grain boundary.The mobility of the grain boundary and dislocation slip were greatly enhanced because of refining the phases.so the plasticity of AZ91-Ti alloy has been enhanced.However, if the Ti contents are above 0.4 (wt%), thick TiAl 3 will solid and gather at the crystal boundary place to destroy the connection between the crystal grain and the crystal grain.That causes the alloy at time of stretching easily to crack in crystal boundary place, finally, the elongation ratio and the ensile strength of the AZ91Ti magnesium alloy are reduced, Even lower than the performance of the original alloy.The fractured surfaces of AZ91-Ti alloy are shown in Fig. (7).It shows brittle fracture feature in which quasicleavage is the main characteristic with obvious quasicleavage steps and tearing ridges.8) is the EDS analysis of tearing ridge and crack respectively.Comparison of Fig. (8a, b), it can be seen that the ingredient of tearing ridge is mainly Mg and Al element (spectrum1), moreover their atomic ratio is 9:1.On the crack (spectrum2), there are not only Mg, Al element, but also Zn element appear.However, the amount of Zn is small, this indicated that pit area is -Mg solid solution with Zn atoms, this also shows the crack has been extended into the -Mg grains.Basing on the EDS analysis to the different position on fracture, it may be obtained that tearing ridges embolden the breaks of phases.When the Ti contents are 0.4 (wt%), the fractures of the alloys were showed tearing ridges clearly, shorter than 0.2 (wt%)Ti, a few circular dimples appear locally, and the characteristics of quasi-cleavage and ductile fracture form as shown in Fig. (7b).From the results of above analysis, the plasticity of the alloys can be enhanced obviously when the Ti contents are 0.4 (wt%).

Effect of Ti Element Addition Content on Corrosion Rates
Fig. (9) illustrates the corrosion rate of AZ91-XTi alloys in the solution of 3. 5% NaCl by static weight loss method.From Fig. (9) it can be seen that the corrosion rate of AZ91 alloy rapidly increased with prolonging corrosion time, and the value of corrosion rate was relatively larger.The addition of Ti remarkably reduces the corrosion rate of AZ91 alloy, and corrosion rate of alloy with 0. 8 (wt%)Ti is minimum value.At the initial stage of corrosion, the corrosion rate of the three alloys with various Ti contents is low, and with corrosion time prolonging, corrosion rate enhances obviously.The corrosion rate of three alloys containing Ti exhibits alternate increasing and decreasing.
In addition, it can be found that, under the same conditions, the corrosion rate of alloys reduced with increasing Ti contents and reached minimum in the case of 0.8 (wt%)Ti.The corrision rate of AZ91-0.4Tialloy is higher than the AZ91-0.8Ti.This phenomenon is proposed to attribute to two kinds of reasons: appearance of pitting corrosion owing to the existence of active anion Cl -, as well as the formation of surface oxide film and thickening.The effect of Cl -gradually reduced with the thickening of surface film, so over a certain time the corrosion rate began declining.Besides the anode dissolved in the process of electrochemical reaction with increasing corrosion time, sapping appeared at some location and resulted in the abscission of the undissolved metallic particles.In this case, the corrosion rate tested by the weight loss method would increase.The pitting action above mentioned intensified dued to the large amount of Cl -in the solution of 3. 5%NaCl.Therefore, the corrosion rate exhibited rapid increasing.But subsequent thickening of surface film could prevent the function of Cl -and decline the corrosion rate.For the various alloys with Ti declining time of corrosion rate were also different, such as immersed after 5 day for AZ91-0.2Tialloy, while immersed after 3 day for AZ91-0.4Tiand 4day for AZ91-0.8Ti alloy, which were related to theirs thickening rate of surface film.During the whole corrosion process, the thickening of surface film and pitting action of Cl -were alternately dominant.The pitting corrosion started immediately and then destroyed the formation of surface film when immersed into the corrosion medium of 3. 5% NaCl.So the corrosion rate increased continuously until 4 th day, the alloys were followed by declining tend, which was probably caused by accumulation of corrosion products on the surface.The corrosion performance of AZ91-XTi alloy depends on the integrated influence of the A1 content in -Mg matrix and the phase barriers.It is knew the corrosion usually appeared in the interior of -Mg grain for AZ91 alloy at the early stage while less corroded at the grain boundary.According to the study [15], higher Al element content in the matrix could effectively depress the corrosion.In as-cast microstructure, Al elements mostly resist in the boundary of phases, after having added Ti element into AZ91 alloy, the amount of phases are low, Al content in -Mg matrix are high.phases as the reinforcing phases in AZ91 alloy play a significant role during the corrosion, especially the content and distributions of phases.If phases non-uniformly distribute (Figs. 1, 2), the barrier effects of phases will reduce even disappear.On the other hand, if phases uniformly distribute (Fig. 3), then phases, corrosion products and TiAl 3 intermetallics (Fig. 4) may act as barrier to prevent corrosion and decrease the corrosion rate of alloys.So addition Ti element results in the massive of netlike phases transformed into discontinuous netlike or dispersed the fine particle and uniformly distributed are advantageous to exert barrier effect of phases.A lower amount of cathodic phases and higher Al content matrix alloy prevent severe corrosion attack.Therefore the result shows the best corrosion resistance for AZ91-xTi alloy.

Effect of Ti Element Addition Content on Electrochemical Behavior
The potentiodynamic polarization curves of different Ti addition content AZ91 alloys in 3.5% NaCl solution are shown in Fig. (11).From Fig. (11) it can be seen that balance potential and corrosion potential of alloys with small content Ti addition can move towards positive direction compared with AZ91 matrix alloy.Namely adding 0.4 (wt%) Ti may

Fig. (
Fig. (8) is the EDS analysis of tearing ridge and crack respectively.Comparison of Fig.(8a, b), it can be seen that the ingredient of tearing ridge is mainly Mg and Al element (spectrum1), moreover their atomic ratio is 9:1.On the crack (spectrum2), there are not only Mg, Al element, but also Zn element appear.However, the amount of Zn is small, this indicated that pit area is -Mg solid solution with Zn atoms, this also shows the crack has been extended into the -Mg grains.Basing on the EDS analysis to the different position on fracture, it may be obtained that tearing ridges embolden the breaks of phases.When the Ti contents are 0.4 (wt%), the fractures of the alloys were showed tearing ridges clearly, shorter than 0.2 (wt%)Ti, a few circular dimples appear locally, and the characteristics of quasi-cleavage and ductile fracture form as shown in Fig.(7b).From the results of above analysis, the plasticity of the alloys can be enhanced obviously when the Ti contents are 0.4 (wt%).

Fig. ( 10
Fig.(10) shows the surface micrographs of AZ91 and AZ91-0.8Tialloy immersed in corrosion medium of 3. 5% NaCl for 6 day.It can be found that AZ91 alloy was