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Processing and tensile properties of A356 composites containing in situ small-sized Al3Ti particulates
Author’s Accepted Manuscript Processing and tensile properties of A356 composites containing in situ small-sized Al 3Ti particulates Zhiwei Liu, Na Cheng, Qiaoling Zheng, Jianhua Wu, Qingyou Han, Zhifu Huang, Jiandong Xing, Yefei Li, Yimin Gao www.elsevier.com/locate/msea
PII: DOI: Reference:
S0921-5093(17)31454-5 https://doi.org/10.1016/j.msea.2017.11.005 MSA35720
To appear in: Materials Science & Engineering A Received date: 10 June 2017 Revised date: 9 September 2017 Accepted date: 3 November 2017 Cite this article as: Zhiwei Liu, Na Cheng, Qiaoling Zheng, Jianhua Wu, Qingyou Han, Zhifu Huang, Jiandong Xing, Yefei Li and Yimin Gao, Processing and tensile properties of A356 composites containing in situ small-sized Al 3Ti p a r t i c u l a t e s , Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2017.11.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Processing and tensile properties of A356 composites containing in situ small-sized Al3Ti particulates Zhiwei Liua*, Na Chenga, Qiaoling Zhenga, Jianhua Wub, Qingyou Hanc, Zhifu Huanga, Jiandong Xinga, Yefei Lia and Yimin Gaoa a State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an 710047, China b Shandong Key Laboratory for High Strength Lightweight Metallic Materials (HLM), Advanced Materials Institute, Shandong Academy of Sciences, Jinan 250014, China c School of Engineering Technology, Purdue University, 401 North Grant Street, West Lafayette, IN 47906, USA Corresponding authors: Tel. +86-18066543603 [email protected] (Liu ZW) Abstract This research proposed an in situ casting method for preparing Al3Tip/A356 composites with high strength and good ductility by adding Ti powders into molten A356 at 800℃. A sampling experiment was firstly carried out to explore the evolution process of Ti powders in the A356 melt. Based on which in situ Al3Tip/A356 composites with 1.5 and 3.0 wt.% Ti additions were fabricated respectively. Both the microstructures and tensile properties of samples after T6 heat treatment were investigated. The results showed that blocky Si-substituted Al3Ti particulates were synthesized in the in situ reaction and the formation of which followed a reaction-peeling model. The utilization ratio of Ti
1
powders in the fabricating process could research 70%. In situ Al3Ti particulates existed inside the α-Al crystals of A356 alloy, with the size smaller than 10μm. Both the equiaxed transition from long columnar dendrite structure and refining of α-Al crystals occurred due to the nucleating effect of Al3Ti. Compared with T6-A356, the yield strength, ultimate tensile strength, and percent elongation of the T6-A356/3.0Ti were improved by 29.2%, 36.0% and 143.6%, respectively. The mechanisms of the improved tensile properties, including the strength and ductility, of the composites were also discussed in this research. Key words: in situ casting; Al3Ti; A356 composites; microstructures; tensile properties 1. Introduction Al-Si casting alloys are widely applied in the automobile industries for the production of automotive components due to their excellent castability, light weight, good wear resistance and low coefficient of thermal expansion [ 1 - 3 ]. As the development of automobile lightweight, Al-Si alloys with improved mechanical properties are required for guaranteeing the safety of vehicles in the service. Adding reinforced particulates，such as SiC [4-5], TiB2 [6-7] into the Al-Si alloys to form Al-Si matrix composites can effectively enhance the mechanical properties of Al-Si alloys. Normally, an improved tensile strength of Al-Si alloys can be obtained by the addition of ceramic reinforcements. The ductility of Al-Si alloys, however, usually degrades due to the segregation of plenty of ceramic particulates at the α-Al grain boundaries [8]. In the casting route, the ceramic particulates can be pushed by α-Al dendrites to the
2
dendrite/grain boundaries during the solidification process because of the poor wettability between ceramic phase and Al, leading to the formation of particulates segregation at the grain boundaries. Preparing particulate reinforced Al-Si composites with high strength and good ductility is always pursued by researchers. In order to achieve the above aim, choosing suitable reinforcements is one of the feasible methods. Recently, Al3Ti particulate formed by in situ reaction, as a potential reinforcement, has attracted considerable attention in the fabrication of particulate reinforced Al alloys [9-10]. Besides its excellent properties [11], such as low density, high Young's modulus and good wettability with Al, Al3Ti is a powerful nucleating agent in the Al alloys [12], i.e., it can refine Al grains. Most importantly, according to Al-Ti phase diagram [13], for the molten Al alloys containing Al3Ti particulates, a C peritectic reaction ( L Al3Ti 665 Al ) will take place when the temperature
cools down to 665℃. Thereby, Al3Ti particulates usually exist inside the α-Al crystals other than the grain boundaries. Accordingly, the ductility degradation of composites resulting from the segregation of reinforced particulates at the grain boundaries can be eliminated efficiently. In situ casting technique has been regarded as a promising method for the commercial production of Al composites due to its low cost, high efficiency and near net forming [14]. In the conventional methods, Al3Ti particulates can be formed in the molten Al alloys by adding K2TiF6 salt. Chen et al. [15] prepared in situ Al3Ti/Al composites by adding K2TiF6 into the Al melt at 1100℃, in which in situ formed Al3Ti
3
particulates were needle-like in morphology with the length exceeding 200µm and width of 10µm. The in situ reaction between K2TiF6-Al, in nature, belongs to interface reaction [16]. At the reaction interface, the amount of [Ti] is abundant, and Al3Ti has the priority growth at <110> direction. Thereby, Al3Ti particulates with large-sized needle-like shape are easily formed. However, the ductility of Al composites could be decreased severely due to the existence of this type of Al3Ti particulates. Reducing the size of in situ formed Al3Ti particulates is the crucial factor to fabricate Al3Tip/Al composites with high strength and good ductility. In the preceding research [17], we proposed an in situ reaction between solid Ti powders and pure Al at low temperature (<800℃) to synthesize Al3Ti particulates, in which in situ formed Al3Ti was blocky in morphology with a size smaller than 10μm. It is clear that adding solid Ti powders into molten Al-Si alloys might provide an opportunity to prepare particulate reinforced Al-Si composites with high strength and good ductility. To our knowledge, the related research is rather limited. In this research, A356 alloy, as a typical Al-Si alloy, was used as the matrix material, and Ti powder were used as the additive. The in situ reaction information, including the evolution process of Ti powder in molten A356 alloy and the holding time for a complete reaction, was explored via a sampling experiment. Furthermore, both the microstructures and tensile properties of A356 composites with different Ti powder contents were investigated. 2. Experimental
4
2.1 Raw materials A356 alloy and Ti powder (99.7% purity, average size of 40μm) were used as the matrix and additive, respectively. The chemical composition of A356 alloy is presented in Table 1. Table 1. Chemical composition of A356 alloy Element
Si
Fe
Mg
Ti
Cu
Zn
Mn
Al
wt.%
6.7-7.2
≤0.12
0.3-0.4
0.1-0.2
≤0.1
≤0.05
≤0.05
Balance
2.2 Sampling experiment In order to explore the evolution process of solid Ti powders in molten A356, a sampling experiment was conducted. An A356 ingot was melted in a graphite crucible in an electrical resistance furnace. When the temperature of molten A356 was stable at 800℃, solid Ti powders wrapped in the aluminum foil were added into the melt, and the amount of which corresponded to the composition of A356-1.5 wt.% Ti. During the reaction process, four small samples were taken out from the melt after 1, 3, 5 and 10 min, and solidified in the air. For simplicity, the four samples were referred as S1, S3, S5 and S10, respectively. 2.3 Fabrication of in situ Al3Tip/A356 composites Similar to the sampling experiment, in situ Al3Tip/A356 composites with 1.5 and 3.0 wt.% Ti additions were fabricated at 800℃ with 10-min reaction time which was determined by the sampling experiment. Before pouring to the graphite mold, the
5
molten A356 containing Al3Ti particulates were stirring by a graphite bar. After solidification the two groups of samples were referred as A356/1.5Ti and A356/3.0Ti , respectively. A control sample without the addition of Ti powders was produced under the same experimental parameters. 2.4 Making tensile specimens Three groups of ingots were cut by using wire-electrode cutting machine according to the dimensions shown in Fig.1 for making the tensile specimens. All the tensile specimens were obtained from the central area of the ingots along the length direction. The surface of the specimens was polished by sand paper to eliminate cutting marks.
Fig.1. Dimensions of the tensile specimen (in mm). 2.5 T6 heat treatment Before measuring the mechanical properties, all samples were treated by using T6 heat treatment: solution treatment at 540 for 2h; quenching in hot water; artificial aging treatment at 170 for 7 h. 2.6 Materials analysis The phases formed in as-cast A356/1.5Ti and A356/3.0Ti were examined by X-ray diffraction (XRD, Bruker D8) using Cu Kα radiation at 40 kV and 40 mA and a scan rate of 0.10°/s. The microstructures of the T6-samples were analyzed by scanning electron microscopy (SEM, s4800) equipped with an energy dispersive spectroscope 6
(EDS) device for identifying the components in the samples. Optical microscopy was used to observe both the evolution processes of solid Ti powder in molten A356 alloy and α-Al crystals in A356 matrix added different Ti additions. In addition, the fracture surfaces of samples for tensile test were observed by using SEM. 2.7 Tensile properties test Tensile test was carried out by using tensile tester (MTS 858, USA) at a speed of 1 mm/min at room temperature to obtain the ultimate tensile strength (UTS), yield strength (YS) and percent elongation (El%) of samples. Three tensile tests were conducted for each group and the average values of UTS, YS and El% were obtained. 3. Results and discussion 3.1 Microstructural analysis of the samplings The evolution processes of solid Ti powders in molten A356 alloy with different holding times were shown clearly by observing samples from S1 to S10, as shown in Fig.2. It is obvious that the solid Ti powders were unable to react with molten A356 alloy completely in 5 min at 800 °C, for some Ti powders covered with reaction layers which were composed by small-sized particles. Also, the thickness of reaction layers was increased as the reaction time was prolonged, which reached around 8, 22 and 26 µm in S1, S3 and S5 samples, respectively. As the reaction time reached 10 min, no unreacted Ti powders were found in the matrix, indicating a complete reaction took place between solid Ti powders and liquid A356 alloy. Thereby, a 10-min reaction time as an experimental parameter was determined for the following fabrication of in situ
7
Al3Tip/A356 composites. It should be noted that the reaction time is associated with the melt temperature and the Ti powder size, which can be estimated by the sampling experiment.
Fig.2. Typical microstructures of the samples obtained in the sampling experiment with different reaction times and higher magnification of areas marked in images: (a) S1: 1 min, (b) S3: 3 min, (c) S5: 5 min, and (d) S10: 10 min. Furthermore, an EDS analysis was carried out in order to clarify the composition of phases existed in the matrix, the results were given in Fig. 3. Fig. 3a was the typical region which included the unreacted Ti powder covered by reaction layer. The in situ formed particulate was identified as Al3Ti phase, in which a small amount of Si element (the content was about 7.8 at.%) was contained, indicating that the Si element participated in the formation process of Al3Ti (Fig. 3b). The detained information about the distribution of the three elements, Al, Ti and Si was provided through the EDS mapping analysis, as shown in Fig. 3c, d and e. It is obvious that the enrichment of Si element existed at the interface between the unreacted Ti and the reaction layer (Fig. 3e), suggesting that the reaction at the interface involved in Al, Ti, and Si elements. Based
8
on the results of the sampling experiment and EDS analysis, it is clear that the formation of Al3Ti particulates containing Si element should follow the reaction-peeling model, which will be explained in detail later.
Fig. 3. EDS analysis of the S5 sample. 3.2 XRD analysis of the as-cast samples Fig. 4 shows the XRD patterns of the control sample (A356) and A356 composites with different Ti additions. It is clear that newly formed phase in A356 composites was detected, and its content increased as the amount of Ti addition increased from 1.5 to 3.0 wt.%. It has been proved that the newly formed phase was Al3Ti in the in situ reaction of solid Ti powder and liquid pure Al [17]. The phase synthesized in this research has peak shift toward the higher angle with respect to the characteristic diffraction peak position of Al3Ti. These peak shifts imply that the interplanar distance of Al3Ti decreases, which means that some alloying element with smaller atomic size than Al element entered into the Al3Ti phase during its formation process in molten 9
A356. It has been reported that [18] up to 15 at.% Al can be replaced by Si in Al3Ti lattice structure and form (Al,Si)3Ti phase. Thereby, the newly formed Al3Ti phase in this research should contain Si element, which was in good agreement with EDS analysis (Fig. 3).
Fig. 4. XRD patterns of the samples. The typical microstructures of T6-A356/1.5Ti and T6-A356/3.0Ti samples were shown in Fig. 5. It is clear that newly formed Al3Ti particulates in both samples were blocky in morphology and most of which located inside the α-Al crystals of A356 alloy. In addition, Al3Ti particulates formed in the in situ casting method were smaller than 10μm and their average size was around 5µm, following the Gaussian distribution well.
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Fig. 5. Typical microstructures of T6-A356/1.5Ti (a) and T6-A356/3.0Ti (b). 3.3 Formation of Al3Ti particulates substituted Si element According to the morphology of reaction layer showed in Fig. 2 and 3, an updated reaction-peeling model was proposed to explain the formation of Al3Ti particulates substituted Si element based on our previous work [17]. After the Ti particles were immersed into the A356 melt, a reactive diffusion took place at the interface between the solid Ti particle and liquid A356 alloy. It is well known that the diffusion rate of Ti atom is much higher than that of Al atom in the Ti (solid)- Al (liquid) system [17], and the initial diffusion of the Ti atoms across the Ti/A356 interface into the liquid A356 can produce a ternary system of Ti-Al-Si. Once the Ti solute gets supersaturated in the Al-rich environment containing Si element, the (Al,Si)3Ti nuclei rather than Al3Ti nucleate on the surface of Ti particle due to the following reason. The pure Al3Ti has a 11
D022 structure (body-centered tetragonal structure). According to the first-principles calculations [19], Si element has a strong preference to substitute Al in the Al3Ti to form (Al,Si)3Ti due to the energy stabilization effect. Also, it was found that the heat of formation of supercells with Si-occupied Al sites is lower than pure Al3Ti until the Si concentration exceeds 12.5 at.%. The content of Si element in the Al3Ti synthesized in this research was about 7.8 at.%. Thereby, Si-substituted Al3Ti could be easily formed in the direct reaction between the solid Ti powders and liquid A356 from the view of thermodynamics. As the reaction time increases, further diffusion of Ti atoms results in the growth of (Al,Si)3Ti. And then the small-sized blocky (Al,Si)3Ti particulates can be easily peeled off from the reaction layer resulting from the combination effects of the growth mode of (Al,Si)3Ti, stress effect and the flow of liquid A356. The related detailed discussion about the peeling process of Al3Ti particulates has been conducted in our preceding research work [14, 17]. 3.4 Utilization ratio of solid Ti powders In order to evaluate the reaction degree between solid Ti powders and molten A356 by using this in situ casting method, the utilization ratios of solid Ti powders (1.5 and 3.0 wt.%) in the fabricating process were calculated, respectively. For simplicity, the participation of Si element in the reaction was ignored. According to the reaction C Al3Ti ), the theoretical volume fractions of newly formed formula ( 3 Al Ti 800
Al3Ti particulates were 3.19 and 6.54% when the additions of solid Ti powders were 1.5 and 3.0 wt. %.
12
The experimental volume fractions of Al3Ti particulates in T6-A356/1.5Ti and T6-A356/3.0Ti samples were measured by using the metallographic analysis software conducted using OM. Five different areas in each sample were measured in order to minimize the experimental error. One of the five groups was shown in Fig.6, in which the red particles were Al3Ti, after statistic analysis, the average experimental volume fractions of Al3Ti particulates in A356/1.5Ti and A356/3.0Ti samples were 2.27 and 4.62%. Thereby, the utilization ratio of Ti powders in the fabricating process was about 70%, meaning that the reaction between solid Ti powders and molten A356 was partly prevented. This might be attributed to the following reason. In this casting route, Al2O3 films were formed on the surface of molten A356 which exposed to the air. Some Ti powders were easily wrapped in the Al2O3 films, and the contact between Ti powders and molten A356 was isolated, leading to the failure of Al3Ti formation. It is reasonable that using inert gas, such as Ar, as a protective measure can effectively improve the utilization ratio of Ti powders. The related work will be carried out in our future research.
Fig.6. In situ formed Al3Ti particulates marked in red by software: (a) T6-A356/1.5Ti, (b) T6-A356/3.0Ti. 13
3.5 Optical microstructural analysis Fig.7. shows the morphology evolution of α-Al crystals in the A356 alloy with different Ti powders additions. It was found that most α-Al crystals in T6-A356 were typical columnar dendritic in shape. The length of some large-sized α-Al crystals could reach several hundred of micrometers and the average equivalent size of α-Al crystals was about 150μm, as shown in Fig. 7a. By adding 1.5 wt.% Ti powders in the alloy, a columnar-to-equiaxed transition took place on the α-Al crystals, and the average α-Al crystal size was decreased to around 100μm (Fig.7b). As the addition of Ti powders was 3.0 wt.%, further spheroidization of α-Al crystals was observed, giving a spherical structure with an average size about 70μm, as shown in Fig. 7c. The equiaxed transition and refining of α-Al crystals mainly attributed to the heterogeneous nucleation effect of Al3Ti particulates during the solidification of the melt. As mentioned, Al3Ti particulates have a good refining effect on the α-Al crystals, i.e. they can work as nuclei of crystallization of α-Al. Thereby, plenty of small α-Al crystals can be formed during the solidification, which can prevent the growth of columnar dendrites of α-Al crystals.
Fig. 7. OM images of samples: (a) T6-A356, (b) T6-A356/1.5Ti, and (c) T6-A356/3.0Ti. 3.6 Tensile properties of T6-samples
14
Fig.8 shows the tensile stress curves of T6-A356, T6-A356/1.5Ti and T6-A356/3.0Ti samples. Based on which the tensile properties of samples, including the YS, UTS and El% were given in Fig. 9. The average values of the YS, UTS and El% of each sample were 195MPa/228MPa/1.95%, 225MPa (improved by 15.4%)/282MPa (improved by 23.7%)/3.44% (improved by 76.4%) and 252MPa (improved by 29.2%)/310MPa (improved by 36.0%)/4.73% (improved by 143.6%), respectively. It is clear that tensile strength and ductility of A356 alloy can be both enhanced effectively by the addition of Ti powders.
Fig.8. Tensile stress curves of T6-samples. In the Al3Ti particulates reinforced A356 composites, although the matrix material undertake the main load during the deformation, the Al3Ti particulates also undertake some load and hinder the distortion of matrix. When dislocations slip to the particulate-matrix interfaces, the movement of dislocations can be blocked, leading to the stress concentration on the particulates. Thereby, an improved tensile strength can be obtained. Moreover, due to the heterogeneous nucleation role of Al3Ti particulates, the 15
size of α-Al crystals in the A356 alloy can be reduced, resulting in the formation of more grain boundaries, which also contribute to the improved tensile strength because of the following reason [ 20 ]. During the plastic deformation of A356 matrix, dislocations move along particular slip lines which change their directions at grain boundaries. As the number of grains increases and grain diameter becomes smaller, dislocations within each grain can travel a smaller distance before they encounter the grain boundary, at which point their movement is terminated (dislocation pileup). It is for this reason that fine-grained materials possess a higher tensile strength. In this research as the content of Al3Ti increased, the number of both particulate-matrix interfaces and grains increased. As a result, the above two strengthening effects became more significant.
Fig.9. Tensile properties of T6-samples. The strengthening mechanisms for YS in the particulate-reinforced Al composites have been reviewed in several publications, and the improved YS of composites was usually attributed to the main following reasons [21]: the load-bearing strengthening
16
(ΔσLoad ), grain refinement strengthening (Δσg ), coefficient of thermal expansion (CTE) mismatch strengthening (∆σCTE ), and Orowan strengthening (ΔσOrowan ). In general, the final increment of YS (∆σ) of the composites is estimated by the above mechanisms, and a related predicted equation has been proposed which is summarized as below [21] Δσ = ΔσLoad + Δσg + ((∆σCTE )2 + (ΔσOrowan )2 )1⁄2
(1)
Orowan strengthening, i.e., second-phase particle strengthening, which describes the strengthening effect originated from the interaction between reinforced particulates and the motion of dislocation in the matrix. Normally, the particulate size should be less than 1μm to initiate this strengthening effect [22]. The size of all Al3Ti particulates formed in this research, however, is larger than 1μm. It is reasonable that the Orowan strengthening effect can be ignored. Accordingly, the Eq (1). predicting the theoretical increment of YS of composites can be described as below: ΔσT = ΔσLoad + Δσg + ∆σCTE
(2)
Load-bearing strengthening The load-bearing strengthening explains the direct strengthening contribution from the presence of reinforcement. The strength improvement by the load-bearing strengthening can be calculated by [23]: 1
ΔσLoad = 2 Vp σm
(3)
where Vp is the volume fraction of reinforced particulates, σm the yield strength of matrix. In this research, σm =195MPa. Thereby, the values of ΔσLoad for A356/1.5Ti and A356/3.0Ti are 2.2MPa and 4.5MPa, respectively.
17
Grain refinement strengthening According to the well-known Hall-Petch equation [24], the increase of YS resulting from the grain refining can be expressed as: Δσg = k(d−1/2 − d0 −1/2 )
(4)
where k is a constant related the material; d and d0 are the average grain size of the composites and unreinforced alloy, respectively. In the present research, k A356 =798 MPa μm1/2[ 25 ], d0 =150μm, dA356/1.5Ti =100μm, dA356/3.0Ti =70μm. Thereby, the values of Δσg for A356/1.5Ti and A356/3.0Ti are 14.6MPa and 30.2MPa, respectively. CTE strengthening The difference between the CTE values of reinforcement (Al3Ti) and matrix (A356 alloy) generates geometrically necessary dislocations and thermally residual stresses. These two effects in the interface resulting from the CTE mismatch make the plastic deformation more difficult, leading to the increase of the YS of composites. The general equation for the CTE mismatch strengthening can be presented by [26]: ∆σCTE = βGm b√ρCTE
(5)
where β is a constant, approximately equal to 1.25; Gm the shear modulus of the matrix; b the Burgers vector of the dislocations; ρCTE the dislocation density induced by the CTE mismatch. Gm and ρCTE can be estimated by the equations below [27]: E
m Gm = 2(1+ν)
(6)
ρCTE = A(αm − αp )ΔTVp / (bdp (1 − Vp ))
(7)
where Em is Young's modulus of alloy matrix; ν is Poisson's ratio of alloy matrix; A
18
is a constant of 12; αm and αp are the CTE of the alloy matrix and Al3Ti, respectively; ΔT the difference between the pouring temperature and room temperatures; dp the average particulate size; and Vp the volume fraction of reinforced particulates. In this research, the values of the above parameters are given below: Em (A356)=75.5GPa;b=0.286nm; ν=0.33; αA356 =24×10-6 K −1 [28];αAl3Ti =13×10-6 K −1 [29]; ΔT=775K; Vp =0.0227 for A356/1.5Ti sample (0.0462 for A356/3.0Ti sample); dp =5μm. Substituting these data into Eqs (4-6). the values of ∆σCTE for T6-A356/1.5Ti and T6-A356/3.0Ti are 13.1MPa and 18.9MPa, respectively. The theoretical strengths calculated from each strengthening mechanism and the measured values of T6-A356/1.5Ti and T6-A356/3.0Ti are summarized in Table 2. A satisfactory agreement between the measured YS and theoretical YS (σT ) was obtained. Furthermore, it is clear that the grain refinement strengthening and the CTE strengthening both contribute to the increment of YS of the A356 composites. Table 2. Contributions of each strengthening mechanism to the YS of A356 composites Δσm
ΔσLoad
Δσg
∆σCTE
σT
Measured YS
(MPa)
(MPa)
(MPa)
(MPa)
(MPa)
(MPa)
T6-A356/1.5Ti
195
2.2
14.6
13.1
224.9
225
T6-A356/3.0Ti
195
4.5
30.2
18.9
248.6
252
It should be noted that the Mg element in the A356 alloy can induce ageing hardening effect through the precipitation of nano-sized Mg2Si phase which can improve the hardness and strength of A356 significantly [30]. It is clear that the Mg2Si phase both exist in the original A356 alloy and A356 composites after T6 treatment, the
19
strengthening effects resulting from the existence of Mg2Si should work for all samples. Since this paper mainly discuss the strengthening effects of the addition of Al3Ti in the A356 on the mechanical properties, the role of Mg2Si phase is not considered in the present work. 3.7 Fracture surface analysis Based on the tensile test results, it is interesting that the addition of in situ formed Al3Ti particulates were able to increase the elongation of A356 alloy, meaning that the ductility of A356 was improved. The evolution of fracture surface of A356 added solid Ti powders is shown in Fig. 10. For the T6-A356 (Fig. 10a), some big holes with smooth and clean surface were found on the fracture surface which meant that some large-sized α-Al crystals were pulled out during the tensile test. These typical features inferred that the intergranular fracture occurred during the tensile test. In addition, some small-sized and shallow dimples were found in the fracture surface. Thereby, it is clear that fracture of T6-A356 attributed to the mixed effect of brittle and plastic fracturing and the brittle fracturing was dominated.
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Fig.10. SEM images of the tensile fracture surfaces of the samples: (a) T6-A356, (b) T6-A356/1.5Ti, and (c) T6-A356/3.0Ti. For the T6-A356/1.5Ti and T6-A356/3.0Ti (Fig.10b and c), it was found that plenty of tear ridges existed on the fracture surface of both samples, suggesting that quasi-cleavage fracture was the main fracture form. Furthermore, the dimples on the surface became deeper and their area became larger, meaning that the ductility of samples was improved. The increment of elongation of A356 reinforced with in situ formed Al3Ti particulates, in nature, attributed to the columnar-to-equiaxed transition and refinement of α-Al crystals[31-32]. During the tensile deformation, materials with small-sized equiaxed grains have more excellent deformation coordination compared 21
with those with large-sized columnar grains, since the collaborative slips between grains and dislocations were much easier. Conclusions A simple casting method was proposed in this research for synthesizing in situ Al3Tip/A356 composites by adding solid Ti powders into the molten A356 alloy, and the mechanical properties of the prepared T6-A356, T6-A356/1.5Ti and T6-A356/3.0Ti samples were investigated as well. The following conclusions are drawn. (1) Si element participated in the formation of Al3Ti particulates by adding Ti powders into molten A356 alloy. When the fabricating temperature was 800°C, in situ formed Al3Ti particulates were blocky in morphology with the size smaller than 10µm, and the utilization of solid Ti powders in this casting route could reach 70%. (2) In situ formed Al3Ti particulates existed inside α-Al crystals of A356, by which the equiaxed transition from long columnar dendrite structure and refining of α-Al crystals occurred. Further spheroidization of α-Al crystals took place when the addition of Ti powders was 3.0 wt. %, giving a spherical structure with a size about 70 μm. (3) Both the strength and ductility of A356 alloy could be enhanced by the addition of Ti powders, since the YS, UTS, and El% of T6-A356/3.0Ti were improved by 29.2%, 36.0% and 143.6% respectively compared with T6-A356. (4) Quantitative analysis of the strengthening mechanism indicated that both the coefficient of thermal expansion (CTE) strengthening and grain refinement strengthening contributed to the increment of the yield strength of composites and the
22
improved ductility of composites mainly attributed to the grain refinement. Acknowledgments This research was supported by the National Natural Science Foundation of China (Grant No. 51604211), the Natural Science Foundation of Shaanxi Province (Grant No. 2016JQ5052), China Postdoctoral Science Foundation (Grant No. 2015M580839), Special Financial Grant from China Postdoctoral Science Foundation (Grant No. 2017T100743),
Shaanxi
Postdoctoral
Science
Foundation
(Grant
No.
2016BSHEDZZ05), and the Natural Science Foundation of Shandong Province (Grant No. BS2015CL016). References [1] B. Dang, Y. Li, F. Liu, Q. Zuo, M. Liu, Effect of T4 heat treatment on microstructure and hardness of A356 alloy refined by Ga+In+Sn mixed alloys, Mater. Des. 57 (2014) 73-78. [2] H. Long, J. Chen, C. Liu, D. Li, The negative effect of solution treatment on the age hardness of A356 alloy, Mater. Sci. Eng. A 566 (2013) 112-118. [3] M. Zhu, Z. Jian, G. yang, Y. Zhou, Effect of T6 treatment on the microstructure, tensile properties, and fracture behavior of the modified A356 alloys, Mater. Des. 36 (2012) 243-249. [4] P.K. Rohatgi, S. Alaraj, R.B. Thakkar, A. Daoud, Variation in fatigue properties of cast A359-SiC composites under total strain controlled conditions: Effects of porosity and inclusions, Composites Part A 38 (2007) 1829-1841. [5] S. Amirkanlou, B. Niroumand, Development of Al356/SiCp composites by injection of SiCp containing composite powders, Mater. Des. 32 (2011) 1895-1902. [6] T.M. Wang, Y.F. Zhao, Z.N. Chen, Y.P. Zheng, H.J. Kang, The bimodal effect of La on the microstructures and mechanical properties of in-situ A356-TiB2 composites, Mater. Des. 85 (2015) 724-732.
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PII: DOI: Reference:
S0921-5093(17)31454-5 https://doi.org/10.1016/j.msea.2017.11.005 MSA35720
To appear in: Materials Science & Engineering A Received date: 10 June 2017 Revised date: 9 September 2017 Accepted date: 3 November 2017 Cite this article as: Zhiwei Liu, Na Cheng, Qiaoling Zheng, Jianhua Wu, Qingyou Han, Zhifu Huang, Jiandong Xing, Yefei Li and Yimin Gao, Processing and tensile properties of A356 composites containing in situ small-sized Al 3Ti p a r t i c u l a t e s , Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2017.11.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Processing and tensile properties of A356 composites containing in situ small-sized Al3Ti particulates Zhiwei Liua*, Na Chenga, Qiaoling Zhenga, Jianhua Wub, Qingyou Hanc, Zhifu Huanga, Jiandong Xinga, Yefei Lia and Yimin Gaoa a State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an 710047, China b Shandong Key Laboratory for High Strength Lightweight Metallic Materials (HLM), Advanced Materials Institute, Shandong Academy of Sciences, Jinan 250014, China c School of Engineering Technology, Purdue University, 401 North Grant Street, West Lafayette, IN 47906, USA Corresponding authors: Tel. +86-18066543603 [email protected] (Liu ZW) Abstract This research proposed an in situ casting method for preparing Al3Tip/A356 composites with high strength and good ductility by adding Ti powders into molten A356 at 800℃. A sampling experiment was firstly carried out to explore the evolution process of Ti powders in the A356 melt. Based on which in situ Al3Tip/A356 composites with 1.5 and 3.0 wt.% Ti additions were fabricated respectively. Both the microstructures and tensile properties of samples after T6 heat treatment were investigated. The results showed that blocky Si-substituted Al3Ti particulates were synthesized in the in situ reaction and the formation of which followed a reaction-peeling model. The utilization ratio of Ti
1
powders in the fabricating process could research 70%. In situ Al3Ti particulates existed inside the α-Al crystals of A356 alloy, with the size smaller than 10μm. Both the equiaxed transition from long columnar dendrite structure and refining of α-Al crystals occurred due to the nucleating effect of Al3Ti. Compared with T6-A356, the yield strength, ultimate tensile strength, and percent elongation of the T6-A356/3.0Ti were improved by 29.2%, 36.0% and 143.6%, respectively. The mechanisms of the improved tensile properties, including the strength and ductility, of the composites were also discussed in this research. Key words: in situ casting; Al3Ti; A356 composites; microstructures; tensile properties 1. Introduction Al-Si casting alloys are widely applied in the automobile industries for the production of automotive components due to their excellent castability, light weight, good wear resistance and low coefficient of thermal expansion [ 1 - 3 ]. As the development of automobile lightweight, Al-Si alloys with improved mechanical properties are required for guaranteeing the safety of vehicles in the service. Adding reinforced particulates，such as SiC [4-5], TiB2 [6-7] into the Al-Si alloys to form Al-Si matrix composites can effectively enhance the mechanical properties of Al-Si alloys. Normally, an improved tensile strength of Al-Si alloys can be obtained by the addition of ceramic reinforcements. The ductility of Al-Si alloys, however, usually degrades due to the segregation of plenty of ceramic particulates at the α-Al grain boundaries [8]. In the casting route, the ceramic particulates can be pushed by α-Al dendrites to the
2
dendrite/grain boundaries during the solidification process because of the poor wettability between ceramic phase and Al, leading to the formation of particulates segregation at the grain boundaries. Preparing particulate reinforced Al-Si composites with high strength and good ductility is always pursued by researchers. In order to achieve the above aim, choosing suitable reinforcements is one of the feasible methods. Recently, Al3Ti particulate formed by in situ reaction, as a potential reinforcement, has attracted considerable attention in the fabrication of particulate reinforced Al alloys [9-10]. Besides its excellent properties [11], such as low density, high Young's modulus and good wettability with Al, Al3Ti is a powerful nucleating agent in the Al alloys [12], i.e., it can refine Al grains. Most importantly, according to Al-Ti phase diagram [13], for the molten Al alloys containing Al3Ti particulates, a C peritectic reaction ( L Al3Ti 665 Al ) will take place when the temperature
cools down to 665℃. Thereby, Al3Ti particulates usually exist inside the α-Al crystals other than the grain boundaries. Accordingly, the ductility degradation of composites resulting from the segregation of reinforced particulates at the grain boundaries can be eliminated efficiently. In situ casting technique has been regarded as a promising method for the commercial production of Al composites due to its low cost, high efficiency and near net forming [14]. In the conventional methods, Al3Ti particulates can be formed in the molten Al alloys by adding K2TiF6 salt. Chen et al. [15] prepared in situ Al3Ti/Al composites by adding K2TiF6 into the Al melt at 1100℃, in which in situ formed Al3Ti
3
particulates were needle-like in morphology with the length exceeding 200µm and width of 10µm. The in situ reaction between K2TiF6-Al, in nature, belongs to interface reaction [16]. At the reaction interface, the amount of [Ti] is abundant, and Al3Ti has the priority growth at <110> direction. Thereby, Al3Ti particulates with large-sized needle-like shape are easily formed. However, the ductility of Al composites could be decreased severely due to the existence of this type of Al3Ti particulates. Reducing the size of in situ formed Al3Ti particulates is the crucial factor to fabricate Al3Tip/Al composites with high strength and good ductility. In the preceding research [17], we proposed an in situ reaction between solid Ti powders and pure Al at low temperature (<800℃) to synthesize Al3Ti particulates, in which in situ formed Al3Ti was blocky in morphology with a size smaller than 10μm. It is clear that adding solid Ti powders into molten Al-Si alloys might provide an opportunity to prepare particulate reinforced Al-Si composites with high strength and good ductility. To our knowledge, the related research is rather limited. In this research, A356 alloy, as a typical Al-Si alloy, was used as the matrix material, and Ti powder were used as the additive. The in situ reaction information, including the evolution process of Ti powder in molten A356 alloy and the holding time for a complete reaction, was explored via a sampling experiment. Furthermore, both the microstructures and tensile properties of A356 composites with different Ti powder contents were investigated. 2. Experimental
4
2.1 Raw materials A356 alloy and Ti powder (99.7% purity, average size of 40μm) were used as the matrix and additive, respectively. The chemical composition of A356 alloy is presented in Table 1. Table 1. Chemical composition of A356 alloy Element
Si
Fe
Mg
Ti
Cu
Zn
Mn
Al
wt.%
6.7-7.2
≤0.12
0.3-0.4
0.1-0.2
≤0.1
≤0.05
≤0.05
Balance
2.2 Sampling experiment In order to explore the evolution process of solid Ti powders in molten A356, a sampling experiment was conducted. An A356 ingot was melted in a graphite crucible in an electrical resistance furnace. When the temperature of molten A356 was stable at 800℃, solid Ti powders wrapped in the aluminum foil were added into the melt, and the amount of which corresponded to the composition of A356-1.5 wt.% Ti. During the reaction process, four small samples were taken out from the melt after 1, 3, 5 and 10 min, and solidified in the air. For simplicity, the four samples were referred as S1, S3, S5 and S10, respectively. 2.3 Fabrication of in situ Al3Tip/A356 composites Similar to the sampling experiment, in situ Al3Tip/A356 composites with 1.5 and 3.0 wt.% Ti additions were fabricated at 800℃ with 10-min reaction time which was determined by the sampling experiment. Before pouring to the graphite mold, the
5
molten A356 containing Al3Ti particulates were stirring by a graphite bar. After solidification the two groups of samples were referred as A356/1.5Ti and A356/3.0Ti , respectively. A control sample without the addition of Ti powders was produced under the same experimental parameters. 2.4 Making tensile specimens Three groups of ingots were cut by using wire-electrode cutting machine according to the dimensions shown in Fig.1 for making the tensile specimens. All the tensile specimens were obtained from the central area of the ingots along the length direction. The surface of the specimens was polished by sand paper to eliminate cutting marks.
Fig.1. Dimensions of the tensile specimen (in mm). 2.5 T6 heat treatment Before measuring the mechanical properties, all samples were treated by using T6 heat treatment: solution treatment at 540 for 2h; quenching in hot water; artificial aging treatment at 170 for 7 h. 2.6 Materials analysis The phases formed in as-cast A356/1.5Ti and A356/3.0Ti were examined by X-ray diffraction (XRD, Bruker D8) using Cu Kα radiation at 40 kV and 40 mA and a scan rate of 0.10°/s. The microstructures of the T6-samples were analyzed by scanning electron microscopy (SEM, s4800) equipped with an energy dispersive spectroscope 6
(EDS) device for identifying the components in the samples. Optical microscopy was used to observe both the evolution processes of solid Ti powder in molten A356 alloy and α-Al crystals in A356 matrix added different Ti additions. In addition, the fracture surfaces of samples for tensile test were observed by using SEM. 2.7 Tensile properties test Tensile test was carried out by using tensile tester (MTS 858, USA) at a speed of 1 mm/min at room temperature to obtain the ultimate tensile strength (UTS), yield strength (YS) and percent elongation (El%) of samples. Three tensile tests were conducted for each group and the average values of UTS, YS and El% were obtained. 3. Results and discussion 3.1 Microstructural analysis of the samplings The evolution processes of solid Ti powders in molten A356 alloy with different holding times were shown clearly by observing samples from S1 to S10, as shown in Fig.2. It is obvious that the solid Ti powders were unable to react with molten A356 alloy completely in 5 min at 800 °C, for some Ti powders covered with reaction layers which were composed by small-sized particles. Also, the thickness of reaction layers was increased as the reaction time was prolonged, which reached around 8, 22 and 26 µm in S1, S3 and S5 samples, respectively. As the reaction time reached 10 min, no unreacted Ti powders were found in the matrix, indicating a complete reaction took place between solid Ti powders and liquid A356 alloy. Thereby, a 10-min reaction time as an experimental parameter was determined for the following fabrication of in situ
7
Al3Tip/A356 composites. It should be noted that the reaction time is associated with the melt temperature and the Ti powder size, which can be estimated by the sampling experiment.
Fig.2. Typical microstructures of the samples obtained in the sampling experiment with different reaction times and higher magnification of areas marked in images: (a) S1: 1 min, (b) S3: 3 min, (c) S5: 5 min, and (d) S10: 10 min. Furthermore, an EDS analysis was carried out in order to clarify the composition of phases existed in the matrix, the results were given in Fig. 3. Fig. 3a was the typical region which included the unreacted Ti powder covered by reaction layer. The in situ formed particulate was identified as Al3Ti phase, in which a small amount of Si element (the content was about 7.8 at.%) was contained, indicating that the Si element participated in the formation process of Al3Ti (Fig. 3b). The detained information about the distribution of the three elements, Al, Ti and Si was provided through the EDS mapping analysis, as shown in Fig. 3c, d and e. It is obvious that the enrichment of Si element existed at the interface between the unreacted Ti and the reaction layer (Fig. 3e), suggesting that the reaction at the interface involved in Al, Ti, and Si elements. Based
8
on the results of the sampling experiment and EDS analysis, it is clear that the formation of Al3Ti particulates containing Si element should follow the reaction-peeling model, which will be explained in detail later.
Fig. 3. EDS analysis of the S5 sample. 3.2 XRD analysis of the as-cast samples Fig. 4 shows the XRD patterns of the control sample (A356) and A356 composites with different Ti additions. It is clear that newly formed phase in A356 composites was detected, and its content increased as the amount of Ti addition increased from 1.5 to 3.0 wt.%. It has been proved that the newly formed phase was Al3Ti in the in situ reaction of solid Ti powder and liquid pure Al [17]. The phase synthesized in this research has peak shift toward the higher angle with respect to the characteristic diffraction peak position of Al3Ti. These peak shifts imply that the interplanar distance of Al3Ti decreases, which means that some alloying element with smaller atomic size than Al element entered into the Al3Ti phase during its formation process in molten 9
A356. It has been reported that [18] up to 15 at.% Al can be replaced by Si in Al3Ti lattice structure and form (Al,Si)3Ti phase. Thereby, the newly formed Al3Ti phase in this research should contain Si element, which was in good agreement with EDS analysis (Fig. 3).
Fig. 4. XRD patterns of the samples. The typical microstructures of T6-A356/1.5Ti and T6-A356/3.0Ti samples were shown in Fig. 5. It is clear that newly formed Al3Ti particulates in both samples were blocky in morphology and most of which located inside the α-Al crystals of A356 alloy. In addition, Al3Ti particulates formed in the in situ casting method were smaller than 10μm and their average size was around 5µm, following the Gaussian distribution well.
10
Fig. 5. Typical microstructures of T6-A356/1.5Ti (a) and T6-A356/3.0Ti (b). 3.3 Formation of Al3Ti particulates substituted Si element According to the morphology of reaction layer showed in Fig. 2 and 3, an updated reaction-peeling model was proposed to explain the formation of Al3Ti particulates substituted Si element based on our previous work [17]. After the Ti particles were immersed into the A356 melt, a reactive diffusion took place at the interface between the solid Ti particle and liquid A356 alloy. It is well known that the diffusion rate of Ti atom is much higher than that of Al atom in the Ti (solid)- Al (liquid) system [17], and the initial diffusion of the Ti atoms across the Ti/A356 interface into the liquid A356 can produce a ternary system of Ti-Al-Si. Once the Ti solute gets supersaturated in the Al-rich environment containing Si element, the (Al,Si)3Ti nuclei rather than Al3Ti nucleate on the surface of Ti particle due to the following reason. The pure Al3Ti has a 11
D022 structure (body-centered tetragonal structure). According to the first-principles calculations [19], Si element has a strong preference to substitute Al in the Al3Ti to form (Al,Si)3Ti due to the energy stabilization effect. Also, it was found that the heat of formation of supercells with Si-occupied Al sites is lower than pure Al3Ti until the Si concentration exceeds 12.5 at.%. The content of Si element in the Al3Ti synthesized in this research was about 7.8 at.%. Thereby, Si-substituted Al3Ti could be easily formed in the direct reaction between the solid Ti powders and liquid A356 from the view of thermodynamics. As the reaction time increases, further diffusion of Ti atoms results in the growth of (Al,Si)3Ti. And then the small-sized blocky (Al,Si)3Ti particulates can be easily peeled off from the reaction layer resulting from the combination effects of the growth mode of (Al,Si)3Ti, stress effect and the flow of liquid A356. The related detailed discussion about the peeling process of Al3Ti particulates has been conducted in our preceding research work [14, 17]. 3.4 Utilization ratio of solid Ti powders In order to evaluate the reaction degree between solid Ti powders and molten A356 by using this in situ casting method, the utilization ratios of solid Ti powders (1.5 and 3.0 wt.%) in the fabricating process were calculated, respectively. For simplicity, the participation of Si element in the reaction was ignored. According to the reaction C Al3Ti ), the theoretical volume fractions of newly formed formula ( 3 Al Ti 800
Al3Ti particulates were 3.19 and 6.54% when the additions of solid Ti powders were 1.5 and 3.0 wt. %.
12
The experimental volume fractions of Al3Ti particulates in T6-A356/1.5Ti and T6-A356/3.0Ti samples were measured by using the metallographic analysis software conducted using OM. Five different areas in each sample were measured in order to minimize the experimental error. One of the five groups was shown in Fig.6, in which the red particles were Al3Ti, after statistic analysis, the average experimental volume fractions of Al3Ti particulates in A356/1.5Ti and A356/3.0Ti samples were 2.27 and 4.62%. Thereby, the utilization ratio of Ti powders in the fabricating process was about 70%, meaning that the reaction between solid Ti powders and molten A356 was partly prevented. This might be attributed to the following reason. In this casting route, Al2O3 films were formed on the surface of molten A356 which exposed to the air. Some Ti powders were easily wrapped in the Al2O3 films, and the contact between Ti powders and molten A356 was isolated, leading to the failure of Al3Ti formation. It is reasonable that using inert gas, such as Ar, as a protective measure can effectively improve the utilization ratio of Ti powders. The related work will be carried out in our future research.
Fig.6. In situ formed Al3Ti particulates marked in red by software: (a) T6-A356/1.5Ti, (b) T6-A356/3.0Ti. 13
3.5 Optical microstructural analysis Fig.7. shows the morphology evolution of α-Al crystals in the A356 alloy with different Ti powders additions. It was found that most α-Al crystals in T6-A356 were typical columnar dendritic in shape. The length of some large-sized α-Al crystals could reach several hundred of micrometers and the average equivalent size of α-Al crystals was about 150μm, as shown in Fig. 7a. By adding 1.5 wt.% Ti powders in the alloy, a columnar-to-equiaxed transition took place on the α-Al crystals, and the average α-Al crystal size was decreased to around 100μm (Fig.7b). As the addition of Ti powders was 3.0 wt.%, further spheroidization of α-Al crystals was observed, giving a spherical structure with an average size about 70μm, as shown in Fig. 7c. The equiaxed transition and refining of α-Al crystals mainly attributed to the heterogeneous nucleation effect of Al3Ti particulates during the solidification of the melt. As mentioned, Al3Ti particulates have a good refining effect on the α-Al crystals, i.e. they can work as nuclei of crystallization of α-Al. Thereby, plenty of small α-Al crystals can be formed during the solidification, which can prevent the growth of columnar dendrites of α-Al crystals.
Fig. 7. OM images of samples: (a) T6-A356, (b) T6-A356/1.5Ti, and (c) T6-A356/3.0Ti. 3.6 Tensile properties of T6-samples
14
Fig.8 shows the tensile stress curves of T6-A356, T6-A356/1.5Ti and T6-A356/3.0Ti samples. Based on which the tensile properties of samples, including the YS, UTS and El% were given in Fig. 9. The average values of the YS, UTS and El% of each sample were 195MPa/228MPa/1.95%, 225MPa (improved by 15.4%)/282MPa (improved by 23.7%)/3.44% (improved by 76.4%) and 252MPa (improved by 29.2%)/310MPa (improved by 36.0%)/4.73% (improved by 143.6%), respectively. It is clear that tensile strength and ductility of A356 alloy can be both enhanced effectively by the addition of Ti powders.
Fig.8. Tensile stress curves of T6-samples. In the Al3Ti particulates reinforced A356 composites, although the matrix material undertake the main load during the deformation, the Al3Ti particulates also undertake some load and hinder the distortion of matrix. When dislocations slip to the particulate-matrix interfaces, the movement of dislocations can be blocked, leading to the stress concentration on the particulates. Thereby, an improved tensile strength can be obtained. Moreover, due to the heterogeneous nucleation role of Al3Ti particulates, the 15
size of α-Al crystals in the A356 alloy can be reduced, resulting in the formation of more grain boundaries, which also contribute to the improved tensile strength because of the following reason [ 20 ]. During the plastic deformation of A356 matrix, dislocations move along particular slip lines which change their directions at grain boundaries. As the number of grains increases and grain diameter becomes smaller, dislocations within each grain can travel a smaller distance before they encounter the grain boundary, at which point their movement is terminated (dislocation pileup). It is for this reason that fine-grained materials possess a higher tensile strength. In this research as the content of Al3Ti increased, the number of both particulate-matrix interfaces and grains increased. As a result, the above two strengthening effects became more significant.
Fig.9. Tensile properties of T6-samples. The strengthening mechanisms for YS in the particulate-reinforced Al composites have been reviewed in several publications, and the improved YS of composites was usually attributed to the main following reasons [21]: the load-bearing strengthening
16
(ΔσLoad ), grain refinement strengthening (Δσg ), coefficient of thermal expansion (CTE) mismatch strengthening (∆σCTE ), and Orowan strengthening (ΔσOrowan ). In general, the final increment of YS (∆σ) of the composites is estimated by the above mechanisms, and a related predicted equation has been proposed which is summarized as below [21] Δσ = ΔσLoad + Δσg + ((∆σCTE )2 + (ΔσOrowan )2 )1⁄2
(1)
Orowan strengthening, i.e., second-phase particle strengthening, which describes the strengthening effect originated from the interaction between reinforced particulates and the motion of dislocation in the matrix. Normally, the particulate size should be less than 1μm to initiate this strengthening effect [22]. The size of all Al3Ti particulates formed in this research, however, is larger than 1μm. It is reasonable that the Orowan strengthening effect can be ignored. Accordingly, the Eq (1). predicting the theoretical increment of YS of composites can be described as below: ΔσT = ΔσLoad + Δσg + ∆σCTE
(2)
Load-bearing strengthening The load-bearing strengthening explains the direct strengthening contribution from the presence of reinforcement. The strength improvement by the load-bearing strengthening can be calculated by [23]: 1
ΔσLoad = 2 Vp σm
(3)
where Vp is the volume fraction of reinforced particulates, σm the yield strength of matrix. In this research, σm =195MPa. Thereby, the values of ΔσLoad for A356/1.5Ti and A356/3.0Ti are 2.2MPa and 4.5MPa, respectively.
17
Grain refinement strengthening According to the well-known Hall-Petch equation [24], the increase of YS resulting from the grain refining can be expressed as: Δσg = k(d−1/2 − d0 −1/2 )
(4)
where k is a constant related the material; d and d0 are the average grain size of the composites and unreinforced alloy, respectively. In the present research, k A356 =798 MPa μm1/2[ 25 ], d0 =150μm, dA356/1.5Ti =100μm, dA356/3.0Ti =70μm. Thereby, the values of Δσg for A356/1.5Ti and A356/3.0Ti are 14.6MPa and 30.2MPa, respectively. CTE strengthening The difference between the CTE values of reinforcement (Al3Ti) and matrix (A356 alloy) generates geometrically necessary dislocations and thermally residual stresses. These two effects in the interface resulting from the CTE mismatch make the plastic deformation more difficult, leading to the increase of the YS of composites. The general equation for the CTE mismatch strengthening can be presented by [26]: ∆σCTE = βGm b√ρCTE
(5)
where β is a constant, approximately equal to 1.25; Gm the shear modulus of the matrix; b the Burgers vector of the dislocations; ρCTE the dislocation density induced by the CTE mismatch. Gm and ρCTE can be estimated by the equations below [27]: E
m Gm = 2(1+ν)
(6)
ρCTE = A(αm − αp )ΔTVp / (bdp (1 − Vp ))
(7)
where Em is Young's modulus of alloy matrix; ν is Poisson's ratio of alloy matrix; A
18
is a constant of 12; αm and αp are the CTE of the alloy matrix and Al3Ti, respectively; ΔT the difference between the pouring temperature and room temperatures; dp the average particulate size; and Vp the volume fraction of reinforced particulates. In this research, the values of the above parameters are given below: Em (A356)=75.5GPa;b=0.286nm; ν=0.33; αA356 =24×10-6 K −1 [28];αAl3Ti =13×10-6 K −1 [29]; ΔT=775K; Vp =0.0227 for A356/1.5Ti sample (0.0462 for A356/3.0Ti sample); dp =5μm. Substituting these data into Eqs (4-6). the values of ∆σCTE for T6-A356/1.5Ti and T6-A356/3.0Ti are 13.1MPa and 18.9MPa, respectively. The theoretical strengths calculated from each strengthening mechanism and the measured values of T6-A356/1.5Ti and T6-A356/3.0Ti are summarized in Table 2. A satisfactory agreement between the measured YS and theoretical YS (σT ) was obtained. Furthermore, it is clear that the grain refinement strengthening and the CTE strengthening both contribute to the increment of YS of the A356 composites. Table 2. Contributions of each strengthening mechanism to the YS of A356 composites Δσm
ΔσLoad
Δσg
∆σCTE
σT
Measured YS
(MPa)
(MPa)
(MPa)
(MPa)
(MPa)
(MPa)
T6-A356/1.5Ti
195
2.2
14.6
13.1
224.9
225
T6-A356/3.0Ti
195
4.5
30.2
18.9
248.6
252
It should be noted that the Mg element in the A356 alloy can induce ageing hardening effect through the precipitation of nano-sized Mg2Si phase which can improve the hardness and strength of A356 significantly [30]. It is clear that the Mg2Si phase both exist in the original A356 alloy and A356 composites after T6 treatment, the
19
strengthening effects resulting from the existence of Mg2Si should work for all samples. Since this paper mainly discuss the strengthening effects of the addition of Al3Ti in the A356 on the mechanical properties, the role of Mg2Si phase is not considered in the present work. 3.7 Fracture surface analysis Based on the tensile test results, it is interesting that the addition of in situ formed Al3Ti particulates were able to increase the elongation of A356 alloy, meaning that the ductility of A356 was improved. The evolution of fracture surface of A356 added solid Ti powders is shown in Fig. 10. For the T6-A356 (Fig. 10a), some big holes with smooth and clean surface were found on the fracture surface which meant that some large-sized α-Al crystals were pulled out during the tensile test. These typical features inferred that the intergranular fracture occurred during the tensile test. In addition, some small-sized and shallow dimples were found in the fracture surface. Thereby, it is clear that fracture of T6-A356 attributed to the mixed effect of brittle and plastic fracturing and the brittle fracturing was dominated.
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Fig.10. SEM images of the tensile fracture surfaces of the samples: (a) T6-A356, (b) T6-A356/1.5Ti, and (c) T6-A356/3.0Ti. For the T6-A356/1.5Ti and T6-A356/3.0Ti (Fig.10b and c), it was found that plenty of tear ridges existed on the fracture surface of both samples, suggesting that quasi-cleavage fracture was the main fracture form. Furthermore, the dimples on the surface became deeper and their area became larger, meaning that the ductility of samples was improved. The increment of elongation of A356 reinforced with in situ formed Al3Ti particulates, in nature, attributed to the columnar-to-equiaxed transition and refinement of α-Al crystals[31-32]. During the tensile deformation, materials with small-sized equiaxed grains have more excellent deformation coordination compared 21
with those with large-sized columnar grains, since the collaborative slips between grains and dislocations were much easier. Conclusions A simple casting method was proposed in this research for synthesizing in situ Al3Tip/A356 composites by adding solid Ti powders into the molten A356 alloy, and the mechanical properties of the prepared T6-A356, T6-A356/1.5Ti and T6-A356/3.0Ti samples were investigated as well. The following conclusions are drawn. (1) Si element participated in the formation of Al3Ti particulates by adding Ti powders into molten A356 alloy. When the fabricating temperature was 800°C, in situ formed Al3Ti particulates were blocky in morphology with the size smaller than 10µm, and the utilization of solid Ti powders in this casting route could reach 70%. (2) In situ formed Al3Ti particulates existed inside α-Al crystals of A356, by which the equiaxed transition from long columnar dendrite structure and refining of α-Al crystals occurred. Further spheroidization of α-Al crystals took place when the addition of Ti powders was 3.0 wt. %, giving a spherical structure with a size about 70 μm. (3) Both the strength and ductility of A356 alloy could be enhanced by the addition of Ti powders, since the YS, UTS, and El% of T6-A356/3.0Ti were improved by 29.2%, 36.0% and 143.6% respectively compared with T6-A356. (4) Quantitative analysis of the strengthening mechanism indicated that both the coefficient of thermal expansion (CTE) strengthening and grain refinement strengthening contributed to the increment of the yield strength of composites and the
22
improved ductility of composites mainly attributed to the grain refinement. Acknowledgments This research was supported by the National Natural Science Foundation of China (Grant No. 51604211), the Natural Science Foundation of Shaanxi Province (Grant No. 2016JQ5052), China Postdoctoral Science Foundation (Grant No. 2015M580839), Special Financial Grant from China Postdoctoral Science Foundation (Grant No. 2017T100743),
Shaanxi
Postdoctoral
Science
Foundation
(Grant
No.
2016BSHEDZZ05), and the Natural Science Foundation of Shandong Province (Grant No. BS2015CL016). References [1] B. Dang, Y. Li, F. Liu, Q. Zuo, M. Liu, Effect of T4 heat treatment on microstructure and hardness of A356 alloy refined by Ga+In+Sn mixed alloys, Mater. Des. 57 (2014) 73-78. [2] H. Long, J. Chen, C. Liu, D. Li, The negative effect of solution treatment on the age hardness of A356 alloy, Mater. Sci. Eng. A 566 (2013) 112-118. [3] M. Zhu, Z. Jian, G. yang, Y. Zhou, Effect of T6 treatment on the microstructure, tensile properties, and fracture behavior of the modified A356 alloys, Mater. Des. 36 (2012) 243-249. [4] P.K. Rohatgi, S. Alaraj, R.B. Thakkar, A. Daoud, Variation in fatigue properties of cast A359-SiC composites under total strain controlled conditions: Effects of porosity and inclusions, Composites Part A 38 (2007) 1829-1841. [5] S. Amirkanlou, B. Niroumand, Development of Al356/SiCp composites by injection of SiCp containing composite powders, Mater. Des. 32 (2011) 1895-1902. [6] T.M. Wang, Y.F. Zhao, Z.N. Chen, Y.P. Zheng, H.J. Kang, The bimodal effect of La on the microstructures and mechanical properties of in-situ A356-TiB2 composites, Mater. Des. 85 (2015) 724-732.
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