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Hot deformation behavior of Cu-bearing antibacterial titanium alloy
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Hot deformation behavior of Cu-bearing antibacterial titanium alloy Zheng Ma a,b,1 , Ling Ren a,1 , M. Babar Shahzad a , Rui Liu a , Ying Zhao b,∗ , Ke Yang a,∗ a b
Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016, China Center for Human Tissues and Organs Degeneration, Shenzhen Institute of Advanced Technology, Chinese Academy of Science, Shenzhen, 518055, China
a r t i c l e
i n f o
Article history: Received 19 July 2017 Received in revised form 17 August 2017 Accepted 19 September 2017 Available online xxx Keywords: Cu-bearing titanium alloy Constitutive equation Thermal deformation Microstructure Processing map
a b s t r a c t We investigated the deformation behavior of a new biomedical Cu-bearing titanium alloy (Ti-645 (Ti6.06Al-3.75V-4.85Cu, in wt%)) to optimize its microstructure control and the hot-working process. The results showed that true stress–true strain curve of Ti-645 alloy was susceptible to both deformation temperature and strain rate. The microstructure of Ti-645 alloy was significantly changed from equiaxed grain to acicular one with the deformation temperature while a notable decrease in grain size was recorded as well. Dynamic recovery (DRV) and dynamic recrystallization (DRX) obviously existed during the thermal compression of Ti-645 alloy. The apparent activation energies in (␣ + ) phase and  single phase regions were calculated to be 495.21 kJ mol−1 and 195.69 kJ mol−1 , respectively. The processing map showed that the alloy had a large hot-working region whereas the optimum window occurred in the strain rate range of 0.001–0.1 s−1 , and temperature range of 900–960 ◦ C and 1000–1050 ◦ C. The obtained results could provide a technological basis for the design of hot working procedure of Ti-645 alloy to optimize the material design and widen the potential application of Ti-645 alloy in clinic. © 2018 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.
1. Introduction As a kind of conventional surgical implant material, titanium alloy has already been widely used in the orthopedic and dental applications [1]. However, absence of any inherent bactericidal ability of Ti-6Al–4 V alloy based implants makes it highly prone to bacterial infections regardless of strict sterile conditions of implantation procedures [2]. Implant related infections not only increase the wound-healing time but also influence the implant effectiveness and even could lead to implant failures, which exerts massive financial burden to the healthcare systems owing to postsurgical complications, re-surgery and longer stays of patients in hospitals. Some serious infections may also result in re-operation, amputation, and even death in extreme cases [3,4]. Therefore, prevention of implants related bacterial infections is major challenge for researchers worldwide. Currently, the problem of bacterial infections has attracted considerable attention and lots of efforts are being made to solve this problem from the materials science point of view. It is well known that Cu ions own inherent antibacterial function, and has
∗ Corresponding authors. E-mail addresses: [email protected] (Y. Zhao), [email protected] (K. Yang). 1 These authors contributed equally to this work.
relatively low toxicity and high biocompatibility [5]. In addition, Cu is an essential trace element for human body, which also involves in metabolic reactions [6]. Thus, Cu alloying in conventional metals/alloys could offer a novel antibacterial function to metal implants. Based on this idea, we had fabricated a medical grade Ti-645 alloy and proved its good antibacterial performance [7,8]. However, along with exhibition of novel antibacterial property and increase in strength with increasing copper content, Cu addition also resulted in some adverse effect on alloy properties. For instance, the plasticity of Cu-bearing alloy relatively reduced contrary to the strength, which could lead to an unfavorable workability as well as poor performance in applications. In this view, the information about deformation behavior of Ti-645 alloy would be of great significance, which unfortunately has not been reported yet. Hence, a comprehensive study of hot deformation behavior of Cu-bearing titanium alloy is highly demanded to better optimize the microstructure control and plastic deformation process. The constitutive model can be described by the correlation between flow stress and working parameters. Another powerful tool to evaluate the deformation mechanism and optimize the processing parameters is the processing map (PM). Based on the dynamic material model (DMM) theory, the deformation process is a power system and the sample is a power dissipater [9]. The total power is composed of two parts: (J) the power dissipated by the metallurgical process and (G) the power dissipated by
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2 Table 1 Composition of Ti-645 alloy (wt%). Alloy
Al
V
Cu
Fe
C
N
O
H
Ti
Ti-645
6.06
3.75
4.85
0.06
0.01
0.002
0.05
0.001
Balance
deformation. Dissipation power coefficient is used to predict and evaluate the microstructure evolution and the hot stability and processability. The PM is constituted by the values of at different deformation strain rates and temperatures at given strain. Different areas in the PM represent different microstructural evolutions, such as dynamic recovering (DRV), dynamic recrystallization (DRX) and superplastic deformation. Normally, the optimum processing parameters and stability region correspond to the with a maximum value. However, during hot deformation, some plastic unstable behaviors, such as adiabatic shear band, flow localization, and cracking may happen, which affect the material properties. Generally, combining with the analysis of the microstructure, the instability and stability regions can be precisely identified by PM. Specifically, in this study, we aimed to investigate the hot deformation characteristics of the Ti-645 alloy using hot compression experiments. Flow stress–strain curves, constitutive equations, processing map and microstructural evolutions were adopted to investigate the hot deformation behavior of Ti-645 alloy. In addition, the apparent activation energy (Q) for Ti-645 alloy at different deformation conditions was calculated. The deformation mechanisms of Ti-645 alloy at different phase regions were clarified through the Q value in comparison to those in ␣ and  Ti alloys. 2. Materials and methods The Ti-645 (Ti-6.06Al-3.75V-4.85Cu, in wt%) alloy was prepared through arc melting, and the detailed chemical composition is listed in Table 1. The -transus temperature of Ti-645 alloy was obtained by metallographic analysis. In detail, the Ti-645 alloy samples were heat treated at 850–1050 ◦ C for 1 h at an interval of 10 ◦ C and then water quenched. The variation of volume fractions of equiaxed ␣ and transformed  with heating temperature showed that the transus temperature of Ti-645 alloy was 930 ◦ C.
Hot compression experiments of the Ti-645 alloy with 12 mm in height and 8 mm in diameter were machined from the forged bar after annealing at 700 ◦ C for 2 h and were carried out on a Gleeble3800 thermo mechanical simulator. Compression experiments were carried out at 800–1050 ◦ C with 50 ◦ C intervals and strain rate between 10−3 and 10 s−1 in order to study the deformation behavior in different phase regions. The Ti-645 alloy samples were held for 5 min at the deformation temperature and then compressed at a compression ratio of 60%. The samples were sectioned parallel to the stress axis prior to observation using optical microscopy (Lecai, MEF4A DMM), field-emission scanning electron microscopy (FE-SEM, SUPRA 55 SAPPHIRE) and transmission electron microscopy (TEM, Tecnai G2 20). 3. Results 3.1. Flow behavior Fig. 1 shows the effects of temperature and strain rate on the flow behavior of Ti-645 alloy. It can be clearly observed that the flow stress is obviously changed with the temperature and strain rate. The stress increased with the strain rate while descended with deformation temperature. In the ␣+ region below -transus temperature (Fig. 1(a–c)), the flow stress increased rapidly and reached a maximum but then dropped to a flat curve. The variation of flow stress was more dramatic at higher strain rate. The work hardening rate in  single phase region (Fig. 1(d, e, f)) was lower than that in ␣+ region whereas the flow curve was smooth. Generally, the flow stress was more stable at high temperature and low strain rate compared to that at low temperature and high strain rate. 3.2. Constitutive analysis Sellars and McTegart [10] proposed the Arrhenius-type equation widely used for constitutive analysis. The effects of temperature, strain rate and Q under different deformation conditions can be described using the Z parameter [11]. These parameters can be described using the following equations: ´ Z = εexp(Q/RT )
Fig 1. True stress–strain curves of Ti-645 alloy at given strain rates and temperatures.
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(1)
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ε´ = A1 n1 exp(-Q/RT )
(2)
ε´ = A2 exp(ˇ)exp(-Q/RT )
(3)
ε´ = A[sinh(˛)]n exp(-Q/RT )
(4)
3
where is the flow stress, ε´ is the strain rate, Q is the apparent activation energy, R is the universal gas constant (8.314 J mol−1 K−1 ), T is the absolute temperature, A, A1 , A2 , n1 , n, ˇ and ␣ (≈ˇ/n1 ) are material constants. As we know, Eq. (2) is used for the low stress level and Eq. (3) can describe the flow stress at the high stress level. However, the hyperbolic sine law Eq. (4) can be used for both low stress and high stress levels [12]. Thus, the material constants ˛, ˇ, n, n1 and Q should be firstly identified for analyzing the flow behavior of the studied titanium alloy. Taking natural logarithm of Eqs. (2) and (3) yields: lnε´ = lnA1 +n1 ln − Q/RT
(5)
lnε´ = lnA2 +ˇ − Q/RT
(6)
The values of n1 and ˇ can be calculated from the slope of ´ ´ and lnε– shown the linear regression curves by plotting lnε–ln in Fig. 2(a) and (b), respectively. The average value of n1 was introduced as 4.22 and 3.76 MPa−1 for (␣+) and  regions, respectively and the average value of ˇ was introduced as 0.046 and 0.09 for (␣+) and  regions, then ␣ was calculated as 0.011 and 0.024 MPa−1 for (␣+) and  regions, respectively. Taking natural logarithm of Eq. (4) yields: lnε´ = lnA + nln(sinh(˛)) − Q/RT
(7)
Eq. (7) can be described as follows: ´ + lnA/n − Q/nRT ln(sinh(˛)) = lnε/n
(8)
Q = nR∂ln(sinh(˛))/∂T −1
(9)
As shown in Fig. 2(c), the material constant n can be obtained by plotting ln(sinh(˛)) –lnε´ at given temperature. The average value of n was calculated as 2.23 for (␣+) region and 3.03 for  region, respectively. The slope of linear regression lines was calculated through plotting ln(sinh(˛))–1/T at constant strain rate, as shown in Fig. 3(a) and (b). Thus, Q value was calculated from the slope. According to Eq. (9), Q values of (␣+) and  phase regions calculated by the obtained values of n and ∂ln(sinh(˛))/∂T−1 are 495.21 kJ mol−1 and 195.69 kJ mol−1 , respectively. As shown in Fig. 4, the values of A calculated from the slope of the ln(sinh(˛))–lnZ, are 8.07 × 1020 for (␣+) region and 1.98 × 107 for  region. The relationship among flow stress, strain rate and temperature can be predicted and analyzed using a constitutive equation as an important mathematical model. These constants ˛, ˇ, n, n1 , Q and A are obtained through a series of constitutive equations, and they are incorporated into the hyperbolic sine equation embracing Zenner– Hollomon parameter (Z parameter) for the Ti-645 alloy as follows: ´ ␣ + region :Z = εexp(495210/RT ) = 8.07 × 1020 (sinh(0.011))2.23 ´ region :Z = εexp(195690/RT ) = 1.98 × 107 (sinh(0.024))3.03 3.3. Processing map The power P absorbed through plastic deformation was described as follows: P = ε´ = J + G
´ (b) –ln ε, ´ and (c) ln(sinh(˛))–lnε´ Fig. 2. Functional relationship of (a) ln–ln ε, in ␣+ and  regions.
(10)
where G is the power dissipated by plastic deformation and J is the power dissipated by microstructure change of workpiece.
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Fig. 3. Functional relationship between ln(sinh(˛)) and temperature in (␣+) and  regions at the strain of 0.6.
Fig. 5. PM of Ti-645 alloy. Fig. 4. Functional relationship between flow stress and Z parameter during hot deformation of Ti-645 alloy.
The strain rate sensitivity parameter, m, is described as follows: ´ (11) m = ∂J/∂G = ε´ ∂/ ∂ε´ = ∂ln/∂lnεWhen m = 1 and ´ the linear dissipator is considered to be an ideal Jmax = ε/2, one. The efficiency of power dissipation can be described as the following equation through normalizing instantaneous J with this maximum value: = J/J max = 2m/(m + 1)
(12)
The parameter indicates how efficiently the material dissipates energy by microstructural evolution in a non-dimensional value. The flow instability criterion is used in this study to evaluate the flow instabilities [13] given as = ∂ ln(m/(m + 1))/∂ lnε´ + m ≤ 0
dissipation () and decreases with the increase of the strain rate. The lower contour numbers obtained in the instability area indicates that the largest number of the plastic power is dissipated in the form of heat in the workpiece [14]. It can be seen that there is a little instability region in the PM, implying a good working ability of Ti-645 alloy. The instability regions are mainly located in temperature range of 800–850, 920–960 and 1025–1050 ◦ C and the high strain rate range of 1–10 s−1 .
(13)
The negative indicates the occurrence of flow instabilities. The flow instability map was established by the variation of the with temperature and strain rate under a given strain. Thus, the PM was generated by superimposing the power dissipation map on the flow instability map. The PM of Ti-645 alloy obtained in the range of 800–1050 ◦ C and 0.001–10 s−1 under true strain 0.6 is shown in Fig. 5. The shaded region corresponds to the flow instability while the value of is negative. The negative denotes the instability area in the PM. The contour number represents the efficiency of power
3.4. Microstructure evolution The microstructures of Ti-645 alloy shown in Fig. 6 indicates a typical Widmannstätten structure and consists of large prior  grains about 500 m and lamellar primary ␣ colony of 1 m thickness. Fig. 7 shows optical microstructures of Ti-645 alloy deformed at 800 ◦ C at a strain rate of 0.001–10 s−1 . Fig. 7(a) shows that equiaxed spherical ␣ phase (white color) distributed in the matrix of transformed  (black color). The grain size was notably decreased after hot deformation compared to the initial Ti-645 alloy shown in Fig. 6. At the same deformation temperature, the ␣ grain size became smaller while the grain boundary got increasingly irregular with the increase of the strain rate (Fig. 7(a–e)). Besides, the shape of grain was changed from spherical to strip feature with the increase of the strain rate. Fig. 8 shows SEM images of Ti-645 alloy deformed at 800 ◦ C at the strain rates of 0.001–10 s−1 . Fig. 8(a) and (b) reveals lots
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Fig. 6. Microstructures of Ti-645 alloy for hot compression tests: (a) at a low magnification, (b) at a high magnification.
Fig. 7. Optical microstructures of Ti-645 alloy deformed at 800 ◦ C with strain rates of (a) 0.001 s−1 , (b) 0.01 s−1 , (c) 0. 1 s−1 , (d) 1 s−1 and (e) 10 s−1.
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Fig. 8. SEM microstructures of Ti-645 alloy deformed at 800 ◦ C with strain rates of (a) 0.001 s−1 , (b) 0.01 s−1 , (c) 0.1 s−1 , (d) 1 s−1 , (e) 10 s−1 , red circle representing the recrystallization zone and red arrows showing the irregular grain boundaries.
of details in both the grey and black areas, where the grey areas represents ␣ phase and the black areas  phase, and very small secondary ␣ phase was deformed in the black  phase. It can be seen that ␣ grain size of the studied alloy deformed at 800 ◦ C gradually was decreased to less than 1 m with the increase of the strain rates from 0.001 s−1 to 10 s−1 . In addition, obvious recrystallization zone (Fig. 8(d)) and dynamically recrystallized  grains surrounded by irregular boundaries can be found after the hot compression deformation of Ti-645 alloy (Fig. 8(c)). Fig. 9 shows SEM images of Ti-645 alloy deformed at strain rate of 0.01 s−1 and different temperatures from 800 ◦ C to 1050 ◦ C. At the same deformation strain rate, the microstructure of Ti-645 alloy was changed significantly from equiaxed grain to acicular one when the deformation temperature exceeded the -transus temperature. This indicates that temperature plays an important role in the microstructure evolution. It is apparent that the progress
of globularization process was depressed with the increase of the deformation temperature. 4. Discussion The hot deformation of Ti-645 alloy was investigated using hot compression experiments in this study. According to the -transus temperature of two phases of Ti-645 alloy (930 ◦ C), the microstructure of Ti-645 alloy was divided into ␣+ below 930 ◦ C and  single phase above 930 ◦ C. The dramatic oscillatory of flow curve at 800–900 ◦ C and 10 s−1 might be attributed to the interaction of hardening mode and softening mode included DRX and DRV. The flow softening behavior at high strain rate might arise from the deformation heat generated during hot deformation and the extent of flow softening was decreased to a less degree with the increase of temperature [15].
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Fig. 9. SEM microstructures of Ti-645 alloy deformed at strain rate of 0.01 s−1 and temperatures of (a) 800 ◦ C, red dotted lines representing the globularized ␣, (b) 850 ◦ C, red dotted line representing elongated globularized grain, (c) 900 ◦ C, (d) 950 ◦ C, red arrows showing the straight grain boundaries, (e) 1000 ◦ C and (f) 1050 ◦ C. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The Q values of Ti-645 alloy were obtained to be 495.21 kJ mol−1 in ␣ +  phase region and 195.69 kJ mol−1 in  phase region. The obtained Q values are much larger than those for the self-diffusion in ␣-Ti (150 kJ mol−1 ) [16] and -Ti (153 kJ mol−1 ) [17]. There exist more active slip systems in the bcc  structure compared to the hcp ␣+ structure, and therefore,  phase of Ti-645 alloy shows higher diffusion coefficient, larger hot deformation ability and lower apparent activation energy. Combining the data and microstructure evolution, it can be seen that the hot deformation mechanism of Ti-645 alloy may be related with the interaction among the kinetic generation and annihilation of dislocations in the DRX and DRV nucleation and growth of precipitates [18]. As we know, the Q value can be influenced by the chemical composition and the microstructure of materials. The obtained Q value of Ti-645 alloy is less than that of Ti-6Al–4 V alloy [15,19,20], suggesting that the addition of Cu reduces the Q and the deformation resistance of Ti-6Al–4 V alloy. The PM is a powerful and effective tool to describe the hot workability of materials and the dynamic response under hot deformation. The PM of Ti-645 alloy obtained indicates that the highest value of can be obtained in the temperature range of 900–960 ◦ C and 1000–1050 ◦ C and the strain rate range of 0.001–0.1 s−1 , which gives the optimum condition for thermo-mechanical processing of the alloy. On the contrary, unstable regions were obtained in the temperature range of 800–850 ◦ C, 920–960 ◦ C and 1025–1050 ◦ C and at the strain rate of 1–10 s−1 . It had been found that the grain size was notably decreased after hot deformation compared to the as-received Ti-645 alloy. Below the -transus temperature, ␣ grain size became smaller with the increase of the strain rate. The reason is that the grain had more adequate time to grow up at lower strain rate than that at higher strain rate. Temperature, strain rate and most the strain would significantly affect the grain size of the ␣ phase [14]. A typical step to produce ␣ phase with desired fine equiaxed grains is globularization, which improves the strength and ductility of alloy at the same time. Hence, the grain size can be effectively adjusted by hot deformation.
The PM is closely connected with the microstructure and is used to predict the hot deformation ability of the metal. Microstructural evolutions of Ti-645 alloy deformed under the conditions of 850 ◦ C/1 s−1 , 950 ◦ C/10 s−1 , 1050 ◦ C/1 s−1 , 800 ◦ C/0.01 s−1 , 950 ◦ C/0.01 s−1 and 1050 ◦ C/0.01 s−1 were investigated, which provided the evidence of stability and instability regions, respectively. Fig. 10 shows the SEM images of Ti-645 alloy deformed in both instability and stability regions in the PM. The microstructures of deformed Ti-645 alloy were investigated by TEM shown in Fig. 11. Fig. 10(a, b and c) shows the SEM images of the alloy deformed at 850 ◦ C/1 s−1 , 950 ◦ C/10 s−1 and 1050 ◦ C/1 s−1 , respectively, indicating different instability regions for the deformed Ti-645 alloy. The SEM image of the alloy deformed at 850 ◦ C/1 s−1 shows that there are obvious distorted and elongated grains after plastic deformation, and the TEM images of the alloy show that there are obvious DRX grains intertwined by plenty of dislocations, as shown in Fig. 11(a) and (b). However, the SEM images of the alloy deformed in the instability region show no obvious defects, although there are some differences in microstructure as marked in Fig. 10(b) and (c) in the deformed Ti-645 alloy. Fig. 10(d, e and f) show the microstructures deformed at 800 ◦ C/0.01 s−1 , 950 ◦ C/0.01 s−1 and 1050 ◦ C/0.01 s−1 , respectively, indicating different stable regions for the deformed Ti-645 alloy. Equiaxed, lath-shaped and staggered lath-shaped grains can be observed accompanying by the processes of DRX and DRV in the stable regions. TEM images also show that there are lath-shaped and staggered lath-shaped grains in the deformed Ti-645 alloy shown in Fig. 11(c) and (d). The deformation mechanisms of the safe domains include DRX, DRV and super-plasticity. Generally, the efficiency values associated with DRV, DRX and super-plasticity are about 0.30, 0.30–0.50, and 0.60, respectively. When the efficiency is less than 0.3, DRV is the dominating softening mechanism during the hot deformation [14]. Therefore, combining with the PM (Fig. 5) and microstructure evolution (Fig. 10), sub-structure and fine grains existed in most of grains, and grain boundaries were saw-teeth shape after deformation, indicating that DRV and DRX occurred obviously after thermal compression of the Ti-645 alloy. Therefore, DRX and DRV should be the dominant restoration mechanisms for hot working
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Fig. 10. Typical microstructures of Ti-645 alloy after hot compression under different conditions, (a) 850 ◦ C/1 s−1 , red dotted lines representing zigzag grain, (b) 950 ◦ C/10 s−1 , (c) 1050 ◦ C/1 s−1 , (d) 800 ◦ C/0.01 s−1 , red circle representing the globularized ␣, (e) 950 ◦ C/0.01 s−1 , red arrows showing the straight grain boundaries, and (f) 1050 ◦ C/0.01 s−1. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 11. TEM images of Ti-645 alloy after hot compression under different conditions: (a) 850 ◦ C/1 s−1 , (b) 950 ◦ C/10 s−1 , (c) 950 ◦ C/0.01 s−1 and (d) 1050 ◦ C/0.01 s−1.
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in the safe process. The maximum of 0.48 in this study represents the optimal processing window. Therefore, the optimum hot working conditions for Ti-645 alloy imply that the material will dissipate more energy for the microstructure alternation under these conditions, which may strongly be associated with the DRX.
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Scheme (No. SGLH20150213143207919), the Basic Research Project of Shenzhen City (No. JCYJ20120616142847342) and Shenzhen Science and Technology Research Funding (JCYJ20160608153641020).) References
5. Conclusion The as-received Ti-645 alloy has a large hot-working region. The optimum window for hot deformation would occur in the strain rate range of 0.001–0.1 s−1 , and temperature range of 900–960 ◦ C and 1000–1050 ◦ C, with a maximum value of 48%. The Q values of Ti-645 alloy in two-phase and single phase regions were obtained to be 495.21 kJ mol−1 and 195.69 kJ mol−1 , respectively. The true stress–true strain curve of the Ti-645 alloy was susceptible to the deformation temperature and strain rate. Generally, the flow stress was decreased with raising deformation temperature and decreasing strain rate. The DRV and DRX existed obviously after thermal compression of the Ti-645 alloy. Acknowledgements We changed the order of 51631009 and added Shenzhen Science and Technology Research Funding (JCYJ20160608153641020). Finally, the acknowledgements changed to (This work was financially supported by the National Natural Science Foundation of China (Nos. 51631009, 81271957, 51501218, and 81572113), the Guangdong Provincial Science and Technology Projects (No. 2014A010105033), the Shenzhen Peacock Programs (Nos. KQCX20140521115045444 and 110811003586331), the Shenzhen-Hong Kong Technology Cooperation Funding
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Hot deformation behavior of Cu-bearing antibacterial titanium alloy Zheng Ma a,b,1 , Ling Ren a,1 , M. Babar Shahzad a , Rui Liu a , Ying Zhao b,∗ , Ke Yang a,∗ a b
Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016, China Center for Human Tissues and Organs Degeneration, Shenzhen Institute of Advanced Technology, Chinese Academy of Science, Shenzhen, 518055, China
a r t i c l e
i n f o
Article history: Received 19 July 2017 Received in revised form 17 August 2017 Accepted 19 September 2017 Available online xxx Keywords: Cu-bearing titanium alloy Constitutive equation Thermal deformation Microstructure Processing map
a b s t r a c t We investigated the deformation behavior of a new biomedical Cu-bearing titanium alloy (Ti-645 (Ti6.06Al-3.75V-4.85Cu, in wt%)) to optimize its microstructure control and the hot-working process. The results showed that true stress–true strain curve of Ti-645 alloy was susceptible to both deformation temperature and strain rate. The microstructure of Ti-645 alloy was significantly changed from equiaxed grain to acicular one with the deformation temperature while a notable decrease in grain size was recorded as well. Dynamic recovery (DRV) and dynamic recrystallization (DRX) obviously existed during the thermal compression of Ti-645 alloy. The apparent activation energies in (␣ + ) phase and  single phase regions were calculated to be 495.21 kJ mol−1 and 195.69 kJ mol−1 , respectively. The processing map showed that the alloy had a large hot-working region whereas the optimum window occurred in the strain rate range of 0.001–0.1 s−1 , and temperature range of 900–960 ◦ C and 1000–1050 ◦ C. The obtained results could provide a technological basis for the design of hot working procedure of Ti-645 alloy to optimize the material design and widen the potential application of Ti-645 alloy in clinic. © 2018 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.
1. Introduction As a kind of conventional surgical implant material, titanium alloy has already been widely used in the orthopedic and dental applications [1]. However, absence of any inherent bactericidal ability of Ti-6Al–4 V alloy based implants makes it highly prone to bacterial infections regardless of strict sterile conditions of implantation procedures [2]. Implant related infections not only increase the wound-healing time but also influence the implant effectiveness and even could lead to implant failures, which exerts massive financial burden to the healthcare systems owing to postsurgical complications, re-surgery and longer stays of patients in hospitals. Some serious infections may also result in re-operation, amputation, and even death in extreme cases [3,4]. Therefore, prevention of implants related bacterial infections is major challenge for researchers worldwide. Currently, the problem of bacterial infections has attracted considerable attention and lots of efforts are being made to solve this problem from the materials science point of view. It is well known that Cu ions own inherent antibacterial function, and has
∗ Corresponding authors. E-mail addresses: [email protected] (Y. Zhao), [email protected] (K. Yang). 1 These authors contributed equally to this work.
relatively low toxicity and high biocompatibility [5]. In addition, Cu is an essential trace element for human body, which also involves in metabolic reactions [6]. Thus, Cu alloying in conventional metals/alloys could offer a novel antibacterial function to metal implants. Based on this idea, we had fabricated a medical grade Ti-645 alloy and proved its good antibacterial performance [7,8]. However, along with exhibition of novel antibacterial property and increase in strength with increasing copper content, Cu addition also resulted in some adverse effect on alloy properties. For instance, the plasticity of Cu-bearing alloy relatively reduced contrary to the strength, which could lead to an unfavorable workability as well as poor performance in applications. In this view, the information about deformation behavior of Ti-645 alloy would be of great significance, which unfortunately has not been reported yet. Hence, a comprehensive study of hot deformation behavior of Cu-bearing titanium alloy is highly demanded to better optimize the microstructure control and plastic deformation process. The constitutive model can be described by the correlation between flow stress and working parameters. Another powerful tool to evaluate the deformation mechanism and optimize the processing parameters is the processing map (PM). Based on the dynamic material model (DMM) theory, the deformation process is a power system and the sample is a power dissipater [9]. The total power is composed of two parts: (J) the power dissipated by the metallurgical process and (G) the power dissipated by
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2 Table 1 Composition of Ti-645 alloy (wt%). Alloy
Al
V
Cu
Fe
C
N
O
H
Ti
Ti-645
6.06
3.75
4.85
0.06
0.01
0.002
0.05
0.001
Balance
deformation. Dissipation power coefficient is used to predict and evaluate the microstructure evolution and the hot stability and processability. The PM is constituted by the values of at different deformation strain rates and temperatures at given strain. Different areas in the PM represent different microstructural evolutions, such as dynamic recovering (DRV), dynamic recrystallization (DRX) and superplastic deformation. Normally, the optimum processing parameters and stability region correspond to the with a maximum value. However, during hot deformation, some plastic unstable behaviors, such as adiabatic shear band, flow localization, and cracking may happen, which affect the material properties. Generally, combining with the analysis of the microstructure, the instability and stability regions can be precisely identified by PM. Specifically, in this study, we aimed to investigate the hot deformation characteristics of the Ti-645 alloy using hot compression experiments. Flow stress–strain curves, constitutive equations, processing map and microstructural evolutions were adopted to investigate the hot deformation behavior of Ti-645 alloy. In addition, the apparent activation energy (Q) for Ti-645 alloy at different deformation conditions was calculated. The deformation mechanisms of Ti-645 alloy at different phase regions were clarified through the Q value in comparison to those in ␣ and  Ti alloys. 2. Materials and methods The Ti-645 (Ti-6.06Al-3.75V-4.85Cu, in wt%) alloy was prepared through arc melting, and the detailed chemical composition is listed in Table 1. The -transus temperature of Ti-645 alloy was obtained by metallographic analysis. In detail, the Ti-645 alloy samples were heat treated at 850–1050 ◦ C for 1 h at an interval of 10 ◦ C and then water quenched. The variation of volume fractions of equiaxed ␣ and transformed  with heating temperature showed that the transus temperature of Ti-645 alloy was 930 ◦ C.
Hot compression experiments of the Ti-645 alloy with 12 mm in height and 8 mm in diameter were machined from the forged bar after annealing at 700 ◦ C for 2 h and were carried out on a Gleeble3800 thermo mechanical simulator. Compression experiments were carried out at 800–1050 ◦ C with 50 ◦ C intervals and strain rate between 10−3 and 10 s−1 in order to study the deformation behavior in different phase regions. The Ti-645 alloy samples were held for 5 min at the deformation temperature and then compressed at a compression ratio of 60%. The samples were sectioned parallel to the stress axis prior to observation using optical microscopy (Lecai, MEF4A DMM), field-emission scanning electron microscopy (FE-SEM, SUPRA 55 SAPPHIRE) and transmission electron microscopy (TEM, Tecnai G2 20). 3. Results 3.1. Flow behavior Fig. 1 shows the effects of temperature and strain rate on the flow behavior of Ti-645 alloy. It can be clearly observed that the flow stress is obviously changed with the temperature and strain rate. The stress increased with the strain rate while descended with deformation temperature. In the ␣+ region below -transus temperature (Fig. 1(a–c)), the flow stress increased rapidly and reached a maximum but then dropped to a flat curve. The variation of flow stress was more dramatic at higher strain rate. The work hardening rate in  single phase region (Fig. 1(d, e, f)) was lower than that in ␣+ region whereas the flow curve was smooth. Generally, the flow stress was more stable at high temperature and low strain rate compared to that at low temperature and high strain rate. 3.2. Constitutive analysis Sellars and McTegart [10] proposed the Arrhenius-type equation widely used for constitutive analysis. The effects of temperature, strain rate and Q under different deformation conditions can be described using the Z parameter [11]. These parameters can be described using the following equations: ´ Z = εexp(Q/RT )
Fig 1. True stress–strain curves of Ti-645 alloy at given strain rates and temperatures.
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(1)
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ε´ = A1 n1 exp(-Q/RT )
(2)
ε´ = A2 exp(ˇ)exp(-Q/RT )
(3)
ε´ = A[sinh(˛)]n exp(-Q/RT )
(4)
3
where is the flow stress, ε´ is the strain rate, Q is the apparent activation energy, R is the universal gas constant (8.314 J mol−1 K−1 ), T is the absolute temperature, A, A1 , A2 , n1 , n, ˇ and ␣ (≈ˇ/n1 ) are material constants. As we know, Eq. (2) is used for the low stress level and Eq. (3) can describe the flow stress at the high stress level. However, the hyperbolic sine law Eq. (4) can be used for both low stress and high stress levels [12]. Thus, the material constants ˛, ˇ, n, n1 and Q should be firstly identified for analyzing the flow behavior of the studied titanium alloy. Taking natural logarithm of Eqs. (2) and (3) yields: lnε´ = lnA1 +n1 ln − Q/RT
(5)
lnε´ = lnA2 +ˇ − Q/RT
(6)
The values of n1 and ˇ can be calculated from the slope of ´ ´ and lnε– shown the linear regression curves by plotting lnε–ln in Fig. 2(a) and (b), respectively. The average value of n1 was introduced as 4.22 and 3.76 MPa−1 for (␣+) and  regions, respectively and the average value of ˇ was introduced as 0.046 and 0.09 for (␣+) and  regions, then ␣ was calculated as 0.011 and 0.024 MPa−1 for (␣+) and  regions, respectively. Taking natural logarithm of Eq. (4) yields: lnε´ = lnA + nln(sinh(˛)) − Q/RT
(7)
Eq. (7) can be described as follows: ´ + lnA/n − Q/nRT ln(sinh(˛)) = lnε/n
(8)
Q = nR∂ln(sinh(˛))/∂T −1
(9)
As shown in Fig. 2(c), the material constant n can be obtained by plotting ln(sinh(˛)) –lnε´ at given temperature. The average value of n was calculated as 2.23 for (␣+) region and 3.03 for  region, respectively. The slope of linear regression lines was calculated through plotting ln(sinh(˛))–1/T at constant strain rate, as shown in Fig. 3(a) and (b). Thus, Q value was calculated from the slope. According to Eq. (9), Q values of (␣+) and  phase regions calculated by the obtained values of n and ∂ln(sinh(˛))/∂T−1 are 495.21 kJ mol−1 and 195.69 kJ mol−1 , respectively. As shown in Fig. 4, the values of A calculated from the slope of the ln(sinh(˛))–lnZ, are 8.07 × 1020 for (␣+) region and 1.98 × 107 for  region. The relationship among flow stress, strain rate and temperature can be predicted and analyzed using a constitutive equation as an important mathematical model. These constants ˛, ˇ, n, n1 , Q and A are obtained through a series of constitutive equations, and they are incorporated into the hyperbolic sine equation embracing Zenner– Hollomon parameter (Z parameter) for the Ti-645 alloy as follows: ´ ␣ + region :Z = εexp(495210/RT ) = 8.07 × 1020 (sinh(0.011))2.23 ´ region :Z = εexp(195690/RT ) = 1.98 × 107 (sinh(0.024))3.03 3.3. Processing map The power P absorbed through plastic deformation was described as follows: P = ε´ = J + G
´ (b) –ln ε, ´ and (c) ln(sinh(˛))–lnε´ Fig. 2. Functional relationship of (a) ln–ln ε, in ␣+ and  regions.
(10)
where G is the power dissipated by plastic deformation and J is the power dissipated by microstructure change of workpiece.
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Fig. 3. Functional relationship between ln(sinh(˛)) and temperature in (␣+) and  regions at the strain of 0.6.
Fig. 5. PM of Ti-645 alloy. Fig. 4. Functional relationship between flow stress and Z parameter during hot deformation of Ti-645 alloy.
The strain rate sensitivity parameter, m, is described as follows: ´ (11) m = ∂J/∂G = ε´ ∂/ ∂ε´ = ∂ln/∂lnεWhen m = 1 and ´ the linear dissipator is considered to be an ideal Jmax = ε/2, one. The efficiency of power dissipation can be described as the following equation through normalizing instantaneous J with this maximum value: = J/J max = 2m/(m + 1)
(12)
The parameter indicates how efficiently the material dissipates energy by microstructural evolution in a non-dimensional value. The flow instability criterion is used in this study to evaluate the flow instabilities [13] given as = ∂ ln(m/(m + 1))/∂ lnε´ + m ≤ 0
dissipation () and decreases with the increase of the strain rate. The lower contour numbers obtained in the instability area indicates that the largest number of the plastic power is dissipated in the form of heat in the workpiece [14]. It can be seen that there is a little instability region in the PM, implying a good working ability of Ti-645 alloy. The instability regions are mainly located in temperature range of 800–850, 920–960 and 1025–1050 ◦ C and the high strain rate range of 1–10 s−1 .
(13)
The negative indicates the occurrence of flow instabilities. The flow instability map was established by the variation of the with temperature and strain rate under a given strain. Thus, the PM was generated by superimposing the power dissipation map on the flow instability map. The PM of Ti-645 alloy obtained in the range of 800–1050 ◦ C and 0.001–10 s−1 under true strain 0.6 is shown in Fig. 5. The shaded region corresponds to the flow instability while the value of is negative. The negative denotes the instability area in the PM. The contour number represents the efficiency of power
3.4. Microstructure evolution The microstructures of Ti-645 alloy shown in Fig. 6 indicates a typical Widmannstätten structure and consists of large prior  grains about 500 m and lamellar primary ␣ colony of 1 m thickness. Fig. 7 shows optical microstructures of Ti-645 alloy deformed at 800 ◦ C at a strain rate of 0.001–10 s−1 . Fig. 7(a) shows that equiaxed spherical ␣ phase (white color) distributed in the matrix of transformed  (black color). The grain size was notably decreased after hot deformation compared to the initial Ti-645 alloy shown in Fig. 6. At the same deformation temperature, the ␣ grain size became smaller while the grain boundary got increasingly irregular with the increase of the strain rate (Fig. 7(a–e)). Besides, the shape of grain was changed from spherical to strip feature with the increase of the strain rate. Fig. 8 shows SEM images of Ti-645 alloy deformed at 800 ◦ C at the strain rates of 0.001–10 s−1 . Fig. 8(a) and (b) reveals lots
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Fig. 6. Microstructures of Ti-645 alloy for hot compression tests: (a) at a low magnification, (b) at a high magnification.
Fig. 7. Optical microstructures of Ti-645 alloy deformed at 800 ◦ C with strain rates of (a) 0.001 s−1 , (b) 0.01 s−1 , (c) 0. 1 s−1 , (d) 1 s−1 and (e) 10 s−1.
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Fig. 8. SEM microstructures of Ti-645 alloy deformed at 800 ◦ C with strain rates of (a) 0.001 s−1 , (b) 0.01 s−1 , (c) 0.1 s−1 , (d) 1 s−1 , (e) 10 s−1 , red circle representing the recrystallization zone and red arrows showing the irregular grain boundaries.
of details in both the grey and black areas, where the grey areas represents ␣ phase and the black areas  phase, and very small secondary ␣ phase was deformed in the black  phase. It can be seen that ␣ grain size of the studied alloy deformed at 800 ◦ C gradually was decreased to less than 1 m with the increase of the strain rates from 0.001 s−1 to 10 s−1 . In addition, obvious recrystallization zone (Fig. 8(d)) and dynamically recrystallized  grains surrounded by irregular boundaries can be found after the hot compression deformation of Ti-645 alloy (Fig. 8(c)). Fig. 9 shows SEM images of Ti-645 alloy deformed at strain rate of 0.01 s−1 and different temperatures from 800 ◦ C to 1050 ◦ C. At the same deformation strain rate, the microstructure of Ti-645 alloy was changed significantly from equiaxed grain to acicular one when the deformation temperature exceeded the -transus temperature. This indicates that temperature plays an important role in the microstructure evolution. It is apparent that the progress
of globularization process was depressed with the increase of the deformation temperature. 4. Discussion The hot deformation of Ti-645 alloy was investigated using hot compression experiments in this study. According to the -transus temperature of two phases of Ti-645 alloy (930 ◦ C), the microstructure of Ti-645 alloy was divided into ␣+ below 930 ◦ C and  single phase above 930 ◦ C. The dramatic oscillatory of flow curve at 800–900 ◦ C and 10 s−1 might be attributed to the interaction of hardening mode and softening mode included DRX and DRV. The flow softening behavior at high strain rate might arise from the deformation heat generated during hot deformation and the extent of flow softening was decreased to a less degree with the increase of temperature [15].
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Fig. 9. SEM microstructures of Ti-645 alloy deformed at strain rate of 0.01 s−1 and temperatures of (a) 800 ◦ C, red dotted lines representing the globularized ␣, (b) 850 ◦ C, red dotted line representing elongated globularized grain, (c) 900 ◦ C, (d) 950 ◦ C, red arrows showing the straight grain boundaries, (e) 1000 ◦ C and (f) 1050 ◦ C. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The Q values of Ti-645 alloy were obtained to be 495.21 kJ mol−1 in ␣ +  phase region and 195.69 kJ mol−1 in  phase region. The obtained Q values are much larger than those for the self-diffusion in ␣-Ti (150 kJ mol−1 ) [16] and -Ti (153 kJ mol−1 ) [17]. There exist more active slip systems in the bcc  structure compared to the hcp ␣+ structure, and therefore,  phase of Ti-645 alloy shows higher diffusion coefficient, larger hot deformation ability and lower apparent activation energy. Combining the data and microstructure evolution, it can be seen that the hot deformation mechanism of Ti-645 alloy may be related with the interaction among the kinetic generation and annihilation of dislocations in the DRX and DRV nucleation and growth of precipitates [18]. As we know, the Q value can be influenced by the chemical composition and the microstructure of materials. The obtained Q value of Ti-645 alloy is less than that of Ti-6Al–4 V alloy [15,19,20], suggesting that the addition of Cu reduces the Q and the deformation resistance of Ti-6Al–4 V alloy. The PM is a powerful and effective tool to describe the hot workability of materials and the dynamic response under hot deformation. The PM of Ti-645 alloy obtained indicates that the highest value of can be obtained in the temperature range of 900–960 ◦ C and 1000–1050 ◦ C and the strain rate range of 0.001–0.1 s−1 , which gives the optimum condition for thermo-mechanical processing of the alloy. On the contrary, unstable regions were obtained in the temperature range of 800–850 ◦ C, 920–960 ◦ C and 1025–1050 ◦ C and at the strain rate of 1–10 s−1 . It had been found that the grain size was notably decreased after hot deformation compared to the as-received Ti-645 alloy. Below the -transus temperature, ␣ grain size became smaller with the increase of the strain rate. The reason is that the grain had more adequate time to grow up at lower strain rate than that at higher strain rate. Temperature, strain rate and most the strain would significantly affect the grain size of the ␣ phase [14]. A typical step to produce ␣ phase with desired fine equiaxed grains is globularization, which improves the strength and ductility of alloy at the same time. Hence, the grain size can be effectively adjusted by hot deformation.
The PM is closely connected with the microstructure and is used to predict the hot deformation ability of the metal. Microstructural evolutions of Ti-645 alloy deformed under the conditions of 850 ◦ C/1 s−1 , 950 ◦ C/10 s−1 , 1050 ◦ C/1 s−1 , 800 ◦ C/0.01 s−1 , 950 ◦ C/0.01 s−1 and 1050 ◦ C/0.01 s−1 were investigated, which provided the evidence of stability and instability regions, respectively. Fig. 10 shows the SEM images of Ti-645 alloy deformed in both instability and stability regions in the PM. The microstructures of deformed Ti-645 alloy were investigated by TEM shown in Fig. 11. Fig. 10(a, b and c) shows the SEM images of the alloy deformed at 850 ◦ C/1 s−1 , 950 ◦ C/10 s−1 and 1050 ◦ C/1 s−1 , respectively, indicating different instability regions for the deformed Ti-645 alloy. The SEM image of the alloy deformed at 850 ◦ C/1 s−1 shows that there are obvious distorted and elongated grains after plastic deformation, and the TEM images of the alloy show that there are obvious DRX grains intertwined by plenty of dislocations, as shown in Fig. 11(a) and (b). However, the SEM images of the alloy deformed in the instability region show no obvious defects, although there are some differences in microstructure as marked in Fig. 10(b) and (c) in the deformed Ti-645 alloy. Fig. 10(d, e and f) show the microstructures deformed at 800 ◦ C/0.01 s−1 , 950 ◦ C/0.01 s−1 and 1050 ◦ C/0.01 s−1 , respectively, indicating different stable regions for the deformed Ti-645 alloy. Equiaxed, lath-shaped and staggered lath-shaped grains can be observed accompanying by the processes of DRX and DRV in the stable regions. TEM images also show that there are lath-shaped and staggered lath-shaped grains in the deformed Ti-645 alloy shown in Fig. 11(c) and (d). The deformation mechanisms of the safe domains include DRX, DRV and super-plasticity. Generally, the efficiency values associated with DRV, DRX and super-plasticity are about 0.30, 0.30–0.50, and 0.60, respectively. When the efficiency is less than 0.3, DRV is the dominating softening mechanism during the hot deformation [14]. Therefore, combining with the PM (Fig. 5) and microstructure evolution (Fig. 10), sub-structure and fine grains existed in most of grains, and grain boundaries were saw-teeth shape after deformation, indicating that DRV and DRX occurred obviously after thermal compression of the Ti-645 alloy. Therefore, DRX and DRV should be the dominant restoration mechanisms for hot working
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Fig. 10. Typical microstructures of Ti-645 alloy after hot compression under different conditions, (a) 850 ◦ C/1 s−1 , red dotted lines representing zigzag grain, (b) 950 ◦ C/10 s−1 , (c) 1050 ◦ C/1 s−1 , (d) 800 ◦ C/0.01 s−1 , red circle representing the globularized ␣, (e) 950 ◦ C/0.01 s−1 , red arrows showing the straight grain boundaries, and (f) 1050 ◦ C/0.01 s−1. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 11. TEM images of Ti-645 alloy after hot compression under different conditions: (a) 850 ◦ C/1 s−1 , (b) 950 ◦ C/10 s−1 , (c) 950 ◦ C/0.01 s−1 and (d) 1050 ◦ C/0.01 s−1.
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in the safe process. The maximum of 0.48 in this study represents the optimal processing window. Therefore, the optimum hot working conditions for Ti-645 alloy imply that the material will dissipate more energy for the microstructure alternation under these conditions, which may strongly be associated with the DRX.
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Scheme (No. SGLH20150213143207919), the Basic Research Project of Shenzhen City (No. JCYJ20120616142847342) and Shenzhen Science and Technology Research Funding (JCYJ20160608153641020).) References
5. Conclusion The as-received Ti-645 alloy has a large hot-working region. The optimum window for hot deformation would occur in the strain rate range of 0.001–0.1 s−1 , and temperature range of 900–960 ◦ C and 1000–1050 ◦ C, with a maximum value of 48%. The Q values of Ti-645 alloy in two-phase and single phase regions were obtained to be 495.21 kJ mol−1 and 195.69 kJ mol−1 , respectively. The true stress–true strain curve of the Ti-645 alloy was susceptible to the deformation temperature and strain rate. Generally, the flow stress was decreased with raising deformation temperature and decreasing strain rate. The DRV and DRX existed obviously after thermal compression of the Ti-645 alloy. Acknowledgements We changed the order of 51631009 and added Shenzhen Science and Technology Research Funding (JCYJ20160608153641020). Finally, the acknowledgements changed to (This work was financially supported by the National Natural Science Foundation of China (Nos. 51631009, 81271957, 51501218, and 81572113), the Guangdong Provincial Science and Technology Projects (No. 2014A010105033), the Shenzhen Peacock Programs (Nos. KQCX20140521115045444 and 110811003586331), the Shenzhen-Hong Kong Technology Cooperation Funding
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