双相钛合金高温变形协调性的CPFEM研究

1. 中国科学院金属研究所 沈阳 110016

2. 中国科学院大学 北京 100049

Crystal Plasticity Finite Element Method Investigation of the High Temperature Deformation Consistency in Dual-Phase Titanium Alloy

LI Xuexiong1,2, XU Dongsheng,1, YANG Rui1

1. Institute of Metal Research, Chinese Academy of Science, Shenyang 110016, China

2. University of Chinese Academy of Sciences, Beijing 100049, China

 基金资助: 国家重点研发计划项目.  No.2016YFB0701304中国科学院信息化专项课题项目.  No.XXH13506-3040中国科学院战略先导科技专项项目.  No.XDC01040100

Corresponding authors: XU Dongsheng, professor, Tel:(024)23971946, E-mail:dsxu@imr.ac.cn

Received: 2018-08-17   Revised: 2019-03-11   Online: 2019-06-26

 Fund supported: National Key Research and Development Program of China.  No.2016YFB0701304Informatization Program of the Chinese Academy of Sciences.  No.XXH13506-3040Strategic Priority Research Program of the Chinese Academy of Sciences.  No.XDC01040100

Abstract

Based on the rate-dependent crystal plasticity constitutive model considering all slip systems, a series of dual-phase polycrystalline models were established using 3D Voronoi tessellation to investigate the high temperature plastic deformation of Ti-6Al-4V alloy with different microstructure features. The spatial distributions and evolution of stress and strain in various grains and phases were calculated in detail, and a new method was proposed to evaluate quantitatively the deformation consistency in the alloy with two phases. Simulations show that grain boundary region responds preferentially in the early stage of deformation. The encircling structure formed between β and α grains can enhance the differences in the local strain distribution. Increasing the aspect ratio of grains and the fractions of heterogeneous phase interface can reduce the local compatibility of deformation. The stress frequency statistics of both α and β phases show a double peak form, with α phase higher in average strain, and β phase higher in stress distribution. Increasing of the volume fractions of α phase may reduce the tensile yield strength, and cause the stress consistency coefficient to decrease, while the strain consistency coefficient decreases first and then increases. As initial α-basal texture intensity increases, both tensile yield strength and stress consistency coefficient increase, while the strain consistency coefficient decreases first and then increases.

Keywords： dual-phase titanium alloy ; Voronoi ; crystal plasticity finite element method (CPFEM) ; distribution of micro stress and strain ; deformation compatibility

Xuexiong LI, Dongsheng XU, Rui YANG. Crystal Plasticity Finite Element Method Investigation of the High Temperature Deformation Consistency in Dual-Phase Titanium Alloy. Acta Metallurgica Sinica[J], 2019, 55(7): 928-938 doi:10.11900/0412.1961.2018.00380

Ti-6Al-4V (质量分数，%)是典型的双相钛合金，应用范围较广，许多钛合金是基于这种合金在不同使役性能需求下的改型[1,2]。该合金在不同加工工艺和热处理条件下均具备丰富的组织类型[3]，对应的力学性能也差异显著，但具有不同组织的钛合金为何性能如此不同、各相对合金的强度贡献如何，各种组织及其变形协调性的关系尚缺乏定量描述，因而研究不同显微组织条件下的强度和变形协调性特征具有重要意义。

1 模拟方法

1.1 率相关晶体塑性模型

$σˆij=Eijkl:Dkl-∑u=1numEijkl:Pklu-(Wiku⋅σkl-σik⋅Wklu)γ˙u$

$γ˙u=γ˙0uτuguτugun-1$

$g˙u=∑v=1numhuvγ˙v$

$huv=qhγ$

1.2 变形协调性统计方法

$CC=1-CV=∑i=1totalfi(xi-μ)2/μ$

图1

Fig.1   Grains and phase distribution in dual-phase polycrystalline titanium alloy

Table 1  Crystal plasticity constitutive parameters of Ti-6Al-4V at 750 ℃[24,25,26,27]

PhaseSlip system type$γ˙0$nqh0 / MPaτ0 / MPaτs / MPa

α-Ti

<a>0.0016.251120.08.218.0
<c+a>0.0016.251120.082.0180.0
β-Ti0.00112.51143.184.396.5

Note: $γ˙0$—strain rate, n—rate sensitivity exponent, q—ratio between self and latent hardening moduli, h0—initial slip hardening moduli, τ0—initial resolved shear stress, τs—saturation flow stress

2 模拟结果与讨论

图2

Fig.2   Polycrystalline microstructures and contour maps of dual-phase titanium alloy after high temperature tensile deformation at 750 ℃ (Fig.2c1 has the same legend of Mises stress with Figs.2a1~a3, Fig.2c2 has the same legend of true strain with Figs.2b1~b3. ○□◇▽ mark the grains in specific region in microstructure)

(a1~a3) Mises strain (20% elongation) (b1~b3) true stain (20% elongation)(c1) Mises strain (10% elongation) (c2) true stain (10% elongation)(c3) maximum Schmid factor (20% elongation)

图3

Fig.3   Contour maps of a group of three grains after high temperature tensile deformation at 750 ℃

(a) intial shape (b) Mises stress (20% elongation) (c) true stain (20% elongation)

图4

Fig.4   Statistics of true strain in selected grains after 20% high temperature tension at 750 ℃(a) average true strain (b) total true strain

图5

Fig.5   Frequency distributions of Mises stress (a) and true strain (b) in the dual-phase titanium alloy

图6

Fig.6   High temperature (750 ℃) tensile stress-strain curves for dual-phase titanium alloys with different volume fractions of α phase

图7

Fig.7   Average true strain, average Mises stress and consistency coefficient for polycrystal with different α fractions at 750 ℃ and 20% elongation

(a) average true strain (b) true strain consistency

(c) average Mises stress (d) Mises stress consistency

图8

Fig.8   High temperature (750 ℃) tensile stress-strain curves for dual-phase titanium alloy (50%α) with different volume fractions of basal-α texture

图9

Fig.9   Effects of volume fraction of basal-α texture on average true strain, average Mises stress and consistency coefficient for dual-phase titanium alloy with 50%α at 750 ℃ and 20% enlongation

(a) average true strain (b) true strain consistency

(c) average Mises stress (d) Mises stress consistency

3 结论

(1) Ti-6Al-4V双相合金高温变形时，晶界及其附近是变形的优先开始区域，且易产生高应力或应变，并由晶界向晶内方向逐渐降低；晶粒轴比越大，周围异相界面越多均使局域变形协调性降低；局域αβ晶粒之间存在包围结构可加剧应变的差异性分配。

(2) 无织构存在时，α相是高应变分配相，β相是高应力分配相。整体应力频率统计曲线呈多峰形态，α相和β相应力频率统计呈双峰形态。

(3) 高温下无织构的双相钛合金组织变形时，随α相含量提高，拉伸屈服强度和应力协调系数降低，整体应变协调系数先降低后升高，并受控于β相应变协调性，α相体积分数为75%时应变协调性最差。

(4) 高温下双相钛合金组织中，随α基面织构体积分数升高，拉伸屈服强度和加工硬化率均升高，整体应力协调系数升高，应变协调系数先降低后升高。α基面织构体积分数为10%时，合金应变差异性分配明显，并且应变协调性系数最低，合金失效风险高。

/

 〈 〉