Search

The globularization behavior of the basketweave microstructure in TC17 alloy is quantitatively analyzed during deformation and subsequent heat treatment. Collaborative behavior in the α lamellae and β phase evolution are proposed to affect globularization based on the transmission electron microscopy (TEM) and high-resolution electron backscatter diffraction (EBSD) observations as follows: (i) the interleaved α lamellae restrain the rotation of neighboring α lamellae and promote the formation of low-angle boundaries (LABs) and grooves, and then (ii) the β phase has a positive effect on facilitating the complete separation of α lamellae during deformation. During heat treatment, (iii) the formation of the LABs in the β grain interiors accelerates the appearance of necklace-like α grains, and (iv) the α lamellae promotes the break-up of β phase while the β phase has a positive effect on facilitating the complete separation of α lamellae. In other words, Collaborative behavior in α lamellae and β phase evolution promotes the globularization of TC17 alloy containing a basketweave microstructure. Except for the boundary splitting and termination migration, the mechanism of substructure rotation also participates in the globularization of TC17 alloy containing a basketweave microstructure, which is different to that of the alloys containing a colony microstructure. [ABSTRACT FROM AUTHOR]

[Display omitted] • The distribution probability of lamellar intersection is modelled to describe the relationship of various α lamellae. • The distributed relationship between lamellar and globular grains is described by the equivalent circle method. • The effect of grain orientation and size is considered compared to the classical grain distribution probability model. • The novelty nearest-surface function is established and used to reveal the uniformity of different morphology grains. A novel probability model of grain morphology distribution considering the orientation and size of non-spherical grains, including P 01 (r) , P 11 (r) , P 02 (r) , P 33 (r) , P 12 (r) , is established based on the nearest-surface function, and it is used to reveal the dispersion and uniformity degree of different morphology grains in titanium alloys. Especially, the distribution probability of lamellar α intersection is introduced instead of the distributed relationship of different α lamellae, and the distributed relationship between lamellar and globular grains is transformed into the distribution of globular grains by the equivalent circle method. Then, several corresponding distribution probabilities are selected in terms of describing different microstructure states (equiaxed, basketweave, duplex microstructures). The model is applied to the quantitative analysis of TC4 alloy with different morphology grains. The results show that the distribution of α grains with different morphology in the β matrix becomes more inhomogeneous with increasing height reduction and cooling rate. Moreover, the larger the globular α size is, the greater the aggregation degree and the smaller the uniformity of other α grains around it is. The reliability of probability model is verified by the comparison between the predicted and the measured frequency distributions for the nearest neighbor surface distance r. [ABSTRACT FROM AUTHOR]

The evolution behavior of α lamellae and evolution relationship between α lamellae and β phase in TC17 alloy during isothermal heating and subsequent compression were investigated. The results show that the α/β interface of TC17 alloy is made up of stable terrace plane and movable ledge plane. The ledge plane migrates along the length direction of α lamellae during isothermal heating, which results in rapid decrease of length and slow decrease of thickness of α lamellae. During compression, the continuous dynamic recrystallization (CDRX) are the main evolution mechanism of α and β phases. The slip systems of β phase are easily activated and the slip in β phase (i.e. (110)[ 1 ̅ 11 ]) will promote the slip in α phase (i.e. (0001)[ 11 2 ̅ 0 ]) at the initial stage of compression. Thus the CDRX of β phase is preferentially initiated by more activated slip and it will promotes the CDRX of α phase, then the HABs are formed in α lamellae. Meanwhile, the β stabilized elements Cr and Mo diffuse into the α/α boundaries in a short-time deformation. The α→β phase transformation is promoted by the enrichment of Cr and Mo elements and finally the α lamellae are separated with further compression. [Display omitted] • The dissolution mechanism of α lamellae during heating and the motion of atoms at α/β interface were investigated. • The CDRX was the main evolution mechanism of α and β phases during high temperature compression. • The easier activated slip system in β phase was beneficial to the slip in α phase at the beginning of compression. • The CDRX of β phase was preferentially initiated and it promoted the CDRX of α phase during compression. • The α→β phase transformation was promoted by the enrichment of β stabilized elements at the HABs of α lamellae. [ABSTRACT FROM AUTHOR]

Abstract: The internal state variable approach nowadays is more and more used to describe the deformation behavior in all of the metallic materials. In this paper, firstly the dislocation density rate and the grain growth rate varying with the processing parameters (deformation temperature, strain rate and strain) are established using the dislocation density rate as an internal state variable. Secondly the flow stress model in high temperature deformation process is analyzed for each phase of titanium alloys, in which the flow stress contains a thermal stress and an athermal stress. A Kock–Mecking model is adopted to describe the thermally activated stress, and an athermal stress model is established using two-parameter internal state variables. Finally, a constitutive model coupling the grain size, volume fraction and dislocation density is developed based on the microstructure and crystal plasticity models. And, the material constants in present model may be identified by a genetic algorithm (GA)-based objective optimization technique. Applying the constitutive model to the isothermal compression of Ti–6Al–4V titanium alloy in the deformation temperature ranging from 1093 to1303K and the strain rate ranging from 0.001 to 10.0s−1, the 20 material constants in those models are identified with the help of experimental flow stress and grain size of prior α phase in the isothermal compression of Ti–6Al–4V titanium alloy. The relative difference between the predicted and experimental flow stress is 6.13%, and those of the sampled and the non-sampled grain size are 6.19% and 7.94%, respectively. It can be concluded that the constitutive model with high prediction precision can be used to describe the high temperature deformation behavior of titanium alloys. [Copyright &y& Elsevier]

Abstract: Effect of the strain on the grain size, the efficiency of power dissipation and the instability parameter are investigated in the isothermal compression of Ti–6Al–4V alloy. And, the processing maps in the isothermal compression of Ti–6Al–4V alloy, Ti–5.6Al–4.8Sn–2.0Zr alloy and Ti–10V–4.5Fe–1.5Al alloy are constructed at a series of strains. Those results show that the grain size of primary α phase of Ti–6Al–4V alloy varies significantly with strain. The effect of strain on the efficiency of power dissipation is significant at strain rates of 0.001s−1 and 10.0s−1. However, the efficiency of power dissipation varies significantly with strain at a strain rate of 0.1s−1 in the α+β two-phase region, but that does slightly in the β single-phase region. And, the instability parameter of Ti–6Al–4V alloy varies significantly with strain at each strain rate and deformation temperature. Effect of the strain on the unstable regions in the processing maps of isothermally compressed Ti–6Al–4V alloy, Ti–5.6Al–4.8Sn–2.0Zr alloy and Ti–10V–4.5Fe–1.5Al alloy are obtained. Compared with the processing maps of different types titanium alloys, it can be found that the effect of strain on the processing map of Ti–10V–4.5Fe–1.5Al alloy (a kind of β type titanium alloy) is most significant, that of Ti–6Al–4V alloy (a kind of α+β type titanium alloy) is secondary and that of Ti–5.6Al–4.8Sn–2.0Zr alloy (a kind of near α type titanium alloy) is less. [Copyright &y& Elsevier]

Abstract: The modeling of constitutive relationships and microstructural variables of the Ti–6.62Al–5.14Sn–1.82Zr alloy during high temperature deformation by using a fuzzy set and artificial neural network (FNN) technique with a back-propagation learning algorithm is the basis of this research. To obtain experimental results for the modeling, the isothermal compression of the titanium alloy in different deformation scenarios was conducted and quantitative metallography was thus obtained. The predicted results of flow stress and microstructural variables, including grain size and volume fraction of the α phase, are compared with the experimental data and the difference is less than 15%. The predicted results are consistent with the experimental data. Furthermore, the comparison between the predicted results of flow stress based on the FNN approach and those by using the regression method has illustrated that the FNN approach is efficient in predicting the flow stress of the alloy. [Copyright &y& Elsevier]

To establish the relationship between mechanical properties and microstructural features, it is necessary to comprehensively describe microstructural characterizations. In present study, an efficient and innovative quantitative method of interleaved degree in lamellar microstructure was developed. Firstly, four types of microstructural variables were defined as key factors that confirmed the microstructural features. Lath length (L) and variation coefficient for distribution of lath angle (CV) were measured to obtain dimension information of laths, and number of lath intersecting points per area (N) and uniformity of lath intersecting points (U) were defined to describe the lath distribution and the lath intersection complexity. Then, the microstructural variables were normalized based on multiple attribute decision-making. Finally, the equation of interleaved degree in lamellar microstructure was established based on entropy weight method. Present quantitative method was applied to analyze the interleaved degree of TC17 alloy containing a basketweave microstructure. It was shown that interleaved degree was higher at a deformation temperature of 930 °C, a height reduction of 40% and a strain rate of 0.1 s−1 and decreased with increasing cooling time. Besides, a model that can theoretically calculate fracture toughness based on tensile properties was utilized to prove the prediction ability of interleaved degree. Unlabelled Image • Equations of microstructural variables are deduced and model of interleaved degree in lamellar microstructure is established. • Interleaved degree is affected by several processing parameters, but mainly depends on cooling time. • Variant selection of α phase causes interleaved degree change and random α variant formation causes high interleaved degree. • Interleaved degree and fracture toughness is proved to be positively related. [ABSTRACT FROM AUTHOR]

Abstract: Isothermal compression tests were conducted for Ti–6Al–4V alloy in the α+β two-phase field, and the effect of deformation parameters on the precipitation of secondary α phase in the subsequent air cooling process following isothermal compression was investigated. The experimental results show that the morphology of secondary α phase transforms gradually from lamellar colony to equiaxed morphology with the increase of strain rate and height reduction and with the decrease of deformation temperature. And the precipitation mechanisms of the secondary α phase under different isothermal compression conditions are explored by noting the effect of the crystal defect (i.e., dislocation, grain boundary and subgrain boundary) density in the β matrix. [Copyright &y& Elsevier]

Abstract: Isothermal compression of Ti–5.6Al–4.8Sn–2.0Zr alloy was carried out on a Thermecmaster-Z simulator at deformation temperatures between 960 and 1060°C, constant strain rate between 0.001 and 10.0s−1, and a maximum height reduction of 70%. The high temperature deformation behavior of Ti–5.6Al–4.8Sn–2.0Zr was characterized based on an analysis of the stress–strain behavior, kinetics and processing map. The activation energy for deformation obtained during high temperature compression of Ti–5.6Al–4.8Sn–2.0Zr alloy are 221.7kJ/mol in the β phase region and 1082.4kJ/mol in the α+β phase region. The constitutive equation proposed to describe the flow stress as a function of strain rate, strain and deformation temperature during high temperature compression of Ti–5.6Al–4.8Sn–2.0Zr alloy enables a close agreement between the predicted and experimental stress–strain curves. [Copyright &y& Elsevier]