This study aims to reveal how microstructure affect the strength and the plasticity of a metastable β titanium alloy Ti–55531 (Ti–5Al–5Mo–5V–3Cr–1Zr). Lamellar microstructure (LM) and bimodal ...microstructure (BM) of Ti–55531 alloy were characterized using transmission electron microscopy, scanning electron microscopy and image analysis software. The deformation mechanisms of LM and BM were systematically investigated by studying dislocation structures of the plastic deformation region (including UDR (uniform deformation region) and NR (necking region)) of tensile specimens. The results indicate that the strength of BM is higher than that of LM. The ductility of LM is equal to or slightly better than that of BM. The deformation of BM is mainly affected by slip and shearing of globular αp (αp, primary α phase). The deformation of LM is primarily controlled by slip, shearing and {101(−) 1}〈112(−) 0〉 twinning of coarsening αs (αs, secondary α phase) lamellae. It seems that twinning can be helpful to improve the ductility of LM to a great extent during deformation. Moreover, the fractographic morphology of LM shows a little more ductile fracture than that of BM.
In order to understand why lamellar microstructure (LM) of Ti–55531 alloy presents a slightly higher ductility level than that of bimodal microstructure (BM) under high strength condition. The dislocation structure of specimens with lamellar and bimodal microstructures after tensile deformation were investigated in detail. The results indicate that the deformation of LM is mainly controlled by slip and twinning of secondary α phase, while the deformation of BM is mainly controlled by slip and shearing of globular αp phase. Display omitted
•Elongation of LM is better than that of BM for Ti-55531 alloy under high strength.•Deformation of LM is mainly controlled by slip and twinning of secondary α phase.•Deformation of BM is mainly controlled by slip and shearing of globular αp phase.•Both LM and BM show microvoid coalescence and intergranular fracture mechanisms.
Isothermal compression of a coarse-grained Ti-55531 titanium alloy was carried out in the β phase region to investigate the hot formability and microstructure evolution of the material. It is found ...that the initial structure does not affect the flow stress after discontinuous yielding. The coarse structure lowers the extent of discontinuous yielding and increases the corresponding strain interval. The microstructural developments are greatly affected by strain rate during β deformation. The substructures are nearly equiaxed at low strain rate and gradually become discontinuous arranged in band in the vicinity of original grain boundaries with the increase of strain rate. At higher strain rate, conventional continuous dynamic recrystallization (CDRX) and a two-step CDRX occur. The two-step CDRX comprises the formation and separation of ribbon grain structure. The high fraction of deformation band (DB) hinders recrystallization at high strain rate. Deformation mechanism is also summarized according to the power dissipation map.
The quest for titanium alloys with an optimal combination of tensile strength and ductility is paramount for their application in critical industries. Additive manufacturing (AM) offers a unique ...avenue to control the alloy's microstructure, thereby modulating its mechanical properties. This study presents a comprehensive investigation into the phase modulation strategies in AM-processed titanium alloys to achieve a synergistic improvement in tensile strength and ductility. The advancements in this work have enabled the progressive evolution of titanium alloy microstructures with addition of β stable elements, transitioning from near α-Ti alloys to α+β alloys, and finally to near β-Ti alloys. This approach has led to a significant enhancement in the tensile strength of as-prepared titanium alloys, complemented by a moderate retention of ductility. The primary contributors to this mechanical property improvement are identified as solid solution strengthening, grain refining strengthening, and grain boundary strengthening, all of which are facilitated by strategic phase modulation. Moreover, the generation of refined α lamellas, equiaxed α phases, and discontinuous grain boundary α phases has been shown to be instrumental in promoting coordinated deformation within the alloy, thus enhancing the overall ductility. Varying fracture modes demonstrated the potential of microstructure tailoring via multi-eutectoid elements alloying, facilitating the in-depth understanding of crack initiation, propagation, and strain-to-failure, shedding the lights on the enhancement of strength-ductility synergy.
The role of stress-induced β → α″ martensitic transformation in contributing to the exceptional strain-hardening behavior of a Ti-11Mo (wt. %) model alloy was studied. The results reveal that the ...α″-martensite plates, undergoing relatively high transformation strains upon formation, act as effective barriers against dislocation motion. As deformation proceeds, the progressive generation of these plates dynamically reduces the dislocation mean free path (dynamic Hall-Petch effect), while simultaneously encouraging the accumulation of dislocations. These dual effects substantially enhance the flow stress with increasing strain, thereby resulting in remarkable strain-hardenability, a crucial factor for maintaining excellent ductility. This study presents innovative experimental evidence elucidating the mechanism of the transformation-induced plasticity effect in metastable β-titanium alloys, while also offering insights for designing novel alloys with superior strain-hardenability and ductility.
Display omitted
The slip transmission across an interface is essential for the mechanical properties of dual-phase alloys like Ti-6Al-4 V. However, the correlation between the dislocation-interface interaction and ...the strength and strain hardening anisotropy remains unclear due to the lack of direct experimental evidence. Via in situ scanning electron microscopy micropillar compression, prismatic plane dislocations were preferentially activated and interacted with an individual α/β interface at different angles. Based on transmission electron microscopy characterization, this study suggests that α/β interface shows a more pronounced strengthening effect when the coordinated slip system is more difficult to be activated and the slip deflection angle is larger. Differently, its higher strain hardening rate is initially determined by the larger Burger vector magnitude of interfacial residual dislocation after slip transmission. These results provide a unique basis for understanding the contribution of the interface to the mechanical properties of dual-phase alloys.
Display omitted
High performance cheaper Ti alloys can be developed based on the addition of several alloying elements, amongst which are Cu, Mn and Al. Studies are available in literature about binary and some ...ternary alloys based on the addition of these elements; however, no quaternary Ti–Cu–Mn–Al alloys were developed. Therefore, in this study quaternary Ti–Cu–Mn–Al alloys were manufactured via powder metallurgy to gain a better understanding of the effect of the alloy composition and the achievable mechanical performance. It is found that the selected quaternary Ti–Cu–Mn–Al alloys are characterised by a lamellar structure, which is progressively refined as the amount of alloying elements increases, and precipitation of the Ti2Cu intermetallic occurs if the Cu content is high enough. The amount of residual porosity increases and changes morphology as more thermal energy is invested in the diffusion of the alloying elements. In terms of mechanical behaviour, the quaternary Ti–Cu–Mn–Al alloys undergo both elastic and plastic deformation upon tensile loading. Strength and hardness linearly increase and the ductility monotonically decreases, which is the result of the compromise between the amount of residual porosity and the several strengthening mechanisms brought about by the actual total amount of alloying elements added.
This paper develops a new technology to improve the anti-friction performance of cutting tools by constructing textured surface with composite lyophilic/lyophobic wettabilities and this method is ...applied to polycrystalline diamond (PCD) tools. Lyophobic micro/nano structures and lyophilic grooves were successively fabricated on the tool surface by a pulsed fiber laser using different processing parameters. Then the influence of un-textured PCD tools, microgrooved tools and textured tools with lyophilic/lyophobic wettabilities on cutting performance and tool wear were investigated during turning of Ti6Al4V titanium alloy bar under minimum quantity lubrication (MQL) environment. The results indicated that textured PCD tools with lyophilic/lyophobic wettabilities reduced the cutting force, average friction coefficient and cutting tool wear in comparison of un-textured tools and microgrooved tools. Furthermore, the anti-friction mechanism of textured tools with lyophilic/lyophobic wettabilities was discussed from the aspects of drag reduction and the regulation of movement of cutting liquid to the impact tool-chip interface. This new technology provides a new way for textured tools to further enhance cutting performance and mitigate tool wear.
A new (α + β) Ti-alloy, Ti-6Al-2V-1Cr-1Fe (wt%), with fine grain sizes, fine precipitates, together with the high strength and excellent ductility in its as-cast state is developed. As compared to ...the as-cast commercial Ti-6Al-4V alloy, the grain size and α lath thickness of the new alloy are substantially refined by 50∼75%, and the yield strength and ductility are increased by 19.7% and 51.8%, respectively. The grain size refinement is achieved by tuning the supercooling capacity of the alloy through alloying with the aid of CALPHAD calculations. In addition, the alloying of Cr and Fe substantially reduces the α lath thickness. This low-cost Ti alloy with the enhanced properties is anticipated to be highly suitable for various structural applications in its as-cast or as-printed state.
Display omitted
For a series of unique properties, titanium alloy has been widely applied in aviation and aerospace fields. However, the poor machinability makes high-speed machining titanium alloy hardly perform as ...expected even with advanced tool materials due to the high cutting temperature. Intermittent cutting could be an effective method to decrease the cutting temperature and improve the cutting performance. As typical intermittent cutting methods, traditional ultrasonic vibration cutting (UVC) and elliptical ultrasonic vibration cutting (EUVC) have achieved significant advancements. However, the critical cutting speed confines them to the field of low speed machining. This article proposed a new type of ultrasonic vibration cutting, i.e. high-speed ultrasonic vibration cutting (HUVC), in which the vibration is always along with the feed direction. The separation of the tool and workpiece can be realized under some certain conditions although the cutting speed exceeds far away from the critical speed of the traditional UVC and EUVC methods. As a consequence, it realized high speed cutting on a macro level and intermittent cutting in the micro, and improved the machinability of titanium alloy. Firstly, a theoretical model of HUVC process was established and both of the separation criteria and the duty cycle of HUVC were fully analyzed. Then the feasibility of HUVC method for cutting Ti-6Al-4V was verified experimentally compared with conventional cutting (CC) and traditional ultrasonic vibration cutting (UVC). The results demonstrated that tool life in HUVC were extended by 300% in an optimal situation due to the significantly tool wear reduction. Besides, the cutting efficiency increased by 90% compared with CC method obviously. Furthermore, significant cutting force reduction about 50% and better surface roughness improvement in a successive cutting process were also achieved.
