Near-β Alloy Research Articles

Investigation of Deposition Parameters for Near-Beta Alloy Ti-55511 Fabricated by Directed Energy Deposition

The directed energy deposition (DED) parameters were determined for near-β alloy Ti-55511 by employing statistical design of experiments (DOEs) methods. Parameters resulting in fully dense freeform deposits were identified using two sequential DOEs. Single laser tracks were printed with several laser power, traverse rate, and powder feed rate settings in an initial DOE to identify promising build parameters. The capture efficiency and effective deposition rate were used to characterize and rank the single track deposits. The best parameters were then used to print a solid cube with various X–Y and Z overlaps (different hatch spacing, HS, and layer thickness, ZS) in a second DOE. Suitable deposition parameters were selected based on the cube density and microstructure and were used to fabricate larger tensile samples for mechanical testing. Multiple parameter sets were found to provide dense Ti-55511 deposits with acceptable mechanical properties and the parametric models showed statistical significance.

Additively manufactured Ti55511 alloy: Microstructure and residual stress effect on mechanical properties

Stress relief (SR) is an effective heat treatment process that potentially changes microstructure and enhances the mechanical properties of additively manufactured parts. In this study, the influence of SR on the microstructure and mechanical properties of Ti-5Al-5Mo-5V-1Cr-1Fe alloy (Ti-55511), a near-β alloy fabricated by laser powder bed fusion (LPBF), was investigated. LPBF-built Ti-55511 samples were characterized and tested under tensile and low cycle fatigue loading in the as-built (AB) and SR conditions. The obtained results show that the LPBF sample was composed of columnar grains in a single bcc β-phase in the AB condition. In contrast, the SR condition resulted in a bi-modal microstructure comprising of α + β phases. This led to significant grain refinement of the β phase and reduced the residual stress (RS) up to 91 % compared to the AB condition. At the same time, the SR treatment increased the yield strength, ultimate tensile strength, and fatigue life at the expense of ductility. The improvement in mechanical properties is attributed to the grain refinement and phase transformation resulting in complex dislocation interactions during deformation.

Characterization of the Interface Between \u03b1 and \u03b2 Titanium Alloys in the Diffusion Couple

The aim of the research was to investigate the microstructural changes caused by diffusion through interface between α and β titanium solid solutions. For this purpose, a diffusion couple composed of two single-phase titanium alloys—α type commercially pure (CP) titanium Grade 2 and near-β Ti-15V-3Al-3Cr-3Sn—was made by annealing at a temperature of 850 °C in an inert atmosphere. The performed heat treatment caused partial diffusion bonding (DB) where the α/β-interface was clearly visible. Based on the results of microscopic (light microscope (LM), scanning electron microscope/electron backscatter diffraction (SEM/EBSD), and transmission electron microscope (TEM)) examination, a significant microstructure evolution of near-β alloy in the region near the interface (diffusion-affected zone) was revealed. It was found that needlelike phases were formed both in α and β solid solutions. Moreover, in the near-β titanium alloy, pores aligned in the Frenkel plane were found. The latter finding indicated that the diffusion of alloying elements of near-β alloy is the most probable reason for the observed microstructural changes. Additionally, the “grooving” phenomenon at the α/β interface was found and it was correlated with faster diffusion through grain boundaries, rather than volume diffusion. Finally, the pore size was measured and numerically modeled. The calculated values were in good agreement with the experimental ones.

Cooling dynamics of two titanium alloys during laser powder bed fusion probed with in situ X-ray imaging and diffraction

Metal parts produced by laser powder bed fusion (LPBF) additive manufacturing exhibit characteristic microstructures comparable to those observed in laser welding. The primary cause of this characteristic microstructure is rapid, localized heating and cooling cycles, which result in extreme thermal gradients where material solidification is followed by fast cooling in the solid state. The final microstructure and mechanical performance are also influenced by pore formation caused by melt pool fluid dynamics. Here, we use high speed, in situ X-ray diffraction to probe the kinetics of cooling and solid-solid phase transitions after laser melting in two aerospace titanium alloys: Ti-6Al-4V, an α + β alloy; and Ti-5Al-5V-5Mo-3Cr, a near-β alloy. We complement these diffraction studies with in situ X-ray imaging to probe melt pool dynamics and pore formation. From these two complementary probes, we quantify pore formation during melting and the subsequent microstructural evolution as the material rapidly cools after solidification. These results are critical for understanding defect formation and residual stress development in different titanium alloys under LPBF conditions and can help inform process models to predict final part performance.

Microstructure and mechanical properties of as-deposited and heat treated Ti–5Al–5Mo–5V–3Cr–1Zr (Ti-55531) alloy fabricated by laser melting deposition

In this work, the first successful attempt of additive manufacturing (AM) Ti-55531 near-β alloy fabricated by laser melting deposition (LMD) is reported. Optimized processing parameters of LMD were used to manufacture high relative density (99.8%) Ti-55531 alloy. It was found that the microstructure of as-deposited sample is mainly composed of β phase with heterogeneous distribution of α phase. The heterogeneity of microstructure of as-deposited sample was caused by the ubiquitous temperature gradient during LMD process. Thus, the as-deposited samples have a significant anisotropic tensile property which may not meet the aeronautical criterion. To overcome the issue of inhomogeneous microstructure and minimize the anisotropy, the microstructure was tailored by two kinds of heat treatment, i.e., β solution treatment subsequent aging (β-STA) and triplex annealing (α/β-STA). A transformed β microstructure was obtained in β-SAT condition, which resulted in the high ultimate tensile strength (UTS) (>1350 MPa) and low ductility (<6.5%). Besides, the bimodal structure was formed in α/β-STA condition and the associated sample shows an excellent balance of strength (UTS>1200 MPa) and ductility (>8%). Furthermore, the formation mechanism of microstructure and the subsequent impact on mechanical properties was systematically discussed. This work demonstrates the potential of fabricating Ti-55531 alloy parts by LMD and paves the way for enhancing LMD processed near β Ti-alloys through heat treatment.

Nucleation driving force for ω-assisted formation of α and associated ω morphology in β-Ti alloys

The structural and chemical changes at ω/β interfaces and the evolution of the morphology of ω in a near-β alloy during isothermal ageing at 573 K were investigated by atom probe tomography and aberration-corrected high-resolution transmission electron microscopy. Ledges and local O enrichment at semi-coherent isothermal ω interfaces are proposed to provide the key driving force for nucleation of ω-assisted α. Following nucleation of α, the morphology of ω evolves from ellipsoidal to rod-like, induced by rapid consumption of ω by α.

Materials Selection of Optimized Titanium Alloys for Aircraft Applications

The aim of the present work was to explore the correlation between the physical metallurgy of titanium alloys and its main attributes to select optimized materials for structural aircraft applications in the landing gear beam. The Ashby's method was employed as the materials selection strategy to consolidate and evaluate the data collected from the current literature in a comprehensive and consistent analysis. Landing gear beam materials are mainly β and near-β alloys. Considering the need for high specific strength and fatigue resistance, the best candidate among them was Ti-3.5Al-5Mo-6V-3Cr-2Sn-0.5Fe alloy. Finally, a brief discussion of additional aspects related to alloy design, microstructural features and their influence on the mechanical properties is presented.

New insights into the phase transformations to isothermal ω and ω-assisted α in near β-Ti alloys

For multicomponent near-β alloys, we have investigated the mechanisms responsible for the β-to-ω and ω-to-α phase transformations upon isothermal ageing at 573 K. Experimental evidence from atom probe tomography and aberration-corrected high-resolution transmission electron microscopy indicates that the formation of isothermal ω involves a structural reconstruction assisted by nanoscale spinodal decomposition of the β matrix, prior to the specific chemistry change required to form ω, rather than a mixed-mode process with structure and chemistry changes occurring simultaneously as has been previously suggested. First, incommensurate embryonic ω evolve via a displacive mechanism within Mo-lean regions created by second-order coherent spinodal decomposition of the β matrix. The subtle spinodal decomposition in β and chemistry of embryonic ω are carefully analysed by an advanced atom probe data analysis algorithm. When the size of embryonic ɷ exceeds a critical value, commensurate isothermal ɷ forms through the exit of the other alloying solutes. O-rich regions present at the isothermal ω/β interface provide potent sites for the formation of α. The concurrent compositional partitioning of solutes in ɷ and α indicates the transformation from ω to α involves both a rapid lattice reconstruction at the ω/α interface and a slow Al diffusion at the α/β, therefore a mixed-mode displacive-diffusive process. This study provides novel experimental evidence to understand the much-disputed transformation processes and elucidate the mechanisms responsible for these important phase transformations.

Transition between mild and severe wear in titanium alloys

The mild to severe wear transition of two titanium alloys, Ti–10V–2Fe–3Al (a near-β titanium alloy) and Ti–6Al–4V (an α+β titanium alloy) was investigated and the correlation between the microstructure and wear micromechanisms was studied. Mild wear was dominated by oxidation via the formation and spallation of mechanically mixed layers (MMLs). The near-β alloy exhibited lower wear compared with α+β alloy in the mild wear region due to more stable MMLs as evidenced by 16% higher thickness of layer. Severe wear was characterized by strain localization and subsurface crack growth. Ti–6Al–4V showed lower transition load and lower resistance to unstable plastic deformation. However, Ti–10V–2Fe–3Al exhibited higher critical shear strain for subsurface crack initiation.

The mechanism of ω-assisted α phase formation in near β-Ti alloys

Partitioning of alloying elements during the ω-to-α phase transformation in a near-β alloy after isothermal ageing at 573 K was measured using atom probe tomography and high-resolution transmission electron microscopy. O-rich regions associated with ω precipitates were observed for the first time, and likely serve as nucleation sites for the α phase. The partitioning behaviours of Al and O, unlike other elements, are different for α and ω, suggesting a mixed-mode mechanism for the ω-to-α phase transformation.

Wear of WC-Co inserts in dry high speed machining of micron-sized particle aeronautical grade nearβtitanium alloy

Current demands of higher damage-tolerance in the Aerospace industry resulted in resurging interest in β, or near β, titanium alloys. The combination of attractive properties of this class of alloys also led to the consideration of beta alloys for castings. Such alloys, however, are more difficult to cut than α-β titanium alloys due to their limited ability to work harden and the effect of β-stabilizers on ductility. These factors affect failure modes and active wear mechanisms of cutting inserts. This paper investigates some of the wear modes exhibited by WC-Co inserts when end milling the α near-β alloy Ti-x. In addition to being of near-β composition, this alloy is characterized by a fine sized microstructure (range of 1−5 μm). The study focuses on SEM and EDS observations of the wear patterns exhibited by two groups of inserts. The first is uncoated WC-Co, whereas the second is coated with multi layers of TiAlN. The results of this post-mortem study are compared to wear patterns, observed under identical conditions, while milling the α-β alloy Ti6Al4V. Results show that inserts used in machining the Ti-x alloy exhibit wear modes that contrast those exhibited when machining Ti64. The paper discusses factors leading to such occurrences and studies the influence of the alloy microstructure on tool effectiveness and failure modes.

A Comparative Analysis of New Ti-Nb-Zr-Ta Orthopedic Alloys

Ti-Nb-Zr-Ta alloys represent a new generation of biomaterials with possible applications in the orthopedic field, being developed in order to eliminate the negative aspects of the current orthopedic biomaterials, which consist mainly in a low biocompatibility with human tissues and high values of modulus of elasticity compared to the human bone. This paper presents a comparative study of new titanium alloys, corresponding to the Ti-Nb-Zr-Ta system: Ti-21Nb-6Zr-15Ta and Ti-25Nb-10Zr-8Ta, which were analyzed by scanning electron microscopy, X-ray diffraction and microindentation. The both alloys are classified as near-β alloys. The addition of alloying elements such as Ta, Nb and Zr represents a good solution for lowering modulus of elasticity, which is an important factor for reducing bone resorption and therefore for preventing implant failure.

Microstructure and Electrochemical Behavior of in Vitro Ti-26Nb, Ti-26Zr and Ti-26Ta Alloys Processed by Levitation Melting Technique

This paper presents a study on microstructure, elasticity modulus and electrochemical behavior of Ti-26Nb, Ti-26Zr and Ti-26Ta binary alloys obtained in an induction furnace equipped with a water cooled copper crucible which can promote levitation of the moltem alloys. The microstructures and eletrochemical behavior were investigated by means of X-ray fluorescence spectroscopy, X-ray diffractometry, optical microscopy, microvickers hardness, open circuit poten- tial, corrosion potential and polarization resistance. The results revealed that the difference in nominal chemical composi- tion related to contamination was extremely low, which indicates that this melting technique is highly appropriate for ob- taining contaminant-free titanium alloy. The addition of niobium was very favorable for obtaining near-β alloy with a low elasticity modulus. Additionally, alloy Ti-26Nb presented the best corrosion resistance combined with low elasticity modulus.

Microstructure Development and Alloying Elements Diffusion during Sintering of Near-β Titanium Alloys

Two near-β alloys, Ti-5Al-5Mo-5V-2Cr-1Fe and Ti-10V-3Fe-3Al, were produced by the blended element powder metallurgy technique. The use of (i) elemental powders with the Al-V master alloy in the case of Ti-5Al-5Mo-5V-2Cr-1Fe and, (ii) the complex Al-Fe-V master alloy in Ti-10V-3Fe-3Al has highlighted the influence of different alloying elements and their combination on microstructure evolution and chemical homogenisation. While Fe has the fastest diffusivity in Ti and its addition improves the density of both sintered alloys it also results in accelerated rates of grain coarsening. The combination of Al and V in the master alloy powder inhibits the diffusion of V into the Ti matrix. The unexpectedly slow diffusion of Cr at the early sintering stage in Ti-5Al-5Mo-5V-2Cr-1Fe was attributed to the formation of surface oxides on the Cr powders.

A Self-Consistent Approach for Modeling the Flow Behavior of the Alpha and Beta Phases in Ti-6Al-4V

The flow behavior of the α and β phases in Ti-6Al-4V was interpreted in the context of a self-consistent modeling formalism. For this purpose, high-temperature compression tests were conducted at various temperatures for a single-phase α alloy (Ti-7Al-1.5V), a variety of near-β alloys, and the two-phase alloy Ti-6Al-4V, each with an equiaxed microstructure. The flow behavior of the α phase in Ti-6Al-4V was deduced from the experimental results of the single-phase α alloy. The flow behavior of the β phase, which was predicted by using the self-consistent approach and the measured flow behaviors of Ti-6Al-4V and Ti-7Al-1.5V, showed good agreement with direct measurements of the various near-β alloys. From these results, it was shown that the strength of the α phase is approximately three times higher than that of the β phase at temperatures between 1088 K and 1223 K (815 °C and 950 °C). It was also concluded that the relative strain rates in the two phases varies significantly with temperature. The usefulness of the approach was confirmed by comparing the predicted and measured flow stresses for other Ti-6Al-4V and near-α alloys.

Production of Ti–13Nb–13Zr alloy for surgical implants by powder metallurgy

The Ti–13Nb–13Zr near-β alloy was developed aiming the replacement of the traditional Ti–6Al–4V alloy in surgical implants owing to its larger biocompatibility. Samples of this alloy were obtained using the blended elemental technique from hydrided powders. The isochronal sintering of the compacts for 2 h was carried out in the range 900–1,400 °C with a heating rate of 20 °C min−1. In this work, the behavior of the elementary powders during sintering and the corresponding microstructural evolution were investigated. The alloy was characterized by means of scanning electron microscopy (SEM) in the backscattered mode, X-ray diffraction, and density measurements. The results indicate that the homogenization of the alloy is diffusion-controlled. With increasing temperature, homogenization of the alloy takes place and a fine plate-like α + β structure is found throughout the microstructure in temperatures above 1,300 °C. The process variables were defined aiming to minimize interstitial pick-up (C, O, and N) and avoiding intensive grain growth.

Thermomechanical processing of Ti–5Al–5Mo–5V–3Cr

The constitutive behaviour and microstructural evolution of the near-β alloy Ti–5Al–5Mo–5V–3Cr in the α + β condition has been characterised during isothermal subtransus forging at a range of temperatures and strain rates. The results indicate that Ti–5Al–5Mo–5V–3Cr has a shallower approach curve, and therefore, offers a more controllable microstructure than the near-β alloy Ti–10V–2Fe–3Al. Flow softening is small in magnitude in both alloys in the α + β condition. The steady state flow stresses obey a Norton–Hoff constitutive law with an activation energy of Q = 183 kJ mol −1, which is similar to the activation energy for self-diffusion in the β phase, suggesting deformation is dominated by dynamic recovery in the β matrix. Good evidence is found for the existence of ω phase after both air cooling and water quenching from above the β transus. In addition, dissolution of the α phase is found to be slow at near-transus temperatures.

Predictions of Titanium Alloy Properties Using Thermodynamic Modeling Tools

Thermodynamic modeling tools have become essential in understanding the effect of alloy chemistry on the final microstructure of a material. Implementation of such tools to improve titanium processing via parameter optimization has resulted in significant cost savings through the elimination of shop/laboratory trials and tests. In this study, a thermodynamic modeling tool developed at CompuTherm, LLC, is being used to predict β transus, phase proportions, phase chemistries, partitioning coefficients, and phase boundaries of multicomponent titanium alloys. This modeling tool includesPandat, software for multicomponent phase equilibrium calculations, andPanTitanium, a thermodynamic database for titanium alloys. Model predictions are compared with experimental results for one α-β alloy (Ti-64) and two near-β alloys (Ti-17 and Ti-10-2-3). The alloying elements, especially the interstitial elements O, N, H, and C, have been shown to have a significant effect on the β transus temperature, and are discussed in more detail herein.