Search

A nearly fully dense β stabilized γ-TiAl alloy was additively manufactured by electron beam melting. The as-fabricated specimen exhibited a fine structure consisting of α2/γ colonies ( 50%) in the sample at 800 °C. Annealing and subsequent stabilization treatments resulted in α2/γ colonies (

A bimodal grain structure consisting of fine grains (d ~ 1–2 µm) and coarse grains (d ~ 10–20 µm) was achieved in the biomedical Co–20Cr–15W–10Ni alloy via medium cold rolling (area reduction of 50%) followed by short-term annealing (15 min) at relatively low temperatures (950–1100 °C). The medium cold rolling induced a heterogeneous microstructure featuring the coexistence of severely deformed regions and weakly deformed domains. With short-term annealing at low temperatures, fine and coarse grains were preferentially recrystallized in severely deformed regions and weakly deformed domains, respectively, resulting a bimodal grain size distribution. During tension, dislocations were more rapidly generated in the fine grains, thus increasing the strain hardening, while they glided longer on the {111} planes in the coarse grains, contributing a high ductility. The combination of these two factors provided well-balanced strength–ductility behavior with an ultimate tensile strength of 1278 MPa, a yield strength of 787 MPa, and an elongation to fracture of 53%, making the alloy suitable for surgical implant and stent applications where the strength and ductility are both important to ensure mechanical reliability in a human body.

The use of selective laser melting (SLM) has become more common with regard to the fabrication of high-strength Ti-6Al-4V components. However, the as-SLMed Ti-6Al-4V alloy parts typically exhibit low ductility because of the formation of acicular and brittle α′ martensites. It is essential to use post-heat treatment to change their microstructures to achieve superior mechanical properties. In this study, a particular multi-step heat treatment (MSHT) was applied to the Ti-6Al-4V alloy samples fabricated via SLM. Conventional post-heat treatments were conducted for comparison. The microstructures and tensile properties of the as-SLMed and heat-treated samples were investigated. The as-SLMed Ti-6Al-4V sample is dominated by plate and acicular α′ martensites, which exhibit high strength (1280 MPa) and low ductility (9.0%). The conventional solution plus aging or stress-relieving treatment cannot result in a good combination of strength and ductility. A single step annealing process at 700 °C for 2 h changes the martensitic structure to a fine lamellar α+β structure, which provides a strong and ductile Ti-6Al-4V with an ultimate tensile strength of 1108 MPa and a total elongation of 17.6%. In contrast, the MSHT process facilitates the globularization of α and, thereby generates a nearly equiaxed structure, resulting in superior ductility (21.8% of total elongation), while maintaining a moderate ultimate tensile strength of 953 MPa.

The present work investigates the microstructural evolution of a biomedical Co-20Cr-15W-10Ni alloy during multi-pass thermomechanical processing (TMP) and clarifies its effect on the tensile properties. The results indicate that multi-pass TMP consisting of repetitive cold groove-rolling followed by annealing is attractive for fabricating the alloy rod for fine wire and mini-tube making. The multi-pass TMP provided significant grain refinement from 100 to 4 μm via five repetition and, can create bimodal grain structures consisting of fine (≈3 μm) and coarse grains (4−16 μm). The bimodal grain structure provided a superior strength-ductility combination, a yield strength of 593−738 MPa and ultimate strength of 1197−1304 MPa with a uniform elongation of 54.7−61.1%. Changing the annealing condition in the final TMP step enables to further refine the grains to 2 μm and also modify the grain size distribution and, the dispersion and fraction of nano-carbides. Thus, the mechanical properties of the Co-20Cr-15W-10Ni alloy can be further optimized, leading to a yield strength of 795−890 MPa and ultimate strength of 1328−1363 MPa with a uniform elongation of 49.4−64.0%.

A study on microstructural evolution and age hardening behavior of a new metastable beta titanium alloy Ti–2Al–9.2Mo–2Fe was undertaken by microscopic observation, Vickers hardness and tensile tests in this research. The result showed that the athermal ω phase was formed in the beta matrix after solution treatment and followed by water quench, but it seemed that the athermal ω phase did not result in a considerable hardening (300 HV in hardness and 670 MPa in YS in ST condition). However, the isothermal ω phases with 10–40 nm and nano-scaled α platelets with 30–100 nm were observed in the alloy aged at lower temperatures (300–450 °C). The nano-scaled ω and α phase led to an attractive hardening effect (400–500 HV in hardness and above 1500 MPa in YS). However, micro-scaled α phase with 0.5–3 μm obtained in the samples aged at temperatures (500–600 °C) showed a moderate hardening (350–450 HV in hardness and 1100–1500 MPa in YS). The hardening went through an under-aging, peak-aging and over-aging due to the continuation of nucleation and growth of the α phases and subsequent coarsening. The coarsen α phases (3–5 μm) and grain boundary α layers (0.1–0.5 μm in thickness) obtained at high temperatures aging (650–750 °C) showed a poor hardening or even a softening (around 300 HV in hardness and below 1000 MPa in YS). The Ti–2Al–9.2Mo–2Fe alloy had a fast aging response, and can be age hardened to high hardness above 400 HV within 30 min at a broad aging temperature region. The age hardening curves can also provide a basic criterion for selecting aging treatment for a given stress level.

The influence of solution and aging parameters on the microstructures and mechanical properties of Ti–8V–1.5Mo–2Fe–3Al alloy was studied through microstructural observation and mechanical property test. It is a newly designed near beta titanium alloy for high strength and toughness on the basis of Ti–10V–2Fe–3Al alloy. According to microstructural observation, the shape and volume fraction of primary α phase is found to be determined by the solution treatment, while aging parameters control the precipitation behavior of secondary α platelets. Moreover, the effect of aging time on tensile properties is slighter compared with that of aging temperature. Tensile test reveals the dependence of mechanical properties on the heat treatment parameters, according to which the tensile strength and elongation can be controlled in a range of 1230–1435 MPa and 9–13%. It is a little superior to the Ti-1023 alloy, the alloy solution treated at 800–820 °C for 1 h and aged at 550–600 °C for 2–8 h has well balanced combination of strength and ductility. At the same strength level, the Ti–8V–1.5Mo–2Fe–3Al alloy has higher toughness and higher ductility than the Ti-1023 alloy, the reason is unknown and should be studied further. Morphological features of the fracture surfaces are consistent with the tensile test result and provide further information on the intragranular fracture behavior when solution treated above the β-transus.

Commercially pure titanium (CP–Ti) has seen major applications in medical and chemical equipment. However, improving its strength in its coarse-grained state is expected to extend its usage in structural applications. This study demonstrated that tailoring bimodal grain structures (in terms of grain size or shape) in CP-Ti offers a new way to enhance its mechanical properties. For this, an extensive effort was made to clarify the relationship between the initial microstructures and the deformation heterogeneities of a CP-Ti (Gr. 2). Samples with initially equiaxed (EQ), bimodal (BM), and lamellar (LM) microstructures were cold-rolled and subjected to annealing at different temperatures. The results showed that variations in initial grain orientation, size, and morphology result in different deformation heterogeneities during the plastic deformation, which in turn play an important role in determining the recrystallization kinetics during annealing as well as the resulting mechanical properties. The initial BM and EQ microstructures yielded better mechanical performances in both their initial and cold-rolled states than the initial LM microstructure. Furthermore, the initial BM microstructure exhibited the best strength–ductility combination in both their partially and fully recrystallized states.

An investigation is given on the influence of heat treatment by the microstructural characteristics and tensile properties of a new high strength alloy Ti–2Al–9.2Mo–2Fe. Both of the α/β and β solution treatment (α/β-ST and β-ST), then aged at temperatures ranging from 400 °C to 600 °C, were prepared. The primary α phases having 2–5 μm are formed during the α/β-ST, and restrain the size of β grains below 10 μm. As a result of the fine β grains, the α/β-ST contributes a higher strength than the β-ST. The co-existence of α″ and athermal ω phase is found in the β-ST and water quenched samples. However, these have little influence on the alloy hardening. After aging, the alloys in the α/β-ST and β-ST condition reveal the phase transformation of β to isothermal ω, and β to α depending on the aging temperature. Although the primary α phase formed during the α/β-ST increases the stability in the β matrix, and the isothermal ω phase also appears to occur during aging at 400 °C and 450 °C for 2 h. These phenomena are less common in beta titanium alloys, when treated in the α/β-ST and aged at lower temperatures. The isothermal ω phase formed in both conditions results in high strength levels (1600 MPa of ultimate strength) with much ductility loss (2.5–4.5% of elongation) as a result of the superior hardening effect and brittleness of ω phase. However, the secondary α phase with the size of 1–3 μm leads to attractive combinations of strength and ductility (1200–1400 MPa of ultimate strength with 7.5–12.5% of elongation). The reason for that is too fine α phase below 1 μm tends to result in ultra-high strength with much ductility loss. As a whole, the alloy can be heat treated to obtain excellent balances of strength and ductility, and provided abundant stress levels with optional ductility as a usable material.

The effect of solution treatment on microstructural characteristics and tensile properties of a new high strength Ti–6Cr–5Mo–5V–4Al alloy was studied. The microstructure–mechanical property relationship is also discussed. The results show that, the Ti–6Cr–5Mo–5V–4Al alloy in solution treated condition has moderate strength levels (800–1000 MPa) with an excellent ductility (−22% of the elongation and 50–60% of the reduction of area). The alloy in the α/β-solution and aged (α/β-STA) condition shows a more attractive combination of strength and ductility than the alloy in the β-solution and aged condition (β-STA). The α/β-STA results in the occurrence of mixed α precipitates at nano-scale (−100 nm) and at micron-scale (1 μm), which contributes to the high ultimate strength levels (1400–1600 MPa). However, the fine β grains below 10 μm and the primary α phase contribute to good ductility (6–13.5% of the elongation and 22–52% of the reduction of area). The β-ST leads to equiaxed and coarsen β grains (40–100 μm), and subsequent aging contributes to the α phase with the size of 1–5 μm. The β-STA microstructure contributes to 1200–1400 MPa of the ultimate strength with 7–10% of the elongation and 22–28% of the reduction of area, which shows less attractive behavior on the properties than that of the α/β-STA.