Influence of ultrasonic vibrations on the phase transformation and strain hardening of a Zr18Nb alloy in tension
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INFLUENCE OF ULTRASONIC VIBRATIONS ON THE PHASE TRANSFORMATION AND STRAIN HARDENING OF A Zr18Nb ALLOY IN TENSION B. М. Mordyuk,1, 2 O. P. Karasevs’ka,1 P. E. Rudoi,1 I. O. Skyba,1 and H. H. Kamins’kyi1 It is established that, in tension of specimens of a Zr18Nb alloy in an ultrasonic field, a decrease in the conventional yield strength σ 0.2 caused by the application of ultrasonic deformations ends with increase in their amplitude as a result of hardening by the disperse ω -phase. It is shown that the strain hardening exponent of the quenched alloy ( n = 0.67) is higher than that for materials with a stable bcc lattice ( n = 0.5) and increases both after preliminary drawing (to n = 0.85) and after the action of ultrasonic deformations in tension (to n = 0.93). These effects are caused by the deformation-induced β → ω transformation and the size and number of ω -precipitates identified by X-ray phase analysis, the results of which correlate with the data of measurements of the electric resistivity. We discuss possible mechanisms of influence of ultrasonic vibrations on the β → ω transformation and size of the ω -phase. The formation of the large amount of the ultradisperse ω -phase located in slip planes of the bcc lattice (the β -matrix) leads to blocking of dislocation, formation of dislocation pile-ups, and increase in the strain hardening exponent and deformation stresses in tension of specimens. Keywords: zirconium alloy, strain hardening, dislocations, deformation-induced phase transformation, ω -phase, ultrasound, electric resistance.
The behavior of bcc metals in tension has been fairly thoroughly studied both at a constant strain rate [1–3] and after application of ultrasonic vibrations (USV) [4–6]. Micromechanisms of plastic deformation have also been clarified. However, the deformation behavior of a metastable bcc phase (e.g., in titanium alloys [8, 9] and zirconium alloys [10]) can substantially depend on the β → α ′′ or β → ω transformations, which proceed under deformation and differ in the sign of the volume effect [11]. In tension of most biocompatible titanium-based alloys with attaining some critical stress, the deformationinduced β → α ′′ phase transformation occurs, which is manifested in a bend on the stress–strain curve in the direction of lower tensile stresses, as a result of which the modulus of pseudoelasticity decreases significantly [12, 13]. This behavior is partially explained by the positive volume effect of the β → α ′′ transformation α ′′ ( ΔΩβ→ = Ωβn /Ωαn ′′ ≤ 1 ), i.e., by the instantaneous elongation of the specimen during the martensitic transn
formation [13]. At the same time, the
β→ω
transformation has a negative volume effect ( ΔΩβ→ω = n
Ωβn /Ωωn > 1 ) in some alloys based on titanium and zirconium [14–16], which must lead to an additional increase in the deformation stress [11]. It was established that the hardness [17–19] and yield strength [10, 20] of alloys containing substantial amounts of the ω -phase in the β -matrix increase, and their maximum values depend on the m
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