Effect of volume fraction of primary alpha(2) on the trigger stress for stress-induced martensitic transformation in two-phase Ti-Al-Nb alloys

Ti-Al-Nb alloys in the composition range studied in the present investigation were found to undergo stress-induced martensitic transformation (SIMT) in alpha(2) + beta heat-treated (two-phase) condition under tensile loading. The effect of different volume fractions of primary alpha(2) on trigger stress for SIM transformation in these alloys was investigated. The trigger stress required for SIM transformation decreased initially when the volume fraction of primary alpha(2) was increased from 0 to 5 pct. However, subsequently, the trigger stress was found to increase with further increase in volume fraction of primary alpha(2) from 5 to 40 pct in all three alloys. An increase in the volume fraction of primary alpha(2) is achieved by changing the solution treatment temperatures, which also results in a change in the beta composition and beta grain size. The observed variation of trigger stress with the volume fraction of primary alpha(2) from 5 to 40 pct was then explained on the basis of the effect of composition and grain size of the beta phase on trigger stress required for stress-induced transformation of beta to martensite

Influence of volume fraction of primary alpha(2) on the fracture toughness of Ti-Al-Nb alloy undergoing stress-induced martensitic transformation

The effect of volume fraction of primary alpha(2) on fracture toughness of Ti-18Al-8Nb alloy, which undergoes stress-induced martensitic transformation (SIMT), was investigated. The fracture toughness of the alloy was found to decrease with an increase in volume fraction of primary alpha(2). The results were explained on the basis of the effect of change in volume fraction of alpha(2) on fracture toughness contributions due to SIMT as well as that due to the subsequent fracture process involving ductile fracture of the beta/martensite/alpha(2) microstructure. (c) 200

Trigger stress for stress-induced martensitic transformation during tensile deformation in Ti-Al-Nb alloys: Effect of grain size

The effect of beta grain size on trigger stress for stress-induced martensitic transformation during tensile deformation in Ti-Al-Nb alloys was investigated. The trigger stress for stress-induced martensitic transformation (SIMT) in Ti-Al-Nb alloys exhibited a U-shaped behavior with variation in grain size. The variation of trigger stress with grain size was explained qualitatively, in terms of the contrasting change in the internal elastic energy stored in matrix due to formation of martensite (Delta E (el) ) and the irreversible work done in overcoming the internal frictional resistance to phase boundary movement (partial derivative E-irr) with beta grain size

On the validity of Hall-Petch equation for single-phase beta Ti-Al-Nb alloys undergoing stress-induced martensitic transformation

The yield strengths of Ti-Al-Nb alloys, which undergo stress-induced martensitic transformation, prior to the onset of plastic deformation during tensile testing, were found to obey the Hall-Petch relationship with grain size. The overall friction stress (sigma(oc)(i)) was observed to decrease with increase in Nb content while it remained more or less unchanged with increase in Al content in these alloys. On the other hand, the overall k(y)(oc), the unpinning constant, which is an index of the efficiency of boundaries as obstacle to dislocation motion, was found to increase with increase in Nb and decrease with increase in Al content in these alloys. (C) 200

Various stages in stress–strain curve of Ti–Al–Nb alloys undergoing SIMT

Ti–Al–Nb alloys in the present range of composition were found to exhibit a typical four-stage behaviour observed in alloys undergoing stress-induced martensitic transformation (SIMT) in β as well as α2–β heat-treated condition. Intermittent unloading–reloading during tensile test was used to measure the apparent modulus at regular strain intervals. This coupled with the observation of microstructure of the samples from tensile tests interrupted at each of the four stages was used to identify the operative mechanism of each stage.© Elsevie

On the Influence of Alloy Composition on the Fracture Toughness of Two-Phase Ti-Al-Nb Alloys Undergoing Stress-Induced Martensitic Transformation

The effect of alloy composition on the fracture toughness of Ti-Al-Nb alloys, which undergo stress-induced martensitic transformation (SIMT), was investigated for a given volume fraction of primary alpha (2). Both Ti-18Al-8Nb and Ti-15Al-12Nb alloys exhibited lower fracture toughness values as compared to Ti-15Al-8Nb alloy, which indicated that higher Al or Nb was detrimental to fracture toughness in these alloys in the presence of primary alpha (2). The results were explained on the basis of the effect of composition on fracture toughness contributions due to SIMT as well as ductile fracture