The Influence of Heat Treatmnet and Stress State on Functional and Mechanical Properties of [001]-Oriented FeNiCoAlTi Single Crystals

The effect of γ'- and β-phase particles on the γ–α' thermoelastic martensitic transformation (MT) during heating/cooling and under stress in [001]-oriented single crystals of a Fe – 28% Ni – 17% Co – 11.5% Al – 2.5% Ti (at.%) alloy is studied in tension and compression. The influence of the ageing mode on the Ms temperature is shown. During a simultaneous precipitation of γ'- and β-phase particles following two-step ageing for 4+4 h at 873 K, a 100 K increase in the Ms temperature is observed compared to one-step ageing for 8 h. No differences (asymmetry) in the stresses σcr for the start of the stress-induced γ–α' MT nor in the values of α = dσcr/dT in tension and compression are observed in the crystals with γ'- and γ'+β-phase particles. The absence of asymmetry of σcr and α = dσcr/dT is due to close values of the shape memory effect (SME) and superelasticity (SE) in tension and compression. Upon precipitation of β-phase particles the values of SME and SE decrease.