Yu. I. Chumlyakov

A comparative study of elastic constants of Ti-Ni-based alloys prior to martensitic transformation

Abstract Single crystal elastic constants of Ti–Ni alloys without (quenched) and with (aged) Ti 3 Ni 4 precipitates were measured systematically by rectangular parallelepiped resonance method as a function of composition and temperature, and compared with Ti–Ni–Cu and Ti–Ni–Fe alloys, in an attempt to answer some long-standing questions as to the origin of the unique monoclinic B19′ martensite, and why composition and thermomechanical treatment greatly changes the path of martensitic transformation. The results showed that softening in c 44 , in additional to c ′, is a common feature for all Ti–Ni binary (both quenched or aged) and ternary alloys. This general feature just corresponds to the fact that all these alloys ultimately transform into B19′, suggesting that softening in c 44 is responsible for the unique B19′ martensite, which found no analogy in other β phase alloys. We also found an interesting correspondence between the temperature dependence of anisotropy factor and transformation path. Prior to B2–B19′ transformation anisotropy shows a decrease with lowering temperature; prior to B2–B19 an anisotropy increase, while prior to B2–R transformation a constant anisotropy. We further showed that three possible martensite candidates (R, B19, B19′) are rooted in anomalies in specific phonon modes and elastic softening. We showed that the multi-stage transformations are restricted by a general rule: multi-stage transformation occurs in the sequence of increasing transformation strain. With this rule we can explain all known transformation paths by considering the effect of alloying addition and fine precipitates/dislocation network on relative stability of different martensites. We further predict that there may exist a new transformation path in Ti–Ni-based alloys: B2–R–B19–B19′.

Understanding the martensitic transformations in TiNi-based alloys by elastic constants measurement

Abstract The origin of the unique monoclinic martensite B19′ in TiNi and TiNi-based alloys has remained obscure for many years. The formation of B19′ martensite also challenged the well-known basal-plane shear/shuffle theory of martensitic transformation in β phase alloys. Recently we proposed that the B2–B19′ transformation stems from a strong coupling between a non-basal-plane shear (c44 shear) with the basal-plane shear (c′ shear), being manifested by a low-lying and decreasing anisotropy factor (c44/c′) towards Ms. This model has gained experimental support from elastic constants measurement on Ti50Ni30Cu20 alloy which exhibits B2–B19 transformation (i.e. absence of the monoclinic shear). In the present study, we attempt to further verify this model by measuring the elastic constants of Ti50Ni40Cu10 alloy which undergoes a two-stage transformation B2–B19–B19′. The results clearly demonstrated once again that whenever the parent phase B2 does not transform directly into B19′, the anisotropy factor exhibits an increase towards Ms, indicating that the c44 shear is not included into the transformation. Therefore, the present study gives additional support to the coupling model of TiNi-based alloys. Furthermore, we tried to understand the multi-stage transformation in terms of coupling strength between the c′ shear and c44 shear.

Effect of disperse Ti3N4 particles on the martensitic transformations in titanium nickelide single crystals

The effect of the size and volume fraction of Ti3N4 particles in Ti-(50.3–51.5) at % Ni single crystals on their martensitic transformation temperatures and temperature hysteresis is studied. Aging at T = 673−823 K leads to a nonmonotonic change in the martensitic transformation temperatures and temperature hysteresis, which is related to a change in the Ni concentration in the matrix, the hardening of the high-temperature phase, a change in the elastic and surface energies generated upon the martensitic transformations, and the internal stresses that appear because of the difference in the lattice parameters of the particles and the matrix. As a result of the high strength of the B2 phase and the high elastic and surface energies that are generated upon the martensitic transformations due to the precipitation of particles of size d < 40 nm at an interparticle distance λ< 50 nm, the martensitic transformation temperatures decrease down to the suppression of the R-B19′ transitions upon cooling to 77 K. A thermodynamic description for the martensitic transformations in heterophase crystals is proposed, and an analogy between the martensitic transformations in heterophase Ti-Ni single crystals with nanoparticles of size d = 20–100 nm and those in single-phase Ti-Ni polycrystals with a grain size of 50–200 nm is found.

Orientational dependence of shape memory effects and superelasticity in CoNiGa, NiMnGa, CoNiAl, FeNiCoTi, and TiNi single crystals

The dependence of deforming stresses, shape memory effect (SME), and superelasticity (SE) on the orientation of the single crystal axis, test temperature, and disperse particle size is examined for CoNiGa, NiMnGa, CoNiAl, FeNiCoTi, and TiNi single crystals. The orientational dependence of SME, SE, and temperature interval of the development of martensitic transformations (MT) under loading and SE is established. The influence of disperse particles on magnitudes of SME, SE, and mechanical hysteresis is discussed.

The shape-memory effect and superelasticity in single-crystal ferromagnetic alloy FeNiCoAlTi

The reversible thermoelastic γ-α′ martensitic transformations under load in new single-crystal ferromagnetic Fe-28%Ni-17%Co-11.5%Al-2.5%Ti (at %) alloy have been studied. It is shown for the first time that precipitation of dispersed particles of the γ′ phase during aging for 7 h at 973 K induces the shape-memory effect and superelasticity in [001] single crystals of Fe-Ni-Co-Al-Ti alloy upon tensile strain. Superelasticity ɛSE = 4.5–6% manifests itself in the temperature range T = 183–333 K.

Orientation and temperature dependence of superelasticity caused by reversible γ-α′ martensitic transformations in FeNiCoAlTa single crystals

Reversible thermoelastic γ-α′ martensite transformations (MTs) have been studied in Fe-28%Ni-17%Co-11.5%Al-2.5%Ta (at. %) single crystals under cooling/heating and loading conditions. It is established that the precipitation of dispersed γ′ phase particles with an average size of d ≤ 5 nm leads to thermoelastic γ-α′ MTs with a small temperature hysteresis (ΔT = 20 K). The [001] oriented crystals favor the attaining of maximum superelasticity (SE) ɛSE = 6.8%, minimum mechanical hysteresis Δσ = 130 MPa, and large SE temperature interval ΔTSE = 130 K. In contrast, in the [111] oriented crystals, small values of ɛSE = 2.0%, high values of Δσ = 350–400 MPa and narrow SE interval ΔTSE = 55 K are found.

Effect of nitrogen and stacking-fault energy on twinning in [\( \bar 1 \)11] single crystals of austenitic stainless steels

AbstractThe dependence of critical shear stresses for twinning τcrtw on the stacking-fault energy γ0, the nitrogen concentration CN (wt %), and the test temperature has been studied for [

The effect of hydrogen on shape memory effect and superelasticity in single-phase nickel titanium single crystals

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