Pseudoelasticity and the strain-memory effect in an Ag-45 at. pct Cd alloy

A study has been made of pseudoelasticity and the strain-memory effect in βAg-Cd alloys having a low temperature martensitic transformation. Tests on single crystals showed that maximum pseudoelasticity occurred in specimens with the tensile axis oriented close to 〈001〉_β at temperatures 20° to 50°C aboveM _s. The habit plane for stress-induced martensite (SIM) was found experimentally to be ~(155)_β and agreed well with a value obtained from the phenomenological theory assuming a (110) [110 ]_β lattice-invariant shear. It is suggested that the macroscopic shear accompanying the transformation gives rise to the pseudoelastic strains, agreement with the experimental strain values being satisfactory. The strain-memory effect was explained as being due to a change from the (011) [011 ]_β lattice-invariant shear associated with thermal martensite to the (110) [110]_β shear associated with SIM. This paper is based upon a thesis submitted by This paper is based upon a thesis submitted by This paper is based upon a thesis submitted by     

The mechanical properties of grain refined β- cuaini strain-memory alloys

A study has been made of the effect of grain refinement on the mechanical and the strain-memory properties of β-CuAlNi alloys. Addition of 0.5 pct Ti to CuAINi decreased the grain growth rate of the beta phase significantly. This appeared to be due mainly to the small fraction of the titanium in solid solution in the β-CuAlNi. By controlled annealing, a grain size as small as 15 μrn could be obtained, though some second phase γ₂ was present due to incomplete precipitate dissolution. Stress-strain curves for most specimens in both the strain-memory and pseudoelastic states showed a three-stage characteristic with a region of lower slope between two regions of higher modulus. It was found that σ₁, (the transition stress between stages 1 and 2) and (dσ/dε@#@) (the slope of stage 2) increased with grain size according to a (g.s.)^-1/2 relationship. The ultimate tensile strength and strain to fracture also followed a similar Hall-Petch relationship. The alloys showed higher strength in the martensitic state than in the pseudoelastic one. The presence of second-phase particles had no significant effect on the mechanical properties and martensite deformation behavior. Fracture strains as high as 7 pct were obtained at the finest grain sizes. It was found that the strain-memory and pseudoelastic recovery properties were not affected significantly by decreasing grain size and the presence of second phase particles. Maximum recovery strains of 6.5 pct were obtained in fine grain samples. Formerly Graduate Student, Department of Metallurgical Engineering, University of British Columbia     

Transformational elasticity in a polycrystalline Cu-Zn-Sn alloy

A study has been made of the transformational elasticity in a polycrystalline Cu-33.6 wt pct Zn-4 wt pct Sn alloy. The magnitude of the transformational elasticity was markedly dependent on the grain size/thickness ratio of the sample. Large reversible strains (<2.5 pct) were realized for samples in which the grains extend through the sample thickness. Samples with a smaller gs/t ratio exhibited very limited transformational elasticity. The gs/t ratio, considered to be a measure of the grain constraint, also affected the M_s, M_b, and A_I temperatures. Although the stress-strain curves appeared quite reproducible after 4 loading-unloading cycles, some non-recoverable deformation was produced with each cycle. This permanent deformation was cumulative during extended cycling, giving rise to a decreasing amount of reversible strain. Increased cycling also reduced the load-unload hysteresis of the stress-strain curves.     

The shape memory effect and pseudoelasticity in polycrystalline Cu-Zn alloys

The shape memory effect associated with the reverse transformation of deformed martensite, pseudoelastic behavior involved in stress-induced martensite formation and the reversion of strained martensite after an applied stress is relaxed aboveA _f have been studied. Grain size and specimen geometry effects have been related to the above phenomena. Although recoverable strains as high as 10.85 pct were observed in coarse-grained (“bamboo” type) specimens, the shape memory effect is restricted in fine-grained specimens because of permanent grain boundary deformation and intergranular fracture which occurs at relatively low strains. A fine grain size also acts to suppress pseudoelastic behavior because permanent, localized deformation is generated concurrent with the formation of stress-induced martensite which inhibits reversion of the latter upon release of stress. The apparent plastic deformation of martensite belowM _f can be restored by transforming back to the original parent phase by heating toA _f (shape memory) or alternatively, can be recovered belowM _f by applying a small stress of opposite sign. Martensite deformed belowM _f with the same stress maintained while heating persists aboveA _f, but reverts to the parent phase in a pseudoelastic manner when the stress is relieved. The athermal thermoelastic martensite, which forms in groups composed of four martensite plate variants, undergoes several morphology changes under deformation. One of the variants within a plate group cluster may grow with respect to the others, and eventually form a single crystalline martensitic region. At a later stage pink colored deformation bands form in the same area and join up with increasing stress, resulting in thermally irreversible kinks. The clusters of plate groups may expand like grain growth or contract as a whole during deformation, or act as immobile “subgrains” which lead to permanent deformation at their boundaries. Stress-induced martensite usually forms as one variant of parallel plates which join up with increasing stress to form single crystalline regions. Further stress leads to pink colored deformation bands, similar to those in the deformed athermal martensite. Other similarities and differences between the stress-induced and athermal martensite have been investigated and are discussed. Formerly with the University of Illinois at Urbana-Champaign     

The shape memory effect and pseudoelasticity in polycrystalline Cu-Zn alloys

The shape memory effect associated with the reverse transformation of deformed martensite, pseudoelastic behavior involved in stress-induced martensite formation and the reversion of strained martensite after an applied stress is relaxed aboveA _f have been studied. Grain size and specimen geometry effects have been related to the above phenomena. Although recoverable strains as high as 10.85 pct were observed in coarse-grained (“bamboo” type) specimens, the shape memory effect is restricted in fine-grained specimens because of permanent grain boundary deformation and intergranular fracture which occurs at relatively low strains. A fine grain size also acts to suppress pseudoelastic behavior because permanent, localized deformation is generated concurrent with the formation of stress-induced martensite which inhibits reversion of the latter upon release of stress. The apparent plastic deformation of martensite belowM _f can be restored by transforming back to the original parent phase by heating toA _f (shape memory) or alternatively, can be recovered belowM _f by applying a small stress of opposite sign. Martensite deformed belowM _f with the same stress maintained while heating persists aboveA _f, but reverts to the parent phase in a pseudoelastic manner when the stress is relieved. The athermal thermoelastic martensite, which forms in groups composed of four martensite plate variants, undergoes several morphology changes under deformation. One of the variants within a plate group cluster may grow with respect to the others, and eventually form a single crystalline martensitic region. At a later stage pink colored deformation bands form in the same area and join up with increasing stress, resulting in thermally irreversible kinks. The clusters of plate groups may expand like grain growth or contract as a whole during deformation, or act as immobile “subgrains” which lead to permanent deformation at their boundaries. Stress-induced martensite usually forms as one variant of parallel plates which join up with increasing stress to form single crystalline regions. Further stress leads to pink colored deformation bands, similar to those in the deformed athermal martensite. Other similarities and differences between the stress-induced and athermal martensite have been investigated and are discussed.     

Improvement of shape memory effect in Fe-Mn-Si-Cr-Ni alloys

Three kinds of Fe-Mn-Si-Cr-Ni alloys, which contain cerium, titanium, and nitrogen, respectively, were employed to study the shape memory effect (SME). Experimental results have shown that the SME can be improved significantly by alloying with Ce, Ti, and N, deforming at a low temperature (77 K), and using a series of heat treatments. By alloying with Ce, the SME can be enhanced greatly, so adding the rare-earth elements is an effective way to improve the SME. When prestrained at 77 K, the alloy containing N shows an excellent SME, with a fully reversible strain of more than 3.2 pct, and 6 pct of the net reversible strain can be obtained under the prestrain of 8 pct. Each alloy has a best solution-treated temperature, at which a better SME can be obtained. All these three alloys exhibit an excellent SME after a series of heat treatments which include aging, resolution treatment after tensile deformation, and thermomechanical training. A small two-way SME has been observed in the alloy containing N.     

The Bauschinger effect in precipitation strengthened aluminum alloys

The Bauschinger effect in precipitation strengthened Al-Cu-Mg, Al-Zn-Mg and Al-Cu polycrystals was measured as a function of applied strain. Alloys heat treated to contain easily shearable precipitates,i.e., GPB, GP and θ″ exhibited a small Bauschinger effect, on the order of that in pure aluminum. In contrast, alloys with nonshearable precipitates, S′, η and θ′ showed an anomolously large effect. A unique hysteresis loop shape, with a region of convex curvature between sharp inflection points, was observed in the nonshear-able precipitate alloys. The large Bauschinger effect and unusual hysteresis loop shape are due to internal elastic or back stresses exerted by the strong precipitates on the matrix. A nonlinear elastic hardening model is proposed in which the overall work harden-ing is partitioned into an elastic back stress component and a frictional dislocation forest hardening term. Plastic relaxation around the precipitates and inhomogeneous deformation in the polycrystal reduces the level of the internal stresses below that predicted theoretically by the Brown and Stobbs hardening theory.     

Fe-Ni-Si基奥氏体时效合金的马氏体相变和形状记忆效应

It is well known that the morphologies of the α martensite formed from the γ phase in ferrous alloys are classified into five types of lath, butterfly, (225)A type plate,lenticular and thin-plate. Among those α martensites, onlythe thinplate martensite, which is characterized by containing a high density of transformation twins, has a potential of exhibiting a perfect shape memory (SM) effect.Recently the present authors found in Fe-Ni-Si alloys that the thin-plate martensite is formed by the introduction of fine and coherent γ-(Ni,Fe)3Si particles with a L12 ordered structure in the austenite matrix due to ausaging. In the present study, the SM properties of the ausaged Fe-Ni-Si alloys with the thin-plate martensite are investigated by a conventional bending-test. The effects of the addition of Co to the Fe-Ni-Si alloys on the martensitic transformation and the SM properties are also investigated. It is shown that while the ausaged Fe-Ni-Si ternary alloys exhibit an imperfect SM effect due to reverse transformation from stress-induced thin-plate martensite to austenite, the SM properties are improved by the addition of Co. An almost perfect SM effect is confirmed in the Fe-Ni-Si-Co alloys by heating to 1 100 ℃ after deformation at -196 ℃.     

Thermoelastic martensite and shape memory effect in ductile Cu-Al-Mn alloys

Ductile shape memory (SM) alloys of the Cu-AI-Mn system have been developed by controlling the degree of order in the β phase. Additions of Mn to the binary Cu-Al alloy stabilize the β phase and widen the single-phase region to lower temperature and lower Al contents. It is shown that Cu-Al-Mn alloys with low Al contents have either the disordered A2 structure or the ordered L2₁ structure with a lower degree of order and that they exhibit excellent ductility. The disordered A2 phase martensitically transforms to the disordered Al phase with a high density of twins. The martensite phase formed from the ordered L2₁ phase has the 18R structure. The SM effect accompanies both the A2 → Al and L2₁ → 18R martensitic transformations. These alloys exhibit 15 pct strain to failure, 60 to 90 pct rolling reduction without cracking, and 80 to 90 pct recovery from bend test in the martensitic condition. Experimental results on the microstructure, crystal structure, mechanical properties, and shape memory behavior in the ductile Cu-AI-Mn alloys are presented and discussed.     

The portevin-le chatelier effect in carburized nickel alloys

From a study of serrated yielding (the Portevin-Le Chatelier Effect) in carburized nickel, it is shown that the expected diffusion coefficient does not fit the approximate Cottrell condition relating strain rate (?e), mobile dislocation density (p), and diffusion coefficient(D). Since serrations begin immediately after yielding, vacancy-enhanced diffusion caused by plastic strain is not responsible for this. Generally, it is proposed that carbide stability controls the temperature at which serrated yielding disappears while carbon diffusion may play a part in the complex process responsible for the appearance of serrated yielding. Cobalt additions to carburized nickel quickly attenuate the effect while copper additions at first accentuate the effect and then remove it. The driving force for carbon to the dislocations responsible for the mobile solute-mobile dislocation interactions is most probably due to the elastic misfit of the carbon atom in solution.     

Stress-strain behavior and shape memory effect in powder metallurgy TiNi alloys

The shape memory properties of the TiNi alloy produced by a powder metallurgical method have been evaluated from tensile stress-strain curves. The contamination of the powders during atomization can be suppressed by applying the Plasma Rotating Electrode Process (P-REP), so that the compact made by Hot Isostatic Pressing (HIP) is expected to exhibit the shape memory effect identical to the typical alloy grown from melt. The fracture behavior of the P/M alloy is also studied, and the improvement of fracture strength of the P/M alloy is attempted.     

The relation between the Portevin-Le chatelier effect and the solid solubility in some binary alloys

Effects of composition and temperature on the occurrence of the Portevin-Le Chatelier effect were investigated in the literature and the experiments in some alloys. These investigations showed that the alloy systems were divided into two groups according to the minimum solute concentration below which the P-L effect did not occur. The first group consisted of alloy systems in which the P-L effects occurred at the solute content less than 1 at.%, and their common feature was that they had small solubilities of solute. In the second group the P-L effects did not occur at the solute content less than 5 at.% and the solute solubilities were large. Based on these results a new theory for the P-L effect that the precipitates or the short range ordered regions interacted with the dislocations, was proposed and compared with the Cottrell theory.     

Experimental study and theoretical simulation of the cross hardening effect in shape memory alloys

The shapes and the relative position of martensitic inelasticity and forward transformation diagrams are experimentally studied. The strain dependences of the stress in loading under martensitic inelasticity conditions after an experiment on the accumulation of the forward transformation–induced strain at a constant or variable stress are investigated on titanium nickelide samples. It is found that the hardening of the martensite part of the representative volume of a shape memory alloy (titanium nickelide) after forward transformation under a nonmonotonically changing stress can be nonuniform. The cross hardening phenomenon is theoretically described in terms of the model of nonlinear deformation of a shape memory alloy during phase and structural transformations.