L. C. Brown

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.

Preparation and properties of fine grain β- CuAlNi strain- memory alloys

A method has been developed to produce grain sizes as small as 5 μm in alloys of β-CuAlNi. The alloys were of eutectoid composition and a procedure was developed for determining the composition of a eutectoid alloy having any required value for transition temperature (Ms). The thermo-mechanical treatment involved two sequential stages of warm rolling followed by recrystallization. The alloys produced were single phase β-type with no second phase being present. Characteristic two-stage stress-strain curves were obtained for most of the specimens. It was generally found that the tensile strength and strain to failure increased with decreasing grain size according to a Hall-Petch type relationship down to a grain size of 5 μm. A fracture strength of 1200 MPa and a fracture strain of 10 pct were obtained in the best alloy. It was found that the major recovery mode, whether pseudoelastic or strain-memory, did not have any significant effect on the total recovery obtained. Recovery properties were not affected significantly by decreasing grain size, and 86 pct recovery could still be obtained at a grain size of around 10 μm. Grain refinement improved the fatigue life considerably, possibly due to the high ultimate fracture stress and ductile fracture mode. A fatigue life of 275,000 cycles could be obtained for an applied stress of 330 MPa and a steady state strain of 0.7 pct. At fine-grain sizes most of the fractures were due to transgranular-type brittle fracture and micro void-type ductile fracture, depending on the alloy composition. It was suggested that the difference between the alloys was due to differences in oxygen segregation at the grain boundaries.

The principle of additivity and the

This study critically examines the principle of additivity and the reason that the proeutectoid ferrite transformation is additive. Austenite-to-proeutectoid ferrite transformation kinetics were measured under isothermal and stepped-isothermal conditions for AISI 1010 and 1020 steel grades using a dilatometer and a Gleeble 1500 thermomechanical simulator. The additive nature of the austenite-to-proeutectoid ferrite transformation was experimentally assessed by measuring transformation kinetics partially at one temperature and after a rapid temperature change to another temperature. Results of the tests on the 1010 steel showed that the proeutectoid ferrite transformation with allotriomorphic morphology is additive. Transformation kinetics were mea- sured for the 1020 steel with the ferrite morphology changing from allotriomorphic to predom- inantly Widmanstätten, and the transformation was additive. However, the stepped-isothermal test in which the ferrite was transformed and equilibrated at the first temperature and then rapidly cooled to the second temperature was not additive. The second part of the study involved de- veloping mathematical models with planar and spherical interface geometries to theoretically assess the additivity of the proeutectoid ferrite transformation. Additivity of the proeutectoid ferrite transformation was tested by predicting the ferrite growth kinetics and the associated carbon gradients under stepped-isothermal conditions. The predictions were consistent with the observed experimental additivity of the proeutectoid ferrite transformation, providing an expla- nation for this behavior, although theory would suggest ferrite reaction to be nonadditive.

The fatigue of pseudoelastic single crystals of α-CuAINi

The fatigue life of single crystal α-CuAINi has been studied on pseudoelastic cyclic loading. In general the fatigue life was quite poor with most specimens having less than 3000 cycles to failure. The fatigue life decreased significantly with increasing stress level. However, the fatigue failure was due primarily to stress-induced martensite formation, since if the stress level on cycling was not sufficient to form martensite, the specimen did not fail. The fatigue life appeared to be largely independent of percent strain, crystal orientation and environment of testing. Surface preparation, however, was very important with an electropolished specimen having a much longer life than an abraded one. Fatigue cracks grew only in the final few hundred cycles of the life of the specimen. Cracks initially grew in the direction of growth of the stress-induced martensite, approximately at 45 deg to the tensile axis. Final failure was due to brittle fracture caused by stress concentration at the tip of the fatigue crack, the α-CuAlNi being very brittle. Scanning electron micrographs of the fracture showed no fatigue striations but rather river markings spreading out from the point of nucleation of the fatigue crack, characteristic of a brittle material.

Growth kinetics of bainite plates and widmanstÄtten needles in theΒ′ phase of a Ag-45at. pct Cd alloy

The morphology of bainite plates and widmanstÄtten needles formed in the Β′ phase of a Ag-45 at. pet Cd alloy at temperatures 160 to 300‡C was studied by optical and scanning electron microscopy. Both precipitate forms were similar in appearance to precipitates reported for Cu-Zn alloys. The growth kinetics of both bainite plates and widmanstÄtten needles were measured by interrupted annealing and scanning electron microscopy. Using the bainite thickening kinetics measured at 160, 200 and 240‡C, the Frank-Zener model for growth of planar precipitates, and supersaturation data obtained from the Ag-Cd metastable phase diagram enabled the effective chemical diffusivities, Deff, to be calculated for the three transformation temperatures. The results were in good agreement with the expected diffusivities. The lengthening kinetics of bainite plates at 160‡C and of widmanstÄtten needles at 240‡C were analyzed using Trivedi’s model for diffusion-controlled growth. Deff obtained from the lengthening kinetics of the needles was in good agreement with the Deff value obtained from the thickening kinetics of the plates, indicating that widmanstÄtten needles lengthened and bainite plates thickened at a rate controlled by volume diffusion. Bainite plates lengthened only in the early stage of growth and at a rate approximately 180 times larger than that permitted by volume diffusion. Possible reasons for such behavior were discussed.

Transitions between stress-induced martensites in Cu Al Ni alloys

Stress-induced martensitic transformations have been studied in β1 Cu Al Ni single crystals at temperatures aboveMs. Close toMsγ′ martensite is formed, well aboveMsβ1′ martensite forms, whilst in an intermediate temperature range β1′ martensite initially forms and then transforms to γ′ on continued stressing and particularly on unloading, γ′ martensite is also formed when the stress-induced β1′ is cooled below a critical temperature. The γ′ martensite has a (101) twinned structure. The morphological and crystallographic aspects of the γ1′→ γ′ transition are discussed in detail. The two twin variants have different habit planes with respect to the β1′ phase, one being (201)γ′ and the other (001)γ′. A thermodynamic argument is presented to explain the γ1′→ γ′ transition, taking into account the relative stabilities of the β1′ and γ′ phases with respect to the β1, and the relative value for the critical driving force to nucleate the stress-induced β1′ and γ′ structures from the β1 phase

The Thermal Effect in Pseudoelastic Single Crystals of β-CuZnSn

The thermal effect associated with the formation and reversion of stress-inducedβ2 martensite has been studied in single crystals of β2-CuZnSn. The martensite in this system forms as isolated plates of β2 which increase in number and ultimately coalesce. The maximum temperature pulse obtained increased with fraction of specimen undergoing transformation and reached a maximum for complete transformation. The temperature pulse was the same for the forward and reverse transformations and the same at different points on the specimen. The maximum temperature change increased with increasing strain rate and reached a constant at high strain rates. Calculations showed that heat loss to an air environment was negligible at high strain rates and the transformation could be considered adiabatic. In tests in water there was always a significant heat loss to the environment and corrections were necessary to allow for this. The temperature rise under adiabatic conditions was used to calculate the entropy change of the transformation and gave ΔSβ2 - β2 = - 1.03 ± 0.07 J/mol K, in excellent agreement with tke value of - 1.00 ± 0.17 J/mol K found from the Clausius-Clapeyron equation. The variation of the entropy change with specimen temperature was small and could be considered constant within experimental error.

The fatigue of pseudoelastic polycrystalline β-cuznsn

The fatigue of polycrystalline pseudoelastic β-CuZnSn has been studied by cycling specimens to fixed stress. The fatigue life was found to decrease with increasing initial strain and decreasing specimen grain size. In both cases the results gave similar stress -vs - fatigue life curves, indicating that stress is the primary parameter controlling fatigue life. The results fitted a curve of the form △ε.NBf= constant, whereβ = 0.32 for the total initial strain, andβ = 0.29 for the initial elastic strain. The fatigue life appeared to be independent of strain rate. Fatigue cracks nucleated in the first cycle at three grain intersections and grew along grain boundaries until adjacent cracks linked up. In the later stages of crack growth, some intergranular cracking occurred when there were no suitably oriented grain boundaries. Both the intergranular and transgranular regions showed somewhat ill-defined fatigue striations.

Shape changes during dissolution of θ -CuAl2

A theoretical study has been made of the shape change of precipitates dissolving under volume diffusion control. It is shown that square and similarly shaped precipitates become circular during dissolution at a rate controlled by the saturation conditions. Shape change is little affected by diffusion fields from adjacent precipitates. Experimental measurements were made of dissolution of the θ phase in aluminum-copper alloys. These had shapes that could be approximated to distorted spheres. It was found that the precipitates dissolved at a rate controlled by volume diffusion and that the precipitates became spherical by the time their radius had decreased 50 pct as indicated by the theory. Both surface and bulk precipitates were studied. There was a much higher density of surface precipitates due to preferential nucleation. However, their dissolution characteristics were the same as bulk precipitates.

Dissolution kinetics of Widmanstätten γ-Ag2AI precipitates

A combination of scanning electron microscope and electron probe studies has been conducted on the dissolution of Widmanstätten γ-Ag2Al plates in Al-15.3 wt pct Ag alloys between the temperatures of 466 °C and 494 °C. Individual rather than averaged kinetic events have been monitored in all cases. The kinetics at the tip were observed to be linear with time and at the broadface to be parabolic with time. The broadface kinetics were slower than expected for volume diffusion control. Electron probe measurements round the dissolving precipitate gave similar diffusion profiles at the tip and the broadface. These indicated a slower dissolution rate at the tip than observed experimentally and a faster rate at the broadface than observed experimentally. A mechanism to explain the discrepancy between the kinetic and probe results is suggested.