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copper based shape memory alloys

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  • Study of corrosion and dry sliding wear resistance for copper based shape memory alloysBy Aseel safee hemzaAl- dulamy B.SC Supervised by Dr. Abdul Raheem . K. Abid Ali

  • Introduction

    Shape-memory alloys are a new class of functional materials have an ability to recover a particular shape with changing temperature. for example Ag-Cd , Au-Cd , Cu-Al-Ni, Cu-Sn , Cu-Zn-(X) ,Ni-Al, Ni-Ti, Fe-Pt, Mn-Cu and Fe-Mn-Si alloys , are a group of metallic materials . The temperature in which phase transformation occurs, is called the transformation temperature.

  • COOLING APPLIED STRESS

    HEATING

  • TypesSmart material May be divide to :-- smart color-changing materials / such as photochromics , thermochromic , mechanochromic and chemochromic. - smart materials- energy changing/ such as Light emitting materials,Electroluminescence, Semiconductor phenomena and Shape Memory Alloys.

  • Mechanism of the Shape Memory Alloys - martensite results from the solid state process of martensitic transformation and the reaction is displacive , no change in chemical composition or atomic diffusion. - Martensitic reactions normally occur athermally and martensite is formed upon cooling from a higher temperature phase called the parent phase. This phenomna come from two shear directions can be applied to the squares to produce two different rhombus variants .

  • Transformation Behavior of Shape Memory Alloys transformation behavior classified according to the type of effect, shape memory effect caused by temperature induced martensitic transformation while pseudo-elasticity (also called superelasticity) happens because of stress induced martensitic transformation (SIMT). Shape memory effect divided to:- One way shape memory effect.Two way shape memory effectpseudo-elasticity

  • One-way shape memory effect Austenite cooled to the Martensite , deformation of the twinned-martensite leads to microstructural changes (detwinning), making possible macroscopic strains of up to 10 %. heating causes the reverse transformation into Austenite. original shape retained during cooling because of accommodation processes (repeated twinning).

  • Two-way shape memory effect Remember both hot and cold shapes. They can be cycled between two different shapes without the need of external stress. Shapes changes occur under the influence of internal stress. Selfaccommodation of the martensite microstructure is lost in the twoway effect due to the presence of these internal stresses.

  • Pseudoelasticity pseudoelasticity describes a mechanical memory has the ability to return to its predetermined shape after a (local) deformation of up to 10 % without heating, just by unloading (rubber-like behavior). observed if the Austenite is stabilized to operation temperature by specific alloying and/or heat treatment. The explanation of this phenomenon is a stress-induced phase transformation during loading and unloading.

  • APPLICATIONAerospace and Naval ApplicationsSmart Bridges Sensors Actuators Medical DentistryTweezersMobile Telephone Clothing

  • Martensitic transformationStress-induced transformations Thermally induced transformations Magnetic eld-induced transformations Electric eld-induced transformations

  • Problems in shape memory alloys Shape memory alloys undergoes to corrosion and wear as shown in the figure by the movement of the fluid in the coil of shape memory alloy as well as the movement of the fluid and the applied pressure causes wear and erosion for the surface of the alloy. Other example the shower valve that is undergoes to corrosion and wear . Memory-safe -antiscald shower valve.Oilcooler bypass valve

  • Technological routine

  • 2 hr at 550 oC 1 hr at 900 oC Temperatur (Oc)Times (hr)

    Sintering process

  • Quenching process1 hr at 850 OC2-4 oCTemoerature (oC)Time (hr)

  • Corrosion test The corrosive behavior of CuAlNi shape memory alloy in solution of (3.5gmNaCl) or (5gm NaOH) at room temperature was studied by means of open circuit potential measurements and investigated by Tafel extrapolation that is used to estimate corrosion current Tafel corrosion curve for alloy (Cu11Al4Ni0.6Cr)in NaOH solution.

  • Wear test wear instrument Dry sliding wear behavior has been studied by using pin on disk concept using (950 rpm) and constant sliding distance(6.3cm) with ( 4.9N and10N) load After a period of time (5,10 and 15 min.)

  • Results: Best compression was (520Mpa) and the XRD results of sintered alloys shown formation (Al4Cu9),(Cu9Al4) and for quenched samples formed (AlCu3)The apparent and true porosity decreases with pressure .Rockwell hardness values for alloys increased with alloying elements and the values of the hardness for martensite phase greater than the hardness for sintered state .DSC thermogram of quenched sample alloy(Cu11Al4Ni0.6Cr) shown that the As ,Af through the range (280oC-320oC). And for quenched sample alloy(Cu11Al4Ni0.6Ag) the As , Af through the range (271.0oC-321OC). SEM microstructure of the quenched alloys surface refer to formation martensites lines

  • Among the alloys, quenched alloy (Copper-13%Aluminum-4% Nickel-0.9% Ag) has the lowest corrosion current (2.7A) then has the highest corrosion resistance in NaCl solution and quenched alloy(Copper-13%Aluminum-4%Nickel-0.9%Cr) has the lowest corrosion current (24.08A)in NaOH solution and quenched alloy(Cu-13%wt.Al-4%wt.Ni-0.9%Cr)has the highest wear resistance and the volume loss was (2.1919mm3) .

  • Conclusions:X-ray diffraction analysis shows that the sintering at 550oC for 2hr. followed by sintering at 900oC for 1hr in vacuum of (10-4 torr) is efficient to satisfy sintering process in which elemental powders are completely transformed into alloy structure. Martensitic transformation is occurred in (Cu-Al-Ni) based shape memory alloys completely by direct quenching form 850oC into ice water(2-4oC ) as clearly shown from x-ray diffraction analysis. The alloys tend to form intermetallic compounds (Al4Cu9), (Cu9Al4) for alloys in sintering state , also (AlCu3)phase formed in quenching state for alloy(2) and alloy (10). Corrosion current of the quenched alloys is less than that for sintered alloys .The EDX analysis revealed the presence of copper ,aluminum ,Nickel , Chromium , Oxides and Chlorides on the alloy surface.

  • The wear resistance for the alloys increases with the increase in alloying elements and weight loss increases with increasing of normal load.The tafel curve can be characterized by one anodic current density peak and the cathodic current density peak. Corrosion current has been decreased with increasing the percent of alloying elements (Cr or Ag) .

  • Suggestions and RecommendationIn order to increase density of sintered compacts, double action press or hot isostatic pressing should be used. Transmission electron microscopy (TEM) and high resolution of (SEM) should be used to reveal the structure of martensite, its form and the degree of ordering for obtained martensite . The solution of corrosion should be tested after corrosion process in order to estimation ions concentration.

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