Vacancy behavior in Cu-26 Zn-4 Al and Cu-14 Al-4 Ni alloys during ageing is investigated and compared by means of positron annihilation (PA) and electrical resistivity measurements. For ageing in the martensitic state after direct quenching, the experimental results show that the S parameter of CuZnAl specimens, measured at liquid nitrogen temperature increases at first and then decreases, while that of CuAlNi specimens remains unchanged. The different behavior of quenched-in vacancies in these two alloys reveals that the clustering of quenched-in vacancies is responsible for the stabilization of martensite at the early stage in CuZnAl alloys. The reason of the insensibility to stabilization for the CuAlNi alloy is discussed.



Das Verhalten von Leerstellen in Cu–26 Zn-4 Al- und Cu-14 Al-4 Ni-Legierungen wahrend des Alterns wird mittels Positronenannihilation und elektrischer Widerstandsmessungen untersucht und verglichen. Bei Altern im Martensit-Zustand nach direktem Abschrecken nimmt der bei der Temperatur des flussigen Stickstoffs gemessene S-Parameter von CuZnAl zunachst zu und dann ab, wahrend in CuAlNi-Proben S unverandert bleibt. Das unterschiedliche Verhalten der Leerstellen in diesen beiden Legierungen zeigt, das Clusterbildung im Anfangszustand fur die Martensit-Stabilisierung in CuZnAl-Legierungen verantwortlich ist. Der Grund fur die Unempfindlichkeit von CuAlNi-Legierungen gegenuber einer Stabilisierung wird diskutiert.

The Behavior of Quenched‐in Vacancies and Stabilization of Martensite in Copper‐Based Shape Memory Alloys

/pdf/laser-spectroscopy-for-the-study-of-exotic-nuclei-e74lz4uc.pdf

Laser spectroscopy for the study of exotic nuclei

In recent years, mechanical property of many SMA has improved in order to introduce these alloysin specific field of industry. Main examples of these alloys are the NiTi, Cu-Zn-Al and Cu-Al-Ni which are usedin many fields of engineering such as aerospace or mechanical systems. Cu-Zn-Al alloys are characterized bygood shape memory properties due to a bcc disordered structure stable at high temperature called ?-phase,which is able to change by means of a reversible transition to a B2 structure after appropriate cooling, andreversible transition from B2 secondary to DO3 order, under other types of cooling. In ?-Cu-Zn-Al shapememory alloys, the martensitic transformation is not in equilibrium at room temperature. It is therefore oftennecessary to obtain the martensitic structure, using a thermal treatment at high temperature followed byquenching. The martensitic phases can be either thermally-induced spontaneous transformation, or stressinduced,or cooling, or stressing the ?- phase. Direct quenching from high temperatures to the martensite phaseis the most effective because of the non-diffusive character of the transformation. The martensite inherits theatomic order from the ?-phase. Precipitation of many kinds of intermetallic phases is the main problem oftreatment on cu-based shape memory alloy. For instance, a precipitation of ?-phase occurs in many lowaluminum copper based SMA alloy and presence of ?-phase implies a strong degradation of shape recovery.However, Cu-Zn-Al SMA alloys characterized by aluminum contents less than 5% cover a good cold machiningand cost is lower than traditional NiTi SMA alloys. In order to improve the SMA performance, it is alwaysnecessary to identify the microstructural changing in mechanical and thermal conditions, using X-Ray analyses.In this work a Cu-Zn-Al SMA alloy obtained in laboratory has been microstructurally and metallographicallycharacterized by means of X-Ray diffraction and Light Optical Microscope (LOM) observations. Furthermorea fatigue crack propagation and fracture surface scanning electron microscope (SEM) observations have beenperformed in order to evaluate the crack path and the main crack micromechanisms.

/pdf/fatigue-crack-behavior-on-a-cu-zn-al-sma-1orqr0jjhq.pdf

Fatigue crack behavior on a Cu-Zn-Al SMA

Positron lifetime spectroscopy is used to study the long time room temperature behavior of defects retained after quench in β-CuZnAl single crystals. Quenching treatments are made in a temperature range from 493 to 1073 K. The decay in the defect concentration with the aging time is attributed to the motion of vacancies to sinks. Assuming the sinks as dislocations, a dislocation density is derived. This density increases with increasing quenching temperature. We find that after long time aging all vacancies disappear for all Tq used in this work except for 773 K. This behavior is associated to the existence of γ-phase precipitates which are known to be present after quench from this temperature.

The post-quench vacancy behavior in β-CuZnAl alloy

By internal friction technique, the behaviour of quenched-in vacancies and their influence on the martensitic transformation of the Cu–11.9Al–2.5Mn alloy has been studied. It is found that the reverse martensitic transformation temperature Tp follows a non-monotonous dependence on quenching temperature Tp, of which the presence of a maximum Tp value is attributed to the different vacancy formation energies in the ordered and disordered phases. The background internal friction decreases with increasing ageing time due to migration of vacancies and resultant pinning effect on the phase interfaces and dislocations. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Z.‐Y. Xu

Papers

The Recovery Behavior of Quenched‐in Vacancies in CuZnAl Alloy

Electromagnetic moments of scandium isotopes and N = 28 isotones in the distinctive 0f7/2 orbit

Z.‐Y. Xu

Papers

The Recovery Behavior of Quenched‐in Vacancies in CuZnAl Alloy

Electromagnetic moments of scandium isotopes and N = 28 isotones in the distinctive 0f7/2 orbit

Electromagnetic moments of scandium isotopes and N = 28 isotones in the distinctive 0f7/2 orbit