POLDI: Pulse Overlap time-of-flight Diffractometer

 

 

 

 

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Case study 2 - Cu/Nb nanocomposites

Introduction

Figure 1. Microstructural composition of the CuNb wires.

The material under investigation is nanofilamentary copper/niobium (Cu/Nb) wires processed via severe plastic deformation. These wires exhibit both a very high strength and high electrical conductivity and are used as windings for resistive coils producing non-destructive magnetic fields over 80T.

The CuNb wires are composed of a conducting multiscale Cu matrix embedding reinforcing Nb nanofilaments. The complex multiscale microstructure is shown in figure 1. More information about the synthesis and microstructure can be found in reference 1. To evaluate the co-deformation behavior allowing optimization of strength and ductility, in-situ neutron diffraction experiments were performed on bulk Cu/Nb wires with diameters between 0.5mm and 1.5mm.

Experimental

Figure 2. Neutron collection during stress relaxation.

In situ tensile tests are performed in two configurations, neutrons scattering at crystallographic planes parallel and perpendicular to the tensile axis, giving, respectively, transverse and axial strains in the Cu and Nb components (click here for a picture of both setups - 122kB).

The neutron measurements were performed at several stress levels. During neutron collection the tensile deformation was interrupted. During this time the stress level is not constant. Therefore neutron collection is only started after the stress approaches its asymptotic level, as shown schematically in figure 2.

Results

Figure 3. Typical diffraction spectrum of nanocomposite CuNb.

Figure 3 displays a typical diffraction spectrum obtained for wires positioned in vertical position (scattering vector perpendicular to the wires axis). In this direction the Cu (220) diffraction peak is very intense which is due to the strong (111) texture for the Cu grains along the wires axis. The Nb diffraction peaks are relatively broad mainly because of the small size of the Nb grains.


The (220) Cu diffraction peak exhibits a clear peak asymmetry and therefore could not be fitted using a single gaussian peak profile. Furthermore during loading the peak asymmetry changes from the high d-side at low stress to the low d-side at higher stresses. This can be understood considering 1) that the size of the Cu grains varies between 400nm for the outer Cu layers down to 50nm for the inner Cu channels (see reference 1 for more details) and 2) that the elastic-plastic response is expected to be different for the different Cu channels.


The Cu (220) diffraction peak could satisfactory be fitted using two gaussian peak profiles with their maxima at different lattice spacing. In other words, the (220) Cu diffraction peaks can be considered as being a superposition from two peaks: the first, from the fine-Cu channels with an initially high axial internal compressive stress, the second, coming from the large-Cu channels, with a lower axial internal compressive stress (see reference 2 for more details). A similar procedure was followed for the Cu (111) diffraction peak in axial direction (i.e. when the wires was placed horizontally such that the scattering vector is lying along the wire axis). The intensity of the other Cu diffraction peaks was too low to perform such a deconvolution so for those diffraction peaks we only obtain an 'average' value. All Nb diffraction peaks could be satisfactory fitted using a single gaussian profile.


Figure 4. Elastic strain as function of applied stress.

Figure 5. Elastic strain as function of applied stress.

Figures 4 and 5 display the elastic strain for the different families as function of applied stress. The transverse strain measurements shown in figure 4 indicate that plasticity starts in the large Cu channels at a stress around 400 MPa (the 111, 200, 311, and 220 large curves), while the fine Cu channels yield around 900 MPa (the 220 fine curve). This behavior is confirmed when inspecting the axial strains. The slope of the Cu (111) curve decrease above 400 MPa indicating a load transfer from the large Cu channels to the fine Cu channels, resulting in a faster increase of the elastic strain with increasing applied stress.

For the Nb phase (figure. 5), the elastic strain starts to increase faster above an applied stress of 400 MPa without leveling off. Such a behavior evidences load transfer from the plastifying Cu matrix into the Nb filaments, the latter remaining in the elastic regime up to sample fracture. This result confirms the whiskerlike behavior of Nb nanofilaments that was observed during in-situ TEM studies. The load transfer from Cu to Nb is also the footprint of the impenetrable character of Cu/Nb interfaces.

References

  1. L. Thilly et al. Philosophical Magazine A 82 (2002) 925
  2. V. Vidal et al. Applied Physics Letters 88 (2006) 191906
  3. L. Thilly et al. Applied Physics Letters 90 (2007) 241907