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## PowerPoint Slideshow about 'a = lattice parameter, h = g -channel width, 70 nm' - elijah

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a = lattice parameter,

h = g-channel width, 70 nm

Mechanisms and Modeling of High-Temperature Anisotropic Deformation of Single Crystal Superalloys Bhaskar S. Majumdar, New Mexico Institute of Mining and Technology, DMR 0413852

Objective: Understand high temperature deformation of single crystal (SX) and directionally solidified (DS) Ni-base superalloys.

Approach: Conduct in situ mechanical tests on a neutron diffractometer to probe elastic strains in the coherent g and g' phases. Complement the elastic strain measurements with TEM analysis and modeling.

Highlight: Here we show results from a 900° C tension test on a DS PWA1422 alloy, where the elastic strain in the g-phase saturates before general yielding of the alloy. We show that the onset of non-linearity in the g-phase occurs because the applied stress is now able to bow dislocations through the vertical channel (V), which have low resolved shear stress (RSS). The CRSS of g from measured strains is obtained as 143 MPa, which is about 4 times larger than the bulk g-phase. The CRSS is consistent with Orowan bowing, illustrated in the sketch. We have measured misfit, and suggest that creep in these superalloys cannot occur if the RSS in the horizontal channel (H) is less than the CRSS. Misfit can be measured by other techniques. Our prediction regarding onset of creep and rafting is in general agreement with published data, and has important application in creep and design of superalloys. Other work in this program include creep and modeling. Recent submissions: two Acta papers.

Texture Memory in NiTi Shape Memory AlloyBhaskar S. Majumdar, New Mexico Institute of Mining and Technology, DMR 0413852

Although this research is not strictly part of the superalloy research, we acknowledge the above NSF funding for this work, which seemed extremely exciting when we first looked at the problem. We anticipate that this will lead to both additional applications of shape memory alloys, as well improved fundamental understanding about martensitic transformations of shape memory alloys in general.

Problem Statement: NiTi alloys are known for their shape memory effect, which enables the alloy to be permanently (plastically) deformed in the martensite state and fully recover the original shape on subsequent heating to austenite. Why then do we observe near-reversible strain hysteresis during thermal cycling of a nominally equiatomic NiTi alloy at a stress that is below the yield strength of the stable martensite? This occurs even in the first cycle, indicating that it is not a training driven strain hysteresis.

Approach: Conduct thermal cycling at stresses below the yield stress of the martensite, and measure texture at different points of the loading cycle. Tests conducted at the Los Alamos neutron diffraction facility, both SMARTS (Inverse Pole Figures, IPF), and HIPPO (Pole Figure, PF)).

Highlight:

Predicted pole-strain map, in excellent agreement with IPF as well as PF

Data at 150 MPa.

Strain range proportional to load below YS of martensite.

Texture memory demonstrated through the above IPFs. Focus on the martensite phase.

Paper just published online, Bing Ye et al, APL, 91, 061918 (2007)

Broader Impact Bhaskar S. Majumdar, New Mexico Institute of Mining and Technology, DMR 0413852

- Students in my group have learnt techniques of Rietveld refinement and use of the GSAS code. These are extremely useful tools for analysis of neutron and XRD spectrum, and we use them routinely now to determine elastic strains, volume fraction of phases, estimates of grain size, and texture analysis.
- Students have been working on developing crystal plasticity code and incorporating into Abaqus FEM. This greatly enhances the modeling capabilities of students studying Materials Science.
- At New Mexico Tech, we have been part of an EPSCoR program aimed at improving nanoscience infrastructure in NM. As part of this program, we have been developing two-week classes for high school teachers, and also organizing workshops. These high school teachers get 2 credits for the class work, 1 credit for the workshop, and 1 credit for developing lesson plans, towards their Master of Science Teachers (MST) degree at NMT. I have developed these courses, and routinely guide teachers through their lesson plans as well as directed-study in nanomaterials.

NM High School Teachers at a Workshop directed by Majumdar at NMT

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