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