skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Shock-induced martensitic transformations in near-equiatomic NiTi alloys

Abstract

Shock-impact generated tensile-stress pulses were used to induce B2-to-monoclinic martensitic transformations in two near-equiatomic NiTi alloys having different martensite transformation start (M{sub s}) temperatures. The NiTi-I alloy (M{sub s} {approx} +27 C) impacted at room temperature at 2.0 and 2.7 GPa tensile stress-pulse magnitude, showed acicular martensite morphology. These martensite needles had a substructure containing microtwins, typical of stress-assisted martensite. The NiTi-II alloy (M{sub s} {approx} {minus}45 C) showed no martensite formation when shocked with tensile-stress pulses of 2 GPa. For tensile stresses of 4.1 GPa, the alloy showed spall initiation near the region of maximum tensile-stress duration. In addition, monoclinic martensite needles, with a well-defined dislocation substructure, typical of strain-induced martensite, were seen clustering around the spall region. No stress-assisted martensite was formed in this alloy due to its very low M{sub s} temperature. The present article documents results of the use of a metallurgical technique for generating large-amplitude tensile stress pulses of finite duration for studies of phase transformations involving changes from a high density to a low density state.

Authors:
 [1];  [2];  [3]
  1. Univ. of Cincinnati, OH (United States). Dept. of Materials Science and Engineering
  2. Georgia Inst. of Tech., Atlanta, GA (United States). School of Materials Science and Engineering
  3. Los Alamos National Lab, NM (United States). Center for Materials Science
Publication Date:
Sponsoring Org.:
National Science Foundation, Washington, DC (United States); USDOE, Washington, DC (United States)
OSTI Identifier:
522358
Resource Type:
Journal Article
Resource Relation:
Journal Name: Metallurgical and Materials Transactions. A, Physical Metallurgy and Materials Science; Journal Volume: 28; Journal Issue: 7; Other Information: PBD: Jul 1997
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; CRYSTAL-PHASE TRANSFORMATIONS; NICKEL ALLOYS; TITANIUM ALLOYS; MARTENSITE; SHOCK WAVES; MONOCLINIC LATTICES; MORPHOLOGY; STRESSES; DISLOCATIONS

Citation Formats

Thakur, A.M., Thadhani, N.N., and Schwarz, R.B. Shock-induced martensitic transformations in near-equiatomic NiTi alloys. United States: N. p., 1997. Web. doi:10.1007/s11661-997-0207-2.
Thakur, A.M., Thadhani, N.N., & Schwarz, R.B. Shock-induced martensitic transformations in near-equiatomic NiTi alloys. United States. doi:10.1007/s11661-997-0207-2.
Thakur, A.M., Thadhani, N.N., and Schwarz, R.B. 1997. "Shock-induced martensitic transformations in near-equiatomic NiTi alloys". United States. doi:10.1007/s11661-997-0207-2.
@article{osti_522358,
title = {Shock-induced martensitic transformations in near-equiatomic NiTi alloys},
author = {Thakur, A.M. and Thadhani, N.N. and Schwarz, R.B.},
abstractNote = {Shock-impact generated tensile-stress pulses were used to induce B2-to-monoclinic martensitic transformations in two near-equiatomic NiTi alloys having different martensite transformation start (M{sub s}) temperatures. The NiTi-I alloy (M{sub s} {approx} +27 C) impacted at room temperature at 2.0 and 2.7 GPa tensile stress-pulse magnitude, showed acicular martensite morphology. These martensite needles had a substructure containing microtwins, typical of stress-assisted martensite. The NiTi-II alloy (M{sub s} {approx} {minus}45 C) showed no martensite formation when shocked with tensile-stress pulses of 2 GPa. For tensile stresses of 4.1 GPa, the alloy showed spall initiation near the region of maximum tensile-stress duration. In addition, monoclinic martensite needles, with a well-defined dislocation substructure, typical of strain-induced martensite, were seen clustering around the spall region. No stress-assisted martensite was formed in this alloy due to its very low M{sub s} temperature. The present article documents results of the use of a metallurgical technique for generating large-amplitude tensile stress pulses of finite duration for studies of phase transformations involving changes from a high density to a low density state.},
doi = {10.1007/s11661-997-0207-2},
journal = {Metallurgical and Materials Transactions. A, Physical Metallurgy and Materials Science},
number = 7,
volume = 28,
place = {United States},
year = 1997,
month = 7
}
  • It was observed that the reverse transformation of stress-induced martensite occurred at a temperature some 20 K higher than that of thermal martensite. The increase in temperature for the reverse transformation was indicative of a stabilization effect. This stabilization effect was attributed to the change in the accommodation morphology of martensite variants from a self-accommodating state for the thermal martensite to an orientated state for the stress-induced martensite. This observation led to the reconsideration of the criteria for pseudoelasticity. Quantitative expressions of criteria on both testing temperature and austenite yield strength were derived which showed satisfactory agreement with experimental observations.
  • High-precision data on phase transformation temperatures in NiTi, including numerical expressions for the effect of Ni on M{sub S}, M{sub F}, A{sub S}, A{sub F} and T{sub 0}, are obtained, and the reasons for the large experimental scatter observed in previous studies are discussed. Clear experimental evidence is provided confirming the predictions of Tang et al. 1999 regarding deviations from a linear relation between the thermodynamic equilibrium temperature and Ni concentration. In addition to affecting the phase transition temperatures, increasing Ni contents are found to decrease the width of thermal hysteresis and the heat of transformation. These findings are rationalizedmore » on the basis of the crystallographic data of Prokoshkin et al. 2004 and the theory of Ball and James. The results show that it is important to document carefully the details of the arc-melting procedure used to make shape memory alloys and that, if the effects of processing are properly accounted for, precise values for the Ni concentration of the NiTi matrix can be obtained.« less
  • The transformation distortions during the ..beta.. ..-->.. ..beta..' and ..beta..' ..-->.. ..beta..'' transformations in near equiatomic Nb--Ru alloys produce phases that have progressively lower symmetry than the CsCl type ..beta.. phase and approach the packing of the hexagonal close packed structure. By the use of a simple approximation, the superposition approximation, a model is developed which explains all of the experimental measurements on this alloy system. The driving force for these transformations is electronic in nature. The Fermi energy ..mu.. of the ..beta.. phase alloys is near a sharp peak in the density of states rho(E). The transformations broaden thismore » peak, lowering rho(..mu..) and the energy of the phase. The significant experimental measurements accounted for by this model are the variation of magnetic susceptibility with phase and Ru content, the variation of transformation temperatures and transformation distortion with Ru composition, and distortion in the ..beta..'' phase with temperature and Ru composition.« less