The Flow-Stress Asymmetry of Ultra-Pure Molybdenum Single Crystals
10.2320/matertrans1989.41.141
Crossref journal-article
Japan Institute of Metals
Materials Transactions, JIM (1149)
Bibliography

Seeger, A., & Hollang, L. (2000). The Flow-Stress Asymmetry of Ultra-Pure Molybdenum Single Crystals. Materials Transactions, JIM, 41(1), 141–151.

Authors 2
  1. A. Seeger (first)
  2. L. Hollang (additional)
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  24. 24) The problem is aggravated if the comparison between tension and compression tests is based not on the critical shear stress but on the stress&ndash;strain curves since the stress axis changes its crystallographic orientation differently in tension tests on rod-shape specimens and in compression tests on disks. The participation of more than one slip system, which for the bcc structure is the rule rather than the exception, brings in uncertainties in the calculation of the Schmid factor &mu; as function of the strain (cf. Sect. 2).
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  28. 28) Crystal orientations with &chi;&lt;0 are less suited for testing the twinning&ndash;anti-twinning hypothesis, since at large negative &chi; the &lang;111&rang;{112} slip system with the largest Schmid factor is not [111](\bar211) but [\bar111](1\bar12), i.e., a system whose slip direction differs from that of [111](\bar101), the {110} system with the largest Schmid factor in the entire interior of the basic stereographic triangle. This causes uncertainties in the interpretation. After strong pre-deformation (as in the present experiments) the parent {110} configuration from which at large negative &chi; the {112} screw-dislocation configuration with the largest Schmid factor is obtained constitutes only a small minority in the dislocation population. This is to be contrasted with the situation at small plastic deformations, where the populations of {112} screw dislocations with <B>b</B>=[111]<I>a</I><SUB>0</SUB>&frasl;2 or [\bar111]<I>a</I><SUB>0</SUB>&frasl;2 are likely to be comparable. Then the two populations impede their movements mutually due to the need to form jogs when the two different types of dislocations intersect. The subsequent jog dragging leads to the generation of vacancies and self-interstitials, a further impediment to slip. These flow-stress&ndash;increasing processes are avoided if both types of screw dislocation glide on their <I>common</I> slip plane, viz. [0\bar11]. Slip on this plane, which in the notation of Fig. 1 corresponds to &psi;=&minus;60&deg; and, therefore, to a rather small Schmid factor, has been dubbed &ldquo;anomalous slip&rdquo;. At low temperatures, where thermal activation is not too helpful in overcoming the energy barriers associated with the processes mentioned above, anomalous slip has indeed been observed on most high-purity refractory bcc metals, including Mo (see, e.g.,). Note that, according to what has been said above on {110} slip, anomalous slip should <I>not</I> give rise to flow-stress asymmetry. The connection between anomalous slip and the metastable {112} core configurations of the [111]<I>a</I><SUB>0</SUB>&frasl;2 screw dislocations in bcc metals will be discussed in more detail elsewhere (A. Seeger, to be published).
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  33. 33) There are indications that transition metals and their alloys with about 7,s/d electrons per atom form exceptions. These are attributed to their particular electronic structure which favours, e.g., in the case of Mn the rather complex &alpha;- and &beta;-Mn structure and in the case of alloys the &sigma;-phase over the bcc structure. If &gamma;<SUB>{112}</SUB> is small enough, &lang;111&rang;<I>a</I><SUB>0</SUB>&frasl;2 dislocations may split into two partial dislocations according to the dissociation reaction [[111]&frasl;2&rarr;[111]&frasl;3+[111]&frasl;6,] with a {112} stacking fault between them. The direction of the splitting is such that the [111]<I>a</I><SUB>0</SUB>&frasl;6 partial dislocation moves away from ist partner in the <I>twinning direction</I>. (Splitting in the opposite direction would give rise to a high-energy fault.) As discussed elsewhere, a criterion for the applicability of these considerations is that in the plastic deformation of a bcc metal or alloy the {112} planes play a similar r&ocirc;le as the {111} planes in the fcc metals and alloys. For the 50Fe&ndash;50Cr alloys, which do have 7,s/d electrons per atom, this criterion is satisfied.
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Dates
Type When
Created 11 years, 5 months ago (March 19, 2014, 11:16 p.m.)
Deposited 3 years, 4 months ago (March 30, 2022, 11:08 a.m.)
Indexed 1 month, 2 weeks ago (July 1, 2025, 9:07 a.m.)
Issued 25 years, 7 months ago (Jan. 1, 2000)
Published 25 years, 7 months ago (Jan. 1, 2000)
Published Print 25 years, 7 months ago (Jan. 1, 2000)
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@article{Seeger_2000, title={The Flow-Stress Asymmetry of Ultra-Pure Molybdenum Single Crystals}, volume={41}, ISSN={2432-471X}, url={http://dx.doi.org/10.2320/matertrans1989.41.141}, DOI={10.2320/matertrans1989.41.141}, number={1}, journal={Materials Transactions, JIM}, publisher={Japan Institute of Metals}, author={Seeger, A. and Hollang, L.}, year={2000}, pages={141–151} }