Key role of transformation twins in comparison of results of crystal geometric and dynamic analysis for thin-plate martensite

Abstract

821 INTRODUCTIONThe crystal geometric approach to describing the macroscopic morphological features of martensite crystals (habit plane, macroshift, orientation relation ships (OR)) was proposed in [1]. The fundamental postulate of the approach declares the habit plane to be the macroscopically invariant one. The most impressive success of this approach was the prediction of the need for crystal twinning and the calculation of the fraction ratio β tw of transformation twins in crys tals with habits {3 10 15} γ , which were observed for the first time in [2] in the alloy Fe-22Ni-0.8C dur ing γ-α martensite transformation (MT). It must be emphasized that, according to [1], a particular habit plane is associated with a well defined (fixed) value of β tw . However, it was shown in experiments [3] that the β tw value varies not only during the transition from crystal to crystal, but also within the same crystal. Actually, this fact is well known by metal physicists and clearly seen in the images of the observed twinned structures of thin plate like crystals, examples of which taken from [3] and [4] are shown in Figs. 1 and 2. Figures 1 and 2 show that, although there is a ten dency toward regularity, the twin structure is not strictly regular, but this fact does not exclude its exist ence as an ideal limiting case.It should be noted that the authors of [3] did not restrict themselves to the fact that β tw varies but also undertook measurements, which evinced that the vol ume fraction of twin component δ tw showed variation between 0.37 < δ tw < 0.46. Since the expected value of δ tw based on [1] Abstract-The experimental data on the ratio of transformation twins fractions for crystals of thin plate α martensite (with the habit planes of the {2 5 9} γ -{3 10 15} γ type) are considered. It is shown that the dynamic theory of the martensite crystals formation is free from the difficulties encountered in the formal crystal geometric approach. The intervals of parameter values that correspond to the observed fractions of the twin component, δ tw , within the same crystal are given. The absence of twinning in thin plate like crystals, which sometimes are observed in experiments on the γ-α martensite transformation induced by strong mag netic field is discussed.

Shifting the Shear Paradigm in the Crystallographic Models of Displacive Transformations in Metals and Alloys

“…Unfortunately, there are few works in metallurgy based on this physical idea. A Russian team lead by Kashchenko and Chashchina [96] are developing the concept of dynamics and wave propagation of martensite in steel, but the physical details (l-waves, s-waves) are not easy to understand for a non-physicist, and would need experimental confirmations.…”

The formation of martensite crystals with a degenerate structure of transformation twins

“…ε 1B = ε * 1B → t γ-α = 1. Similarly, for the case of bcc-fct transformation instead of (15) we find expression for t reciprocal to ratio (17):…”

The rules for selection of dislocation nucleation centers of martensite in the dynamic theory of reconstructive martensitic transformations

“…Высокая скорость образования кристаллов мартенсита указывает на существование волнового механизма управления ростом. К хорошему соответствию с данными эксперимента приводит двухволновая модель управления [1][2][3][4]6], в которой волновые нормали n 1,2 квазипродольных волн, несущих пороговую деформацию типа растяжение-сжатие в ортогональных направлениях, выбираются близкими к ориентациям собственных векторов ξ 1 и ξ 2 тензора упругой деформации в локальной области с дисторсией (1). Поэтому в качестве дополнительного критерия выступает требование близости вектора N w (нормали к габитусу мартенситного кристалла в волновой модели) к экспериментально наблюдаемой нормали N Э к габитусной плоскости, где N w выбирается из бинарного спектра:…”

“…напр. [1][2][3][4]). В рамках этой парадигмы вместо традиционного представления о квазиравновесных зародышах (приемлемого для медленно протекающих превращений) вводится понятие начального возбужденного состояния (НВС), возникающего в упругих полях дислокационных центров зарождения (ДЦЗ).…”