冶金反应工程学中反应过程动力学的研究方法探究

冶金物理化学中反应动力学的方法已经成功地在冶金学(如黑色冶金、有色冶金等)中得到了应用,并成为传统的冶金学学科体系中一个重要的理论基础,其研究方法已经非常的完善,形成了独自的结构和体系.冶金反应工程学的重要任务之一是要对冶金工程进行模拟,要准确地得到反应过程的机理和不同控制环节转换时间点,因此,冶金物理化学中冶金反应动力学的研究方法和结果并不完全适合于冶金反应工程学.为给在冶金反应工程学中求解传输和化学反应方程的定解条件提供必要的传输与反应过程动力学参数和不同控制环节的转换时间点,提出了用分段尝试法研究冶金反应过程动力学的新思路,并对比了2种方法和得到的结果及对分段尝试法进一步的发展进行了讨论.分段尝试法作为冶金反应工程学中研究反应过程动力学的方法,既可满足冶金反应工程学的自身需要,又能为建立独立的冶金反应工程学学科体系提供必要的保障.     

Incomplete reaction phenomenon

Bainite forms by a displacive transformation mechanism and exhibits an incomplete reaction phenomenon. The kinetics and mechanism of bainite transformation have been studied in high- strength Fe-C-Si-Mn and Fe-C-Si-Ni steels using high-speed dilatometry. The new experiments reported here confirm the incomplete reaction phenomenon, with the bainite transformation stop- ping well before paraequilibrium is achieved. These results show that bainite probably grows without diffusion, but soon afterwards, excess carbon is partitioned into the residual austenite.     

The pearlite reaction

A critical appraisal of theory and experiments for both isothermal and forced velocity pearlite is presented. It is concluded for binary systems that both the theoretical models for volume diffusion and boundary diffusion control are well-advanced and adequate for the purposes of experimental test. However, some ambiguity remains in the boundary diffusion model with respect to the thermodynamics of the boundary ”phase” region, so it is still not possible to predict absolute rates of transformation. The theoretical problem for ternary pearlites is also well understood, although rigorous theory seems intractable. A new perturbation procedure for definition of the optimal steady-state spacing is presented and amplified for both isothermal and forced velocity pearlite, and for both volume and boundary diffusion models. In terms of the critical spacing S_c for isothermal pearlite and the spacing at minimum undercooling S_m for forced velocity pearlite the predicted stability points are as follows: {fx2777-1} For isothermal pearlite these perturbation results correspond closely to the state of maximum entropy production rate while for forced velocity pearlite the correspondence is also satisfactory. A detailed analysis of the data leads us to reaffirm the author’s conclusions that the eutectoid reactions in Cu-12 pct Al and some related ternary alloys reported by Asundi and West are controlled by volume diffusion and that the eutectoid reaction in Al-78 Zn reported by Cheetham and Ridley is controlled by boundary diffusion. We conclude further after careful analysis that the pearlite reaction in Fe-0.8 C is controlled for the higher temperatures by volume diffusion of carbon in austenite. We are also led to state that the pearlite transformations in Fe-C-Mn and Fe-C-Ni occur for the most part in a nopartition regime and are therefore controlled by volume diffusion of carbon in austenite, while the transformations in Fe-C-Cr and Fe-C-Mo, being forced by thermodynamics to sustain partition of chromium and molybdenum, are controlled by phase boundary diffusion of the latter elements. nt]mis|M. P. PULS, formerly Postdoctoral Fellow, Department of Metallurgy and Materials Science, McMaster University, Hamilton, Ontario, Canada This paper is based on a presentation made at a symposium on “The Cellular and the Pearlite Reactions,” held at the Detroit Meeting of The Metallurgical Society of AIME, October 20, 1971, under the sponsorship of the IMD Heat Treatment Committee.     

The peritectic reaction

Evidence supporting the occurrence of a peritectic reaction prior to the peritectic transformation is re-examined. It is concluded that the peritectic reaction cannot occur at the peritectic temperature for thermodynamic reasons and does not occur in most peritectic systems because crystallization of the peritectic phase occurs directly from the liquid. However, in peritectic systems where the liquidus lines of the pro-peritectic and peritectic phases co-incide a peritectic reaction can occur at temperatures below the peritectic temperature. Evidence is provided by microstructures produced by directional solidification of a CuSn alloy.     

Thymoma-associated cutaneous graft-versus-host-like reaction

In spite of the high incidence of pilonidal disease at young population and prolonged disabling period that it demands, colorectal surgeons have not reached consensus about etiology or best treatment for this condition. The authors intend to analyse results from 154 cases operated on at HCFMUSP through a retrospective chart analysis regarding patients' sex, age and race, operative technique, complications and recurrence. Half patients were male. Eighty-three percent were between 11 and 30 yr-old. Incision and curettage was offered to 74.7% of the patients. Excision technique was used in 25.3%. In patients who underwent excision technique, there were no recurrences and a cicatrization defect was diagnosed in 2 (5.1%) patients. For patients who underwent incision and curettage, recurrence was 3.5% and cicatrization defect was seen in 1 (0.9%) case. Excision or incision plus curettage techniques seem to be both effective for treatment of chronic pilonidal disease although the former may produce healing defect more commonly. Patients who present with complex lesions may benefit from excisional techniques associated to skin or myocutaneous flaps in a first attempt instead of conservative approaches.     

Response force and reaction time in a simple reaction task under time pressure

The aim of this study was to examine the effect of effort on timing and force of simple reactions. To do this, we performed an experiment, arranged like a video-game, in which these variables were measured under different time-pressure conditions. In accordance with our expectations reaction time was shorter and force amplitude larger when the time for responses was limited. These findings are discussed within the framework of Sanders' (1983) model of stress.     

On the relation of mean reaction time and intraindividual reaction time variability.

Researchers often statistically control for means when examining individual or age-associated differences in variances, assuming that the relation between the 2 is linear and invariant within and across individuals and age groups. We tested this assumption in the domain of working memory by applying variance-heterogeneity multilevel models to reaction times in the n-back task. Data are from the COGITO study, which comprises 101 younger and 103 older adults assessed in over 100 daily sessions. We found that relations between means and variances vary reliably across age groups and individuals, thereby contradicting the invariant linearity assumption. We argue that statistical control approaches need to be replaced by theoretical models that simultaneously estimate central tendency and dispersion of latencies and accuracies and illustrate this claim by applying the diffusion model to the same data. Finally, we note that differences in reliability between estimates for means and variances need to be considered when comparing their unique contributions to developmental outcomes. (PsycINFO Database Record (c) 2010 APA, all rights reserved)