根據相圖，多數(shu)合(he)金元(yuan)素(su)(su)在固(gu)(gu)相中(zhong)的(de)(de)溶(rong)(rong)解度要低于液(ye)相，因此在凝(ning)固(gu)(gu)過程(cheng)中(zhong)溶(rong)(rong)質(zhi)(zhi)(zhi)原子不(bu)斷被(bei)排出到(dao)液(ye)相，這種固(gu)(gu)液(ye)界面(mian)兩側溶(rong)(rong)質(zhi)(zhi)(zhi)濃度的(de)(de)差異導(dao)致合(he)金凝(ning)固(gu)(gu)后溶(rong)(rong)質(zhi)(zhi)(zhi)元(yuan)素(su)(su)成(cheng)分(fen)(fen)不(bu)均(jun)勻性，稱作(zuo)偏(pian)(pian)(pian)(pian)(pian)析(xi)(xi)。溶(rong)(rong)質(zhi)(zhi)(zhi)元(yuan)素(su)(su)分(fen)(fen)布不(bu)均(jun)勻性發生(sheng)在微(wei)觀(guan)(guan)結構形(xing)(xing)成(cheng)范(fan)圍(wei)內（有10~100μm的(de)(de)樹(shu)狀枝晶(jing)），此時為(wei)微(wei)觀(guan)(guan)偏(pian)(pian)(pian)(pian)(pian)析(xi)(xi)。溶(rong)(rong)質(zhi)(zhi)(zhi)元(yuan)素(su)(su)通過對(dui)(dui)流傳質(zhi)(zhi)(zhi)等(deng)(deng)質(zhi)(zhi)(zhi)量(liang)傳輸，將導(dao)致大(da)(da)范(fan)圍(wei)內成(cheng)分(fen)(fen)不(bu)均(jun)勻性，即(ji)形(xing)(xing)成(cheng)了宏(hong)觀(guan)(guan)偏(pian)(pian)(pian)(pian)(pian)析(xi)(xi)。宏(hong)觀(guan)(guan)偏(pian)(pian)(pian)(pian)(pian)析(xi)(xi)可以認(ren)為(wei)是由凝(ning)固(gu)(gu)過程(cheng)中(zhong)液(ye)體和(he)固(gu)(gu)體相對(dui)(dui)運動和(he)溶(rong)(rong)質(zhi)(zhi)(zhi)再(zai)分(fen)(fen)配(pei)過程(cheng)共同導(dao)致的(de)(de)。此外，在凝(ning)固(gu)(gu)早(zao)期所(suo)形(xing)(xing)成(cheng)的(de)(de)固(gu)(gu)體相或(huo)非金屬夾雜的(de)(de)漂浮(fu)和(he)下沉也會造成(cheng)宏(hong)觀(guan)(guan)偏(pian)(pian)(pian)(pian)(pian)析(xi)(xi)。一(yi)般認(ren)為(wei)在合(he)金鑄件或(huo)鑄錠內，從幾(ji)毫米到(dao)幾(ji)厘米甚(shen)至幾(ji)米范(fan)圍(wei)內濃度變化(hua)為(wei)宏(hong)觀(guan)(guan)偏(pian)(pian)(pian)(pian)(pian)析(xi)(xi)。因為(wei)溶(rong)(rong)質(zhi)(zhi)(zhi)在固(gu)(gu)態中(zhong)的(de)(de)擴散系數(shu)很(hen)低，而成(cheng)分(fen)(fen)不(bu)均(jun)勻性范(fan)圍(wei)又很(hen)大(da)(da)，所(suo)以在凝(ning)固(gu)(gu)完成(cheng)后，宏(hong)觀(guan)(guan)偏(pian)(pian)(pian)(pian)(pian)析(xi)(xi)很(hen)難(nan)通過加(jia)工處理(li)來消除(chu)，因此抑制宏(hong)觀(guan)(guan)偏(pian)(pian)(pian)(pian)(pian)析(xi)(xi)的(de)(de)產生(sheng)主要是對(dui)(dui)工藝參數(shu)進行(xing)優化(hua)，如控制合(he)金成(cheng)分(fen)(fen)、施加(jia)外力場（磁場等(deng)(deng)）、優化(hua)鑄錠幾(ji)何形(xing)(xing)狀、適當加(jia)大(da)(da)冷卻速率等(deng)(deng)。

  宏觀(guan)偏(pian)(pian)析(xi)(xi)(xi)(xi)是(shi)大范圍內的(de)(de)(de)(de)成(cheng)分不均勻(yun)現象，按(an)其表現形(xing)(xing)式可分為正(zheng)偏(pian)(pian)析(xi)(xi)(xi)(xi)、反(fan)偏(pian)(pian)析(xi)(xi)(xi)(xi)和(he)(he)(he)比(bi)重(zhong)(zhong)偏(pian)(pian)析(xi)(xi)(xi)(xi)等(deng)(deng)。①. 正(zheng)偏(pian)(pian)析(xi)(xi)(xi)(xi)：對平衡分配系(xi)數(shu)o<1的(de)(de)(de)(de)合金(jin)系(xi)鑄錠(ding)(ding)先凝(ning)(ning)(ning)固(gu)的(de)(de)(de)(de)部(bu)分，其溶(rong)(rong)(rong)(rong)質(zhi)(zhi)(zhi)含量低于(yu)(yu)后凝(ning)(ning)(ning)固(gu)的(de)(de)(de)(de)部(bu)分。對ko>1的(de)(de)(de)(de)合金(jin)系(xi)則正(zheng)好相(xiang)(xiang)反(fan)，其偏(pian)(pian)析(xi)(xi)(xi)(xi)程度(du)與凝(ning)(ning)(ning)固(gu)速率、液(ye)體(ti)對流(liu)以及(ji)溶(rong)(rong)(rong)(rong)質(zhi)(zhi)(zhi)擴(kuo)散(san)等(deng)(deng)條件有關(guan)。②. 反(fan)偏(pian)(pian)析(xi)(xi)(xi)(xi)：在ko<1的(de)(de)(de)(de)合金(jin)鑄錠(ding)(ding)中(zhong)，其外層(ceng)溶(rong)(rong)(rong)(rong)質(zhi)(zhi)(zhi)元素(su)高于(yu)(yu)內部(bu)，和(he)(he)(he)正(zheng)偏(pian)(pian)析(xi)(xi)(xi)(xi)相(xiang)(xiang)反(fan)，故稱為反(fan)偏(pian)(pian)析(xi)(xi)(xi)(xi)。③. 比(bi)重(zhong)(zhong)偏(pian)(pian)析(xi)(xi)(xi)(xi)：是(shi)由(you)合金(jin)凝(ning)(ning)(ning)固(gu)時形(xing)(xing)成(cheng)的(de)(de)(de)(de)初(chu)(chu)晶(jing)(jing)相(xiang)(xiang)和(he)(he)(he)溶(rong)(rong)(rong)(rong)液(ye)之(zhi)間的(de)(de)(de)(de)比(bi)重(zhong)(zhong)顯著差別引起的(de)(de)(de)(de)一(yi)種宏觀(guan)偏(pian)(pian)析(xi)(xi)(xi)(xi)，主要(yao)存在于(yu)(yu)共晶(jing)(jing)系(xi)和(he)(he)(he)偏(pian)(pian)晶(jing)(jing)系(xi)合金(jin)中(zhong)。如圖2-49所示，由(you)于(yu)(yu)溶(rong)(rong)(rong)(rong)質(zhi)(zhi)(zhi)元素(su)濃度(du)相(xiang)(xiang)對低的(de)(de)(de)(de)等(deng)(deng)軸(zhou)(zhou)(zhou)晶(jing)(jing)沉積導(dao)(dao)致(zhi)在鑄錠(ding)(ding)的(de)(de)(de)(de)底部(bu)出(chu)現負偏(pian)(pian)析(xi)(xi)(xi)(xi)；由(you)于(yu)(yu)浮力和(he)(he)(he)在凝(ning)(ning)(ning)固(gu)的(de)(de)(de)(de)最后階段收縮所引起的(de)(de)(de)(de)晶(jing)(jing)間流(liu)動(dong)，在頂(ding)部(bu)會出(chu)現很嚴重(zhong)(zhong)的(de)(de)(de)(de)正(zheng)偏(pian)(pian)析(xi)(xi)(xi)(xi)（頂(ding)部(bu)偏(pian)(pian)析(xi)(xi)(xi)(xi)）。A型(xing)偏(pian)(pian)析(xi)(xi)(xi)(xi)是(shi)溶(rong)(rong)(rong)(rong)質(zhi)(zhi)(zhi)富(fu)集的(de)(de)(de)(de)等(deng)(deng)軸(zhou)(zhou)(zhou)晶(jing)(jing)帶(dai)，由(you)溶(rong)(rong)(rong)(rong)質(zhi)(zhi)(zhi)受浮力作用流(liu)動(dong)穿過柱(zhu)狀晶(jing)(jing)區，其方(fang)向與等(deng)(deng)溫線移(yi)動(dong)速度(du)方(fang)向一(yi)致(zhi)但速率更快所導(dao)(dao)致(zhi)。A型(xing)偏(pian)(pian)析(xi)(xi)(xi)(xi)形(xing)(xing)狀與流(liu)動(dong)類型(xing)有關(guan)。V型(xing)偏(pian)(pian)析(xi)(xi)(xi)(xi)位(wei)于(yu)(yu)鑄錠(ding)(ding)中(zhong)心(xin)，源于(yu)(yu)中(zhong)心(xin)形(xing)(xing)成(cheng)等(deng)(deng)軸(zhou)(zhou)(zhou)晶(jing)(jing)區和(he)(he)(he)容易斷裂(lie)的(de)(de)(de)(de)連接疏松(song)的(de)(de)(de)(de)網狀物的(de)(de)(de)(de)形(xing)(xing)成(cheng)，之(zhi)后裂(lie)紋沿切(qie)應力面展(zhan)開為V型(xing)，并(bing)且(qie)充滿了(le)富(fu)集元素(su)的(de)(de)(de)(de)液(ye)相(xiang)(xiang)。而沿鑄錠(ding)(ding)側壁(bi)分布的(de)(de)(de)(de)帶(dai)狀偏(pian)(pian)析(xi)(xi)(xi)(xi)則是(shi)由(you)凝(ning)(ning)(ning)固(gu)過程初(chu)(chu)期的(de)(de)(de)(de)不穩定傳熱和(he)(he)(he)流(liu)動(dong)導(dao)(dao)致(zhi)的(de)(de)(de)(de)。

  對(dui)于(yu)宏(hong)觀(guan)(guan)偏(pian)析(xi)的(de)(de)(de)研究主要(yao)有(you)實驗(yan)檢(jian)(jian)測(ce)和模(mo)擬計(ji)算(suan)兩種手段。實驗(yan)檢(jian)(jian)測(ce)包括硫印檢(jian)(jian)驗(yan)法(fa)(fa)、原位分(fen)析(xi)法(fa)(fa)、火(huo)花放電原子發射光譜法(fa)(fa)、鉆(zhan)孔取(qu)樣法(fa)(fa)以及(ji)化(hua)學分(fen)析(xi)法(fa)(fa)等(deng)。模(mo)擬計(ji)算(suan)是通(tong)過(guo)(guo)數(shu)值求(qiu)解能量、動(dong)量以及(ji)溶質(zhi)傳(chuan)輸等(deng)數(shu)學模(mo)型，進而(er)探討元素成(cheng)分(fen)不均勻性的(de)(de)(de)方法(fa)(fa)；進入20世(shi)紀后，人們對(dui)凝固(gu)(gu)過(guo)(guo)程(cheng)中的(de)(de)(de)宏(hong)觀(guan)(guan)偏(pian)析(xi)現(xian)象進行了(le)大量系統的(de)(de)(de)研究。Flemings研究表明鑄錠(ding)中多種不同的(de)(de)(de)宏(hong)觀(guan)(guan)偏(pian)析(xi)都可由(you)凝固(gu)(gu)時的(de)(de)(de)傳(chuan)熱(re)、流動(dong)和傳(chuan)質(zhi)過(guo)(guo)程(cheng)來(lai)定量描述，從而(er)為宏(hong)觀(guan)(guan)偏(pian)析(xi)的(de)(de)(de)定量計(ji)算(suan)提(ti)供可能性，隨著計(ji)算(suan)機計(ji)算(suan)能力迅(xun)猛提(ti)升(sheng)，宏(hong)觀(guan)(guan)偏(pian)析(xi)的(de)(de)(de)模(mo)擬計(ji)算(suan)得到(dao)了(le)迅(xun)速(su)發展，主要(yao)分(fen)為多區域法(fa)(fa)和連(lian)續介質(zhi)法(fa)(fa)等(deng)。

  對于高氮不銹鋼，改善氮偏析以及消除氣孔等凝固缺陷，優化制備工藝制度，是高氮奧氏體不銹鋼制備技術中亟待解決的難題之一。氮作為重要合金元素之一，其偏析程度對材料強度、韌性、抗蠕變性、耐磨性和耐腐蝕等性能的均勻性至關重要，直接影響材料的服役壽命。與高氮不銹鋼中鉻、錳等其他元素相比，氮的分配系數較小，氮偏析嚴重，易形成氮氣泡，凝固末了殘留在鑄錠中形成氮氣孔等凝固缺陷，甚至導致材料直接報廢，因此氮偏析的控制對高氮不銹鋼制備而言至關重要。不同壓力和不同初始氮含量下21.5Cr5Mn1.5Ni0.25N含氮雙相鋼中氮偏析導致氮氣孔的形貌如圖2-50所示，其中D1、D3和D5分別在0.04MPa、0.1MPa和0.13MPa下完成凝固，不同氮質量分數的D2(0.25%N)、D3(0.26%N)和D4(0.29%N)均在0.1MPa下凝固。